Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to Hilcorp Alaska, LLC Oil and Gas Activities in Cook Inlet, Alaska

Federal RegisterJul 24, 2025

Ask Donna

What actually matters in this document.

Text

DEPARTMENT OF COMMERCE

National Oceanic and Atmospheric Administration

50 CFR Part 217

[Docket No. 250721-0127]

RIN 0648-BN57

Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to Hilcorp Alaska, LLC Oil and Gas Activities in Cook Inlet, Alaska

AGENCY:

National Marine Fisheries Service (NMFS), National Oceanic and Atmospheric Administration (NOAA), Commerce.

ACTION:

Proposed rule; request for comments.

SUMMARY:

NMFS has received a request from Hilcorp Alaska, LLC (Hilcorp) for regulations allowing for the take of marine mammals incidental to activities conducted in support of oil and gas exploration, development, production, and decommissioning in Cook Inlet, Alaska, over the course of 5 years (2025-2030). As required by the Marine Mammal Protection Act (MMPA), NMFS is proposing regulations to govern the requested take, and requests comments on the proposed regulations. NMFS will consider public comments prior to making any final decision on the requested MMPA regulations. Agency responses to received comments will be summarized in the final rule, if issued.

DATES:

Comments and information must be received no later than August 25, 2025.

ADDRESSES:

A plain language summary of this proposed rule is available at

https://www.regulations.gov/docket/NOAA-NMFS-2025-0052.

Submit all electronic public comments via the Federal e-Rulemaking Portal. Go to

https://www.regulations.gov

and type NOAA-NMFS-2025-0052 in the Search box (note: copying and pasting the FDMS Docket Number directly from this document may not yield search results). Click on the “Comment” icon, complete the required fields, and enter or attach your comments. Comments sent by any other method, to any other address or individual, or received after the end of the comment period, may not be considered by NMFS. All comments received are a part of the public record and will generally be posted for public viewing on

https://www.regulations.gov

without change. All personal identifying information (

e.g.,

name, address, etc.), confidential business information, or otherwise sensitive information submitted voluntarily by the sender will be publicly accessible. NMFS will accept anonymous comments (enter “N/A” in the required fields if you wish to remain anonymous).

Electronic copies of the application and supporting documents, as well as a list of the references cited in this document, may be obtained online at:

https://www.fisheries.noaa.gov/national/marine-mammal-protection/incidental-take-authorizations-oil-and-gas.

In case of problems accessing these documents, please call the contact listed below.

FOR FURTHER INFORMATION CONTACT:

Jaclyn Daly, Office of Protected Resources, NMFS, (301) 427-8401.

SUPPLEMENTARY INFORMATION:

Background

The MMPA prohibits the “take” of marine mammals, with certain exceptions. Sections 101(a)(5)(A) and (D) of the MMPA (16 U.S.C. 1361

et seq.

) direct the Secretary of Commerce (as delegated to NMFS) to allow, upon request, the incidental, but not intentional, taking of small numbers of marine mammals by U.S. citizens who engage in a specified activity (other than commercial fishing) within a specified geographical region if certain findings are made and either regulations are promulgated and a Letter of Authorization (LOA) is issued or an incidental harassment authorization (IHA) is issued.

Authorization for incidental takings shall be granted if NMFS finds that the taking will have a negligible impact on the species or stock(s) and will not have an unmitigable adverse impact on the availability of the species or stock(s) for taking for subsistence uses (where relevant). If such findings are made, NMFS must prescribe the permissible methods of taking and other “means of effecting the least practicable adverse impact” on the affected species or stocks and their habitat, paying particular attention to rookeries, mating grounds, and areas of similar significance, and on the availability of the species or stocks for taking for certain subsistence uses (referred to in shorthand as “mitigation”); and set forth requirements pertaining to the monitoring and reporting of the takings. The definitions of applicable MMPA statutory terms are provided directly below or included in the relevant sections of this proposed rule.

•

U.S. citizen

—individual U.S. citizens or any corporation or similar entity if it is organized under the laws of the United States or any governmental unit defined in 16 U.S.C. 1362(13); 50 CFR 216.103);

•

Take

—to harass, hunt, capture, or kill, or attempt to harass, hunt, capture, or kill any marine mammal (16 U.S.C. 1362(13);

•

Incidental harassment, incidental taking, and incidental, but not intentional, taking

—an accidental taking. This does not mean that the taking is unexpected, but rather it includes those takings that are infrequent, unavoidable or accidental (50 CFR 216.103);

•

Level A harassment

—any act of pursuit, torment, or annoyance which has the potential to injure a marine mammal or marine mammal stock in the wild (16 U.S.C. 1362(18); 50 CFR 216.3); and

•

Level B harassment

—any act of pursuit, torment, or annoyance which has the potential to disturb a marine mammal or marine mammal stock in the wild by causing disruption of behavioral patterns, including, but not limited to, migration, breathing, nursing, breeding, feeding, or sheltering (16 U.S.C. 1362(18); 50 CFR 216.3).

Purpose of Regulatory Action

NMFS received an application from Hilcorp requesting 5-year regulations and a letter of authorization (LOA) that would authorize the take of 12 marine mammal species, comprising 15 stocks, by Level B harassment, and take by Level A harassment of 9 of those 12 species, comprising 12 stocks, incidental to activities conducted by Hilcorp in support of oil and gas exploration, development, production, and decommissioning. No serious injury or mortality is anticipated or proposed for authorization.

The proposed regulations would provide a framework for authorizing the take of marine mammals incidental to specified activities associated with Hilcorp's oil and gas exploration, development, production, and decommissioning activities in Cook Inlet, Alaska.

Legal Authority for the Proposed Action

Section 101(a)(5)(A) of the MMPA (16 U.S.C. 1371(a)(5)(A)) directs the Secretary of Commerce to allow, upon request, the incidental, but not intentional taking of small numbers of marine mammals by U.S. citizens who engage in a specified activity (other than commercial fishing) within a specified geographical region for up to 5 years if, after notice and public comment, the agency makes certain findings and promulgates regulations that set forth permissible methods of taking pursuant to that activity and other means of effecting the “least practicable adverse impact” on the affected species or

stocks and their habitat (see the discussion below in the Proposed Mitigation section), as well as monitoring and reporting requirements. Section 101(a)(5)(A) of the MMPA and the implementing regulations at 50 CFR part 216, subpart I provide the legal basis for issuing this proposed rule containing 5-year regulations and for any subsequent Letter(s) of Authorization (LOA).

Summary of Major Provisions Within the Proposed Rule

The major provisions of this proposed rule are:

• Allowing NMFS to authorize, through an LOA, the take of small numbers of marine mammals by Level A harassment and/or Level B harassment incidental to Hilcorp's specified activities (no mortality or serious injury of any marine mammal would be authorized);

• Avoiding activities that may result in take of Cook Inlet beluga whales (CIBWs) within 16 kilometers (km) (10 miles, mi) of the Mean Higher High Water (MHHW) line of the Susitna Delta (Beluga River to the Little Susitna River) between April 15 and November 15 to avoid and minimize impacts when CIBWs are more likely engaging in foraging behavior;

• Requiring NMFS-approved protected species observers (PSOs) and delaying commencement of or shutting down certain activities should a marine mammal be detected within identified clearance or shutdown zones to minimize the amount and severity of take;

• Requiring a soft start for impact pile driving to allow marine mammals the opportunity to leave the area prior to being exposed to higher noise levels; and

• Requiring submission of monitoring reports including, but not limited to, a summary of marine mammal species and behavioral observations, construction shutdowns or delays, and construction work completed.

National Environmental Policy Act

To comply with the National Environmental Policy Act of 1969 (NEPA; 42 U.S.C. 4321

et seq.

) and NOAA Administrative Order (NAO) 216-6A, NMFS must review our proposed action (

i.e.,

promulgation of regulations and subsequent issuance of an LOA thereunder) with respect to potential impacts on the human environment. Accordingly, NMFS has prepared a draft Environmental Assessment (EA) to evaluate the environmental impacts associated with the proposed issuance of the regulations and LOA. NMFS' draft EA is available at

https://www.fisheries.noaa.gov/national/marine-mammal-protection/incidental-take-authorizations-oil-and-gas.

NMFS will review all comments submitted in response to this notice prior to concluding our NEPA process or making a final decision on this request.

Summary of Request

On October 30, 2024, NMFS received an application from Hilcorp requesting authorization to take marine mammals incidental to oil and gas exploration, development, production, and decommissioning activities in Cook Inlet, Alaska. Specifically, Hilcorp plans to conduct necessary work, including use of tugs towing, holding, or positioning a jack-up rig, pile driving, and pipeline replacement/installation activities. NMFS requested additional information from Hilcorp regarding their request on November 19, 2024, which Hilcorp provided on January 2, 2025. A final request from NMFS for information was sent to Hilcorp on January 22, 2025. Hilcorp provided all necessary information on February 10, 2025, and NMFS deemed Hilcorp's application adequate and complete on February 18, 2025 (note that NMFS' Notice of Receipt of Hilcorp's application erroneously described this date as being February 10, 2025). On March 13, 2025, NMFS published a notice of receipt (NOR) of Hilcorp's adequate and complete application in the

Federal Register

(90 FR 11951), requesting comments and soliciting information related to Hilcorp's request during a 30-day public comment period. NMFS did not receive public comments. Subsequently, on March 14, 2025, Hilcorp submitted a revised application that corrected minor details but did not substantively modify the description of the specified activities or the type or amount of take requested incidental to those activities. This revised application is available at:

https://www.fisheries.noaa.gov/action/incidental-take-authorization-hilcorp-alaska-llcs-oil-and-gas-activities-cook-inlet-alaska.

The requested regulations, if promulgated, would be valid for 5 years, from approximately September 23, 2025, through September 22, 2030. The exposure of marine mammals occurring in the vicinity to underwater noise generated by the activities could result in incidental take of marine mammals by Level A and/or Level B harassment. Therefore, Hilcorp requests authorization to incidentally take marine mammals.

Hilcorp's Incidental Take Authorization (ITA) History

NMFS previously issued multiple ITAs to Hilcorp. Initially, NMFS issued 5-year Incidental Take Regulations (ITR) to Hilcorp for a suite of oil and gas activities in Cook Inlet, Alaska (84 FR 37442, July 31, 2019) and three 1-year Letters of Authorization (LOAs) under the ITR. The 2019 ITR allowed for the take of marine mammals incidental to two-dimensional (2D) and three-dimensional (3D) geophysical surveys, vibratory sheet pile driving, and drilling of exploratory wells.

On September 17, 2019, Cook Inletkeeper and the Center for Biological Diversity filed suit in Federal district court in Alaska challenging the 2019 ITR and LOAs and supporting documents (the EA and Endangered Species Act (ESA) Biological Opinion). In a decision issued on March 30, 2021, the court ruled largely in NMFS' favor, but found a lack of adequate support in NMFS' record for the agency's determination that tug towing of drill rigs in connection with production activity would not cause take of CIBWs and remanded the rulemaking back to NMFS for further analysis of tug use under the MMPA, ESA, and NEPA. Hilcorp notified NMFS that all activities covered by the 2019 ITR had already been completed or would not be completed in the remaining effective period of the ITR. As a result, the only remaining activity was the use of tugs towing a jack-up rig. NMFS considered the specific circumstances relating to Hilcorp's request for take authorization for tug towing of a jack-up rig, which had not been covered under the 2019 ITR, and issued two sequential Incidental Harassment Authorizations (IHAs) (87 FR 62364, October 14, 2022). In 2024, NMFS issued an additional IHA to Hilcorp, at its request and in consideration of the specific circumstances, for production drilling support in Cook Inlet, Alaska, which included the use of tugs towing, holding, and positioning a jack-up rig (89 FR 79529, September 30, 2024). Hilcorp complied with all the requirements (

e.g.,

mitigation, monitoring, and reporting) of the previous LOAs and IHAs, and information regarding their monitoring results may be found in the Potential Effects of Specified Activities on Marine Mammals and their Habitat section of this notice.

Description of Proposed Activity

Overview

Hilcorp plans to continue oil and gas exploration, development, production,

and decommissioning activities in Cook Inlet, Alaska, for the reasonably foreseeable future. Over the course of the five years considered here, this work includes up to 54 days of tugs towing, holding, or positioning a jack-up rig in support of production drilling at existing platforms in middle Cook Inlet and Trading Bay; up to 70 days of pile driving in support of production well development at the Tyonek Platform in middle Cook Inlet; up to 6 days of tugs towing, holding, or positioning a jack-up rig and up to 18 days of pile driving in support of exploration drilling at two locations in the Middle Ground Shoal Unit in middle Cook Inlet and one location between the Anna and Bruce platforms on the northern border of Trading Bay; and up to 22 days of pipeline replacement/installation, involving either pipe pulling or anchor handling or a combination of both, at up to two locations in middle Cook Inlet and/or Trading Bay. Hilcorp requests authorization of take by Level B harassment for 12 marine mammal species (including CIBWs (

Delphinapterus leucas

)), and additionally by Level A harassment for a subset of 9 of these species.

Dates and Duration

The specified activities analyzed in this proposed rule are anticipated to begin in September 2025 and extend through December 2029. However, the proposed rule and LOA, if issued, would be effective through September 2030 (5 years) to allow for any delays in project activities. Table 1 provides a summary of Hilcorp's anticipated timings and durations for their planned activities; however, the schedule may shift such that actual activities occur in different years than specified below.

Table 1—Summary of Hilcorp's Planned Activities

Project activity

Cook Inlet region

Seasonal

timing

Year(s) planned

Anticipated duration of sound-producing activity

Anticipated sound sources

Tugs under Load with a Jack-Up Rig in support of Production Drilling

Middle Cook Inlet

April—December

Years 1, 3, and 5 (2025, 2027, 2029)

12 days (2 days each: 1 mobilization, 4 location-location moves, 1 demobilization, up to 12 total pinning events)

3 to 4 tugs towing, holding, and positioning a jack-up rig.

Year 2 (2026)

10 days (2 days each: 1 mobilization, 3 location-location moves, 1 demobilization, up to 10 total pinning events)

Year 4 (2028)

8 days (2 days each: 1 mobilization, 2 location-location moves, 1 demobilization, up to 8 total pinning events)

Pile Driving in Support of Production Well Development at the Tyonek Platform

Middle Cook Inlet

Mid-November—Mid-April

Year 1-Year 5 (2025-2029)

14 days (7 days per pile (intermittent); 8 hour (hr) per day; 2 piles per year)

Impact pile driving.

Tugs under Load with a Jack-Up Rig and Pile Driving in Support of Exploratory Drilling

Trading Bay (between Anna and Bruce platforms)

April—December

Year 2 (2026)

2 days tugs under load with a jack-up rig (1 location-location move, up to 2 total pinning events); 6 days intermittent pile driving (1 well, 1 pile each well)

Impact pile driving, 3 to 4 tugs towing, holding, and positioning a jack-up rig.

Middle Cook Inlet (MGS Unit)

April—December

Year 4 (2028)

4 days tugs under load with a jack-up rig (2 location-location moves, up to 4 total pinning events); 12 days intermittent pile driving (2 wells, 1 pile each well)

Impact pile driving, 3 to 4 tugs towing, holding, and positioning a jack-up rig

Pipeline Replacement/Installation

Middle Cook Inlet/Trading Bay

April—November

Year 2 (2026)

Scenario 1: 11 days using lay barge methods

Scenario 2: 22 days using lay barge methods (11 days per project, 2 projects)

Scenario 1: Anchor handling.

Scenario 2: Anchor handling.

April—November

Year 4 (2028)

Scenario 1: 8 days using pipe pull methods

Scenario 2: no pipeline replacement/installation

Scenario 1: 2 tugs engaged in pipe pulling, bottom impact sounds of pipe connecting with seafloor.

Scenario 2: none.

The specific years activities are planned to occur may or may not coincide with the actual year of execution.

One exploratory well between Anna and Bruce is analyzed to occur in Year 2 and two exploratory wells in the Middle Ground Shoal Unit are analyzed to occur in Year 4; however, the exploratory wells may be developed in any separate years during the proposed authorization period.

Two pipeline scenarios are analyzed to occur: Scenario 1 comprises one project using lay barge methods in Year 2 and one project using pipe pull methods in Year 4; Scenario 2 comprises two projects using lay barge methods in Year 2 and no additional projects thereafter. A maximum of two pipeline projects will occur during the proposed authorization period. Pipeline projects may occur simultaneously in any one year or in separate years during the proposed authorization period. However, only lay barge methodology can be utilized in the same year (

i.e.,

Scenario 2).

Specified Geographical Region

Hilcorp's planned activities would occur in Cook Inlet, Alaska, which is the specified geographical region. Specifically, activities would occur in middle Cook Inlet and Trading Bay, Alaska (figure 1) from a point on the eastern shoreline approximately 12 km (7.5 mi) south of the East Foreland to a

point approximately 16 km (10 mi) south of Point Possession on the west side, to the northernmost production platform in middle Cook Inlet (Tyonek, located in the North Cook Inlet Unit) to a point that is 3.5 km (2.2 mi) north of the village of Tyonek near the mouth of the Chuitna River. From there the area extends south to a point along the western shoreline approximately 15 km (9.3 mi) south of the West Foreland, and across the inlet back to a point on the eastern shoreline approximately 12 km (7.5 mi) south of the East Foreland. The geographic area of all activity covers a total of approximately 1,865 square kilometers (km

) (460,850 acres) (within Cook Inlet in State of Alaska waters). For the purpose of this proposed rule, middle Cook Inlet refers to waters north of the East and West Forelands and south of Threemile River in the west and Point Possession in the east, and Trading Bay refers to waters from approximately Granite Point in the north to the West Foreland in the south. Upper Cook Inlet refers to waters north and east of Beluga River in the west and Point Possession in the east.

BILLING CODE 3510-22-P

EP24JY25.001

BILLING CODE 3510-22-C

Detailed Description of the Specified Activity

Hilcorp's ITR petition includes four stages of oil and gas activities: production, exploration, development, and decommissioning.

Production Drilling and Well Development

—Hilcorp routinely conducts production drilling activities at offshore platforms to meet production needs; all Hilcorp platforms have the potential for production drilling activity. Drilling activities are accomplished using conventional drilling equipment from a variety of rig configurations and occur through existing well slots or wellbores located in legs of the existing platforms. Hilcorp plans to conduct production drilling in middle Cook Inlet and Trading Bay during the open water season, which generally runs from April to November but may extend into December depending on ice conditions. Drilling activities would span up to 240 days (table 1), with tugboats towing, holding, or positioning a jack-up rig for a total of up to 54 days across the 5-year proposed authorization period (table 1). In addition to production drilling activities, 10 drilling conductor pipes (piles) would be driven into the sediment to support future well slots for production well development on the Tyonek Platform at an installation rate not to exceed two per year (table 1). Pile driving would occur intermittently between mid-November and mid-April. Hilcorp has requested, and NMFS proposes to authorize, take associated with tug use (to tow, hold, and position a jack-up rig) and pile driving at the Tyonek Platform to support production drilling and well development.

Tugs under load with a jack-up rig in support of production drilling.

Some platforms in Cook Inlet have permanent drilling rigs installed that operate using power provided by the platform power generation systems; other platforms do not have drill rigs or the platform drill rig is not sufficient for the work needing to be done, and the use of a mobile drill rig is required. Mobile offshore drill rigs may be powered by the platform power generation system (if compatible with the platform power generation system) or may self-generate power with the use of diesel-powered generators.

Hilcorp proposes to conduct production drilling using the Spartan 151 jack-up drill rig (or similar). A jack-up rig is a type of mobile offshore drill unit used in offshore oil and gas drilling activities. It is comprised of a buoyant mobile platform or hull with moveable legs that are adjusted to raise and lower the hull over the surface of the water. Spartan 151 is a 150 H class independent-leg, cantilevered jack-up drill rig with a drilling depth capability of 7,620 meters (m) (25,000 feet (ft)) that can operate in maximum water depths up to 46 m (151 ft). To maintain safety and work efficiency, the jack-up rig would be equipped with the following:

• Either a 5,000-, 10,000-, or 15,000-pound-per-square-inch (psi) blowout preventer (BOP) stack for drilling in higher pressure formations found at greater depths in Cook Inlet;

• Sufficient variable deck load to accommodate the increased drilling loads, tubular frame for deeper drilling;

• Reduced draft characteristics to enable the rig to easily access shallow water locations;

• Riser tensioning system to adequately deal with the extreme tides and currents in up to 46 m (151 ft) water depth;

• Steel hull designed according to United States Coast Guard (USCG) specifications (inspected by USCG prior to entering the water); and

• Ability to cantilever over existing platforms for working on development wells.

The jack-up rig would be stocked with most of the drilling supplies required to complete a full summer program each year, including both production and exploratory drilling. Deliveries of the remaining items, including crew transfers, would be performed by support vessels and helicopters. The majority of the oilfield support services contractors have offices, shops, and additional equipment located in Anchorage, Kenai, and Nikiski that would support their remote field operations. Tugs would be used to mobilize or move the jack-up rig and would be released once the rig is in place.

Jack-up rig equipment would use diesel fuel or electricity from generators. Personnel associated with fuel delivery, transfer, and handling would be knowledgeable of industry Best Management Practices (BMPs) related to fuel transfer and handling, drum labeling, secondary containment guidelines, and the use of liners/drip trays. The jack-up rig would take on a maximum fuel load prior to operations to reduce fuel transfers during production or exploratory drilling. T

The jack-up rig would have a diesel burn rate of approximately 9,464 liters (2,500 gallons) per day. The jack-up rig would need to be refueled on location one time per well via an International Organization for Standardization (ISO) tank or directly from a supply boat. Commercial tank farms in the Nikiski or Kenai area would supply fuel transported by workboats as needed. The Rig Barge Master would oversee re-fueling and fluid transfers between the rig and fuel workboats, and subsequent transfers between tanks on the rig.

Three ocean-going tugs would be used to safely pull the Spartan-151 (or similar jack-up rig) to drilling locations and to position the jack-up rig to appropriately secure it on the sea floor. The most common configuration while traveling with the jack-up rig during the proposed moves is two tugs positioned side by side (approximately 30 to 60 m apart [98 to 197 ft]), pulling from the front of the jack-up rig, and one tug approximately 200 m (656 ft) behind the front tugs positioned behind the jack-up rig, applying tension on the line as needed for steering and straightening. While positioning the jack-up rig on a platform, the tugs may be fanned out around the jack-up rig to provide the finer control of movement necessary to safely position the jack-up rig on the platform. A fourth tug would be available on standby in the event that mechanical issues occur with one of the tugs. Additionally, the fourth tug may be used minimally (for approximately 1 hr) to help with positioning the jack-up rig. The horsepower (hp) of each of the tugs may range between 4,000 and 8,000. Details of the proposed tugs, or similar, are provided in table 2.

Table 2—Description of Tugs (or Similar) Planned for Use for Towing, Holding, and Positioning the Jack-Up Rig

Vessel

Activity

Length

(m)

Width

(m)

Gross

tonnage

Bering Wind (or similar)

Towing, holding, and positioning the jack-up rig

144

Stellar Wind (or similar)

Towing, holding, and positioning the jack-up rig

160

Glacial Wind (or similar)

Towing, holding, and positioning the jack-up rig

196

Dr. Hank Kaplan (or similar)

Standby tug used only for positioning the jack-up rig, if needed

196

This is not intended to be a specific list of tugs. Rather, tugs would be the same or similar such that potential effects of their use would be equivalent to what is analyzed herein.

The jack-up rig would be moved in a manner to minimize any potential safety risks as well as cultural or environmental impacts. While under tow to a well site, rig operations would be monitored by Hilcorp and the drilling contractor. Very high frequency (VHF) radio, satellite, and cellular phone communication systems would also be used while the jack-up rig is under tow. Helicopter transport would be available to move personnel to and from the rig and platforms.

The amount of time that tugs would be under load transiting and holding or positioning the jack-up rig in Cook Inlet would be tide-dependent. The amount of operational effort (

i.e.,

power output) the tugs use for transiting would depend on whether the tugs are towing with or against the tide and could vary across a tide cycle as the current increases or decreases in speed over time. Hilcorp would make every effort to maximize transit with the tide (which would require lower power output) and minimize transit against the tide (which would require higher power output). See the Estimated Take of Marine Mammals section of this proposed notice of issuance for more detail on assumptions related to power output.

To mobilize the jack-up rig each year, a high slack tide is necessary for the tugs to approach close enough to shore to attach and pull the jack-up rig off Rig Tenders Dock. The same conditions would be required for demobilization when the jack-up rig is returned to Rig Tenders Dock at the end of the open water season and to position the jack-up rig on existing platforms or well sites. The relatively slow current and calm conditions at a slack tide would enable the tugs to perform the fine movements necessary to safely position the jack-up rig within several feet of the platform. Additionally, positioning and securing the jack-up rig at high slack tide rather than low slack tide would allow for the legs to be pinned down (jack the legs down onto the sea floor) at an adequate height to ensure that the hull of the jack-up rig remains above the water level of the subsequent incoming high tide. Because 12 hrs elapse between each high slack tide, tugs are generally under load for those 12 hrs, even if the towed distance is small, as high slack tides are preferred to both attach and detach the jack-up rig from the tugs. Once the tugs are on location with the jack-up rig at high slack tide (12 hours from the previous departure), there is a 1 to 2-hour window when the tide is slow enough (

e.g.,

1 to 2 knots [kt]) for the tugs to initiate positioning the jack-up rig and pin the legs to the seafloor on location. The tugs are estimated to be under load, generally at half-power conditions or less, for up to 14 hours from the time of departure through the initial positioning attempt of the jack-up rig. An additional fourth tug, may engage during positioning activities to assist with fine movements necessary to place the jack-up rig. The fourth tug would engage with the three tugs during a positioning attempt for up to approximately 1 hour at half power.

If the first positioning attempt takes longer than anticipated, the increasing current speed (approximately 3 to 4 kt) would prevent the tugs from safely positioning the jack-up rig on location. If the first positioning attempt is not successful, the jack-up rig would be pinned down at a nearby location and the tugs would be released from the jack-up rig and no longer be under load. The tugs would remain nearby, generally floating with the current. Approximately one hour before the next high slack tide, the tugs would re-attach to the jack-up rig and reattempt positioning over a period of 2 to 3 hours. Positioning activities would generally be performed at half power. If a second attempt is needed, the tugs would be under load holding or positioning the jack-up rig on a second day for up to 5 hours. Typically, the jack-up rig can be successfully positioned over the platform in one or two attempts.

During a location-to-location transport (

e.g.,

platform-to-platform), the tugs would transport the jack-up rig, traveling with the tide in nearly all circumstances except in situations that threaten the safety of humans and/or infrastructure integrity. In a north-to-south transit, the tugs would tow the jack-up rig with the outgoing tide and would typically arrive at their next location to position the jack-up rig on the low slack tide, requiring half power or a lower power output during the transport. In a south-to-north transit, Hilcorp would prefer to pull the jack-up rig from the platform on a low slack tide to begin transiting north following the incoming tide. This would maximize their control over the jack-up rig and would require half power or a lower power output. There may be a situation where the tugs pulling the jack-up rig begin transiting with the tide to their next location, miss the tide window to safely set the jack-up rig on the platform or pin it nearby, and so have to transport the jack-up rig against the tide to a safe harbor. Tugs may also need to transport the jack-up rig against the tide if large pieces of ice or extreme wind events threaten the stability of the jack-up rig on the platform. All tug towing, holding, or positioning would be done in a manner implementing best management practices to preserve water quality, and no work would occur around creek mouths or river systems where prey abundance could be affected.

Although the variability in power output from the tugs can range from an estimated 20 to 90 percent when they are under load with the jack-up rig, as described above, the majority of the hours (spent transiting, holding, and positioning) would occur at half power (

i.e.,

50 percent) or less. Scenarios in which power output may be greater than 50 percent could include small periods of time (

i.e.,

minutes during positioning to counteract the tide (up to 90 percent power output); Durham, pers. comm. 2022).

Production Well Development at Tyonek Platform.

Hilcorp plans to install ten 76.2-centimeter (cm; 30-inch [in]) diameter (or smaller) steel piles immediately adjacent to three of the four existing legs of the Tyonek Platform in middle Cook Inlet during the proposed 5-year authorization period. The piles would be delivered to the platform via a supply vessel from Nikiski and pile driving operations would be conducted using an existing crane on the Tyonek platform. Each pile would be arranged in a concentric configuration around the outside of legs 1, 2, and 3. Each leg would have up to

four piles with a maximum of 10 piles total between all three legs. Pile driving would be intermittent to weld additional pile sections onto the driven pile approximately every 12.2 m (40 ft). Once installation is complete, each pile would extend approximately 53 m (175 ft) or 91 m (300 ft) below the mudline. The piles would be driven to target depth using an APE 20-5 hydraulic impact hammer with a ram weight of 18,144 kilograms (kg; 40,000 pounds [lb]) or an APE D80-42 single acting diesel impact hammer with a ram weight of 18,144 kg (40,000 lb) or a similar impact hammer. Impact hammering is anticipated to occur intermittently over weeks for 8 hours per day for up to seven days per pile, and a total of up to 14 days per season. Pile driving at the Tyonek Platform would occur between mid-November and mid-April.

Exploration Drilling-

Hilcorp plans to drill one exploratory well between the Anna and Bruce platforms near the northern border of Trading Bay and two exploratory wells in the MGS Unit in middle Cook Inlet, based on mapping of subsurface structures from previously collected two-dimensional (2D) and three-dimensional (3D) seismic data and historical well information (table 1). For all three wells, drilling would begin after the jack-up rig has already mobilized to middle Cook Inlet and before it has demobilized back to Rig Tenders Dock. The exact start dates for drilling the wells are currently unknown and would be dependent upon availability of the jack-up rig. Hilcorp anticipates that each well would take approximately 40 to 60 days to drill and test with 2 days of tugs towing a jack-up rig, and 6 days of impact pile driving. After testing, the wells would undergo plug and abandonment (P&A) for the following 14 to 90 days.

Tugs under load with a jack-up rig in support of exploration drilling

—Tugging activity in support of exploratory drilling is the same as described above for production drilling. In Year 2, tugs would tow, hold, or position the jack-up rig for up to 2 days at one exploratory well site between the Anna and Bruce platforms. In Year 4, tugs would tow, hold, or position the jack-up rig for up to 4 days at two exploratory well sites within the MGS Unit.

Drilling Program and Well Operations

—A drive pipe is a relatively short, large-diameter pipe driven into the sediment prior to the drilling of oil wells. The drive pipe serves to support the initial sedimentary part of the well, preventing the loose surface layer from collapsing and obstructing the wellbore. Drive pipes (piles) for each well would be installed using impact pile driving techniques. At each well site, Hilcorp proposes to drive a 76.2-cm (30-in) diameter pile to approximately 120 m (394 ft) using an APE Model 15-4 hydraulic impact hammer (or similar) with a ram weight of 13,608 kg (30,001 lb). Pile driving would be discontinuous and average 0.3 m (1 ft) per min with a 1-hour break for cooling and maintenance after approximately every 40 min. For each well, the assumed maximum impact hammering in one 24-hour period is 12 hours and is anticipated to last up to 6 days at each well site, although actual hammering of the pile would occur intermittently over the whole period.

Once piles are installed and ready for drilling, smaller diameter conductor pipes would be inserted into the 76.2-cm (30-in) diameter piles to transport drill cuttings to the surface. These small diameter pipes would be drilled and not hammered, and the drilling sounds would not be in direct contact with the water column. As a result, no take is expected to result from this activity.

The drilling program for one exploratory well between the Anna and Bruce platforms and for two wells in the MGS Unit would be described in detail in the request for a permit to drill submitted to the Alaska Oil and Gas Conservation Commission (AOGCC). When planned and permitted operations are completed, the wells would be suspended according to State of Alaska regulations. All drilling waste, wastewater, recyclables, hazardous waste, and municipal solid waste would be stored, transported, and disposed of in accordance with local, state, and Federal regulations. Drilling waste from each well including drilling fluids, mud, and rock cuttings would be circulated from downhole to the jack-up mud pit system. Non-hydrocarbon-based drilling wastes would be discharged to Cook Inlet under an approved Alaska Pollutant Discharge Elimination System general permit or sent to an approved waste disposal facility. Hydrocarbon-based drilling wastes which would be delivered to an onshore permitted location for disposal. Hilcorp would follow BMPs and all stipulations of the applicable permits for this activity. More information on oil production can be found in Hilcorp's application.

Pipeline Installation and/or Replacement.

Hilcorp proposes to execute two pipeline replacement or installation projects in any year. The acoustic sources associated with pipeline replacement/installation activities for which Hilcorp has requested incidental take authorization include tugs engaged in anchor handling and/or pipe pulling activities (table 3). The project timelines are subject to weather conditions and equipment readiness. Each project's scope entails the installation or replacement of pipeline in either middle Cook Inlet or Trading Bay or a combination of both. The specific methodology of the pipeline replacement or installation is pending finalization, with both methods—pipe pulling and lay barge positioning—under consideration for implementation. Both methods include replacing or installing approximately 2,286 m (7,500 ft) of pipeline. Pipeline replacement and installation is driven by the need to mitigate corrosion, pipeline fatigue, and abrasion leaks, ensuring alignment with requirements of the Pipeline and Hazardous Materials Safety Administration. Installation of new gas pipelines would also address the growing consumer demand for natural gas in Southcentral Alaska by allowing larger quantities of natural gas to be extracted for use.

Table 3—Summary of Pipeline Activities, Purposes, Durations, and Anticipated Sound Sources

Activity

Purpose

Duration per project

Anticipated sound sources

Lay Barge Method

Anchor Setting

Set 8-point anchor system

2 days

2 AHTs*, 1 assist tug.

Pipelay

Lay out 2,286 (7,500 ft) of pipeline

8 days

2 AHTs.

Anchor Recovery

Recover 8 Anchors

1 day

2 AHTs.

Pipe Pull Method

Pipelay

Pull out 2,286 m (7,500 ft) of pipeline

8 days

1 installation tug, 1 assist tug, seafloor bottom impact sounds.

Note:

AHT is an acronym for anchor handling tugs.

Pipeline Replacement Activities Using Lay Barge Methodology.

Hilcorp is considering employing lay barge methods for pipeline replacement/installation. This approach would involve the use of anchor handling tugs (AHTs) and anchor systems to maintain the optimal stability and alignment of a specialized vessel, referred to as a lay barge, while laying pipeline on the seafloor. Additional pre-mobilization needs for replacement/installation activities using lay barge methods include procurement and transport to the worksite of project materials and vessels. All activities involving sound generation in the day-to-day activities, such as anchor handling and pipelaying, would follow uniform procedures for both pipeline replacement and installation as detailed in the subsequent sections.

Subsea Pipeline Operations.

Pipeline activities utilizing lay barge methods require support from two AHTs, a pipelay barge, and one assist tug (see table 4 for examples of anticipated vessel specifications). The pipelay barge would be towed by an AHT to the initial anchor setting location. To anchor the barge, eight anchors would be set, one at a time using one AHT during slack tide. During anchor setting, a tug would handle each anchor, one at a time. Setting each anchor during slack tide may require 1 hour each, intermittently, over a 2-day period (

i.e.,

4 hours per day for all eight anchors). During an incoming or outgoing tide, anchors would not be set, rather one AHT and one assist tug would hold the pipelay barge in a stationary position until the next slack tide (

i.e.,

4 to 5 hours).

Table 4—Example Types of Tugs and Barges Used in Lay Barge Operations

Vessel

Operational Use

Length

(m)

Beam

(m)

Barge

Ninilchik

Lay barge to be positioned and anchored by tugs using up to eight anchors and serve as an above-water work platform

N/A

Assist Tug

Bering Wind

Assist tug used to assist the AHTs in holding the pipelay barge in place against an incoming or outgoing tide during initial barge positioning

5,080

Dr. Hank Kaplan

5,380

Anchor Handling Tugs

Denise Foss

AHTs used to hold the barge in place during incoming or outgoing tides when anchor setting or retrieving; re-position anchors along the pipeline route; and operate in tandem during pipelay

7,268

Resolve Pioneer

5,750

This is not intended to be a specific list of tugs. Rather, tugs would be the same or similar such that potential effects of their use would be equivalent to what is analyzed herein.

Vessel length and beam width are rounded to the nearest whole number.

Tugs may range in power from 2,000 to 8,000 horsepower (hp).

Pipeline segments would be installed approximately every 305 m (1,000 ft) from the pipelay barge along the proposed routes. To lay the pipeline in place, the pipelay barge would be moved in a sequence along the pipeline route by moving the eight anchors one at a time to shift the position of the barge forward. To move anchors, two AHTs would operate one at a time in sequence and move a single anchor at a time (

i.e.,

a single tug would move an individual anchor).

Laying pipe from the lay barge would engage a U-shaped stinger roller assembly that would be affixed to the pipelay barge to guide the pipeline into the water. This assembly is specifically engineered to regulate the curvature of the pipeline during the laying process and safely lay pipe while preventing damage from excessive bending. On the deck of the pipelay barge, segments of pipeline would be inspected and hydrotested and coatings would be verified prior to installation in the water.

Anchor Setting.

To secure the pipelay barge, each of the eight anchors would be set one at a time using one tug (C. Burvee, Blackfin, Pers. comm., March 13, 2023). Setting one anchor would take approximately 1 hour during slack tide. There are four slack tides per day; therefore, four anchors would be set in 1 day. Setting all eight anchors is expected to take 2 days (

i.e.,

during each of the four slack tides per day). There are approximately 4 to 5 hours between slack tides (

i.e.,

between low tide and high tide).

During an incoming or outgoing tide, an assist tug would work simultaneously with an AHT to hold the pipelay barge in place against the tide. During this 4-to-5-hour period, the two tugs would average 50 percent power output (C. Burvee, Blackfin, Pers.

comm., March 13, 2023). During an incoming or outgoing tide, the other AHT would be idle. Therefore, during a 24-hour period of setting four anchors, a single AHT would operate at an average of 50 percent power for 4 hours (during each slack tide) to set anchors, followed by a 4-to-5-hour period when the assist tug and the second AHT would operate at an average of 50 percent power holding the barge between each slack tide. This pattern would continue until all eight anchors are set over 2 days. During anchor setting, only one tug would be anchor handling at a time, operating at 50 percent power. Once all eight anchors are set, the assist tug would depart the pipelay site, leaving only the two AHTs for pipelay. Setting an anchor requires the tug captain to aim for an X, Y coordinate on the seafloor. Due to the strong tides and currents in Cook Inlet and the need to aim for a specific location, setting anchors is more complex and requires more time than anchor retrieval.

Pipelaying.

Once all eight anchors are set, the barge would be moved approximately every 305 m (1,000 ft) along the pipeline route. Each time that the barge needs to be repositioned, a single tug would be used at half power (50 percent) for anchor handling. Each of the eight anchors would be repositioned in the new location, one anchor at a time. Two bow anchors would typically be repositioned first (one at a time), then each set of port and starboard anchors (

i.e.,

two on each side) would be repositioned one at a time, finishing with the two stern anchors one at a time to move the barge. The two tugs would work in sequence to relocate a single anchor at a time to “crawl” the barge into the new position approximately 305 m (1,000 ft) from the previous position. To execute this, the bow anchor cables of the pipelay barge would be tightened to slowly pull it to the new position as the stern anchors are slowly released. It is estimated it will take 8 days to complete the pipelaying portion of replacement/installation activity using the lay barge method.

Each anchor weighs 9,071 kg (20,000 lb) and has approximately 4.6 m (15 ft) of chain and 915 m (3,002 ft) of wire cable. All wire cables would be under tension when in the water. During pipelay, each anchor move would take about 15 minutes and would be an intermittent process as each anchor is moved individually.

Anchor Retrieval.

Anchor retrieval is only possible during slack tides. The process would involve pulling eight anchors one by one using a single tug from a fixed, stationary position. While one tug is engaged in retrieving an anchor, the second tug would remain idle. Between slack tides, when the tide is either incoming or outgoing, both AHTs would hold the barge in place for approximately 4 to 5 hours, operating at an average power output of 50 percent. The process of retrieving anchors would be swifter compared to the initial anchoring procedure due to the relative ease of lifting the anchor from its fixed position on the seafloor as detailed in the previous section. Within one slack tide period, two anchors could be successfully retrieved. Consequently, all eight anchors could be pulled up within a 24-hr span.

Vessels for Pipeline Replacement/Installation Activities.

Pipeline activities utilizing lay barge methods would require support from two AHTs, a pipelay barge, and one assist tug as shown in table 4. The two AHTs would be involved in replacement/installation operations, specifically during anchor handling. An assist tug from within Cook Inlet (

Bering Wind

Dr. Hank Kaplan,

or similar) would hold the barge between slack tides along with one of the AHTs. It is important to note their availability may not be guaranteed once project timelines are finalized. In such cases, a comparable vessel would be chosen for the intended activity.

Pipeline Replacement Activities Using Pipe Pull Methodology.

Hilcorp is also considering implementing a pipe pull method for pipeline replacement/installation during the proposed ITR period. For this approach, the pipeline would be assembled on land in 305-m (1000-ft) sections and subsequently towed to sea one section at a time along the seafloor. This process would be executed by an installation tug with assistance from an assist tug. A large tug will tow the project spools into position.

During the initial phases of pipe-pulling, a pull wire would be connected to the winch on the installation tug; the tug would then pull the pipe towards its stern while remaining on anchor. As the towing of the pull wire begins, buoyancy assemblies would be installed from shore along the pull wire to lift the wire out of the mud. This added buoyancy would act to reduce drag and would also reduce the pull force required by the installation tug. Onshore, the pull wire would be attached to the buoyed pull head of the first pipeline spool and the first pipeline segment would be pulled into the water during high tide.

After the installation tug has pulled the pipeline, placing the tailhead about 91 m (300 ft) offshore, it would stop pulling and continue to release wire as it moves to a specified location farther offshore. Once it reached this new location, the tug would resume pulling the pipeline segment using the attached pull wire and remove the buoyancy assemblies as the pull wire is reeled in. Next, the pipeline segment would be guided into position within a predetermined 3 m by 3 m (10 ft by 10 ft) target area near the platform; the installation vessel would then detach the pull wire and attach a buoy to a 45.7-m (150-ft) pennant secured at the front of the pipeline.

Following positioning of the first pipeline spool, the process would be repeated by pulling the second spool within a 3 m by 3 m (10 ft by 10 ft) target area at the tailhead of the first spool, and then pull the third spool within a 3 m by 3 m (10 ft by 10 ft) target area at the tailhead of the second spool and so on until all spools are laid out 2,286 m (7,500 ft). The assist tug would help keep the installation tug on the correct bearing throughout each pull and will assist in final positioning of the spools. The estimated duration to position all the spools is approximately 8 days, with one spool being pulled per day. The total anticipated tug operation time, operating at 50 percent to 85 percent power, is expected to be 3 hours per day.

A separate tugboat would help the installation tug maintain the planned route during tidal changes. A shallow-water support vessel would ferry the messenger rope from the installation tug to the beach and assist in any onshore to offshore operations. The messenger rope is a wire used to transfer the larger pull wire from/to the floating asset to/from the beach. This is a light, usually floating line that can be messengered by a small craft. Messenger wire would only be needed if the weight, due to length or required diameter, of the actual pulling wire would be unmanageable by a small craft. Divers would remove the 45.7-m (150-ft) pennant wire and buoy from the pull head, flood each pipeline segment, and assist with post tie-in operations. See table 5 for examples of vessel sizes and function details for this activity.

Table 5—Example Types of Tugs and Barges Used in Pipe Pull Operations

Vessel

Operational Use

Length

Beam

Horsepower

Installation Tug

Resolve Pioneer

Main tug for installation and is responsible for pulling spools into position

5,750

Assist Tug

Steller Wind

Assist tug supports the Resolve Pioneer, or similar, in maneuvering the spool of pipe to its designated seabed position, particularly when contending with tidal currents

3,500

This is not intended to be a specific list of tugs. Rather, tugs would be the same or similar such that potential effects of their use would be commensurate with what is analyzed herein.

Vessel length and beam width are rounded to the nearest whole number.

Tugs may range in power from 2,000 to 8,000 hp.

The risk of interaction or entanglement with gear or equipment during pipeline replacement/installation activities is avoided due to the small area occupied by the cables relative to the marine mammals' habitat in Cook Inlet, use of taut lines, and mitigation and monitoring measures described in the Proposed Mitigation and Proposed Monitoring and Reporting sections of this notice. Vessel strikes or other encounters are also not anticipated as a result of anchor handling activities. No other interactions are anticipated.

Proposed mitigation, monitoring, and reporting measures are described in detail later in this document (please see Proposed Mitigation and Proposed Monitoring and Reporting).

Description of Marine Mammals in the Area of Specified Activities

Sections 3 and 4 of the application summarize available information regarding status and trends, distribution and habitat preferences, and behavior and life history of the potentially affected species. NMFS fully considered all of this information, and we refer the reader to these descriptions, instead of reprinting the information. Additional information regarding population trends and threats may be found in NMFS' Stock Assessment Reports (SARs;

https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments

) and more general information about these species (

e.g.,

physical and behavioral descriptions) may be found on NMFS' website (

https://www.fisheries.noaa.gov/find-species

Table 6 lists all species or stocks for which take is expected and proposed to be authorized for this activity and summarizes information related to the population or stock, including regulatory status under the MMPA and ESA and potential biological removal (PBR), where known. PBR is defined by the MMPA as the maximum number of animals, not including natural mortalities, that may be removed from a marine mammal stock while allowing that stock to reach or maintain its optimum sustainable population (as described in NMFS' SARs). While no mortality or serious injury (M/SI) is anticipated or proposed to be authorized here, PBR and M/SI rates from anthropogenic sources are included here as gross indicators of the status of the species or stocks and other threats.

Marine mammal abundance estimates presented in table 6 represent the total number of individuals that make up a given stock or the total number estimated within a particular study or survey area. NMFS' stock abundance estimates for most species represent the total estimate of individuals within the geographic area, if known, that comprises that stock. For some species, this geographic area may extend beyond U.S. waters. All values presented in table 6 are the most recent available at the time of publication (including from the draft 2024 SARs; 90 FR 13344, March 21, 2025) and are available online at:

https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments.

Table 6—Species With Estimated Take From the Specified Activities

Common name

Scientific name

Stock

ESA/MMPA status; strategic (Y/N)

Stock abundance

(CV, N

min

, most recent

abundance survey)

PBR

Annual M/SI

Order Artiodactyla—Cetacea—Mysticeti (baleen whales)

Family Eschrichtiidae:

Gray Whale

Eschrichtius robustus

Eastern N Pacific

-, -, N

26,960 (0.05, 25,849, 2016)

801

131

Family Balaenidae

Family Balaenopteridae (rorquals):

Fin Whale

Balaenoptera physalus

Northeast Pacific

E, D, Y

3,168 (0.26, 2,554, 2013)

UND

0.6

Humpback Whale

Megaptera novaeangliae

Hawai'i

-, -, N

11,278 (0.56, 7,265, 2020)

127

27.09

Mexico-North Pacific

T, D, Y

N/A

(N/A, N/A, 2006)

UND

0.57

Western North Pacific

E, D, Y

1,084 (0.088, 1,007, 2006)

3.4

5.82

Minke Whale

Balaenoptera acutorostrata

Alaska

-, -, N

N/A

(N/A, N/A, N/A)

UND

Odontoceti (toothed whales, dolphins, and porpoises)

Family Delphinidae:

Killer Whale

Orcinus orca

Eastern North Pacific Alaska Resident

-, -, N

1,920 (N/A, 1,920, 2019)

1.3

Eastern North Pacific Gulf of Alaska, Aleutian Islands and Bering Sea Transient

-, -, N

587 (N/A, 587, 2012)

5.9

0.8

Pacific White-Sided Dolphin

Lagenorhynchus obliquidens

North Pacific

-, -, N

26,880 (N/A, N/A, 1990)

UND

Family Monodontidae (white whales):

Beluga Whale

Delphinapterus leucas

Cook Inlet

E, D, Y

331 (0.076, 311, 2022)

0.62

Family Phocoenidae (porpoises):

Dall's Porpoise

Phocoenoides dalli

Alaska

-, -, N

UND

(UND, UND, 2015)

UND

Harbor Porpoise

Phocoena

Gulf of Alaska

-, -, Y

31,046 (0.21, N/A, 1998)

UND

Order Carnivora—Pinnipedia

Family Otariidae (eared seals and sea lions):

California Sea Lion

Zalophus californianus

U.S.

-, -, N

257,606 (N/A, 233,515, 2014)

14,011

>321

Steller Sea Lion

Eumetopias jubatus

Western

E, D, Y

49,837

(N/A, 49,837, 2020)

299

267

Family Phocidae (earless seals):

Harbor Seal

Phoca vitulina

Cook Inlet/Shelikof Strait

-, -, N

28,411 (N/A, 26,907, 2018)

807

107

Information on the classification of marine mammal species can be found on the web page for The Society for Marine Mammalogy's Committee on Taxonomy (

https://marinemammalscience.org/science-and-publications/list-marine-mammal-species-subspecies/

Endangered Species Act (ESA) status: endangered (E), threatened (T)/MMPA status: depleted (D). A dash (-) indicates that the species is not listed under the ESA or designated as depleted under the MMPA. Under the MMPA, a strategic stock is one for which the level of direct human-caused mortality exceeds PBR or which is determined to be declining and likely to be listed under the ESA within the foreseeable future. Any species or stock listed under the ESA is automatically designated under the MMPA as depleted and as a strategic stock.

NMFS marine mammal stock assessment reports online at:

https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessment-reports-region.

CV is coefficient of variation; N

min

is the minimum estimate of stock abundance.

These values, found in NMFS' SARs, represent annual levels of human-caused mortality plus serious injury from all sources combined (

e.g.,

commercial fisheries, ship strike). Annual M/SI often cannot be determined precisely and is in some cases presented as a minimum value or range. A CV associated with estimated mortality due to commercial fisheries is presented in some cases.

The values presented here based on the 2020 SAR and are an underestimate for the entire stock because it is based on surveys which covered only a small portion of the stock's range.

Abundance estimates are currently considered unknown.

Reliable population estimates are not available for this stock. Please see Friday

et al.,

(2013) and Zerbini

et al.,

(2006) for additional information on numbers of minke whales in Alaska.

The best available abundance estimate is likely an underestimate for the entire stock because it is based upon a survey that covered only a small portion of the stock's range.

Nest is best estimate of counts, which have not been corrected for animals at sea during abundance surveys.

As indicated above, all 12 species (with 15 managed stocks) in table 6 temporally and spatially co-occur with the activity to the degree that take is reasonably likely to occur. In addition, the northern sea otter may be found in Cook Inlet, Alaska. However, northern sea otters are managed by the U.S. Fish and Wildlife Service and are not considered further in this rulemaking.

Gray Whale

The stock structure for gray whales in the Pacific has been studied for a number of years and remains uncertain as of the most recent (2022) Pacific SARs (Carretta

et al.,

2023). Gray whale population structure is not determined by simple geography and may be in flux due to evolving migratory dynamics (Carretta

et al.,

2023). Currently, the SARs delineate a western North Pacific (WNP) gray whale stock and an eastern North Pacific (ENP) stock based on genetic differentiation (Carretta

et al.,

2023). WNP gray whales are not known to feed in or travel to upper Cook Inlet (Conant and Lohe, 2023; Weller

et al.,

2023). Therefore, we assume that gray whales near the project area are members of the ENP stock.

An Unusual Mortality Event (UME) for gray whales along the U.S. West Coast and in Alaska occurred from December 17, 2018, through November 9, 2023. During that time, 146 gray whales stranded off the coast of Alaska. The investigative team concluded that the preliminary cause of the UME was localized ecosystem changes in the whale's Subarctic and Arctic feeding areas that led to changes in food, malnutrition, decreased birth rates, and increased mortality (see

https://www.fisheries.noaa.gov/national/marine-life-distress/2019-2023-gray-whale-unusual-mortality-event-along-west-coast-and

for more information).

Gray whales are infrequent visitors to Cook Inlet, but may be seasonally present during spring and fall in the lower inlet (Bureau of Ocean Energy Management (BOEM), 2021). Migrating gray whales pass through the lower inlet during their spring and fall migrations to and from their primary summer feeding areas in the Bering, Chukchi, and Beaufort seas (Swartz, 2018; Silber

et al.,

2021; BOEM, 2021). Several surveys and monitoring programs have sighted gray whales in lower Cook Inlet (Shelden

et al.,

2013; Owl Ridge, 2014; Lomac-MacNair

et al.,

2013, 2014; Kendall

et al.,

2015, as cited in Weston and SLR, 2022). Gray whales are occasionally seen in mid- and upper Cook Inlet, Alaska, but they are not common. During NMFS aerial surveys conducted in June 1994, 2000, 2001, 2005, and 2009 gray whales were observed in Cook Inlet near Port Graham and Elizabeth Island as well as near Kamishak Bay, with one gray whale observed as far north as the Beluga River (Shelden

et al.,

2013). Gray whales were also observed offshore of Cape Starichkof in 2013 by marine mammal observers monitoring Buccaneer's Cosmopolitan drilling project (Owl Ridge, 2014) and in middle Cook Inlet in 2014 during the 2014 Apache 2D seismic survey (Lomac-MacNair

et al.,

2015). Several projects performed in Cook Inlet in recent years reported no observations of gray whales. These project activities included the SAExploration seismic survey in 2015

(Kendall and Cornick, 2015), the 2018 Cook Inlet Pipeline (CIPL) Extension Project (Sitkiewicz

et al.,

2018), the 2019 Hilcorp seismic survey in lower Cook Inlet (Fairweather Science, 2020), Hilcorp's 2022, 2023, and 2024 aerial and rig-based monitoring efforts (Horsley and Larson, 2023, 2024).

In 2020, a young male gray whale was stranded in the Twentymile River near Girdwood for over a week before swimming back into Turnagain Arm. The whale did not survive and was found dead in west Cook Inlet later that month (NMFS, 2020). One gray whale was sighted in Knik Arm near the Don Young Port of Alaska (POA) in Anchorage in upper Cook Inlet in May of 2020 during observations conducted during construction of the Petroleum and Cement Terminal project (61N Environmental, 2021). The sighting occurred less than a week before the reports of the gray whale stranding in the Twentymile River and was likely the same animal. In 2021, one small gray whale was sighted in Knik Arm near Ship Creek, south of the POA (61N Environmental, 2022a). Although some sightings have been documented in the middle and upper Inlet, the gray whale range typically only extends into the lower Cook Inlet region.

Humpback Whale

The 2022 NMFS Alaska and Pacific SARs described a revised stock structure for humpback whales which modifies the previous designated stocks to align more closely with the ESA-designated Distinct Population Segments (DPSs) (Carretta

et al.,

2023; Young

et al.,

2023). Specifically, the three previous North Pacific humpback whale stocks (Central and Western North Pacific stocks and a CA/OR/WA stock) were replaced by five stocks, largely corresponding with the ESA-designated DPSs. These include Western North Pacific and Hawaii stocks and a Central America/Southern Mexico-California (CA)/Oregon (OR)/Washington (WA) stock (which corresponds with the Central America DPS). The remaining two stocks, corresponding with the Mexico DPS, are the Mainland Mexico-CA/OR/WA and Mexico-North Pacific stocks (Carretta

et al.,

2023; Young

et al.,

2023). The former stock is expected to occur along the west coast from California to southern British Columbia, while the latter stock may occur across the Pacific, from northern British Columbia through the Gulf of Alaska and Aleutian Islands/Bering Sea region to Russia.

The Hawaii stock consists of one demographically independent population (DIP) (Hawaii-Southeast Alaska/Northern British Columbia DIP) and the Hawaii-North Pacific unit, which may or may not be composed of multiple DIPs (Wade

et al.,

2021). The DIP and unit are managed as a single stock at this time, due to the lack of data available to separately assess them and lack of compelling conservation benefit to managing them separately (Martien et al., 2021, NMFS 2019, 2023a). The DIP is delineated based on two strong lines of evidence: genetics and movement data (Wade

et al.,

2021). Whales in the Hawaii-Southeast Alaska/Northern British Columbia DIP winter off Hawaii and largely summer in Southeast Alaska and Northern British Columbia (Wade

et al.,

2021). The group of whales that migrate from Russia, western Alaska (Bering Sea and Aleutian Islands), and central Alaska (Gulf of Alaska excluding Southeast Alaska) to Hawaii have been delineated as the Hawaii-North Pacific unit (Wade

et al.,

2021). There are a small number of whales that migrate between Hawaii and southern British Columbia/Washington, but current data and analyses do not provide a clear understanding of which unit these whales belong to (Wade

et al.,

2021; Carretta

et al.,

2023; Young

et al.,

2023).

The Mexico-North Pacific stock is likely composed of multiple DIPs, based on movement data (Martien

et al.,

2021, Wade 2021, Wade

et al.,

2021). However, because currently available data and analyses are not sufficient to delineate or assess DIPs within the unit, it was designated as a single stock (NMFS 2019, 2022d, 2023a). Whales in this stock winter off Mexico and the Revillagigedo Archipelago and summer primarily in Alaska waters (Martien

et al.,

2021; Carretta

et al.,

2023; Young

et al.,

2023).

The Western North Pacific stock consists of two units—the Philippines/Okinawa-North Pacific unit and the Marianas/Ogasawara-North Pacific unit. The units are managed as a single stock at this time, due to a lack of data available to separately assess them (NMFS 2019, 2022d, 2023a). Recognition of these units is based on movements and genetic data (Oleson

et al.,

2022). Whales in the Philippines/Okinawa-North Pacific unit winter near the Philippines and in the Ryukyu Archipelago and migrate to summer feeding areas primarily off the Russian mainland (Oleson

et al.,

2022). Whales that winter off the Mariana Archipelago, Ogasawara, and other areas not yet identified and then migrate to summer feeding areas off the Commander Islands, and to the Bering Sea and Aleutian Islands comprise the Marianas/Ogasawara-North Pacific unit.

The most comprehensive photo-identification data available suggest that approximately 89 percent of all humpback whales in the Gulf of Alaska are from the Hawaii stock, 11 percent are from the Mexico stock, and less than 1 percent are from the Western North Pacific stock (Wade, 2021). Individuals from different stocks are known to intermix in feeding grounds. There is no designated critical habitat for humpback whales in or near the project area (86 FR 21082, April 21, 2021), nor does the project overlap with any known biologically important areas.

Humpback whales are encountered regularly in lower Cook Inlet and occasionally in mid-Cook Inlet; sightings are rare in upper Cook Inlet. Eighty-three groups containing an estimated 187 humpbacks were sighted during the CIBW aerial surveys conducted by NMFS from 1994 to 2012 (Shelden

et al.,

2013). Surveys conducted north of the forelands have documented small numbers in middle Cook Inlet. During the 2014 Apache seismic surveys in Cook Inlet, five groups (six individuals) were reported, with three groups north of the forelands on the east side of the inlet (Lomac-MacNair

et al.,

2014). In 2015, during the construction of the Furie Operating Alaska, LLC (Furie) platform and pipeline, four groups of humpback whales were documented. Another group of 6 to 10 unidentified whales, thought to be either humpback or gray whales, was sighted approximately 15 km (9 mi) northeast of the Julius R. Platform. Large cetaceans were visible near the project (

i.e.,

whales or blows were visible) for 2 hours out of the 1,275 hours of observation conducted (Jacobs, 2015). During SAExploration's 2015 seismic program, three humpback whales were observed in Cook Inlet, including two near the Forelands and one in lower Cook Inlet (Kendall

et al.,

2015 as cited in Weston and SLR, 2022). Hilcorp did not record any sightings of humpback whales in middle Cook Inlet from their aerial or rig-based monitoring efforts in 2022, 2023, or 2024 (Horsley and Larson, 2023, 2024). The POA has observed humpbacks in Knik Arm (2 sightings of likely the same individual near Ship Creek in 2017 (ABR 2017) and 1 at the POA in 2022 (61N Environmental 2022b). Based on these observations, humpback whales may be infrequent visitors to middle Cook Inlet.

Minke Whale

Two stocks of minke whales occur within U.S. waters: Alaska and California/Oregon/Washington (Muto

et al.,

2022). The Alaskan stock of minke whales is considered migratory, as they

are speculated to migrate seasonally from the Bering and Chukchi Seas in fall to areas of the central North Pacific Ocean (Delarue

et al.,

2013). Although they are likely migratory in Alaska, minke whales have been observed off Cape Starichkof and Anchor Point year-round (Muto

et al.,

2017).

Minke whales are most abundant in the Gulf of Alaska during summer and occupy localized feeding areas (Zerbini

et al.,

2006). During the NMFS annual and semiannual surveys of Cook Inlet, minke whales were observed near Anchor Point in 1998, 1999, 2006, and 2021 (Shelden

et al.,

2013, 2015b, 2017, 2022; Shelden and Wade, 2019) and near Ninilchik and the middle of lower Cook Inlet in 2021 (Shelden

et al.,

2022). Minkes were sighted southeast of Kalgin Island and near Homer during Apache's 2014 survey (Lomac-MacNair

et al.,

2014), and one was observed near Tuxedni Bay in 2015 (Kendall

et al.,

2015, as cited in Weston and SLR, 2022). During Hilcorp's seismic survey in lower Cook Inlet in the fall of 2019, eight minke whales were observed (Fairweather Science, 2020). In 2018, no minke whales were observed during observations conducted for the CIPL project near Tyonek (Sitkiewicz

et al.,

2018). Minke whales were also not recorded during Hilcorp's aerial or rig-based monitoring efforts in 2022, 2023, or 2024 (Horsley and Larson, 2023, 2024), however, one sighting of one minke whale was recorded during Hilcorp's spring marine vibroseis seismic survey offshore from Anchor Point in April 2024 (Hanks

et al.,

2024).

Fin Whale

In U.S. Pacific waters, fin whales are seasonally found in the Gulf of Alaska, and Bering Sea and as far north as the northern Chukchi Sea (Muto

et al.,

2021). Several surveys have been conducted to assess the distribution and habitat preferences of fin whales within parts of their range in the North Pacific. In coastal waters of the Aleutians and the Alaska Peninsula, they were found primarily from the Kenai Peninsula to the Shumagin Islands, with a higher abundance near the Semidi Islands and Kodiak Island (Zerbini

et al.,

2006). An opportunistic survey in the Gulf of Alaska revealed that fin whales were concentrated west of Kodiak Island, in Shelikof Strait, and in the southern Cook Inlet region, with smaller numbers observed over the shelf east of Kodiak to Prince William Sound (Alaska Fisheries Science Center [AFSC], 2003). Muto

et al.

(2021) reported visual sightings and acoustic detections in the northeastern Chukchi Sea have been increasing, suggesting that the stock may be re-occupying habitat used prior to large-scale commercial whaling. Delarue

et al.

(2013) also detected fin whale calls in the northeastern Chukchi Sea from July through October in a 3-year acoustic study.

Fin whales’ range extends into lower Cook Inlet; however, their sightings are infrequent, and they are mostly spotted near the inlet's entrance. Fin whales are usually observed as individuals traveling alone, although they are sometimes observed in small groups. Rarely, large groups of 50 to 300 fin whales can travel together during migrations (NMFS, 2010). Fin whales in Cook Inlet have only been observed as individuals or in small groups. From 2000 to 2022, 10 sightings of 26 estimated individual fin whales in lower Cook Inlet were observed during NMFS aerial surveys (Shelden

et al.,

2013, 2015b, 2017, 2022; Shelden and Wade, 2019). No fin whales were observed during the 2018 Harvest's CIPL Extension Project Acoustic Monitoring Program in middle Cook Inlet (Sitkiewicz

et al.,

2018). In September and October 2019, Castellote

et al.

(2020) detected fin whales acoustically in lower Cook Inlet during three-dimensional (3D) seismic surveys, which coincided with the Hilcorp lower Cook Inlet seismic survey. During this period, 8 sightings of 23 individual fin whales were reported, indicating the offshore waters of lower Cook Inlet may be more heavily used than previously believed, especially during the fall season (Fairweather Science, 2020). Hilcorp did not record any sightings of fin whales from their aerial or rig-based monitoring efforts in 2022, 2023, or 2024 (Horsley and Larson, 2023, 2024).

Beluga Whale

Five stocks of beluga whales are recognized in Alaska: the Beaufort Sea stock, eastern Chukchi Sea stock, eastern Bering Sea stock, Bristol Bay stock, and Cook Inlet stock (Young

et al.,

2023). The Cook Inlet stock is geographically and genetically isolated from the other stocks (O'Corry-Crowe

et al.,

1997; Laidre

et al.,

2000) and resides year-round in Cook Inlet (Laidre

et al.,

2000; Castellote

et al.,

2020). Only the Cook Inlet stock inhabits the area where Hilcorp would conduct activities. CIBWs were designated as depleted under the MMPA in 2000 (65 FR 34950, May 31, 2000), and as a DPS and listed as endangered under the ESA in October 2008 (73 FR 62919, October 10, 2008) when the species failed to recover following a moratorium on subsistence harvest. Between 2008 and 2018, CIBWs experienced a decline of about 2.3 percent per year (Wade

et al.,

2019). The decline partially overlapped with the northeast Pacific marine heatwave that occurred from 2014 to 2016 in the Gulf of Alaska, significantly impacting the marine ecosystem (Suryan

et al.,

2021, as cited in Goetz

et al.,

2023).

In June 2023, NMFS released an updated abundance estimate for CIBWs in Alaska that incorporates aerial survey data from June 2021 and 2022 and accounted for visibility bias (Goetz

et al.,

2023). This report estimated that CIBW abundance is between 290 and 386, with a median best estimate of 331. Goetz

et al.

(2023) also present an analysis of population trends for the most recent 10-year period (2012-2022). The addition of data from the 2021 and 2022 survey years in the analysis resulted in a 65.1 percent probability that the CIBW population is now increasing at 0.9 percent per year (95 percent prediction interval of −3 to 5.7 percent). This increase drops slightly to 0.2 percent per year (95 percent prediction interval of −1.8 to 2.6 percent) with a 60 percent probability that the CIBW population is increasing more than 1 percent per year when data from 2021, which had limited survey coverage due to poor weather, are excluded from the analysis.For management purposes, NMFS has determined that the carrying capacity of Cook Inlet is 1,300 CIBWs (65 FR 34590, May 31, 2000) based on historical CIBW abundance estimated by Calkins (1989).

Threats that have the potential to impact this stock and its habitat include the following: catastrophic events (

e.g.,

natural disasters, spills, mass strandings); disease agents (

e.g.,

pathogens, parasites, and harmful algal blooms), habitat loss or degradation, reduction in prey, changes in prey availability due to natural environmental variability, ocean acidification, and commercial fisheries; climatic changes affecting habitat; predation by killer whales; contaminants; noise; ship strikes; waste management; urban runoff; construction projects; and physical habitat modifications that may occur as Cook Inlet becomes increasingly urbanized (Moore

et al.,

2000; Hobbs

et al.,

2015; NMFS, 2016b). Another source of CIBW mortality in Cook Inlet is predation by transient-type (mammal-eating) killer whales (NMFS, 2016b; Shelden

et al.,

2003). No human-caused mortality or serious injury of CIBWs through interactions with commercial, recreational, and subsistence fisheries, takes by subsistence hunters, and or human-caused events (

e.g.,

entanglement in marine debris, ship strikes) has been recently documented,

and harvesting of CIBWs has not occurred since 2008 (NMFS, 2008b).

Critical Habitat.

On April 11, 2011, NMFS designated two areas of critical habitat for CIBW (76 FR 20179). The designation includes 7,800 km

(3012 square mi, mi

) of marine and estuarine habitat within Cook Inlet, encompassing approximately 1,909 km

(737 mi

) in Area 1 and 5,891 km

(2275 mi

) in Area 2 (see figure 1 in 76 FR 20179). Area 1 of the CIBW critical habitat encompasses all marine waters of Cook Inlet north of a line connecting Point Possession and the mouth of Three Mile Creek, including waters of the Susitna, Little Susitna, and Chickaloon Rivers below Mean Higher High Water (MHHW). From spring through fall, Area 1 critical habitat has the highest concentration of CIBWs due to its important foraging and calving habitat. Critical Habitat Area 2 encompasses some of the fall and winter feeding grounds in middle Cook Inlet. This area has a lower concentration of CIBWs in spring and summer but is used by CIBWs in fall and winter. More information on CIBW critical habitat can be found at

https://www.fisheries.noaa.gov/action/critical-habitat-cook-inlet-beluga-whale.

The designation identified the following Primary Constituent Elements, essential features important to the conservation of the CIBW:

(1) Intertidal and subtidal waters of Cook Inlet with depths of less than 9 m (30 ft) Mean Lower-Low Water (MLLW) and within 8 km (5 mi) of high- and medium-flow anadromous fish streams;

(2) Primary prey species, including four of the five species of Pacific salmon (chum (

Oncorhynchus keta

), sockeye (

Oncorhynchus nerka

), Chinook (

Oncorhynchus tshawytscha

), and coho (

Oncorhynchus kisutch

)), Pacific eulachon (

Thaleichthys pacificus

), Pacific cod (

Gadus macrocephalus

), walleye Pollock (

Gadus chalcogrammus

), saffron cod (

Eleginus gracilis

), and yellowfin sole (

Limanda aspera

)

;

(3) The absence of toxins or other agents of a type or amount harmful to CIBWs;

(4) Unrestricted passage within or between the critical habitat areas; and

(5) The absence of in-water noise at levels resulting in the abandonment of habitat by CIBWs.

Biologically Important Areas.

Wild

et al.

(2023) delineated a Small and Resident Population Biologically Important Area (BIA) in Cook Inlet that is active year-round and overlaps where Hilcorp would conduct the specified activities. The authors assigned the BIA an importance score of 2, an intensity score of 2, a data support score of 3, and a boundary certainty score of 2 (scores range from 1 to 3, with a higher score representing an area of more concentrated or focused use and higher confidence in the data supporting the BIA; Harrison

et al.,

2023). These scores indicate that the BIA is of moderate importance and intensity, the authors have high confidence that the population is small and resident and in the abundance and range estimates of the population, and the boundary certainty is medium (see Harrison

et al.

(2023) for additional information about the scoring process used to identify BIAs). The boundary of the CIBW BIA is consistent with NMFS' critical habitat designation (Wild

et al.,

2023).

Foraging ecology

CIBWs feed on a wide variety of prey species, particularly those that are seasonally abundant. From late spring through summer, most CIBW stomachs sampled contained salmon, which corresponded to the timing of fish runs in the area. Anadromous smolt and adult fish aggregate at river mouths and adjacent intertidal mudflats (Calkins, 1989). All five Pacific salmon species (

i.e.,

Chinook, pink (

Oncorhynchus gorbuscha

), coho, sockeye, and chum) spawn in rivers throughout Cook Inlet (Moulton, 1997; Moore

et al.,

2000). Overall, Pacific salmon represent the highest percent frequency of occurrence of prey species in CIBW stomachs. This suggests that their spring feeding in upper Cook Inlet, principally on fat-rich fish such as salmon and eulachon, is important to the energetics of these animals (NMFS, 2016b).

The nutritional quality of Chinook salmon in particular is unparalleled, with an energy content four times greater than that of a Coho salmon. It is suggested the decline of the Chinook salmon population has left a nutritional void in the diet of the CIBWs that no other prey species can fill in terms of quality or quantity (Norman

et al.,

2020, 2022).

In fall, as anadromous fish runs begin to decline, CIBWs return to consume fish species (cod and bottom fish) found in nearshore bays and estuaries. Stomach samples from CIBWs are not available for winter (December through March), although dive data from CIBWs tagged with satellite transmitters suggest that they feed in deeper waters during winter (Hobbs

et al.,

2005), possibly on such prey species as flatfish, cod, sculpin, and pollock.

Habitat Use

The CIBW stock remains within Cook Inlet throughout the year, showing only small seasonal shifts in distribution (Goetz

et al.,

2012a; Lammers

et al.,

2013; Castellotte

et al.,

2015; Shelden

et al.,

2015a, 2018; Lowry

et al.,

2019). The ecological range of CIBWs has contracted significantly since the 1970s. From late spring to fall, nearly the entire population is now found in the upper inlet north of the forelands, with a range reduced to approximately 39 percent of the size documented in the late 1970s (Goetz

et al.,

2023). The recent annual and semiannual aerial surveys (since 2008) found that approximately 83 percent of the population inhabits the area between the Beluga River and Little Susitna River during the survey period, typically conducted in early June. Some aerial survey counts were performed in August, September, and October, finding minor differences in the numbers of belugas in the upper inlet compared to June, reinforcing the importance of the upper inlet habitat area (Young

et al.,

2023).

During spring and summer, CIBWs generally aggregate near the warmer waters of river mouths along the northern shores of middle and upper Cook Inlet where prey availability is high and predator occurrence is low (Moore

et al.,

2000; Shelden and Wade, 2019; McGuire

et al.,

2020). In particular, CIBW groups are seen in the Susitna River Delta, the Beluga River and along the shore to the Little Susitna River, Knik Arm, and along the shores of Chickaloon Bay. Large groups of CIBWs have been observed in the Susitna River Delta, with sizes ranging from 200 to 300 individuals and including a mix of adults, juveniles, and neonates (McGuire

et al.,

2014, 2020). Small groups have been recorded farther south in Kachemak Bay, Redoubt Bay (Big River), and Trading Bay (McArthur River) prior to 1996, but rarely thereafter. Since the mid-1990s, most CIBWs (96 to 100 percent) aggregate in shallow areas near river mouths in upper Cook Inlet, and they are only occasionally sighted in the central or southern portions of Cook Inlet during summer (Hobbs

et al.,

2008). Almost the entire population can be found in northern Cook Inlet from late spring through the summer and into the fall (Muto

et al.,

2020), shifting into deeper waters in middle Cook Inlet in winter (Hobbs

et al.,

2008).

Data from tagged whales (14 tags deployed July 2000 through March 2003) show that CIBWs use upper Cook Inlet intensively between summer and late autumn (Hobbs

et al.,

2005). CIBWs tagged with satellite transmitters continue to use Knik Arm, Turnagain Arm, and Chickaloon Bay as late as October, but some range into lower

Cook Inlet to Chinitna Bay, Tuxedni Bay, and Trading Bay (McArthur River) in fall (Hobbs

et al.,

2005, 2012). From September through November, CIBWs move between Knik Arm, Turnagain Arm, and Chickaloon Bay (Hobbs

et al.,

2005; Goetz

et al.,

2012b). By December, CIBWs are distributed throughout the upper to mid-inlet. Data from recent monitoring suggests lower Cook Inlet (

e.g.,

Tuxedni Bay and Kenai and Kasilof waters) may also be important spring, fall, and/or winter habitat (Castellote

et al.,

2023, 2024; Kumar, 2024). From January into March, they move as far south as Kalgin Island and slightly beyond in central offshore waters. However, CIBWs make occasional excursions into Knik Arm and Turnagain Arm in February and March in spite of ice cover (Hobbs

et al.,

2005). Although tagged CIBWs move widely around Cook Inlet throughout the year, there is no indication of seasonal migration in and out of Cook Inlet (Hobbs

et al.,

2005). Data from NMFS aerial surveys, opportunistic sighting reports, and corrected satellite-tagged CIBWs confirm that they are more widely dispersed throughout Cook Inlet during winter (November-April), with animals found between Kalgin Island and Point Possession. Generally fewer observations of CIBWs are reported from the Anchorage and Knik Arm area from November through April (76 FR 20179, April 11, 2011; Rugh

et al.,

2000, 2004). Later in winter (January into March), belugas were sighted near Kalgin Island and in deeper waters offshore. However, even when ice cover exceeds 90 percent in February and March, belugas travel into Knik Arm and Turnagain Arm (Hobbs

et al.,

2005).

The NMFS AFSC has conducted long-term passive acoustic monitoring demonstrating seasonal shifts in CIBW concentrations throughout Cook Inlet. Castellote

et al.

(2015) conducted long-term acoustic monitoring at 13 locations throughout Cook Inlet between 2008 and 2015: North Eagle Bay, Eagle River Mouth, South Eagle Bay, Six Mile, Point MacKenzie, Cairn Point, Fire Island, Little Susitna, Beluga River, Trading Bay, Kenai River, Tuxedni Bay, and Homer Spit; the former 6 stations being located within Knik Arm. In general, the observed seasonal distribution is in accordance with descriptions based on aerial surveys and satellite telemetry: CIBW detections are higher in the upper inlet during summer, peaking at Little Susitna, Beluga River, and Eagle Bay, followed by fewer detections at those locations during winter. Higher detections in winter at Trading Bay, Kenai River, and Tuxedni Bay suggest a broader CIBW distribution in the lower inlet during winter. That is, CIBWs spend a considerable amount of time outside of middle Cook Inlet where Hilcorp project activities would occur (Castellote

et al.,

2024). Acoustic studies also provide evidence that the Susitna Delta is a crucial habitat for CIBWs, especially during the summer and fall months. An acoustic recorder in the Little Susitna River detected peak CIBW activity from late May to early June and again from July through August (Castellote

et al.,

2016). In the Beluga River, three peaks in activity were recorded: the first from mid-February to early April, the strongest peak from June to mid-July, and a third peak from mid-November to mid-December. The bimodal distribution of these detections is thought to be related to the known availability of the two main anadromous summer prey species for CIBWs, eulachon and Pacific salmon.

Goetz

et al.

(2012b) modeled habitat preferences using NMFS' 1994-2008 June abundance survey data. In large areas, such as the Susitna Delta (Beluga to Little Susitna Rivers) and Knik Arm, there was a high probability that CIBWs were in larger groups. CIBW presence and acoustic foraging behavior also increased closer to rivers with Chinook salmon runs, such as the Susitna River (

e.g.,

Castellote

et al.,

2021). Movement has been correlated with the peak discharge of seven major rivers emptying into Cook Inlet. Boat-based surveys (McGuire and Stephens, 2017) and results from passive acoustic monitoring across the entire inlet (Castellote

et al.,

2015) also support seasonal patterns observed with other methods. Based on long-term passive acoustic monitoring, foraging behavior was more prevalent during summer, particularly at upper inlet rivers, than during winter. The foraging index was highest at Little Susitna, with a peak in July‐August and a secondary peak in May, followed by Beluga River and then Eagle Bay; monthly variation in the foraging index indicates CIBWs shift their foraging behavior among these three locations from April through September. The location of the towing routes are areas of predicted low density in the summer months.

CIBWs are believed to mostly calve in the summer, and breed between late spring and early summer (NMFS, 2016b), primarily in upper Cook Inlet. The only known observed occurrence of calving occurred on July 20, 2015, in the Susitna Delta area (T. McGuire, pers. comm., March 27, 2017). The first neonates encountered during each field season from 2005 through 2015 were always seen in the Susitna River Delta in July. The photographic identification team's documentation of the dates of the first neonate of each year indicate that calving begins in mid-late July/early August, generally coinciding with the observed timing of annual maximum group size. Probable mating behavior of CIBWs was observed in April and May of 2014, in Trading Bay. Young CIBWs are nursed for 2 years and may continue to associate with their mothers for a considerable time thereafter (Colbeck

et al.,

2013). Important calving grounds are thought to be located near the river mouths of upper Cook Inlet.

CIBWs have been observed during marine mammal monitoring efforts in support of industry and research projects. During Apache's seismic test program in 2011 along the west coast of Redoubt Bay, lower Cook Inlet, a total of 33 CIBWs were sighted during the survey (Lomac-MacNair

et al.,

2013). During Apache's 2012 seismic program in mid-inlet, a total of 151 groups consisting of an estimated 1,463 CIBWs were observed (note individuals were likely observed more than once) (Lomac-MacNair

et al.,

2014). During SAExploration's 2015 seismic program, a total of eight groups of 33 estimated individual CIBWs were visually observed during this time period and there were two acoustic detections of CIBWs in upper Cook Inlet (Kendall

et al.,

2015). During Harvest Alaska's CIPL project on the west side of Cook Inlet in between Ladd Landing and Tyonek Platform, a total of 143 CIBW groups (814 individuals) were observed almost daily from May 31 to July 11, even though observations spanned from May 9 through September 15 (Sitkiewicz

et al.,

2018). There were two CIBW carcasses observed by the project vessels in the 2019 Hilcorp lower Cook Inlet seismic survey in the fall which were reported to the NMFS Marine Mammal Stranding Network (Fairweather Science, 2020). Both carcasses were moderately decomposed when they were sighted by the PSOs. Daily aerial surveys specifically for CIBWs were flown over the lower Cook Inlet region, but no CIBWs were observed. Aerial surveys during Hilcorp rig moves in June 2021, and June and September 2022 reported sightings of 11, more than 25, and 20 individual CIBWs, respectively; some were within the aerial survey area and some outside. Rig moves also occurred in June and July of 2023; aerial observers reported 37 sightings of 281 individuals observed both in and out of the survey area (Horsley and Larson, 2023). No CIBWs were sighted from vessel-based PSOs during these rig moves. In May 2024

during Hilcorp's jack-up rig move, two opportunistic sightings of approximately 25 CIBWs were recorded outside of the designated aerial survey area. No additional observations were recorded by aerial or vessel-based PSOs (Horsley and Larson, 2024). Furthermore, three additional CIBWs were observed near the Tyonek Platform by vessel-based PSOs during the pre-clearance monitoring period for Hilcorp's October 2024 jack-up rig move (Horsley et al,. 2024). In November 2024, no sightings of CIBWs were reported during the rig move conducted under the operatorship of Furie Operating Alaska, LLC (S. Vercelline, pers. comm., December 9, 2024).

Killer Whale

Along the west coast of North America, seasonal and year-round occurrence of killer whales has been noted along the entire Alaska coast (Braham and Dahlheim, 1982), in British Columbia and Washington inland waterways (Bigg

et al.,

1990), and along the outer coasts of Washington, Oregon, and California (Green

et al.,

1992; Barlow 1995, 1997; Forney

et al.,

1995). Killer whales from these areas have been labeled as “resident,” “transient,” and “offshore” type killer whales (Bigg

et al.,

1990; Ford

et al.,

2000; Dahlheim

et al.,

2008) based on aspects of morphology, ecology, genetics, and behavior (Ford and Fisher, 1982; Baird and Stacey, 1988; Baird

et al.,

1992; Hoelzel

et al.,

1998, 2002; Barrett-Lennard, 2000; Dahlheim

et al.,

2008). Based on data regarding association patterns, acoustics, movements, and genetic differences, eight killer whale stocks are now recognized within the U.S. Pacific, two of which have the potential to be found in Cook Inlet: the Eastern North Pacific Alaska Resident stock and the Gulf of Alaska, Aleutian Islands, and the Bering Sea Transient stock. Both stocks occur in lower Cook Inlet, but rarely in middle and upper Cook Inlet (Shelden

et al.,

2013). While stocks overlap the same geographic area, they maintain social and reproductive isolation and feed on different prey species. Resident killer whales are primarily fish-eaters, while transients primarily hunt and consume marine mammals, such as harbor seals, Dall's porpoises, harbor porpoises, CIBWs and sea lions. Killer whales are not harvested for subsistence in Alaska. Potential threats most likely to result in direct human-caused mortality or serious injury of killer whales in this region include oil spills, vessel strikes, and interactions with fisheries.

Killer whales have been sighted near Homer and Port Graham in lower Cook Inlet (Shelden

et al.,

2003, 2022; Rugh

et al.,

2005). Resident killer whales from pods often sighted near Kenai Fjords and Prince William Sound have been occasionally photographed in lower Cook Inlet (Shelden

et al.,

2003). The availability of salmon influences when resident killer whales are more likely to be sighted in Cook Inlet. Killer whales were observed in the Kachemak and English Bay three times during aerial surveys conducted between 1993 and 2004 (Rugh

et al.,

2005). Passive acoustic monitoring efforts throughout Cook Inlet documented killer whales at the Beluga River, Kenai River, and Homer Spit, although they were not encountered within Knik Arm (Castellote

et al.,

2016). These detections were likely resident killer whales. Transient killer whales likely have not been acoustically detected due to their propensity to move quietly through waters to track prey (Small, 2010; Lammers

et al.,

2013). Transient killer whales were increasingly reported to feed on belugas in the middle and upper Cook Inlet in the 1990s.

During the 2015 SAExploration seismic program near the North Foreland, two killer whales were observed (Kendall

et al.,

2015, as cited in Weston and SLR, 2022). Killer whales were observed in lower Cook Inlet in 1994, 1997, 2001, 2005, 2010, 2012, and 2022 during the NMFS aerial surveys (Shelden

et al.,

2013, 2022). Eleven killer whale strandings have been reported in Turnagain Arm: 6 in May 1991 and 5 in August 1993. During the Hilcorp lower Cook Inlet seismic survey in the fall of 2019, 21 killer whales were documented (Fairweather Science, 2020). Throughout 4 months of observation in 2018 during the CIPL project in middle Cook Inlet, no killer whales were observed (Sitkiewicz

et al.,

2018). In September 2021, two killer whales were documented in Knik Arm in upper Cook Inlet, near the POA (61N Environmental, 2022a). One killer whale was observed during Hilcorp's pilot marine vibroseis seismic survey in lower Cook Inlet in October of 2023. During the 2024 marine vibroseis seismic survey, a group of four individuals was recorded nearshore Clam Gulch (Hanks

et al.,

2024). Hilcorp did not record any sightings of killer whales from their aerial or rig-based monitoring efforts in 2022, 2023, or 2024 (Horsley and Larson, 2023, 2024).

Pacific White-Sided Dolphin

The Pacific white-sided dolphin is divided into three stocks within U.S. waters. The North Pacific stock includes the coast of Alaska, including the project area. Pacific white-sided dolphins are common in the Gulf of Alaska's pelagic waters and Alaska's nearshore areas, British Columbia, and Washington (Ferrero and Walker, 1996, as cited in Muto

et al.,

2022). They do not typically occur in Cook Inlet, but in 2019, Castellote

et al.

(2020) documented short durations of Pacific white-sided dolphin presence using passive acoustic recorders near Iniskin Bay (6 minutes) and at an offshore mooring located approximately midway between Port Graham and Iniskin Bay (51 minutes). Detections of vocalizations typically lasted on the order of minutes, suggesting the animals did not remain in the area and/or continue vocalizing for extended durations. Visual monitoring conducted during the same period by marine mammal observers on seismic vessels near the offshore recorder did not detect any Pacific white-sided dolphins (Fairweather Science, 2020). These observational data, combined with anecdotal information, indicate that there is a small potential for Pacific white-sided dolphins to occur in the Project area. On May 7, 2014, Apache Alaska observed three Pacific white-sided dolphins during an aerial survey near Kenai. This is one of the only recorded visual observations of Pacific white-sided dolphins in Cook Inlet; they have not been reported in groups as large as those estimated in other parts of Alaska (Muto

et al.,

2022).

Harbor Porpoise

In the eastern North Pacific Ocean, harbor porpoise range from Point Barrow, along the Alaska coast, and down the west coast of North America to Point Conception, California. The 2022 Alaska SARs describe a revised stock structure for harbor porpoises (Young

et al.,

2023). Previously, NMFS had designated three stocks of harbor porpoises: the Bering Sea stock, the Gulf of Alaska stock, and the Southeast Alaska stock (Muto

et al.,

2022; Zerbini

et al.,

2022). The 2022 Alaska SARs splits the Southeast Alaska stock into three separate stocks, resulting in five separate stocks in Alaskan waters for this species. This update better aligns harbor porpoise stock structure with genetics, trends in abundance, and information regarding discontinuous distribution trends (Young

et al.,

2023). Harbor porpoises found in Cook Inlet are likely to be members of the Gulf of Alaska stock (Young

et al.,

2023).

Harbor porpoises occur most frequently in waters less than 100 m deep (Hobbs and Waite, 2010) and are common in nearshore areas of the Gulf of Alaska, Shelikof Strait, and lower Cook Inlet (Dahlheim

et al.,

2000).

Harbor porpoises are often observed in lower Cook Inlet in Kachemak Bay and from Cape Douglas to the West Foreland (Rugh

et al.,

2005). They can be opportunistic foragers but consume primarily schooling forage fish (Bowen and Siniff, 1999). Given their shallow water distribution, harbor porpoise are vulnerable to physical modifications of nearshore habitats resulting from urban and industrial development (including waste management and nonpoint source runoff) and activities such as construction of docks and other over-water structures, filling of shallow areas, dredging, and noise (Linnenschmidt

et al.,

2013). Subsistence users have not reported any harvest from the Gulf of Alaska harbor porpoise stock since the early 1900s (Shelden

et al.,

2014). Calving occurs from May to August; however, this can vary by region. Harbor porpoises are often found traveling alone, or in small groups of less than 10 individuals (Schmale, 2008).

Harbor porpoises occur throughout Cook Inlet, with passive acoustic detections being more prevalent in lower Cook Inlet. Although harbor porpoises have been frequently observed during aerial surveys in Cook Inlet (Shelden

et al.,

2014), most sightings are of single animals and are concentrated at Chinitna and Tuxedni bays on the west side of lower Cook Inlet (Rugh

et al.,

2005), with smaller numbers observed in upper Cook Inlet between April and October. The occurrence of larger numbers of porpoise in the lower Cook Inlet may be driven by greater availability of preferred prey and possibly less competition with CIBWs, as CIBWs move into upper inlet waters to forage on Pacific salmon during the summer months (Shelden

et al.,

2014).

An increase in harbor porpoise sightings in upper Cook Inlet was observed over recent decades (

e.g.,

61N Environmental, 2021, 2022a; Shelden

et al.,

2014). Small numbers of harbor porpoises have been consistently reported in upper Cook Inlet between April and October (Prevel-Ramos

et al.,

2008). The overall increase in the number of harbor porpoise sightings in upper Cook Inlet is unknown, although it may be an artifact of increased studies and marine mammal monitoring programs in upper Cook Inlet. It is also possible that the contraction in the CIBW's range has opened up previously occupied CIBW range to harbor porpoises (Shelden

et al.,

2014).

During Apache's 2012 seismic program in middle Cook Inlet, 137 groups of harbor porpoises comprising 190 individuals were documented between May and August (Lomac-MacNair

et al.,

2013). Lomac-MacNair

et al.

(2014) identified 13 groups of harbor porpoises totaling 77 individuals during Apache's 2014 Cook Inlet seismic survey, both from vessels and aircraft, in May. In June 2012, Shelden

et al.

(2015b) documented 65 groups of 129 individual harbor porpoises during an aerial survey, none of which were in upper Cook Inlet. Kendall

et al.

(2015, as cited in Weston and SLR, 2022) documented 52 groups comprising 65 individuals north of the Forelands during SAExploration's 2015 seismic survey. Shelden

et al.

(2017, 2019, and 2022) also conducted aerial surveys in June and July over Cook Inlet in 2016, 2018, 2021, and 2022 and recorded 65 individuals. Observations occurred in middle and lower Cook Inlet with a majority in Kachemak Bay. A total of 29 groups (44 individuals) were observed north of the Forelands from May to September during the CIPL Extension Project (Sitkiewicz

et al.,

2018). There were two sightings of three harbor porpoises observed during the 2019 Hilcorp lower Cook Inlet seismic survey in the fall (Fairweather Science, 2020). Four monitoring events were conducted at the POA in Anchorage between April 2020 and August 2022, during which 42 groups of harbor porpoises comprising 50 individual porpoises were documented over 285 days of observation (61N Environmental 2021, 2022a, 2022b, and 2022c). An additional 16 harbor porpoises were observed near the POA during their North Extension Stabilization—Step 1 (NES1) project (61N Environmental 2025). During jack-up rig moves in 2021, a PSO observed an individual harbor porpoise in middle Cook Inlet in July and another in October (Horsley and Larson 2023). During a jack-up rig move in June 2023, a PSO also observed an individual harbor porpoise in middle Cook Inlet (Horsley and Larson 2023). In 2023 Hilcorp conducted a pilot marine vibroseis seismic survey in October where two sightings of two harbor porpoises were recorded offshore from Clam Gulch. In April, the survey was conducted once again and one harbor seal sighting of one individual was reported in the same area (Hanks

et al.,

2024). Recent passive acoustic research in Cook Inlet by Alaska Department of Fish and Game (ADF&G) and AFSC have indicated harbor porpoises occur more frequently than expected, particularly in the West Foreland area in spring, although overall numbers are unknown at this time (Castellote

et al.,

2016).

Dall's Porpoise

Dall's porpoises are found throughout the North Pacific, from southern Japan to southern California north to the Bering Sea. All Dall's porpoises in Alaska are of the Alaska stock. This species can be found in offshore, inshore, and nearshore habitat. The Dall's porpoise range in Alaska includes lower Cook Inlet, but very few sightings have been reported in upper Cook Inlet. Observations have been documented near Kachemak Bay and Anchor Point (Owl Ridge, 2014; BOEM, 2015). Shelden

et al.

(2013) and Rugh

et al.

(2005) collated data from aerial surveys conducted between 1994 and 2012 and documented 9 sightings of 25 individuals in the lower Cook Inlet during June and/or July 1997, 1999, and 2000. No Dall's porpoise were observed on subsequent surveys in June and/or July 2014, 2016, 2018, 2021, and 2022 (Shelden

et al.,

2015b, 2017, and 2022; Shelden and Wade, 2019). During Apache's 2014 seismic survey, two groups of three Dall's porpoises were observed in Upper and middle Cook Inlet (Lomac-MacNair

et al.,

2014). In August 2015, one Dall's porpoise was reported in the mid-inlet north of Nikiski in middle Cook Inlet during SAExploration's seismic program (Kendall

et al.,

2015). During aerial surveys in Cook Inlet, they were observed in Iniskin Bay, Barren Island, Elizabeth Island, and Kamishak Bay (Shelden

et al.,

2013). No Dall's porpoises were observed during the 2018 CIPL Extension Project Acoustic Monitoring Program in middle Cook Inlet (Sitkiewicz

et al.,

2018); however, 30 individuals in 10 groups were sighted during a lower Cook Inlet seismic project in the fall 2019 (Fairweather Science, 2020). Hilcorp recorded three sightings of Dall's porpoises in 2021 and one sighting of a Dall's porpoise in 2023 from their rig-based monitoring efforts in the project area (Korsmo

et al.,

2022; Horsley and Larson, 2023). One Dall's porpoise was observed near the POA during the NES1 project, but it is possible this was misidentified (61N Environmental 2025). This higher number of sightings suggests Dall's porpoise may use portions of middle Cook Inlet in greater numbers than previously expected but would still be considered infrequent in middle and upper Cook Inlet.

Steller Sea Lion

Two DPSs of Steller sea lion occur in Alaska: the western DPS and the eastern DPS. The western DPS includes animals that occur west of Cape Suckling, Alaska, and therefore includes

individuals within the project area. The western DPS was listed under the ESA as threatened in 1990 (55 FR 49204, November 26, 1990), and its continued population decline resulted in a change in listing status to endangered in 1997 (62 FR 24345, May 5, 1997). Since 2000, studies indicate that the population east of Samalga Pass (

i.e.,

east of the Aleutian Islands) has increased and is potentially stable (Young

et al.,

2023).

There is uncertainty regarding threats currently impeding the recovery of Steller sea lions, particularly in the Aleutian Islands. Many factors have been suggested as causes of the steep decline in abundance of western Steller sea lions observed in the 1980s, including competitive effects of fishing, environmental change, disease, contaminants, killer whale predation, incidental take, and illegal and legal shooting (Atkinson

et al.,

2008; NMFS, 2008a). A number of management actions have been implemented since 1990 to promote the recovery of the Western U.S. stock of Steller sea lions, including 5.6-km (3-nautical mile) no-entry zones around rookeries, prohibition of shooting at or near sea lions, and regulation of fisheries for sea lion prey species (

e.g.,

walleye pollock, Pacific cod, and Atka mackerel (

Pleurogrammus monopterygius

)) (Sinclair

et al.,

2013; Tollit

et al.,

2017). Additionally, potentially deleterious events, such as harmful algal blooms (Lefebvre

et al.,

2016) and disease transmission across the Arctic (VanWormer

et al.,

2019) that have been associated with warming waters, could lead to potentially negative population-level impacts on Steller sea lions.

NMFS designated critical habitat for Steller sea lions on August 27, 1993 (58 FR 45269), including portions of the southern reaches of lower Cook Inlet. The critical habitat designation for the Western DPS of was determined to include a 37-km (20-nautical mile) buffer around all major haul-outs and rookeries, and associated terrestrial, atmospheric, and aquatic zones, plus three large offshore foraging areas, none of which occurs in the project area. There is no designated critical habitat for Steller sea lions in the mid- or upper inlet, nor are there any known BIAs for Steller sea lions within the project area. Rookeries and haul out sites in lower Cook Inlet include those near the mouth of the inlet, which are approximately 56 km or more south of the closest action area.

Steller sea lions are opportunistic predators, feeding primarily on a wide variety of seasonally abundant fishes and cephalopods, including Pacific herring (

Clupea pallasi

), walleye pollock, capelin (

Mallotus villosus

), Pacific sand lance (

Ammodytes hexapterus

), Pacific cod, salmon (

Oncorhynchus spp.

), and squid (

Teuthida spp.

); (Jefferson

et al.,

2008; Wynne

et al.,

2011). Steller sea lions do not generally eat every day, but tend to forage every 1-2 days and return to haulouts to rest between foraging trips (Merrick and Loughlin, 1997; Rehberg

et al.,

2009). Steller sea lions feed largely on walleye pollock, salmon, and arrowtooth flounder during the summer, and walleye pollock and Pacific cod during the winter (Sinclair and Zeppelin, 2002).

Most Steller sea lions in Cook Inlet occur south of Anchor Point on the east side of lower Cook Inlet, with concentrations near haulout sites at Shaw Island and Elizabeth Island and by Chinitna Bay and Iniskin Bay on the west side (Rugh

et al.,

2005). Steller sea lions are rarely seen in upper Cook Inlet (Nemeth

et al.,

2007). About 3,600 sea lions use haulout sites in the lower Cook Inlet area (Sweeney

et al.,

2017), with additional individuals venturing into the area to forage.

Several surveys and monitoring programs have documented Steller sea lions throughout Cook Inlet, including in upper Cook Inlet in 2012 (Lomac-MacNair

et al.,

2013), near Cape Starichkof in 2013 (Owl Ridge, 2014), in middle and lower Cook Inlet in 2015 (Kendall

et al.,

2015, as cited in Weston and SLR, 2022), in middle Cook Inlet in 2018 (Sitkiewicz

et al.,

2018), in lower Cook Inlet in 2019 (Fairweather Science, 2020), and near the POA in Anchorage in 2020, 2021, 2022, and 2025 (61N Environmental, 2021, 2022a, 2022b, and 2022c, 2025). During NMFS CIBW aerial surveys from 2000 to 2016, 39 sightings of 769 estimated individual Steller sea lions in lower Cook Inlet were recorded (Shelden

et al.,

2017). Sightings of large congregations of Steller sea lions during NMFS aerial surveys occurred outside the specific geographic region, on land in the mouth of Cook Inlet (

e.g.,

Elizabeth and Shaw Islands). In 2012, during Apache's 3D seismic surveys, three sightings of approximately four individuals in upper Cook Inlet were recorded (Lomac-MacNair

et al.,

2013). PSOs associated with Buccaneer's drilling project off Cape Starichkof observed seven Steller sea lions in summer 2013 (Owl Ridge, 2014), and another four Steller sea lions were observed in 2015 in Cook Inlet during SAExploration's 3D Seismic Program. Of the three 2015 sightings, one sighting occurred between the West and East Forelands, one occurred near Nikiski, and one occurred northeast of the North Foreland in the center of Cook Inlet (Kendall and Cornick, 2015). One sighting of two individuals occurred during the CIPL Extension Project in 2018 in middle Cook Inlet (Sitkiewicz

et al.,

2018). Additionally, five sightings of five Steller sea lions were recorded during Hilcorp's lower Cook Inlet seismic survey in the fall of 2019 (Fairweather Science, 2020). At the end of July 2022, while conducting a waterfowl survey an estimated 25 Steller sea lions were observed hauled-out at low tide in the Lewis River, on the west side of Cook Inlet. (K. Lindberg, pers. comm., August 15, 2022). Hilcorp did not record any sightings of Steller sea lions from their aerial or rig-based monitoring efforts in 2022, 2023, or 2024 (Horsley and Larson, 2023, 2024).

Harbor Seal

Harbor seals inhabit waters all along the western coast of the United States, British Columbia, and north through Alaska waters to the Pribilof Islands and Cape Newenham. NMFS currently identifies 12 stocks of harbor seals in Alaska based largely on genetic structure (Young

et al.,

2023). Harbor seals potentially affected by Hilcorp's specified activities are members of the Cook Inlet/Shelikof stock, which ranges from the southwest tip of Unimak Island east along the southern coast of the Alaska Peninsula to Elizabeth Island off the southwest tip of the Kenai Peninsula, including Cook Inlet, Knik Arm, and Turnagain Arm. Distribution of the Cook Inlet/Shelikof stock extends from Unimak Island, in the Aleutian Islands archipelago, north through all of upper and lower Cook Inlet (Young

et al.,

2023).

Harbor seals inhabit the coastal and estuarine waters of Cook Inlet and are observed in both upper and lower Cook Inlet throughout most of the year (Boveng

et al.,

2012; Shelden

et al.,

2013). High-density areas include Kachemak Bay, Iniskin Bay, Iliamna Bay, Kamishak Bay, Cape Douglas, and Shelikof Strait. Up to a few hundred seals seasonally occur in middle and upper Cook Inlet (Rugh

et al.

2005), with the highest concentrations found near the Susitna River and other tributaries within upper Cook Inlet during eulachon and salmon runs (Nemeth

et al.,

2007; Boveng

et al.,

2012), but most remain south of the forelands (Boveng

et al.,

2012).

Harbor seals haul out on rocks, reefs, beaches, and drifting glacial ice (Young

et al.,

2023). Their movements are influenced by tides, weather, season, food availability, and reproduction, as well as individual sex and age class (Lowry

et al.,

2001; Small

et al.,

2003;

Boveng

et al.,

2012). The results of past and recent satellite tagging studies in Southeast Alaska, Prince William Sound, Kodiak Island, and Cook Inlet are also consistent with the conclusion that harbor seals are non-migratory (Lowry

et al.,

2001; Small

et al.,

2003; Boveng

et al.,

2012). However, some long-distance movements of tagged animals in Alaska have been recorded (Pitcher and McAllister, 1981; Lowry

et al.,

2001; Small

et al.,

2003; Womble, 2012; Womble and Gende, 2013). Strong fidelity of individuals for haulout sites during the breeding season has been documented in several populations (Härkönen and Harding, 2001), including in Cook Inlet (Pitcher and McAllister, 1981; Small

et al.,

2005; Boveng

et al.,

2012; Womble, 2012; Womble and Gende, 2013). Harbor seals usually give birth to a single pup between May and mid-July; birthing locations are dispersed over several haulout sites and not confined to major rookeries (Klinkhart

et al.,

2008). More than 200 haulout sites are documented in lower Cook Inlet (Montgomery

et al.,

2007) and 18 in middle and upper Cook Inlet (London

et al.,

2015). Of the 18 in middle and upper Cook Inlet, nine are considered “key haulout” locations where aggregations of 50 or more harbor seals have been documented. Seven key haulouts are in the Susitna River delta, and two are near the Chickaloon River.

Recent research on satellite-tagged harbor seals observed several movement patterns within Cook Inlet (Boveng

et al.,

2012), including a strong seasonal pattern of more coastal and restricted spatial use during the spring and summer (breeding, pupping, molting) and more wide-ranging movements within and outside of Cook Inlet during the winter months, with some seals ranging as far as Shumagin Islands. During summer months, movements and distribution were mostly confined to the west side of Cook Inlet and Kachemak Bay, and seals captured in lower Cook Inlet generally exhibited site fidelity by remaining south of the Forelands in lower Cook Inlet after release (Boveng

et al.,

2012). In the fall, a portion of the harbor seals appeared to move out of Cook Inlet and into Shelikof Strait, northern Kodiak Island, and coastal habitats of the Alaska Peninsula. The western coast of Cook Inlet had higher usage by harbor seals than eastern coast habitats, and seals captured in lower Cook Inlet generally exhibited site fidelity by remaining south of the Forelands in lower Cook Inlet after release (south of Nikiski; Boveng

et al.,

2012).

Harbor seals have been sighted in Cook Inlet during every year of the aerial surveys conducted by NMFS and during all recent mitigation and monitoring programs in lower, middle, and upper Cook Inlet (61N Environmental, 2021, 2022a, 2022b, 2022c, 2025; Fairweather Science, 2020; Kendall

et al.,

2015 as cited in Weston and SLR, 2022; Lomac-MacNair

et al.,

2013, 2014; Sitkiewicz

et al.,

2018). In 2018 Harvest Alaska conducted marine mammal monitoring in middle Cook Inlet within the same geographic area as Hilcorp's proposed action area and reported 313 sightings comprised of 316 harbor seal individuals (Sitkiewicz

et al.,

2018). During Hilcorp's June 2023 jack-up rig move, PSOs observed two separate sightings of harbor seals in middle Cook Inlet: one just north of Nikiski, and the other closer to the Tyonek Platform (Horsley and Larson, 2023). Two separate sightings of harbor seals in middle Cook Inlet also occurred during Hilcorp's May 2024 jack-up rig move, one occurring near the Tyonek Platform and the other approximately halfway between the Tyonek Platform and OSK Dock (Horsley and Larson, 2024).

California Sea Lion

California sea lions live along the Pacific coastline spanning an area from central Mexico to Southeast Alaska and typically breed on islands located in southern California, western Baja California, and the Gulf of California (Carretta

et al.,

2020). Five genetically distinct geographic populations are known to exist: Pacific Temperate, Pacific Subtropical, Southern Gulf of California, Central Gulf of California, and Northern Gulf of California (Schramm

et al.,

2009).

Few observations of California sea lions have been reported in Alaska and most observations have been limited to solitary individuals, typically males that are known to migrate long distances. Occasionally, California sea lions can be found in small groups of two or more and are usually associated with Steller sea lions at their haul outs and rookeries (Maniscalco

et al.,

2004). The few California sea lions observed in Alaska typically do not travel further north than Southeast Alaska. They are often associated with Steller sea lion haulouts and rookeries (Maniscalco

et al.,

2004). Sightings in Cook Inlet are rare, with two documented during the Apache 2012 seismic survey (Lomac-MacNair

et al.,

2013) and anecdotal sightings in Kachemak Bay. No California sea lions were sighted during the 2019 Hilcorp lower Cook Inlet seismic survey (Fairweather Science, 2020), the CIPL project in 2018 (Sitkiewicz

et al.,

2018), or the 2022, 2023, or 2024 Hilcorp aerial or rig-based monitoring efforts (Horsley and Larson, 2023, 2024).

Marine Mammal Hearing

Hearing is the most important sensory modality for marine mammals underwater, and exposure to anthropogenic sound can have deleterious effects. To appropriately assess the potential effects of exposure to sound, it is necessary to understand the frequency ranges marine mammals are able to hear. Not all marine mammal species have equal hearing capabilities (

e.g.,

Richardson

et al.,

1995; Wartzok and Ketten, 1999; Au and Hastings, 2008). To reflect this, Southall

et al.

(2007, 2019) recommended that marine mammals be divided into hearing groups based on directly measured (behavioral or auditory evoked potential techniques) or estimated hearing ranges (behavioral response data, anatomical modeling,

etc.

). Subsequently, NMFS (2018, 2024) described generalized hearing ranges for these marine mammal hearing groups. Generalized hearing ranges were chosen based on the ~65-decibel (dB) threshold from composite audiograms, with the exception for lower limits for low-frequency cetaceans where the lower bound was deemed to be biologically implausible and the lower bound from Southall

et al.

(2007) retained. Frequency is expressed in hertz (Hz) or kilohertz (kHz). We note that the names of two hearing groups and the generalized hearing ranges of all marine mammal hearing groups were recently updated (NMFS 2024) as reflected below in table 7. Of the species potentially present in the action area, gray whales, fin whales, minke whales, and humpback whales are considered low-frequency (LF) cetaceans, CIBWs, pacific white-sided dolphins, and killer whales are considered mid-frequency (MF) cetaceans, harbor porpoises and Dall's porpoises are considered high-frequency (HF) cetaceans, Steller sea lions and California sea lions are otariid pinnipeds (OW), and harbor seals are phocid pinnipeds (PW).

Table 7—Marine Mammal Hearing Groups (NMFS, 2024)

Hearing group

Generalized hearing range *

Low-frequency (LF) cetaceans (baleen whales)

7 Hz to 36 kHz.

High-frequency (HF) cetaceans (dolphins, toothed whales, beaked whales, bottlenose whales)

150 Hz to 160 kHz.

Very high-frequency (VHF) cetaceans (true porpoises,

Kogia,

river dolphins, Cephalorhynchid,

Lagenorhynchus cruciger

L. australis

)

200 Hz to 165 kHz.

Phocid pinnipeds (PW) (underwater) (true seals)

40 Hz to 90 kHz.

Otariid pinnipeds (OW) (underwater) (sea lions and fur seals)

60 Hz to 68 kHz.

* Represents the generalized hearing range for the entire group as a composite (

i.e.,

all species within the group), where individual species' hearing ranges may not be as broad. Generalized hearing range chosen based on ~65 dB threshold from composite audiogram, previous analysis in NMFS 2018, and/or data from Southall

et al.,

2007; Southall

et al.,

2019. Additionally, animals are able to detect very loud sounds above and below that “generalized” hearing range.

For more detail concerning these groups and associated frequency ranges, please see NMFS (2024) for a review of available information.

Potential Effects of Specified Activities on Marine Mammals and Their Habitat

This section provides a discussion of the ways in which components of the specified activity may impact marine mammals and their habitat. The Estimated Take of Marine Mammals section later in this document includes a quantitative analysis of the number of individuals that are expected to be taken by this activity. The Negligible Impact Analysis and Determination section considers the content of this section, the Estimated Take of Marine Mammals section, and the Proposed Mitigation section, to draw conclusions regarding the likely impacts of these activities on the reproductive success or survivorship of individuals and whether those impacts are reasonably expected to, or reasonably likely to, adversely affect the species or stock through effects on annual rates of recruitment or survival.

There are a variety of types and degrees of effects to marine mammals, prey species, and habitat that could occur as a result of Hilcorp's specified activities. In this section, NMFS provides a brief description of the types of sound sources that would be generated by the specified activities of the project, and a description of the ways marine mammals may be generally affected by these activities including in the form of mortality, physical injury, sensory impairment (permanent threshold shifts (PTS), TTS, acoustic masking), physiological responses (particular stress responses), behavioral disturbance, and habitat effects. The Estimated Take of Marine Mammals section also discusses how the potential effects on marine mammals from non-impulsive and impulsive sources relate to the MMPA definitions of Level A harassment and Level B harassment, and quantifies those effects that rise to the level of a take. The Preliminary Analysis and Negligible Impact Determination section assesses whether the proposed authorized take would have a negligible impact on the affected species and stocks.

Background on Sound

This section contains a brief technical background on sound, on the characteristics of certain sound types, on metrics used relevant to the specified activity, and to a discussion of the potential effects of the specified activity on marine mammals found later in this document. For general information on sound and its interaction with the marine environment, please see: Erbe and Thomas (2022); Au and Hastings (2008); Richardson

et al.

(1995); Urick (1983); as well as the Discovery of Sound in the Sea website at

https://dosits.org/.

Sound is a vibration that travels as an acoustic wave through a medium such as a gas, liquid, or solid. Sound waves alternately compress and decompress the medium as the wave travels. In water, sound waves radiate in a manner similar to ripples on the surface of a pond and may be either directed in a beam (narrow beam or directional sources) or sound may radiate in all directions (omnidirectional sources), as is the case for sound produced by tugs under load with a jack-up rig considered here. The compressions and decompressions associated with sound waves are detected as changes in pressure by marine mammals and human-made sound receptors such as hydrophones.

Sound travels more efficiently in water than almost any other form of energy, making the use of sound as a primary sensory modality ideal for inhabitants of the aquatic environment. In seawater, sound travels at roughly 1,500 meters per second (m/s). In air, sound waves travel much more slowly at about 340 m/s. However, the speed of sound in water can vary by a small amount based on characteristics of the transmission medium such as temperature and salinity.

The basic characteristics of a sound wave are frequency, wavelength, velocity, and amplitude. Frequency is the number of pressure waves that pass by a reference point per unit of time and is measured in Hz or cycles per second. Wavelength is the distance between two peaks or corresponding points of a sound wave (length of one cycle). Higher frequency sounds have shorter wavelengths than lower frequency sounds, and typically attenuate (decrease) more rapidly with distance, except in certain cases in shallower water. The amplitude of a sound pressure wave is related to the subjective “loudness” of a sound and is typically expressed in dB, which are a relative unit of measurement that is used to express the ratio of one value of a power or pressure to another. A sound pressure level (SPL) in dB is described as the ratio between a measured pressure and a reference pressure, and is a logarithmic unit that accounts for large variations in amplitude; therefore, a relatively small change in dB corresponds to large changes in sound pressure. For example, a 10-dB increase is a 10-fold increase in acoustic power. A 20-dB increase is then a 100-fold increase in power and a 30-dB increase is a 1,000-fold increase in power. However, a 10-fold increase in acoustic power does not mean that the sound is perceived as being 10 times louder. The dB is a relative unit comparing two pressures; therefore, a reference pressure must always be indicated. For underwater sound, this is 1 microPascal (μPa). For in-air sound, the reference pressure is 20 μPa. The amplitude of a sound can be presented in various ways; however, NMFS typically considers three metrics: sound exposure level (SEL), root-mean-square (RMS) SPL, and peak SPL (defined below). The source level represents the SPL referenced at a standard distance from the source, typically 1 m (Richardson

et al.,

1995; American National Standards Institute (ANSI), 2013), while the received level is the SPL at the receiver's position. For

tugging activities, the SPL is typically referenced at 1 m.

SEL (represented as dB referenced to 1 micropascal squared second (re 1 μPa

-s)) represents the total energy in a stated frequency band over a stated time interval or event, and considers both intensity and duration of exposure. SEL can also be a cumulative metric; it can be accumulated over a single pulse (

i.e.,

during activities such as impact pile driving) or calculated over periods containing multiple pulses. Cumulative SEL (SEL

cum

) represents the total energy accumulated by a receiver over a defined time window or during an event. The SEL metric is useful because it allows sound exposures of different durations to be related to one another in terms of total acoustic energy. The duration of a sound event and the number of pulses, however, should be specified as there is no accepted standard duration over which the summation of energy is measured.

RMS SPL is equal to 10 times the logarithm (base 10) of the ratio of the mean-square sound pressure to the specified reference value, and given in units of dB (International Organization for Standardization (ISO), 2017). RMS is calculated by squaring all of the sound amplitudes, averaging the squares, and then taking the square root of the average (Urick, 1983). RMS accounts for both positive and negative values; squaring the pressures makes all values positive so that they may be accounted for in the summation of pressure levels (Hastings and Popper, 2005). This measurement is often used in the context of discussing behavioral effects, in part because behavioral effects, which often result from auditory cues, may be better expressed through averaged units than by peak SPL. For impulsive sounds, RMS is calculated by the portion of the waveform containing 90 percent of the sound energy from the impulsive event (Madsen, 2005).

Peak SPL (also referred to as zero-to-peak sound pressure or 0-pk) is the maximum instantaneous sound pressure measurable in the water, which can arise from a positive or negative sound pressure, during a specified time, for a specific frequency range at a specified distance from the source, and is represented in the same units as the RMS sound pressure (ISO, 2017). Along with SEL, this metric is used in evaluating the potential for permanent threshold shift (PTS) and temporary threshold shift (TTS) associated with impulsive sound sources.

Sounds are also characterized by their temporal components. Continuous sounds are those whose sound pressure level remains above that of the ambient or background sound with negligibly small fluctuations in level (ANSI, 2005) while intermittent sounds are defined as sounds with interrupted levels of low or no sound (National Institute for Occupational Safety and Health (NIOSH), 1998). A key distinction between continuous and intermittent sound sources is that intermittent sounds have a more regular (predictable) pattern of bursts of sounds and silent periods (

i.e.,

duty cycle), which continuous sounds do not. Tugs under load are considered sources of continuous sound.

Sounds may be either impulsive or non-impulsive (defined below). The distinction between these two sound types is important because they have differing potential to cause physical effects, particularly with regard to noise-induced hearing loss (

e.g.,

Ward, 1997 in Southall

et al.,

2007). Please see NMFS (2018) and Southall

et al.

(2007, 2019) for an in-depth discussion of these concepts.

Impulsive sound sources (

e.g.,

explosions, gunshots, sonic booms, seismic airgun shots, impact pile driving) produce signals that are brief (typically considered to be less than 1 second), broadband, atonal transients (ANSI, 1986, 2005; NIOSH, 1998) and occur either as isolated events or repeated in some succession. Impulsive sounds are all characterized by a relatively rapid rise from ambient pressure to a maximal pressure value followed by a rapid decay period that may include a period of diminishing, oscillating maximal and minimal pressures, and generally have an increased capacity to induce physical injury as compared with sounds that lack these features. Impulsive sounds are intermittent in nature. The duration of such sounds, as received at a distance, can be greatly extended in a highly reverberant environment.

Non-impulsive sounds can be tonal, narrowband, or broadband, brief or prolonged, and may be either continuous or non-continuous (ANSI, 1995; NIOSH, 1998). Some of these non-impulsive sounds can be transient signals of short duration but without the essential properties of impulses (

e.g.,

rapid rise time). Examples of non-impulsive sounds include those produced by vessels (including tugs under load), aircraft, machinery operations, such as drilling or dredging, and vibratory pile driving, and active sonar systems.

Even in the absence of sound from the specified activity, the underwater environment is characterized by sounds from both natural and anthropogenic sound sources. Ambient sound is defined as a composite of naturally-occurring (

i.e.,

non-anthropogenic) sound from many sources both near and far (ANSI, 1995). Background sound is similar, but includes all sounds, including anthropogenic sounds, minus the sound produced by the proposed activities (NMFS, 2012, 2016a). The sound level of a region is defined by the total acoustical energy being generated by known and unknown sources. These sources may include physical (

e.g.,

wind and waves, earthquakes, ice, atmospheric sound), biological (

e.g.,

sounds produced by marine mammals, fish, and invertebrates), and anthropogenic (

e.g.,

vessels, dredging, construction) sound.

A number of sources contribute to background and ambient sound, including wind and waves, which are a main source of naturally occurring ambient sound for frequencies between 200 Hz and 50 kHz (Mitson, 1995). In general, background and ambient sound levels tend to increase with increasing wind speed and wave height. Precipitation can become an important component of total sound at frequencies above 500 Hz, and possibly down to 100 Hz during quiet times. Marine mammals can contribute significantly to background and ambient sound levels, as can some fish and snapping shrimp. The frequency band for biological contributions is from approximately 12 Hz to over 100 kHz. Sources of background sound related to human activity include transportation (surface vessels), dredging and construction, oil and gas drilling and production, geophysical surveys, sonar, and explosions. Vessel noise typically dominates the total background sound for frequencies between 20 and 300 Hz. In general, the frequencies of many anthropogenic sounds, particularly those produced by construction activities, are below 1 kHz (Richardson

et al.,

1995). When sounds at frequencies greater than 1 kHz are produced, they generally attenuate relatively rapidly (Richardson

et al.,

1995), particularly above 20 kHz due to propagation losses and absorption (Urick, 1983).

Transmission loss (TL) defines the degree to which underwater sound has spread in space and lost energy after having moved through the environment and reached a receiver. It is defined as the reduction in a specified level between two specified points that are within an underwater acoustic field (ISO, 2017). Careful consideration of TL and appropriate propagation modeling is a crucial step in determining the impacts of underwater sound, as it helps to define the ranges (isopleths) to which

impacts are expected and depends significantly on local environmental parameters such as seabed type, water depth (bathymetry), and the local speed of sound. Geometric spreading laws are powerful tools, which provide a simple means of estimating TL, based on the shape of the sound wave front in the water column. For a sound source that is equally loud in all directions and in deep water, the sound field takes the form of a sphere, as the sound extends in every direction uniformly. In this case, the intensity of the sound is spread across the surface of the sphere, and thus we can relate intensity loss to the square of the range (as area = 4

). When TL = 20

Log

(range), this situation is known as spherical spreading. In shallow water, the sea surface and seafloor will bound the shape of the sound, leading to a more cylindrical shape, as the top and bottom of the sphere is truncated by the largely reflective boundaries. This situation is termed cylindrical spreading, and is given by TL = 10

Log

(range) (Urick, 1983). An intermediate scenario may be defined by the equation TL = 15

Log

(range), and is referred to as practical spreading. Though these geometric spreading laws do not capture many often important details (scattering, absorption,

etc.

), they offer a reasonable and simple approximation of how sound decreases in intensity as it is transmitted. Cook Inlet is a particularly complex acoustic environment with strong currents, large tides, variable sea floor and generally changing conditions.

The sum of the various natural and anthropogenic sound sources at any given location and time depends not only on the source levels, but also on the propagation of sound through the environment. Sound propagation is dependent on the spatially and temporally varying properties of the water column and sea floor, and is frequency-dependent. As a result of the dependence on a large number of varying factors, background and ambient sound levels can be expected to vary widely over both coarse and fine spatial and temporal scales. Sound levels at a given frequency and location can vary by 10 to 20 dB from day to day (Richardson

et al.,

1995). The result is that, depending on the source type and its intensity, sound from a specified activity may be a negligible addition to the local environment or could form a distinctive signal that may affect marine mammals.

Description of Sound Sources for the Specified Activities

In-water activities associated with the project that have the potential to incidentally take marine mammals through exposure to sound include impact pile driving, tugs under load with a jack-up rig, and tugs involved in anchor handling and pipe pulling. Impact hammers typically operate by repeatedly dropping and/or pushing a heavy piston onto a pile to drive the pile into the substrate. Sound generated by impact hammers is impulsive, characterized by rapid rise times and high peak levels, a potentially injurious combination (Hastings and Popper, 2005). Sound energy associated with tug use is produced by vessel propeller cavitation, a non-impulsive sound source. Bow thrusters, also a non-impulsive sound source, would be occasionally used for a short duration (20 to 30 seconds) to either push or pull a vessel in or away from a dock or platform. Other sound sources include onboard diesel generators and sound from the main engine, but both are subordinate to the thruster and main propeller blade rate harmonics (Gray and Greeley, 1980). The various tug scenarios that may occur during this project include tugs in a stationary mode positioning the drill rig and pulling the jack-up rig at nearly full power against strong tides, and tugs engaged in anchor handling and pipe pulling activities. Our assessments of the likelihood for harassment of marine mammals incidental to Hilcorp's tug activities specified here and in its take authorization request are conservative in light of the general Level B harassment exposure thresholds, the fact that NMFS is still in the process of developing analyses of the impact that non-quantitative contextual factors have on the likelihood of Level B harassment occurring, and the nature and duration of the particular tug activities analyzed here. Potential non-acoustic stressors could result from the physical presence of the equipment and personnel; however, given there are no known pinniped haul-out sites in the vicinity of the specified activity, visual and other non-acoustic stressors would be limited, and any impacts to marine mammals are expected to primarily be acoustic in nature.

Potential Effects of Underwater Sound on Marine Mammals

The introduction of underwater anthropogenic noise into the aquatic environment from impact pile driving and tugs towing, holding, and positioning a jack-up rig or engaging in pipe pulling or anchor handling is the primary means by which marine mammals may be disturbed from Hilcorp's specified activity. In general, animals exposed to natural or anthropogenic sound may experience physical and psychological effects, ranging in magnitude from none to severe (Southall

et al.,

2007, 2019). Exposure to anthropogenic noise has the potential to result in auditory threshold shifts and behavioral reactions (

e.g.,

avoidance, temporary cessation of foraging and vocalizing, changes in dive behavior). In addition to auditory implications, there exists the potential for non-auditory physical effects. Prolonged exposure to intense underwater sound associated with industrial activities may trigger physiological responses in marine mammals that are not observable to the eye, including stress, neurological effects, bubble formation, resonance effects, and various forms of organ or tissue damage (Richardson

et al.,

1995). Additional noise in a marine mammal's habitat can mask acoustic cues used to carry out daily functions, such as communication and predator and prey detection. The effects of noise on marine mammals are dependent on several factors, including but not limited to sound type (

e.g.,

impulsive vs. non-impulsive), the species, age and sex class (

e.g.,

adult male vs. mother with calf), duration of exposure, the distance between the vessel and the animal, received levels, behavior at time of exposure, and previous history with exposure (Wartzok

et al.,

2004; Southall

et al.,

2007). Here we provide additional detail regarding potential impacts on marine mammals and their habitat from noise in general, starting with hearing impairment, as well as from Hilcorp's specified activities, to the degree available.

Hearing Threshold Shifts

Marine mammals, like all mammals, develop increased hearing thresholds over time due to age-related degeneration of auditory pathways and sensory cells of the inner ear. This natural, age-related hearing loss is contrasted with noise-induced hearing loss (Møller, 2013). Marine mammals exposed to high-intensity sound or to lower-intensity sound for prolonged periods can experience a noise-induced hearing threshold shift (TS), which NMFS defines as a change, usually an increase, in the threshold of audibility at a specified frequency or portion of an individual's hearing range above a previously established reference level as a result of noise exposure (NMFS, 2018, 2024). The amount of threshold shift is customarily expressed in dB. Noise-induced hearing TS can be temporary (TTS) or permanent (PTS), with higher-energy sound exposures (which considers both intensity and duration)

are more likely to cause PTS or other auditory injury. As described in NMFS (2018, 2024) there are numerous factors to consider when examining the consequence of TS, including but not limited to the signal temporal pattern (

e.g.,

impulsive or non-impulsive), likelihood an individual would be exposed for a long enough duration or to a high enough level to induce a TS, the magnitude of the TS, time to recovery (seconds to minutes or hours to days), the frequency range of the exposure (

i.e.,

spectral content), the hearing frequency range of the exposed species relative to the signal's frequency spectrum (

i.e.,

how animal uses sound within the frequency band of the signal;

e.g.,

Kastelein

et al.,

2014), and the overlap between the animal and the source (

e.g.,

spatial, temporal, and spectral).

Auditory Injury (AUD INJ).

NMFS (2024) defines AUD INJ as damage to the inner ear that can result in destruction of tissue, such as the loss of cochlear neuron synapses or auditory neuropathy (Houser, 2021; Finneran, 2024). AUD INJ may or may not result in a PTS. PTS is defined as a permanent, irreversible increase in the threshold of audibility at a specified frequency or portion of an individual's hearing range above a previously established reference level (NMFS, 2024). PTS does not generally affect more than a limited frequency range, and an animal that has incurred PTS has some level of hearing loss at the relevant frequencies; thus typically animals with PTS or other AUD INJ are not functionally deaf (Au and Hastings, 2008; Finneran, 2016). For marine mammals, AUD INJ is considered possible when sound exposures are sufficient to produce 40 dB of TTS measured after exposure (Southall

et al.,

2007, 1019). AUD INJ levels for marine mammals are estimates, as with the exception of a single study unintentionally inducing PTS in a harbor seal (Kastak

et al.,

2008; Reichmuth

et al.,

2019), there are no empirical data measuring AUD INJ in marine mammals largely due to the fact that, for various ethical reasons, experiments involving anthropogenic noise exposure at levels inducing AUD INJ are not typically pursued or authorized (NMFS, 2024).

Temporary Threshold Shift (TTS).

TTS is the mildest form of hearing impairment that can occur during exposure to sound. TTS is a temporary, reversible increase in the threshold of audibility at a specified frequency or portion of an individual's hearing range above a previously established reference level (NMFS, 2024) that represents primarily tissue fatigue (Henderson

et al.,

2008), and is not considered an AUD INJ. Based on data from marine mammal TTS measurements (see Southall

et al.,

2007, 2019), a TTS of 6 dB is considered the minimum threshold shift clearly larger than any day-to-day or session-to-session variation in a subject's normal hearing ability (Finneran

et al.,

2000, 2002; Schlundt

et al.,

2000). While experiencing TTS, the hearing threshold rises, meaning that a sound must be at a higher level in order to be heard. As described in Finneran (2015), marine mammal studies have shown the amount of TTS increases with SEL

cum

in an accelerating fashion: at low exposures with lower SEL

cum

, the amount of TTS is typically small and the growth curves have shallow slopes. At exposures with higher SEL

cum

, the growth curves become steeper and approach linear relationships with the noise SEL.

In terrestrial and marine mammals, TTS can last from minutes or hours to days (

i.e.,

there is recovery back to baseline/pre-exposure levels), can occur within a specific frequency range (

i.e.,

an animal might only have a temporary loss of hearing sensitivity within a limited frequency band of its auditory range), and can be of varying amounts (

e.g.,

an animal's hearing sensitivity might be reduced by only 6 dB or reduced by 30 dB). In many cases, hearing sensitivity recovers rapidly after exposure to the sound ends. While there are data on sound levels and durations necessary to elicit mild TTS for marine mammals, recovery is complicated to predict and dependent on multiple factors.

Depending on the degree (elevation of threshold in dB), duration (

i.e.,

recovery time), and frequency range of TTS, and the context in which it is experienced, TTS can have effects on marine mammals ranging from discountable to serious (similar to those discussed in auditory masking, below). For example, a marine mammal may be able to readily compensate for a brief, relatively small amount of TTS in a non-critical frequency range that takes place during a time when the animal is traveling through the open ocean, where ambient noise is lower and there are not as many competing sounds present. Alternatively, a larger amount and longer duration of TTS sustained during time when communication is critical for successful mother/calf interactions could have more serious impacts. We note that reduced hearing sensitivity as a simple function of aging has been observed in marine mammals, as well as humans and other taxa (Southall

et al.,

2007), so we can infer that strategies exist for coping with this condition to some degree, though likely not without cost.

Many studies have examined noise-induced hearing loss in marine mammals (see Finneran (2015) and Southall

et al.

(2019) for summaries). TTS is the mildest form of hearing impairment that can occur during exposure to sound (Kryter, 2013). While experiencing TTS, the hearing threshold rises, and a sound must be at a higher level in order to be heard. In terrestrial and marine mammals, TTS can last from minutes or hours to days (in cases of strong TTS). In many cases, hearing sensitivity recovers rapidly after exposure to the sound ends. For cetaceans, published data on the onset of TTS are limited to captive bottlenose dolphin (

Tursiops truncatus

), beluga whale, harbor porpoise, and Yangtze finless porpoise (

Neophocoena asiaeorientalis

) (Southall

et al.,

2019). For pinnipeds in water, measurements of TTS are limited to harbor seals, elephant seals (

Mirounga angustirostris

), bearded seals (

Erignathus barbatus

), and California sea lions (Kastak

et al.,

1999, 2007; Kastelein

et al.,

2019b, 2019c, 2021, 2022a, 2022b; Reichmuth

et al.,

2019; Sills

et al.,

2020). TTS was not observed in spotted (

Phoca largha

) and ringed (

Pusa hispida

) seals exposed to single airgun impulse sounds at levels matching previous predictions of TTS onset (Reichmuth

et al.,

2016). These studies examine hearing thresholds measured in marine mammals before and after exposure to intense or long-duration sound exposures. The difference between the pre-exposure and post-exposure thresholds can be used to determine the amount of threshold shift at various post-exposure times.

The amount and onset of TTS depends on the exposure frequency. Sounds below the region of best sensitivity for a species or hearing group are less hazardous than those near the region of best sensitivity (Finneran and Schlundt, 2013). At low frequencies, onset-TTS exposure levels are higher compared to those in the region of best sensitivity (

i.e.,

a low frequency noise would need to be louder to cause TTS onset when TTS exposure level is higher), as shown for harbor porpoises and harbor seals (Kastelein

et al.,

2019a, 2019c). Note that in general, harbor seals and harbor porpoises have a lower TTS onset than other measured pinniped and cetacean species (Finneran, 2015). In addition, TTS can accumulate across multiple exposures, but the resulting TTS will be less than the TTS from a single, continuous exposure with the same SEL (Mooney

et al.,

2009; Finneran

et al.,

2010;

Kastelein

et al.,

2014, 2015). This means that TTS predictions based on the total, cumulative SEL will overestimate the amount of TTS from intermittent exposures, such as sonars and impulsive sources. Nachtigall

et al.

(2018) describe measurements of hearing sensitivity of multiple odontocete species (bottlenose dolphin, harbor porpoise, beluga, and false killer whale (

Pseudorca crassidens

)) when a relatively loud sound was preceded by a warning sound. These captive animals were shown to reduce hearing sensitivity when warned of an impending intense sound. Based on these experimental observations of captive animals, the authors suggest that wild animals may dampen their hearing during prolonged exposures or if conditioned to anticipate intense sounds. Another study showed that echolocating animals (including odontocetes) might have anatomical specializations that might allow for conditioned hearing reduction and filtering of low-frequency ambient noise, including increased stiffness and control of middle ear structures and placement of inner ear structures (Ketten

et al.,

2021). Data available on noise-induced hearing loss for mysticetes are currently lacking (NMFS, 2018). Additionally, the existing marine mammal TTS data come from a limited number of individuals within these species.

Relationships between TTS and PTS thresholds have not been studied in marine mammals, and there is no PTS data for cetaceans, but such relationships are assumed to be similar to those in humans and other terrestrial mammals. PTS typically occurs at exposure levels at least several decibels above that inducing mild TTS (

e.g.,

a 40-dB threshold shift approximates PTS onset (Kryter

et al.,

1966; Miller, 1974), while a 6-dB threshold shift approximates TTS onset (Southall

et al.,

2007, 2019). Based on data from terrestrial mammals, a precautionary assumption is that the PTS thresholds for impulsive sounds are at least 6 dB higher than the TTS threshold on a peak-pressure basis and PTS cumulative sound exposure level thresholds are 15 to 20 dB higher than TTS cumulative sound exposure level thresholds (Southall

et al.,

2007, 2019). Given the higher level of sound or longer exposure duration necessary to cause PTS as compared with TTS, it is considerably less likely that PTS could occur.

Non-acoustic Stressors.

Hilcorp's specified activities could also involve non-acoustic stressors. Potential non-acoustic stressors could result from the physical presence of the equipment (

e.g.,

tug and vessel configuration, pile driving equipment) and personnel; however, given there are no known pinniped haul-out sites in the vicinity of the project site, visual and other non-acoustic stressors would be limited, and any impacts to marine mammals are expected to primarily be acoustic in nature.

Behavioral Disturbance.

Exposure to noise also has the potential to behaviorally disturb marine mammals to a level that rises to the definition of Level B harassment under the MMPA. Behavioral disturbance may include a variety of effects, including subtle changes in behavior (

e.g.,

minor or brief avoidance of an area or changes in vocalizations), more conspicuous changes in similar behavioral activities, and more sustained and/or potentially severe reactions, such as displacement from or abandonment of high-quality habitat. Behavioral responses may include changing durations of surfacing and dives, changing direction and/or speed; reducing/increasing vocal activities; changing/cessation of certain behavioral activities (such as socializing or feeding); eliciting a visible startle response or aggressive behavior (such as tail/fin slapping or jaw clapping); and avoidance of areas where sound sources are located (Erbe

et al.,

2019). In addition, pinnipeds may increase their haul out time, possibly to avoid in-water disturbance (Thorson and Reyff, 2006).

Behavioral responses to sound are highly variable and context-specific and any reactions depend on numerous intrinsic and extrinsic factors (

e.g.,

species, state of maturity, experience, current activity, reproductive state, auditory sensitivity, time of day), as well as the interplay between factors (

e.g.,

Richardson

et al.,

1995; Wartzok

et al.,

2004; Southall

et al.,

2007, 2019; Weilgart, 2007; Archer

et al.,

2010; Erbe

et al.,

2019). Behavioral reactions can vary not only among individuals but also within an individual, depending on previous experience with a sound source, context, and numerous other factors (Ellison

et al.,

2012), and can vary depending on characteristics associated with the sound source (

e.g.,

whether it is moving or stationary, number of sources, distance from the source). For example, animals that are resting may show greater behavioral change in response to

This text is long and has been trimmed here. Open the source document for the complete record.

This is a copy of a public record, reproduced as it was published. It is not legal advice, and it may not be the version a court would rely on. Check the official source before you cite it.