# Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to the Hydaburg Seaplane Base Refurbishment Project in Hydaburg, Alaska

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URL: https://www.frixlaw.com/law-library/documents/fr%3A2023-14939

## Record

- **Collection:** Federal Register
- **Document type:** Notice
- **Published:** July 17, 2023
- **Citation:** 88 FR 45774

## Text

DEPARTMENT OF COMMERCE
National Oceanic and Atmospheric Administration
[RTID 0648-XD052]
Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to the Hydaburg Seaplane Base Refurbishment Project in Hydaburg, Alaska

AGENCY:

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

ACTION:

Notice; proposed incidental harassment authorization; request for comments on proposed authorization and possible renewal.

SUMMARY:

NMFS has received a request from the Alaska Department of Transportation and Public Facilities (DOT&PF) for authorization to take marine mammals incidental to the Hydaburg Seaplane Base Refurbishment Project in Hydaburg, Alaska. Pursuant to the Marine Mammal Protection Act (MMPA), NMFS is requesting comments on its proposal to issue an incidental harassment authorization (IHA) to incidentally take marine mammals during the specified activities. NMFS is also requesting comments on a possible one-time, 1-year renewal that could be issued under certain circumstances and if all requirements are met, as described in Request for Public Comments at the end of this notice. NMFS will consider public comments prior to making any final decision on the issuance of the requested MMPA authorization and agency responses will be summarized in the final notice of our decision.

DATES:

Comments and information must be received no later than August 16, 2023.

ADDRESSES:

Comments should be addressed to Jolie Harrison, Chief, Permits and Conservation Division, Office of Protected Resources, National Marine Fisheries Service, and should be submitted via email to
ITP.tyson.moore@noaa.gov
. 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-construction-activities
. In case of problems accessing these documents, please call the contact listed below.

Instructions:
NMFS is not responsible for comments sent by any other method, to any other address or individual, or received after the end of the comment period. Comments, including all attachments, must not exceed a 25-megabyte file size. All comments received are a part of the public record and will generally be posted online at
https://www.fisheries.noaa.gov/national/marine-mammal-protection/incidental-take-authorizations-construction-activities
without change. All personal identifying information (
e.g.,
name, address) voluntarily submitted by the commenter may be publicly accessible. Do not submit confidential business information or otherwise sensitive or protected information.

FOR FURTHER INFORMATION CONTACT:

Reny Tyson Moore, 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 proposed or, if the taking is limited to harassment, a notice of a proposed IHA is provided to the public for review.

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). Further, 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 requirements pertaining to the mitigation, monitoring and reporting of the takings are set forth. The definitions of all applicable MMPA statutory terms cited above are included in the relevant sections below.

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.,
the issuance of an IHA) with respect to potential impacts on the human environment.

This action is consistent with categories of activities identified in Categorical Exclusion B4 (IHAs with no anticipated serious injury or mortality) of the Companion Manual for NAO 216-6A, which do not individually or cumulatively have the potential for significant impacts on the quality of the human environment and for which we have not identified any extraordinary circumstances that would preclude this categorical exclusion. Accordingly, NMFS has preliminarily determined that the issuance of the proposed IHA qualifies to be categorically excluded from further NEPA review.

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

Summary of Request

On June 28, 2022, NMFS received a request from DOT&PF for an IHA to take marine mammals incidental to the Hydaburg Seaplane Base Refurbishment Project in Hydaburg, Alaska. Following NMFS' review of the application, and multiple discussions between DOT&PF and NMFS, DOT&PF submitted responses to NMFS questions on December 15, 2022 and a revised application on February 22, 2023. The application was deemed adequate and complete on March 13, 2023. DOT&PF's request is for take of nine species of marine mammals by Level B harassment and, for a subset of these species (
i.e.,
harbor seal (
Phoca vitulina
), northern elephant seal (
Mirounga angustirostris
), harbor porpoise (
Phocoena phocoena
), Dall's porpoise (
Phocoenoides dalli
), humpback whale (
Megaptera novaeangliae
), and minke whale (
Balaenoptera acutorostrata
)), Level A harassment. Neither DOT&PF nor NMFS expect serious injury or mortality to result from this activity and, therefore, an IHA is appropriate.

Description of Proposed Activity

Overview

DOT&PF, in cooperation with the Federal Aviation Administration, is proposing maintenance improvements to the existing Hydaburg Seaplane Base as part of the Hydaburg Seaplane Base Refurbishment Project. The existing facility has experienced deterioration in recent years, and DOT&PF has conducted several repair projects. The facility is near the end of its useful life,

and replacement of the existing float structures is required to continue safe operation in the future. The in-water portion of the project would include the removal of five existing steel piles and installation of eight permanent steel piles to support replacement of the floating dock structure. Up to 10 temporary steel piles would be installed to support permanent pile installation and would be removed following completion of permanent pile installation. Proposed activities included as part of the project with potential to affect marine mammals include vibratory removal, down-the-hole (DTH) installation, and vibratory and impact installation of steel pipe piles.

Dates and Duration

The proposed IHA would be effective from September 15, 2023, through September 14, 2024. Construction of the proposed project is anticipated to occur over approximately 2 months beginning in early fall 2023. Pile installation and removal will be intermittent during this period, depending on weather, construction and mechanical delays, protected species shutdowns, and other potential delays and logistical constraints. Pile installation will occur intermittently during the work period for durations of minutes to hours at a time. Pile installation and removal will occur over 26 nonconsecutive days within the 2-month construction window. DOT&PF plans to conduct all work during daylight hours.

Specific Geographic Region

The project site is located in the City of Hydaburg, on Prince of Wales Island, approximately 76 kilometers (km) west of Ketchikan, in southeast Alaska. The Hydaburg Seaplane Base is located at the south end of Hydaburg, attached to the Hydaburg city dock on the north shore of the Sukkwan Strait (Figure 1).

Hydaburg is located along the Sukkwan Strait on the southwest side of Prince of Wales Island. A series of passes and straits lead to the open Pacific Ocean; however, Hydaburg is tucked in a relatively calm and secluded area. Sukkwan Strait is generally characterized by semidiurnal tides with mean tidal ranges of around 5 meters (m). Freshwater inputs to Sukkwan Strait include multiple anadromous streams: Hydaburg River, Saltery Creek, and two streams originating from unnamed lakes. The bathymetry of the bay is variable depending on location and proximity to shore, islands, or rocks. Depths approach 76 m within Sukkwan Strait and up to 37 m in South Pass.

Ongoing vessel activities near Hydaburg, as well as land-based industrial and commercial activities, result in elevated in-air and underwater acoustic conditions in the project area that likely increase with proximity to the project site. Background sound levels likely vary seasonally, with elevated levels during summer when the commercial and fishing industries are at their peaks. Hydaburg has no cruise ship or ferry facilities, so only commercial and fishing vessels visit Hydaburg regularly (Miller
et al.,
2019).

BILLING CODE 3510-22-P

EN17JY23.000

BILLING CODE 3510-22-C
Figure 1—Location of Seaplane Base in Hydaburg, Alaska

Detailed Description of the Specified Activity

The DOT&PF proposed project would involve the removal of five existing cantilever steel pipe piles (16-inch (40.64-centimeter (cm)) diameter) that support the existing multiple-float structure. The multiple-float timber structure, which covers 372 square m (m
2
), would also be removed. A new 446-m
2
single-float timber structure would be installed in the same general location. Four 24-inch (60.96-cm) and four 20-inch (50.80-cm) permanent steel pipe piles would be installed vertically to act as restraints for the new seaplane float. Up to 10 temporary 24-inch (60.96 cm) steel pipe piles would be installed to support pile installation and would be removed following completion of construction. Rock sockets and tension anchors would be required on all 24-inch (60.96 cm) piles and two 20-inch (50.80 cm) piles. Rock sockets would also be potentially required on five of the temporary piles. See Table 1 for a summary of the numbers and types of piles to be installed and removed, as well as the estimated durations of each activity.

Table 1—Summary of Piles To Be Installed and Removed

Pile diameter and type
Number of piles

Number of rock
sockets

Number of tension
anchors

Impact strikes per pile

Vibratory duration per pile
(minutes)

Rock socket
DTH pile
Installation,
duration
per pile,
minutes
(range)

Tension
anchor
DTH pile
installation,
duration
per pile,
minutes (range)

Total
duration of
activity
per pile,
hours

Typical
production
rate in
piles per
day (range)

Days of
installation
or removal

Pile Installation

24″ Steel Plumb Piles (Permanent)
4
4
4
50
15
240 (60-480)
120 (60-240)
6.75
0.5 (0-1)
8

20″ Steel Plumb Piles (Permanent)
4
2
2
50
15
240 (60-480)
120 (60-240)

1
0.75/6.75

0.5 (0-1)
8

24″ Steel Piles (Temporary)
10
5
N/A
N/A
15
240 (60-480)
N/A
4.25
2.5 (1-10)
4

Pile Removal

16″ Steel Cantilevered Piles
5
N/A
N/A
N/A
30
N/A
N/A
0.5
2.5 (2-4)
2

24″ Steel Piles (Temporary)
10
N/A
N/A
N/A
30
N/A
N/A
0.5
2.5 (2-4)
2

Totals
23
11
6
N/A
N/A
N/A
N/A
N/A
N/A
26

1
Two of the 20-inch plumb piles will include vibratory and impact installation in addition to rock sockets and tension anchors, estimated at 6.75 hours duration total, and two will only use vibratory and impact, estimated at 0.75 hours duration total.

DTH pile installation would involve drilling rock sockets into the bedrock to support installation of piles. A rock socket is a pile inserted into a drilled hole in the underlying bedrock after the pile has been driven through the overlying softer sediments to refusal by vibratory or impact methods. The pile is advanced farther into the drilled hole to properly secure the bottom portion of the pile into the rock. The depth of the rock socket varies, but up to 6 m may be required for this project. The diameter of the rock socket is slightly larger than the pile being driven. Rock sockets are constructed using a DTH device that consists of a drill bit that drills through the bedrock using both rotary and percussion mechanisms. This breaks up the rock to allow removal of the fragments and insertion of the pile. The pile is advanced at the same time that drilling occurs. Drill cuttings are expelled from the top of the pile using compressed air. It is estimated that drilling rock sockets into the bedrock may take on average 4 hours per pile.

Tension anchors would be installed in six of the permanent piles (four 24-inch (60.96-cm) and two 20-inch (50.80-cm) piles). Tension anchors are installed within piles that are drilled into the bedrock below the elevation of the pile tip after the pile has been driven through the sediment layer to refusal. A 6- or 8-inch (15.24- or 20.32-cm) diameter steel pipe casing would be inserted inside the larger diameter production pile. A rock drill would be inserted into the casing, and a 6- to 8-inch (15.24- to 20.32-cm) diameter hole would be drilled into bedrock with rotary and percussion drilling methods. The drilling work is contained within the steel pile casing and the steel pipe pile. The typical depth of the drilled tension anchor hole varies, but 6-9 m is common. Rock fragments would be removed through the top of the casing with compressed air. A steel rod would then be grouted into the drilled hole and affixed to the top of the pile. The purpose of a tension anchor is to secure the pile to the bedrock to withstand uplift forces. It is estimated that tension anchor installation will take about 1-4 hours per pile. Hereafter, DTH pile installation refers to both rock socket drilling and tension anchor installation unless specified. See Figure 1-3 in the DOT&PF's application for a schematic of DTH pile installation and tension anchor techniques.

Pile removal would be conducted using a vibratory hammer. Pile installation would be conducted using both a vibratory and an impact hammer and DTH pile installation methods. Piles would be advanced to refusal using a vibratory hammer. After DTH pile installation, the final approximately 3 m of driving would be conducted using an impact hammer so that the structural capacity of the pile embedment could be verified. The pile installation methods used would depend on sediment depth and conditions at each pile location. Pile installation and removal would occur in waters approximately 6-7 m in depth.

Actual numbers and sizes of piles, installation times, numbers of impact strikes, and other design and construction details and methods may vary slightly from the estimates outlined in this document. The DOT&PF does not anticipate that the project will change such that potential impacts on marine mammals will change or vary from those described here.

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 DOT&PF's 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, referenced here, instead of reprinting the information. Additional information regarding population trends and threats may be found in NMFS' Stock Assessment Reports (SARs;
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 2 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 Endangered Species Act (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 serious injury or mortality is expected to occur, PBR and annual serious injury and mortality 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 this document 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 stocks managed under the MMPA in this region are assessed in NMFS' U.S. Alaska and Pacific SARs (
e.g.,
Carretta,
et al.,
2022; Muto
et al.,
2022). All values presented in Table 2 are the most recent available at the time of publication (including from the draft 2022 SARs, Young
et al.,
2022) and are available online at:
www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessments
).

Table 2—Species
4
Likely Impacted by the Specified Activities

Common name
Scientific name
Stock

ESA/MMPA status; strategic

(Y/N)
1

Stock abundance (CV, N
min
, most recent abundance

survey)
2

PBR

Annual M/SI
3

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 Balaenopteridae (rorquals):

Humpback Whale

Megaptera novaeangliae

Central N Pacific
-, -, Y
10,103 (0.3, 7,891, 2006)
3.4
4.46

Minke Whale

Balaenoptera acutorostrata

Alaska
-, -, N
N/A (N/A, N/A, N/A)
UND
0

Odontoceti (toothed whales, dolphins, and porpoises)

Family Physeteridae:

Sperm Whale

Physeter macrocephalus

N Pacific
E, D, Y
UND (UND, UND, 2015)
UND
3.5

Family Delphinidae:

Killer Whale

Orcinus orca

Eastern North Pacific Alaska Resident
-, -, N
1,920 (N/A, 1,920, 2019)
19
1.3

Killer Whale

Orcinus orca

Eastern Northern Pacific Northern Resident
-, -, N
302 (N/A, 302, 2018)
2.2
0.2

Killer Whale

Orcinus orca

West Coast Transient
-, -, N
349 (N/A, 349, 2018)
3.5
0.4

Pacific White-Sided Dolphin

Lagenorhynchus obliquidens

N Pacific
-, -, N
26,880 (N/A, N/A, 1990)
UND
0

Family Phocoenidae (porpoises):

Dall's Porpoise

Phocoenoides dalli

Alaska
-, -, N
UND (UND, UND, 2015)
UND
37

Harbor Porpoise

Phocoena

Southeast Alaska
-, -, Y
UND (UND, UND, 2019)
UND
34

Order Carnivora—Pinnipedia

Family Otariidae (eared seals and sea lions):

Steller Sea Lion

Eumetopias jubatus

Eastern
-, -, N
43,201 (N/A, 43,201, 2017)
2,592
112

Family Phocidae (earless seals):

Harbor Seal

Phoca vitulina

Dixon/Cape Decision
-, -, N
23,478 (N/A, 21,453, 2015)
644
69

Northern Elephant Seal

Mirounga angustirostris

CA Breeding
-, -, N
187,386 (N/A, 85,369, 2013)
5,122
13.7

1
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.

2
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. In some cases, CV is not applicable (N/A)

3
These values, found in NMFS's SARs, represent annual levels of human-caused mortality plus serious injury from all sources combined (
e.g.,
commercial fisheries, ship strike). Annual human caused mortality and serious injury (M/SI) often cannot be determined precisely and is in some cases presented as a minimum value or range.

4
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/;
Committee on Taxonomy (2022)).

On January 24, 2023, NMFS published the draft 2022 SARs (
https://www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-stock-assessment-reports-region
). The Alaska and Pacific SARs include a proposed update to the humpback whale stock structure and the Alaska SAR includes a proposed update to the Southeast Alaska harbor porpoise stock structure. These new structures, if finalized, would modify the MMPA-designated humpback stocks to align more closely with the ESA-designated distinct population segments (DPSs), and for harbor porpoise to align with genetics, trends in abundance, and discontinuous distribution NMFS has proposed as supporting the delineation of two demographically independent populations. Please refer to the draft 2022 Alaska and Pacific SARs for additional information.

NMFS Office of Protected Resources, Permits and Conservation Division has generally considered peer-reviewed data in draft SARs (relative to data provided in the most recent final SARs), when available, as the best available science, and has done so here for all species and stocks, with the exception of the new proposal to revise humpback whale and harbor porpoise stock structure. Given that the proposed changes to the stock structures involve application of NMFS' Guidance for Assessing Marine Mammals Stocks and could be revised following consideration of public comments, it is more appropriate to conduct our analysis in this proposed authorization based on the status quo stock structure identified in the most recent final SARs for those species (Carretta
et al.,
2022; Muto
et al.,
2022).

All species that could potentially occur in the proposed survey areas are included in Table 2 of the IHA application. While gray whale and sperm whale have occurred in northern Southeast Alaska in recent years, they are highly unlikely to occur in the proposed project area. The temporal and/or spatial occurrence of these species is such that take is not expected to occur, and they are not discussed further. The remaining 9 species (with 11 managed stocks) in Table 2 temporally and spatially co-occur with the activity to the degree that take is reasonably likely to occur, and we have proposed authorizing it.

Steller Sea Lion

Steller sea lions are found throughout the northern Pacific Ocean, including coastal and inland waters from Russia (Kuril Islands and the Sea of Okhotsk), east to Alaska, and south to central California (Año Nuevo Island). Steller sea lions were listed as threatened range-wide under the ESA on November 26, 1990 (55 FR 49204); they were subsequently partitioned into the western and eastern DPSs (and MMPA stocks) in 1997 (62 FR 24345, May 5, 1997). The eastern DPS remained classified as threatened (62 FR 24345) until it was delisted in November 2013, while the western DPS (those individuals west of 144° W longitude or Cape Suckling, Alaska) was upgraded to endangered status following separation of the DPSs, and it remains endangered today. There is regular movement of both DPSs across this 144° W longitude boundary (Jemison
et al.,
2013), however, due to the distance from this DPS boundary, it is likely that only eastern DPS Steller sea lions are present in the project area. Therefore, animals potentially affected by the project are assumed to be part of the eastern DPS.

Steller sea lions are opportunistic predators, feeding primarily on a wide variety of fishes and cephalopods, including Pacific herring (
Clupea pallasi
), walleye pollock (
Gadus chalcogramma
), capelin (
Mallotus villosus
), Pacific sand lance (
Ammodytes hexapterus
), Pacific cod (
Gadus macrocephalus
), 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 are not common in the project area and systematic counts or surveys have not been completed in the area directly surrounding Hydaburg. The nearest documented haulout is Point Islet (Point Rock), about 13 km southeast of Hydaburg (see Figure 4-1 in the DOT&PF's application). No Steller sea lions were present during aerial surveys over Point Islet that occurred during 2013, 2015, or 2017 (Fritz
et al.,
2016b; Sweeney
et al.,
2017), and it was not surveyed in 2019 (Sweeney
et al.,
2019). Anecdotal evidence provided by local residents indicates that Steller sea lions are rare and do not occur regularly near the project area. However, Steller sea lion presence could be higher during the late summer and early fall salmon runs.

Harbor Seal

Harbor seals range from Baja California north along the west coasts of California, Oregon, Washington, British Columbia, and Southeast Alaska; west through the Gulf of Alaska, Prince William Sound, and the Aleutian Islands; and north in the Bering Sea to Cape Newenham and the Pribilof Islands. In 2010, harbor seals in Alaska were partitioned into 12 separate stocks based largely on genetic structure (Allen and Angliss, 2010). Harbor seals present near Hydaburg are recognized as part of the Dixon/Cape Decision stock.

Harbor seals haul out on rocks, reefs, beaches, and drifting glacial ice, and feed in marine, estuarine, and occasionally fresh waters (Muto
et al.,
2022). Harbor seals generally are non-migratory, with local movements associated with such factors as tides, weather, season, food availability, and reproduction (Scheffer and Slipp, 1944; Fisher 1952; Bigg, 1969, 1981; Hastings
et al.,
2004). 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 (Swain
et al.,
1996; 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).

Harbor seals usually give birth to a single pup between May and mid-July. Birthing locations are often dispersed over several haulout sites and not confined to major rookeries (Klinkhart
et al.,
2008). Strong fidelity of individuals for haul-out sites during the breeding season though have been documented in several populations (Härkönen and Harding, 2001), including some regions in Alaska such as Kodiak Island, Prince William Sound, Glacier Bay/Icy Strait, and Cook Inlet (Pitcher and McAllister, 1981; Small
et al.,
2005; Boveng
et al.,
2012; Womble, 2012; Womble and Gende, 2013).

Harbor seals forage on fish and invertebrates (Orr
et al.,
2004) including capelin, eulachon (
Thaleichthys pacificus
), cod, pollock, flatfish, shrimp, octopus, and squid (Wynne, 2012). They are opportunistic feeders that forage in marine, estuarine, and occasionally freshwater habitat, adjusting their foraging behavior to take advantage of prey that are locally and seasonally abundant (Payne and Selzer, 1989). Depending on prey availability, research has demonstrated that harbor seals conduct both shallow and deep dives while foraging (Tollit
et al.,
1997).

Harbor seals are commonly sighted in the waters of the inside passages throughout Southeast Alaska. Surveys have been rarely carried out on Dixon/Cape Decision, with the last surveys taking place between 2007 to 2011 and 2015. The NMFS Alaska Fisheries

Science Center identifies two “key” haulouts, or haulouts that have had 50 or more harbor seals documented during surveys, in Sukkwan Strait and four additional “not key” haulouts, those with fewer than 50 harbor seals documented during surveys, near the proposed project area (see Figure 4-2 in the DOT&PF's application) (NOAA, 2021). NMFS aerial survey data indicate that as few as 0 to as many as 157 harbor seals were sighted near the project area during surveys between 2003 and 2011 (Areas BD28 and BD30; NOAA, 2022). However, local residents report that only a few (two to four) harbor seals are regularly observed near Hydaburg. These individuals are generally observed near the small boat harbor outside of the proposed project area and during peak salmon runs in late summer and early fall. Harbor seals are known to be curious and may approach novel activity, so it is possible that some may enter the proposed project area during pile installation and removal.

Northern Elephant Seal

Northern elephant seals are wide-ranging throughout the North Pacific, spending as much as 80 percent of their time at sea (Hindell and Perrin, 2009). Populations of northern elephant seals in the U.S. and Mexico have recovered after being nearly hunted to extinction (Stewart
et al.,
1994). Northern elephant seals underwent a severe population bottleneck and loss of genetic diversity when the population was reduced to an estimated 10-30 individuals (Hoelzel
et al.,
2002). Since 1998, northern elephant seals have been undergoing a large population increase, estimated at 3.1 percent annually (Lowry
et al.,
2020). There are two demographically isolated breeding populations: the California breeding population and the Baja California population. No international agreements exist for the joint management of this species by the U.S. and Mexico. The California breeding population is considered to be a separate stock. Any northern elephant seals observed near Hydaburg would be considered part of the California breeding stock.

Spatial segregation in foraging areas between males and females is evident from satellite tag data (Le Beouf
et al.,
2000). Males migrate to the Gulf of Alaska and western Aleutian Islands along the continental shelf to feed on benthic prey, while females migrate to pelagic areas in the Gulf of Alaska and the central North Pacific to feed on pelagic prey (Le Beouf
et al.,
2000). Elephant seals spend a majority of their time at sea (average of 74.7 days during post breeding migration and an average of 218.5 days during the post-molting migration; Robinson
et al.,
2012). Although northern elephant seals are known to visit the Gulf of Alaska to feed on benthic prey, they rarely occur on the beaches of Alaska.

Northern elephant seals breed and give birth in California and Baja Mexico, primarily on offshore islands (Stewart
et al.,
1994, from December to March (Stewart and Huber, 1993)) before dispersing widely across the North Pacific (Le Boeuf
et al.,
2000). Although movement and genetic exchange continues between rookeries, most elephant seals return to natal rookeries when they start breeding (Huber
et al.,
1991). Gestation in elephant seals lasts 11 months, with births taking place onshore when seals are at the breeding colony (Stewart
et al.,
1994).

There is a low probability that northern elephant seals would occur in the proposed project area. Northern elephant seals generally feed along the continental shelf break (Le Boeuf
et al.,
2000) and are not expected to spend time in shallow areas like the Sukkwan Strait. No sightings of elephant seals have been documented near Hydaburg; however, protected species observers (PSOs) at a DOT&PF project site in Ketchikan (located approximately 76 km east of Hydaburg) reported sightings of a northern elephant seal on multiple days (C. Gentemann, personal communication, April 8, 2022). Additional sightings of northern elephant seals around the state concurrent to the Ketchikan sighting were reported in Seward, King Cove, and Kodiak (L. Davis, personal communication, April 14, 2022). Given the recent increase in sightings, including sightings in Southeast Alaska, it is assumed that a few northern elephant seals could be present in Hydaburg during construction of the proposed project.

Harbor Porpoise

In the eastern North Pacific Ocean, the harbor porpoise ranges from Point Barrow, along the Alaska coast, and down the west coast of North America to Point Conception, California. In Alaska, harbor porpoises are currently divided into three stocks, based primarily on geography: the Bering Sea stock, the Southeast Alaska stock, and the Gulf of Alaska stock. Harbor porpoises near Hydaburg are currently recognized as members of the Southeast Alaska stock. The Southeast Alaska stock ranges from Cape Suckling to the Canada boundary (Muto
et al.,
2022).

Harbor porpoises primarily frequent coastal waters in southeast Alaska (Dahlheim
et al.,
2009) and occur most frequently in waters less than 100 m deep (Hobbs and Waite, 2010). Harbor porpoises forage in waters less than 200 m deep on small pelagic schooling fishes such as herring, cod, pollock, octopus, smelt, and bottom-dwelling fish, occasionally feeding on squid and crustaceans (Bjørge and Tolley, 2009; Wynne
et al.,
2011).

Calving occurs from May to August; however, this can vary by region. Harbor porpoises are often found traveling alone, or in small groups less than 10 individuals (Schmale, 2008). According to aerial surveys of harbor porpoise abundance in southeast Alaska conducted in 1991-1993, mean group size was calculated to be 1.2 animals (Dahlheim
et al.,
2000).

Studies of harbor porpoises reported no evidence of seasonal changes in distribution for the inland waters of southeast Alaska (Dahlheim
et al.,
2009). Their small overall size, lack of a visible blow, low dorsal fins and overall low profile, and short surfacing time make them difficult to observe (Dahlheim
et al.,
2015), likely reducing identification and reporting of this species, and these estimates therefore may be low.

Although there have been no systematic studies or observations of harbor porpoises specific to Hydaburg or Sukkwan Strait, there is potential for them to occur in the proposed project area. Abundance data for harbor porpoises in southeast Alaska were collected during 18 seasonal surveys spanning 22 years, from 1991 to 2012 (Dahlheim
et al.,
2015). During that study, a total of 81 harbor porpoises were observed in the southern inland waters of southeast Alaska; however, the survey terminated 80 km southeast of Hydaburg and did not include Sukkwan Strait as part of the survey. There does not appear to be any seasonal variation in harbor porpoise density in the inland waters of southeast Alaska (Dahlheim
et al.,
2015). Harbor porpoises have not been reported by local residents.

Dall's Porpoise

Dall's porpoises are found throughout the North Pacific, from southern Japan to southern California and north to the Bering Sea. All Dall's porpoises in Alaska are members of the Alaska stock, and those off California, Oregon, and Washington are part of a separate stock. Dall's porpoises can be found in offshore, inshore, and nearshore habitat, but they are most commonly found in waters deeper than 183 m (Dahlheim
et al.,
2009; Jefferson, 2009).

Common prey of Dall's porpoise include a variety of small, schooling fishes (such as herrings and mackerels)

and cephalopods. Dall's porpoises may migrate between inshore and offshore areas and make latitudinal movements or short seasonal migrations, but these movements are generally not consistent (Jefferson, 2009).

Dall's porpoises generally occur in groups of 2 to 20 individuals but have also been recorded in groups numbering in the hundreds. The mean group size in southeast Alaska is estimated at approximately three individuals (Dahlheim
et al.,
2009; Jefferson, 2019). However, Dall's porpoises are reported to typically occur in groups of 10-15 animals near Ketchikan Alaska, which is located approximately 76 km east of Hydaburg, with an estimated maximum group size of 20 animals (Freitag 2017, 83 FR 37473, August 1, 2018).

No systematic studies of Dall's porpoise abundance or distribution have occurred in Sukkwan Strait; however, Dall's porpoises have been observed in Cordova Bay 30 km south of Hydaburg during a summer 2011 survey (Jefferson
et al.,
2019). Despite generalized water depth preferences, Dall's porpoises may occur in shallow waters. Moran
et al.
(2018) recently mapped Dall's porpoise distributions in bays, shallow water, and nearshore areas of Prince William Sound, habitats not typically utilized by this species. If Dall's porpoises occur in the proposed project area, they will likely be present in March or April, given the strong seasonal patterns observed in nearby areas of southeast Alaska (Dahlheim
et al.,
2009). No local residents have described seeing Dall's porpoises within Sukkwan Strait.

Pacific White-Sided Dolphin

Pacific white-sided dolphins are a pelagic species inhabiting temperate waters of the North Pacific Ocean and along the coasts of California, Oregon, Washington, and Alaska (Muto
et al.,
2022). Despite their distribution mostly in deep, offshore waters, they may also be found over the continental shelf and in nearshore waters, including inland waters of southeast Alaska (Ferrero and Walker, 1996). Pacific white-sided dolphins are managed as two distinct stocks: the California/Oregon/Washington stock and the North Pacific stock (north of 45° N, including Alaska). Pacific white-sided dolphins present near the project area are recognized as being members of the North Pacific stock, which ranges from Canada into Alaska (Muto
et al.,
2022).

Pacific white-sided dolphins prey on squid and small schooling fish such as capelin, sardines, and herring (Morton, 2006). They are known to work in groups to herd schools of fish and can dive underwater for up to 6 minutes to feed (Morton, 2006). Group sizes have been reported to range from 40 to over 1,000 animals, but groups of between 10 and 100 individuals (Stacey and Baird, 1991) occur most commonly. Seasonal movements of Pacific white-sided dolphins are not well understood, but there is evidence of both north-south seasonal movement (Leatherwood
et al.,
1984) and inshore-offshore seasonal movement (Stacey and Baird, 1991).

Pacific white-sided dolphins do not generally occur in the shallow, inland waterways of southeast Alaska. Scientific studies and data are lacking relative to the presence or abundance of Pacific white-sided dolphins in or near Sukkwan Strait. When Pacific white-sided dolphins have been observed, sighting rates were highest in spring and decreased throughout summer and fall (Dahlheim
et al.,
2009).

Most observations of Pacific white-sided dolphins occur off the outer coast or in inland waterways near entrances to the open ocean. According to Muto
et al.
(2022), aerial surveys in 1997 sighted one group of 164 Pacific white-sided dolphins in Dixon Entrance to the southeast of Hydaburg. These observational data, combined with anecdotal information, indicate that there is a small potential for Pacific white-sided dolphins to occur in the proposed project area. NMFS previously estimated that a group of up to 92 individuals (median between 20 and 164 individuals) could be present at Metlakatla, Alaska (86 FR 43190, August 6, 2021), which is located approximately 80 km east of Hydaburg.

Killer Whale

Killer whales have been observed in all the world's oceans, but the highest densities occur in colder and more productive waters found at high latitudes (NMFS, 2016a). Killer whales occur along the entire Alaska coast, in British Columbia and Washington inland waterways, and along the outer coasts of Washington, Oregon, and California (NMFS, 2016a).

Based on data regarding association patterns, acoustics, movements, and genetic differences, eight killer whale stocks are now recognized within the Pacific U.S. exclusive economic zone. Only individuals from the Eastern North Pacific Alaska Resident stock (Alaska Resident stock), Eastern North Pacific Northern Resident stock (Northern Resident stock), and West Coast Transient stock may occur in the proposed project area (Muto
et al.,
2022).

There are three distinct ecotypes, or forms, of killer whales recognized: resident, transient, and offshore. The three ecotypes differ morphologically, ecologically, behaviorally, and genetically. Surveys between 1991 and 2007 encountered resident killer whales during all seasons throughout southeast Alaska. Both residents and transients were common in a variety of habitats and all major waterways, including protected bays and inlets. There does not appear to be strong seasonal variation in abundance or distribution of killer whales, but there was substantial variability between years during this study (Dahlheim e
t al.,
2009). Spatial distribution has been shown to vary among the different ecotypes, with resident and, to a lesser extent, transient killer whales more commonly observed along the continental shelf, and offshore killer whales more commonly observed in pelagic waters (Rice
et al.,
2021).

Transient killer whales hunt and feed primarily on marine mammals, while residents forage primarily on fish. Transient killer whales feed primarily on harbor seals, Dall's porpoises, harbor porpoises, and sea lions. Resident killer whale populations in the eastern North Pacific feed mainly on salmonids, showing a strong preference for Chinook salmon (NMFS, 2016a).

Transient killer whales are often found in long-term stable social units (pods) of 1 to 16 whales. Average pod sizes in southeast Alaska were six in spring, five in summer, and four in fall (Dahlheim
et al.,
2009). Pod sizes of transient whales are generally smaller than those of resident social groups. Resident killer whales occur in pods ranging from 7 to 70 whales that are seen in association with one another more than 50 percent of the time (Dahlheim
et al.,
2009; NMFS 2016b). In southeast Alaska, resident killer whale mean pod size was approximately 21.5 in spring, 32.3 in summer, and 19.3 in fall (Dahlheim
et al.,
2009).

No systematic studies of killer whales have been conducted in or around Sukkwan Strait. Dahlheim
et al.
(2009) observed transient killer whales within Lynn Canal, Icy Strait, Stephens Passage, Frederick Sound, and upper Chatham Strait. Anecdotal local information suggests that killer whales are rarely seen near the Hydaburg area, but a pod may be seen occasionally every few months.

Humpback Whale

Humpback whales are found throughout southeast Alaska in a variety of marine environments, including open ocean, nearshore waters, and areas with strong tidal currents (Dahlheim
et al.,
2009). Most humpback whales are migratory and spend winters in the

breeding grounds off either Hawaii or Mexico. Humpback whales generally arrive in southeast Alaska in March and return to their wintering grounds in November. Some humpback whales depart late or arrive early to feeding grounds, and therefore the species occurs in southeast Alaska year-round (Straley, 1990; Straley
et al.,
2018). Current threats to humpback whales include vessel strikes, spills, climate change, and commercial fishing operations (Muto
et al.,
2022).

Humpback whales worldwide were designated as “endangered” under the Endangered Species Conservation Act in 1970 and had been listed as a species under the ESA since its inception in 1973. On September 8, 2016, NMFS published a final decision that changed the status of humpback whales under the ESA (81 FR 62259), effective on October 11, 2016. The decision recognized the existence of 14 DPSs based on distinct breeding areas in tropical and temperate waters. Five of the 14 DPSs were classified under the ESA (4 endangered and 1 threatened), while the other 9 DPSs were delisted. Humpback whales found in the project area are predominantly members of the Hawaii DPS, which is not listed under the ESA. However, based on a comprehensive photo-identification study, members of the Mexico DPS, which is listed as threatened, are known to occur in southeast Alaska. Members of different DPSs are known to intermix on feeding grounds; therefore, all waters off the coast of Alaska should be considered to potentially have ESA-listed humpback whales. Approximately 2 percent of all humpback whales encountered in southeast Alaska and northern British Columbia are expected to be members of the Mexico DPS, while all others are expected to be members of the Hawaii DPS (Wade
et al.,
2021).

The DPSs of humpback whales that were identified through the ESA listing process do not necessarily equate to the existing MMPA stocks. The stock delineations of humpback whales under the MMPA are currently under review. Until this review is complete, NMFS considers humpback whales in southeast Alaska to be part of the Central North Pacific stock, with a status of endangered under the ESA and designations of strategic and depleted under the MMPA (Muto
et al.,
2022).

Southeast Alaska is considered a biologically important area (BIA) for feeding humpback whales between May and September (Wild
et al.,
2023), though not currently designated as critical habitat (86 FR 21082, April 21, 2021). Most humpback whales migrate to other regions during winter to breed, but over-wintering (non-breeding) humpback whales have been noted and may be increasingly common and attributable to staggered migration (Straley, 1990, Straley
et al.,
2018). It is thought that those humpbacks that remain in southeast Alaska do so in response to the availability of winter schools of fish prey, which primarily includes overwintering herring (Straley
et al.,
2018). In Alaska, humpback whales filter feed on tiny crustaceans, plankton, and small fish such as walleye pollock, Pacific sand lance, herring (
Clupea pallasii
), eulachon (
Thaleichthys pacificus
), and capelin (Witteveen
et al.,
2012). It is common to observe groups of humpback whales cooperatively bubble feeding. Group sizes in southeast Alaska generally range from one to four individuals (Dahlheim
et al.,
2009).

No systematic studies have documented humpback whale abundance near Hydaburg. Anecdotal information from local residents suggests that humpback whales' utilization of the area is intermittent year-round. Their abundance, distribution, and occurrence are dependent on and fluctuate with fish prey. Local residents estimate that one to two humpback whales may be present in the Sukkwan Strait on a weekly basis. Elsewhere in southeast Alaska, marine mammal monitoring for projects in Tongass Narrows, Ketchikan, Alaska, indicate that humpback whales are present in that area most regularly from May through October (DOT&PF, 2021; 2022) and may occur in lower numbers in winter, which we would expect to be the case for Hydaburg.

Minke Whale

Minke whales are found throughout the northern hemisphere in polar, temperate, and tropical waters (Jefferson
et al.,
2008). The population status of minke whales is considered stable throughout most of their range. Historically, commercial whaling reduced the population size of this species, but given their small size, they were never a primary target of whaling and did not experience severe population declines as did larger cetaceans.

The International Whaling Commission has identified three minke whale stocks in the North Pacific: one near the Sea of Japan, a second in the rest of the western Pacific, and a third, less concentrated, stock throughout the eastern Pacific. NMFS further splits this third stock between Alaska whales and resident whales of California, Oregon, and Washington (Muto
et al.,
2022). Minke whales in southeast Alaska are part of the Alaska stock (Muto
et al.,
2022). Minke whales are found in all Alaskan waters. There are no population estimates for minke whales in southeast Alaska. Surveys in southeast Alaska have consistently identified individuals throughout inland waters in low numbers (Dahlheim
et al.,
2009).

In Alaska, the minke whale diet consists primarily of euphausiids and walleye pollock. Minke whales are generally found in shallow, coastal waters within 200 m of shore (Zerbini
et al.,
2006) and are almost always solitary or in small groups of two to three. Rarely, loose aggregations of up to 400 animals have been associated with feeding areas in Arctic latitudes. In Alaska, seasonal movements are associated with feeding areas that are generally located at the edge of the pack ice (NMFS, 2014).

There are no known occurrences of minke whales within the project area. Dedicated surveys for cetaceans in southeast Alaska found that minke whales were scattered throughout inland waters from Glacier Bay and Icy Strait to Clarence Strait, with small concentrations near the entrance of Glacier Bay (Dahlheim
et al.,
2009). All sightings were of single minke whales, except for a single sighting of multiple minke whales. Surveys took place in spring, summer, and fall, and minke whales were present in low numbers in all seasons and years. NMFS is not aware of information on the winter occurrence of minke whales in southeast Alaska.

Anecdotal observations suggest that minke whales are not seen near Hydaburg and so are expected to occur rarely in the project area. However, NMFS has previously estimated that a group of up to three individuals could be present at nearby Metlakatla, Alaska over 4 months (86 FR 43190, August 6, 2021). Since their ranges extend into the project area and they have been observed in southeast Alaska, including in Clarence Strait (Dahlheim
et al.,
2009), it is possible the species could occur near the project area.

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 or hear over the same frequency range (
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.
). Note that no direct measurements of hearing ability have been successfully completed for mysticetes (
i.e.,
low-frequency cetaceans). Subsequently, NMFS (2018) described generalized hearing ranges for these marine mammal hearing groups. Generalized hearing ranges were chosen based on the approximately 65 decibel (dB) threshold from the normalized 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. Marine mammal hearing groups and their associated hearing ranges are provided in Table 3.

Table 3—Marine Mammal Hearing Groups
[NMFS, 2018]

Hearing group

Generalized
hearing range *

Low-frequency (LF) cetaceans (baleen whales)
7 Hz to 35 kHz.

Mid-frequency (MF) cetaceans (dolphins, toothed whales, beaked whales, bottlenose whales)
150 Hz to 160 kHz.

High-frequency (HF) cetaceans (true porpoises,
Kogia,
river dolphins, Cephalorhynchid,
Lagenorhynchus cruciger
&
L. australis
)

275 Hz to 160 kHz.

Phocid pinnipeds (PW) (underwater) (true seals)
50 Hz to 86 kHz.

Otariid pinnipeds (OW) (underwater) (sea lions and fur seals)
60 Hz to 39 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 are typically not as broad. Generalized hearing range chosen based on ~65 dB threshold from normalized composite audiogram, with the exception for lower limits for LF cetaceans (Southall
et al.,
2007) and PW pinniped (approximation).

The pinniped hearing group was modified from Southall
et al.
(2007) on the basis of data indicating that phocid species have consistently demonstrated an extended frequency range of hearing compared to otariids, especially in the higher frequency range (Hemilä
et al.,
2006; Kastelein
et al.,
2009; Reichmuth
et al.,
2013).

For more detail concerning these groups and associated generalized hearing ranges, please see NMFS (2018) 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 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 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.

Acoustic effects on marine mammals during the specified activity are expected to potentially occur from impact pile installation, vibratory pile installation, and DTH systems. The effects of underwater noise from the DOT&PF's proposed activities have the potential to result in Level B harassment of marine mammals in the action area, and, for some species as a result of certain activities, Level A harassment

Background on Sound

This section contains a brief technical background on sound, on the characteristics of certain sound types, and on metrics used in this proposal in as much as the information is 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,
e.g.,
Erbe and Thomas (2022); Au and Hastings (2008); Richardson
et al.
(1995); Urick (1983) as well as the Discovery of Sound in the Sea (DOSITS) 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 the construction activities 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 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 hertz (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 micropascal (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 from 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 pile driving activities, the SPL is typically referenced at 10 m.

SEL (represented as dB referenced to 1 micropascal squared per second (re 1 μPa
2
-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. The per-pulse SEL (
e.g.,
single strike or single shot SEL) is calculated over the time window containing the entire pulse (
i.e.,
100 percent of the acoustic energy). SEL can also be a cumulative metric; it can be accumulated over a single pulse (for pile driving this is the same as single-strike SEL, above; SELss), or calculated over periods containing multiple pulses (SELcum). Cumulative SEL (SELcum) 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 10 times the logarithm (base 10) of the ratio of the mean-square sound pressure to the specified reference value, in dB (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 (International Organization for Standardization (ISO), 2017) at a specified distance from the source, and is represented in the same units as the RMS sound pressure. Along with SEL, this metric is used in evaluating the potential for PTS (permanent threshold shift) and TTS (temporary threshold shift) associated with impulsive sound sources.

Sounds are also characterized by their temporal component. 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.

Sounds can 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
et al.
(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 one second), broadband, atonal transients (ANSI, 1986; NIOSH, 1998; ANSI 2005) 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, aircraft, machinery operations such as drilling or dredging, vibratory pile driving, and active sonar systems.

Even in the absence of sound from the specified activity, the underwater environment is typically loud due to 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 sounds produced by the proposed activity (NMFS, 2012; NOAA, 2016b). 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 kilohertz (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, 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 by the ISO as the reduction in a specified level between two specified points that are within an underwater acoustic field (ISO 2017). Careful consideration of transmission loss 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*pi*r
2
). When expressing logarithmically in dB as TL, we find that TL = 20*Log
10
(range), for the case of 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
10
(range) (Urick, 1983). An intermediate scenario may be defined by the equation TL = 15*Log
10
(range), and is referred to as practical spreading. Though these two geometric spreading laws defined above 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. In the absence of measured data indicating the level of transmission loss at a given site for a specific activity, NMFS recommends practical spreading (
i.e.,
15*Log
10
(range)) to model acoustic propagation for construction activities in most nearshore environments.

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 10-20 dB from day to day (Richardson et al., 1995). The result is that, depending on the source type and its intensity, sound from the specified activity may be a negligible addition to the local environment or could form a distinctive signal that may affect marine mammals.

Ongoing marine vessel traffic, seaplane traffic and associated activities throughout the Sukkwan Strait area, as well as land-based industrial and commercial activities, result in elevated in-air and underwater sound conditions in the project area that increase with proximity to the project site. Sound levels likely vary seasonally, with elevated levels during summer, when the commercial and fishing industries are at their peaks.

Description of Sound Sources for the Specified Activities

In-water construction activities associated with the project would include impact pile installation, vibratory pile installation and removal, and DTH installation. Impact hammers 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 characterized by rapid rise times and high peak levels, a potentially injurious combination (Hastings and Popper, 2005). Vibratory hammers install piles by vibrating them and allowing the weight of the hammer to push them into the sediment. Vibratory hammers typically produce less sound (
i.e.,
lower levels) than impact hammers. Peak SPLs may be 180 dB or greater, but are generally 10 to 20 dB lower than SPLs generated during impact pile driving of the same-sized pile (Oestman
et al.,
2009). The rise time is slower, reducing the probability and severity of injury, and the sound energy is distributed over a greater amount of time (Nedwell and Edwards, 2002; Carlson
et al.,
2005).

DTH systems would also be used during the proposed construction to install rock sockets and tension anchors. A DTH hammer is essentially a drill bit that drills through the bedrock using a rotating function like a normal drill, in concert with a hammering mechanism operated by a pneumatic (or sometimes hydraulic) component integrated into the DTH hammer to increase speed of progress through the substrate (
i.e.,
it is similar to a “hammer drill” hand tool). The sounds produced by the DTH methods contain both a continuous non-impulsive component from the drilling action and an impulsive component from the hammering effect. Therefore, NMFS treats DTH systems as both impulsive and continuous, non-impulsive sound source types simultaneously.

The likely or possible impacts of the DOT&PF's proposed activities on marine mammals could involve both non-acoustic and acoustic stressors. 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 proposed 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.

Acoustic Impacts

The introduction of anthropogenic noise into the aquatic environment from pile driving or drilling is the primary means by which marine mammals may be harassed from the DOT&PF'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). In general, exposure to pile driving or drilling 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). Exposure to anthropogenic noise can also lead to non-observable physiological responses, such an increase in stress hormones. Additional noise in a marine mammal's habitat can mask acoustic cues used by marine mammals to carry out daily functions, such as communication and predator and prey detection. The effects of pile driving or drilling 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. mom with calf), duration of exposure, the distance

between the pile 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 discuss physical auditory effects (threshold shifts) followed by behavioral effects and potential impacts on habitat.

NMFS defines a noise-induced threshold shift (TS) 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 (NMFS, 2018). The amount of threshold shift is customarily expressed in dB. A TS can be permanent or temporary. As described in NMFS (2018a), 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 and vocalization 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). When considering auditory effects for the DOT&PF's proposed activities, vibratory pile driving is considered a non-impulsive source, while impact pile driving is treated as an impulsive source. DTH systems are considered to have both non-impulsive and impulsive components.

Permanent Threshold Shift (PTS)
—NMFS defines PTS 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, 2018). PTS does not generally affect more than a limited frequency range, and an animal that has incurred PTS has incurred some level of hearing loss at the relevant frequencies; typically animals with PTS are not functionally deaf (Richardson
et al.,
1995; Au and Hastings, 2008). Available data from humans and other terrestrial mammals indicate that a 40 dB threshold shift approximates PTS onset (see Ward
et al.,
1958, 1959; Ward, 1960; Kryter
et al.,
1966; Miller, 1974; Ahroon
et al.,
1996; Henderson
et al.,
2008). PTS criteria for marine mammals are estimates, as with the exception of a single study unintentionally inducing PTS in a harbor seal (Kastak
et al.,
2008), there are no empirical data measuring PTS in marine mammals largely due to the fact that, for various ethical reasons, experiments involving anthropogenic noise exposure at levels inducing PTS are not typically pursued or authorized (NMFS, 2018).

Temporary Threshold Shift (TTS)
—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, 2018). Based on data from cetacean 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 (Schlundt
et al.,
2000; Finneran
et al.,
2000, 2002). As described in Finneran (2015), marine mammal studies have shown the amount of TTS increases with SELcum in an accelerating fashion: at low exposures with lower SELcum, the amount of TTS is typically small and the growth curves have shallow slopes. At exposures with higher SELcum, the growth curves become steeper and approach linear relationships with the noise SEL.

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 (
Delphinapterus leucas
), 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, bearded seals (
Erignathus barbatus
), and California sea lions (
Zalophus californianus
) (Kastak
et al.,
1999; 2007; Kastelein
et al.,
2019b; 2019c; Reichmuth
et al.,
2019; Sills
et al.,
2020; Kastelein
et al.,
2021; 2022a; 2022b). 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 at low frequencies, well below the region of best sensitivity for a species or hearing group, are less hazardous than those at higher frequencies, 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; Kastelein
et al.,
2019c). Note that in general, harbor seals and harbor porpoises have a lower TTS onset than other measured pinniped or 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 (a 40-dB threshold shift approximates PTS onset;
e.g.,
Kryter
et al.,
1966; Miller, 1974) that inducing mild TTS (a 6-dB threshold shift approximates TTS onset;
e.g.,
Southall
et al.,
2007). Based on data from terrestrial mammals, a precautionary assumption is that the PTS thresholds for impulsive sounds (such as impact pile driving pulses as received close to the source) 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). 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.

Behavioral Harassment
—Exposure to noise from pile driving and drilling also has the potential to behaviorally disturb marine mammals to a level that rises to the definition of harassment under the MMPA. Generally speaking, NMFS considers a behavioral disturbance that rises to the level of harassment under the MMPA a non-minor response—in other words, not every response qualifies as behavioral disturbance, and for responses that do, those of a higher level, or accrued across a longer duration, have the potential to affect foraging, reproduction, or survival. 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. Disturbance may result in 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); avoidance of areas where sound sources are located. 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; Weilgart, 2007; Archer
et al.,
2010, Southall
et al.,
2021). 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). In general, pinnipeds seem more tolerant of, or at least habituate more quickly to, potentially disturbing underwater sound than do cetaceans, and generally seem to be less responsive to exposure to industrial sound than most cetaceans. Please see Appendices B and C of Southall
et al.
(2007) and Gomez
et al.
(2016) for reviews of studies involving marine mammal behavioral responses to sound.

Habituation can occur when an animal's response to a stimulus wanes with repeated exposure, usually in the absence of unpleasant associated events (Wartzok
et al.,
2004). Animals are most likely to habituate to sounds that are predictable and unvarying. It is important to note that habituation is appropriately considered as a “progressive reduction in response to stimuli that are perceived as neither aversive nor beneficial,” rather than as, more generally, moderation in response to human disturbance (Bejder
et al.,
2009). The opposite process is sensitization, when an unpleasant experience leads to subsequent responses, often in the form of avoidance, at a lower level of exposure.

As noted above, behavioral state may affect the type of response. For example, animals that are resting may show greater behavioral change in response to disturbing sound levels than animals that are highly motivated to remain in an area for feeding (Richardson
et al.,
1995; National Research Council (NRC), 2003; Wartzok
et al.,
2004). Controlled experiments with captive marine mammals have showed pronounced behavioral reactions, including avoidance of loud sound sources (Ridgway
et al.,
1997; Finneran
et al.,
2003). Observed responses of wild marine mammals to loud pulsed sound sources (typically seismic airguns or acoustic harassment devices) have been varied but often consist of avoidance behavior or other behavioral changes suggesting discomfort (Morton and Symonds, 2002; see also Richardson
et al.,
1995; Nowacek
et al.,
2007).

Available studies show wide variation in response to underwater sound; therefore, it is difficult to predict specifically how any given sound in a particular instance might affect marine mammals perceiving the signal. If a marine mammal does react briefly to an underwater sound by changing its behavior or moving a small distance, the impacts of the change are unlikely to be significant to the individual, let alone the stock or population. However, if a sound source displaces marine mammals from an important feeding or breeding area for a prolonged period, impacts on individuals and populations could be significant (
e.g.,
Lusseau and Bejder, 2007; Weilgart, 2007; NRC, 2005). However, there are broad categories of potential response, which we describe in greater detail here, that include alteration of dive behavior, alteration of foraging behavior, effects to breathing, interference with or alteration of vocalization, avoidance, and flight.

Changes in dive behavior can vary widely and may consist of increased or decreased dive times and surface intervals as well as changes in the rates of ascent and descent during a dive (
e.g.,
Frankel and Clark, 2000; Costa
et al.,
2003; Ng and Leung, 2003; Nowacek
et al.,
2004; Goldbogen
et al.,
2013a,b). Variations in dive behavior may reflect interruptions in biologically significant activities (
e.g.,
foraging) or they may be of little biological significance. The impact of an alteration to dive behavior resulting from an acoustic exposure depends on what the animal is doing at

the time of the exposure and the type and magnitude of the response.

Disruption of feeding behavior can be difficult to correlate with anthropogenic sound exposure, so it is usually inferred by observed displacement from known foraging areas, the appearance of secondary indicators (
e.g.,
bubble nets or sediment plumes), or changes in dive behavior. As for other types of behavioral response, the frequency, duration, and temporal pattern of signal presentation, as well as differences in species sensitivity, are likely contributing factors to differences in response in any given circumstance (
e.g.,
Croll
et al.,
2001; Nowacek
et al.,
2004; Madsen
et al.,
2006; Yazvenko
et al.,
2007). A determination of whether foraging disruptions incur fitness consequences would require information on or estimates of the energetic requirements of the affected individuals and the relationship between prey availability, foraging effort and success, and the life history stage of the animal.

Variations in respiration naturally vary with different behaviors and alterations to breathing rate as a function of acoustic exposure can be expected to co-occur with other behavioral reactions, such as a flight response or an alteration in diving. However, respiration rates in and of themselves may be representative of annoyance or an acute stress response. Various studies have shown that respiration rates may either be unaffected or could increase, depending on the species and signal characteristics, again highlighting the importance in understanding species differences in the tolerance of underwater noise when determining the potential for impacts resulting from anthropogenic sound exposure (
e.g.,
Kastelein
et al.,
2001, 2005, 2006; Gailey
et al.,
2007).

Marine mammals vocalize for different purposes and across multiple modes, such as whistling, echolocation click production, calling, and singing. Changes in vocalization behavior in response to anthropogenic noise can occur for any of these modes and may result from a need to compete with an increase in background noise or may reflect increased vigilance or a startle response. For example, in the presence of potentially masking signals, humpback whales and killer whales have been observed to increase the length of their songs or vocalizations, respectively (Miller
et al.,
2000; Fristrup
et al.,
2003; Foote
et al.,
2004), while right whales (
Eubalaena glacialis
) have been observed to shift the frequency content of their calls upward while reducing the rate of calling in areas of increased anthropogenic noise (Parks
et al.,
2007). In some cases, animals may cease sound production during production of aversive signals (Bowles
et al.,
1994).

Avoidance is the displacement of an individual from an area or migration path as a result of the presence of a sound or other stressors, and is one of the most obvious manifestations of disturbance in marine mammals (Richardson
et al.,
1995). For example, gray whales are known to change direction—deflecting from customary migratory paths—in order to avoid noise from seismic surveys (Malme
et al.,
1984). Avoidance may be short-term, with animals returning to the area once the noise has ceased (
e.g.,
Bowles
et al.,
1994; Goold, 1996; Stone
et al.,
2000; Morton and Symonds, 2002; Gailey
et al.,
2007). Longer-term displacement is possible, however, which may lead to changes in abundance or distribution patterns of the affected species in the affected region if habituation to the presence of the sound does not occur (
e.g.,
Blackwell
et al.,
2004; Bejder
et al.,
2006; Teilmann
et al.,
2006).

A flight response is a dramatic change in normal movement to a directed and rapid movement away from the perceived location of a sound source. The flight response differs from other avoidance responses in the intensity of the response (
e.g.,
directed movement, rate of travel). Relatively little information on flight responses of marine mammals to anthropogenic signals exist, although observations of flight responses to the presence of predators have occurred (Connor and Heithaus, 1996, Bowers
et al.,
2018). The result of a flight response could range from brief, temporary exertion and displacement from the area where the signal provokes flight to, in extreme cases, marine mammal strandings (England, 2001). However, it should be noted that response to a perceived predator does not necessarily invoke flight (Ford and Reeves, 2008), and whether individuals are solitary or in groups may influence the response.

Behavioral disturbance can also impact marine mammals in more subtle ways. Increased vigilance may result in costs related to diversion of focus and attention (
i.e.,
when a response consists of increased vigilance, it may come at the cost of decreased attention to other critical behaviors such as foraging or resting). These effects have generally not been demonstrated for marine mammals, but studies involving fishes and terrestrial animals have shown that increased vigilance may substantially reduce feeding rates (
e.g.,
Beauchamp and Livoreil, 1997; Fritz
et al.,
2002; Purser and Radford, 2011). In addition, chronic disturbance can cause population declines through reduction of fitness (
e.g.,
decline in body condition) and subsequent reduction in reproductive success, survival, or both (
e.g.,
Harrington and Veitch, 1992; Daan
et al.,
1996; Bradshaw
et al.,
1998). However, Ridgway
et al.
(2006) reported that increased vigilance in bottlenose dolphins exposed to sound over a 5-day period did not cause any sleep deprivation or stress effects.

Many animals perform vital functions, such as feeding, resting, traveling, and socializing, on a diel cycle (24-hour cycle). Disruption of such functions resulting from reactions to stressors such as sound exposure are more likely to be significant if they last more than one diel cycle or recur on subsequent days (Southall
et al.,
2007). Consequently, a behavioral response lasting less than 1 day and not recurring on subsequent days is not considered particularly severe unless it could directly affect reproduction or survival (Southall
et al.,
2007). Note that there is a difference between multi-day substantive behavioral reactions and multi-day anthropogenic activities. For example, just because an activity lasts for multiple days does not necessarily mean that individual animals are either exposed to activity-related stressors for multiple days or, further, exposed in a manner resulting in sustained multi-day substantive behavioral responses.

Stress Responses
—An animal's perception of a threat may be sufficient to trigger stress responses consisting of some combination of behavioral responses, autonomic nervous system responses, neuroendocrine responses, or immune responses (
e.g.,
Selye, 1950; Moberg, 2000). In many cases, an animal's first and sometimes most economical (in terms of energetic costs) response is behavioral avoidance of the potential stressor. Autonomic nervous system responses to stress typically involve changes in heart rate, blood pressure, and gastrointestinal activity. These responses have a relatively short duration and may or may not have a significant long-term effect on an animal's fitness.

Neuroendocrine stress responses often involve the hypothalamus-pituitary-adrenal system. Virtually all neuroendocrine functions that are affected by stress—including immune competence, reproduction, metabolism, and behavior—are regulated by pituitary hormones. Stress-induced changes in the secretion of pituitary hormones have been implicated in failed reproduction, altered metabolism, reduced immune competence, and behavioral disturbance

(
e.g.,
Moberg, 1987; Blecha, 2000). Increases in the circulation of glucocorticoids are also equated with stress (Romano
et al.,
2004).

The primary distinction between “stress” (which is adaptive and does not normally place an animal at risk) and “distress” is the cost of the response. During a stress response, an animal uses glycogen stores that can be quickly replenished once the stress is alleviated. In such circumstances, the cost of the stress response would not pose serious fitness consequences. However, when an animal does not have sufficient energy reserves to satisfy the energetic costs of a stress response, energy resources must be diverted from other functions. This state of distress will last until the animal replenishes its energetic reserves sufficient to restore normal function.

Relationships between these physiological mechanisms, animal behavior, and the costs of stress responses are well-studied through controlled experiments and for both laboratory and free-ranging animals (
e.g.,
Holberton
et al.,
1996; Hood
et al.,
1998; Jessop
et al.,
2003; Krausman
et al.,
2004; Lankford
et al.,
2005). Stress responses due to exposure to anthropogenic sounds or other stressors and their effects on marine mammals have also been reviewed (Fair and Becker, 2000; Romano
et al.,
2002b) and, more rarely, studied in wild populations (
e.g.,
Romano
et al.,
2002a). For example, Rolland
et al.
(2012) found that noise reduction from reduced ship traffic in the Bay of Fundy was associated with decreased stress in North Atlantic right whales. These and other studies lead to a reasonable expectation that some marine mammals will experience physiological stress responses upon exposure to acoustic stressors and that it is possible that some of these would be classified as “distress.” In addition, any animal experiencing TTS would likely also experience stress responses (NRC, 2003), however distress is an unlikely result of this project based on observations of marine mammals during previous, similar construction projects.

Auditory Masking
—Since many marine mammals rely on sound to find prey, moderate social interactions, and facilitate mating (Tyack, 2008), noise from anthropogenic sound sources can interfere with these functions, but only if the noise spectrum overlaps with the hearing sensitivity or vocal ranges of the marine mammal (Southall
et al.,
2007; Clark
et al.,
2009; Hatch
et al.,
2012). Chronic exposure to excessive, though not high-intensity, noise could cause masking at particular frequencies for marine mammals that utilize sound for vital biological functions (Clark
et al.,
2009). Acoustic masking is when other noises such as from human sources interfere with an animal's ability to detect, recognize, or discriminate between acoustic signals of interest (
e.g.,
those used for intraspecific communication and social interactions, prey detection, predator avoidance, navigation) (Richardson
et al.,
1995; Erbe
et al.,
2016). Therefore, under certain circumstances, marine mammals whose acoustical sensors or environment are being severely masked could also be impaired from maximizing their performance fitness in survival and reproduction. The ability of a noise source to mask biologically important sounds depends on the characteristics of both the noise source and the signal of interest (
e.g.,
signal-to-noise ratio, temporal variability, direction), in relation to each other and to an animal's hearing abilities (
e.g.,
sensitivity, frequency range, critical ratios, frequency discrimination, directional discrimination, age, or TTS hearing loss), and existing ambient noise and propagation conditions.

Under certain circumstances, marine mammals experiencing significant masking could also be impaired from maximizing their performance fitness in survival and reproduction. Therefore, when the coincident (masking) sound is human-made, it may be considered harassment when disrupting or altering critical behaviors. It is important to distinguish TTS and PTS, which persist after the sound exposure, from masking, which occurs during the sound exposure. Because masking (without resulting in TS) is not associated with abnormal physiological function, it is not considered a physiological effect, but rather a potential behavioral effect.

The frequency range of the potentially masking sound is important in determining any potential behavioral impacts. For example, low-frequency signals may have less effect on high-frequency echolocation sounds produced by odontocetes but are more likely to affect detection of mysticete communication calls and other potentially important natural sounds such as those produced by surf and some prey species. The masking of communication signals by anthropogenic noise may be considered as a reduction in the communication space of animals (
e.g.,
Clark
et al.,
2009) and may result in energetic or other costs as animals change their vocalization behavior (
e.g.,
Miller
et al.,
2000; Foote
et al.,
2004; Parks
et al.,
2007; Di Iorio and Clark, 2010; Holt
et al.,
2009). Masking can be reduced in situations where the signal and noise come from different directions (Richardson
et al.,
1995), through amplitude modulation of the signal, or through other compensatory behaviors (Houser, 2014). Masking can be tested directly in captive species (
e.g.,
Erbe, 2008), but in wild populations it must be either modeled or inferred from evidence of masking compensation. There are few studies addressing real-world masking sounds likely to be experienced by marine mammals in the wild (
e.g.,
Branstetter
et al.,
2013).

Marine mammals near the proposed project site are exposed to anthropogenic noise which may lead to some habituation, but is also a source of masking. Vocalization changes may result from a need to compete with an increase in background noise and include increasing the source level, modifying the frequency, increasing the call repetition rate of vocalizations, or ceasing to vocalize in the presence of increased noise (Hotchkin and Parks, 2013).

Masking is more likely to occur in the presence of broadband, relatively continuous noise sources. Energy distribution of pile driving covers a broad frequency spectrum, and sound from pile driving would be within the audible range of pinnipeds and cetaceans present in the proposed action area. While some construction during the DOT&PF's activities may mask some acoustic signals that are relevant to the daily behavior of marine mammals, the short-term duration and limited areas affected make it very unlikely that survival would be affected.

Airborne Acoustic Effects
—Pinnipeds that occur near the project site could be exposed to airborne sounds associated with construction activities that have the potential to cause behavioral harassment, depending on their distance from these activities. Airborne noise would primarily be an issue for pinnipeds that are swimming or hauled out near the project site within the range of noise levels elevated above airborne acoustic criteria. Although pinnipeds are known to haul-out regularly on man-made objects, incidents of take resulting solely from airborne sound are unlikely due to the sheltered proximity between the proposed project area and the known haulout sites (the closest known pinniped haulout is for harbor seals, which is located 4.5 km (2.8 mi) southeast of the proposed project site, but blocked by a land shadow). Cetaceans are not expected to be exposed to airborne sounds that would result in harassment as defined under the MMPA.

We recognize that pinnipeds in the water could be exposed to airborne sound that may result in behavioral harassment when looking with their heads above water. Most likely, airborne sound would cause behavioral responses similar to those discussed above in relation to underwater sound. For instance, anthropogenic sound could cause hauled-out pinnipeds to exhibit changes in their normal behavior, such as reduction in vocalizations, or cause them to temporarily abandon the area and move further from the source. However, these animals would previously have been “taken” because of exposure to underwater sound above the behavioral harassment thresholds, which are in all cases larger than those associated with airborne sound. Thus, the behavioral harassment of these animals is already accounted for in these estimates of potential take. Therefore, we do not believe that authorization of incidental take resulting from airborne sound for pinnipeds is warranted, and airborne sound is not discussed further here.

Potential Effects on Marine Mammal Habitat

The proposed project will occur within the same footprint as existing marine infrastructure. The nearshore and intertidal habitat where the proposed project will occur is an area of relatively high marine vessel traffic. Most marine mammals do not generally use the area within the footprint of the project area. Temporary, intermittent, and short-term habitat alteration may result from increased noise levels within the Level A and Level B harassment zones. Effects on marine mammals will be limited to temporary displacement from pile installation and removal noise, and effects on prey species will be similarly limited in time and space.

Water Quality
—Temporary and localized reduction in water quality will occur as a result of in-water construction activities. Most of this effect will occur during the installation and removal of piles and bedrock removal when bottom sediments are disturbed. The installation and removal of piles and bedrock removal will disturb bottom sediments and may cause a temporary increase in suspended sediment in the project area. During pile extraction, sediment attached to the pile moves vertically through the water column until gravitational forces cause it to slough off under its own weight. The small resulting sediment plume is expected to settle out of the water column within a few hours. Studies of the effects of turbid water on fish (marine mammal prey) suggest that concentrations of suspended sediment can reach thousands of milligrams per liter before an acute toxic reaction is expected (Burton, 1993).

Impacts to water quality from DTH hammers are expected to be similar to those described for pile driving. Impacts to water quality would be localized and temporary and would have negligible impacts on marine mammal habitat. Effects to turbidity and sedimentation are expected to be short-term, minor, and localized. Since the currents are strong in the area, following the completion of sediment-disturbing activities, suspended sediments in the water column should dissipate and quickly return to background levels in all construction scenarios. Turbidity within the water column has the potential to reduce the level of oxygen in the water and irritate the gills of prey fish species in the proposed project area. However, turbidity plumes associated with the project would be temporary and localized, and fish in the proposed project area would be able to move away from and avoid the areas where plumes may occur. Therefore, it is expected that the impacts on prey fish species from turbidity, and therefore on marine mammals, would be minimal and temporary. In general, the area likely impacted by the proposed construction activities is relatively small compared to the available marine mammal habitat in southeast Alaska.

Potential Effects on Prey
—Sound may affect marine mammals through impacts on the abundance, behavior, or distribution of prey species (
e.g.,
crustaceans, cephalopods, fish, zooplankton). Marine mammal prey varies by species, season, and location and, for some, is not well documented. Studies regarding the effects of noise on known marine mammal prey are described here.

Fish utilize the soundscape and components of sound in their environment to perform important functions such as foraging, predator avoidance, mating, and spawning (
e.g.,
Zelick and Mann, 1999; Fay, 2009). Depending on their hearing anatomy and peripheral sensory structures, which vary among species, fishes hear sounds using pressure and particle motion sensitivity capabilities and detect the motion of surrounding water (Fay
et al.,
2008). The potential effects of noise on fishes depends on the overlapping frequency range, distance from the sound source, water depth of exposure, and species-specific hearing sensitivity, anatomy, and physiology. Key impacts to fishes may include behavioral responses, hearing damage, barotrauma (pressure-related injuries), and mortality.

Fish react to sounds that are especially strong and/or intermittent low-frequency sounds. Short duration, sharp sounds can cause overt or subtle changes in fish behavior and local distribution. The reaction of fish to noise depends on the physiological state of the fish, past exposures, motivation (
e.g.,
feeding, spawning, migration), and other environmental factors. Hastings and Popper (2005) identified several studies that suggest fish may relocate to avoid certain areas of sound energy. Additional studies have documented effects of pile driving on fishes; several are based on studies in support of large, multiyear bridge construction projects (
e.g.,
Scholik and Yan, 2001, 2002; Popper and Hastings, 2009). Several studies have demonstrated that impulse sounds might affect the distribution and behavior of some fishes, potentially impacting foraging opportunities or increasing energetic costs (
e.g.,
Fewtrell and McCauley, 2012; Pearson
et al.,
1992; Skalski
et al.,
1992; Santulli
et al.,
1999; Paxton
et al.,
2017). However, some studies have shown no or slight reaction to impulse sounds (
e.g.,
Peña
et al.,
2013; Wardle
et al.,
2001; Jorgenson and Gyselman, 2009; Cott
et al.,
2012). More commonly, though, the impacts of noise on fishes are temporary.

SPLs of sufficient strength have been known to cause injury to fishes and fish mortality (summarized in Popper
et al.,
2014). However, in most fish species, hair cells in the ear continuously regenerate and loss of auditory function likely is restored when damaged cells are replaced with new cells. Halvorsen
et al.
(2012a) showed that a TTS of 4-6 dB was recoverable within 24 hours for one species. Impacts would be most severe when the individual fish is close to the source and when the duration of exposure is long. Injury caused by barotrauma can range from slight to severe and can cause death, and is most likely for fish with swim bladders. Barotrauma injuries have been documented during controlled exposure to impact pile driving (Halvorsen
et al.,
2012b; Casper
et al.,
2013).

Essential fish habitat (EFH) has been designated in the proposed project area for all five species of salmon
(i.e.,
chum salmon, pink salmon, coho salmon, sockeye salmon, and Chinook salmon; NMFS 2017), which are common prey of marine mammals. Many creeks flowing into Sukkwan Strait and nearby areas are known to contain salmonids, including three primary creeks: Hydaburg River, Natzuhini River, and Saltery Creek (Giefer and Blossom

2020); however, adverse effects on EFH in this area are not expected.

Fish populations in the proposed project area that serve as marine mammal prey could be temporarily affected by noise from pile installation and removal. The frequency range in which fish generally perceive underwater sounds is 50 to 2,000 Hz, with peak sensitivities below 800 Hz (Popper and Hastings, 2009). Fish behavior or distribution may change, especially with strong and/or intermittent sounds that could harm fish. High underwater SPLs have been documented to alter behavior, cause hearing loss, and injure or kill individual fish by causing serious internal injury (Hastings and Popper, 2005).

The greatest potential impact to fishes during construction would occur during impact pile driving and DTH excavation. In-water construction activities would only occur during daylight hours allowing fish to forage and transit the project area in the evening. Vibratory pile driving would possibly elicit behavioral reactions from fishes such as temporary avoidance of the area but is unlikely to cause injuries to fishes or have persistent effects on local fish populations. In general, impacts on marine mammal prey species are expected to be minor, localized, and temporary.

In-Water Construction Effects on Potential Foraging Habitat

The proposed activities would not result in permanent impacts to habitats used directly by marine mammals. The total seafloor area affected by pile installation and removal is a very small area compared to the vast foraging area available to marine mammals outside this project area. Construction would have minimal permanent and temporary impacts on benthic invertebrate species, a marine mammal prey source. In addition, although southeast Alaska in its entirety is listed as a BIA for humpback whales (Wild
et al.,
2023), the proposed project area does not contain particularly high-value habitat and is not unusually important for this species or any of the other species potentially impacted by the DOT&PF's proposed activities. Therefore, impacts of the project are not likely to have adverse effects on marine mammal foraging habitat in the proposed project area.

The area impacted by the proposed project is relatively small compared to the available habitat just outside the project area, and there are no areas of particular importance that would be impacted by this project. Any behavioral avoidance by fish of the disturbed area would still leave significantly large areas of fish and marine mammal foraging habitat in the nearby vicinity. As described in the preceding, the potential for the DOT&PF's construction to affect the availability of prey to marine mammals or to meaningfully impact the quality of physical or acoustic habitat is considered to be insignificant.

Estimated Take

This section provides an estimate of the number of incidental takes proposed for authorization through this IHA, which will inform both NMFS' consideration of “small numbers,” and the negligible impact determinations.

Harassment is the only type of take expected to result from these activities. Except with respect to certain activities not pertinent here, section 3(18) of the MMPA defines “harassment” as any act of pursuit, torment, or annoyance, which (i) has the potential to injure a marine mammal or marine mammal stock in the wild (Level A harassment); or (ii) 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 (Level B harassment).

Authorized takes would primarily be by Level B harassment, as use of the acoustic source (
i.e.,
vibratory pile driving, impact pile driving, and DTH systems) has the potential to result in disruption of behavioral patterns for individual marine mammals. There is also some potential for auditory (Level A harassment) to result, primarily for mysticetes and high frequency species and phocids because predicted auditory injury zones are larger than for mid-frequency species and otariids. Auditory injury is unlikely to occur for mid-frequency species or otariids. The proposed mitigation and monitoring measures are expected to minimize the severity of the taking to the extent practicable. As described previously, no serious injury or mortality is anticipated or proposed to be authorized for this activity. Below we describe how the proposed take numbers are estimated.

For acoustic impacts, generally speaking, we estimate take by considering: (1) acoustic thresholds above which NMFS believes the best available science indicates marine mammals will be behaviorally harassed or incur some degree of permanent hearing impairment; (2) the area or volume of water that will be ensonified above these levels in a day; (3) the density or occurrence of marine mammals within these ensonified areas; and, (4) the number of days of activities. We note that while these factors can contribute to a basic calculation to provide an initial prediction of potential takes, additional information that can qualitatively inform take estimates is also sometimes available (
e.g.,
previous monitoring results or average group size). Below, we describe the factors considered here in more detail and present the proposed take estimates.

Acoustic Thresholds

NMFS recommends the use of acoustic thresholds that identify the received level of underwater sound above which exposed marine mammals would be reasonably expected to be behaviorally harassed (equated to Level B harassment) or to incur PTS of some degree (equated to Level A harassment).

Level B Harassment
—Though significantly driven by received level, the onset of behavioral disturbance from anthropogenic noise exposure is also informed to varying degrees by other factors related to the source or exposure context (
e.g.,
frequency, predictability, duty cycle, duration of the exposure, signal-to-noise ratio, distance to the source), the environment (
e.g.,
bathymetry, other noises in the area, predators in the area), and the receiving animals (hearing, motivation, experience, demography, life stage, depth) and can be difficult to predict (
e.g.,
Southall
et al.,
2007, 2021, Ellison
et al.,
2012). Based on what the available science indicates and the practical need to use a threshold based on a metric that is both predictable and measurable for most activities, NMFS typically uses a generalized acoustic threshold based on received level to estimate the onset of behavioral harassment. NMFS generally predicts that marine mammals are likely to be behaviorally harassed in a manner considered to be Level B harassment when exposed to underwater anthropogenic noise above root-mean-squared pressure received levels (RMS SPL) of 120 dB re 1 μPa for continuous (
e.g.,
vibratory pile-driving, drilling) and above RMS SPL 160 dB re 1 μPa for non-explosive impulsive (
e.g.,
seismic airguns) or intermittent (
e.g.,
scientific sonar) sources. Generally speaking, Level B harassment take estimates based on these behavioral harassment thresholds are expected to include any likely takes by TTS as, in most cases, the likelihood of TTS occurs at distances from the source less than those at which behavioral harassment is likely. TTS of a sufficient degree can manifest as behavioral harassment, as reduced hearing sensitivity and the

potential reduced opportunities to detect important signals (conspecific communication, predators, prey) may result in changes in behavior patterns that would not otherwise occur.

The DOT&PF's proposed activity includes the use of continuous (vibratory pile driving) and intermittent (impact pile driving) sources, and therefore the RMS SPL thresholds of 120 and 160 dB re 1 μPa are applicable. DTH systems have both continuous, non-impulsive, and impulsive components as discussed in the
Description of Sound Sources
section above. When evaluating Level B harassment, NMFS recommends treating DTH as a continuous source and applying the RMS SPL thresholds of 120 dB re 1 μPa.

Level A Harassment
—NMFS' Technical Guidance for Assessing the Effects of Anthropogenic Sound on Marine Mammal Hearing (Version 2.0) (Technical Guidance, 2018) identifies dual criteria to assess auditory injury (Level A harassment) to five different marine mammal groups (based on hearing sensitivity) as a result of exposure to noise from two different types of sources (impulsive or non-impulsive). The DOT&PF's proposed construction includes the use of impulsive (impact pile driving) and non-impulsive (vibratory pile driving) sources. As described above, DTH includes both impulsive and non-impulsive characteristics. When evaluating Level A harassment, NMFS recommends treating DTH as an impulsive source.

The thresholds used to identify the onset of PTS are provided in Table 4. The references, analysis, and methodology used in the development of the thresholds are described in NMFS' 2018 Technical Guidance, which may be accessed at:
www.fisheries.noaa.gov/national/marine-mammal-protection/marine-mammal-acoustic-technical-guidance.

Table 4—Thresholds Identifying the Onset of Permanent Threshold Shift

Hearing Group

PTS onse

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Source: Frix Law Library, https://www.frixlaw.com/law-library/documents/fr%3A2023-14939. Public record. Not legal advice.