2024-22724. Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to a Marine Geophysical Survey of the Chain Transform Fault in the Equatorial Atlantic Ocean

Table 1—Species Likely Impacted by the Specified Activities

Common name	Scientific name	Stock	ESA/MMPA status; strategic (Y/N) 1	Modeled abundance 2
Order Artiodactyla—Cetacea—Mysticeti (baleen whales)
Family Balaenopteridae (rorquals):
Blue Whale	Balaenoptera musculus	NA	E, D, Y	2  191/2,4  300
( print page 80216)
Fin Whale	Balaenoptera physalus	NA	E, D, Y	11,672
Humpback Whale	Megaptera novaeangliae	NA	-, -, N	2  4,990/5  42,000
Common Minke Whale	Balaenoptera acutorostrata	NA	-, -, N	13,784
Antarctic Minke Whale	Balaenoptera bonaerensis	NA	-, -, N	3  515,000
Sei Whale	Balaenoptera borealis	NA	E, D, Y	19,530
Bryde's Whale	Balaenoptera edeni	NA	-, -, N	536
Odontoceti (toothed whales, dolphins, and porpoises)
Family Physeteridae:
Sperm Whale	Physeter macrocephalus	NA	E, D, Y	64,015
Family Kogiidae:
Pygmy Sperm Whale	Kogia breviceps	NA	-, -, N	7  26,043
Dwarf Sperm Whale	Kogia sima	NA	-, -, N
Family Ziphiidae (beaked whales):
Blainville's Beaked Whale	Mesoplodon densirostris	NA	-, -, N	8  65,069
Cuvier's Beaked Whale	Ziphius cavirostris	NA	-, -, N
Gervais' Beaked Whale	Mesoplodon europaeus	NA	-, -, N
Family Delphinidae:
Killer Whale	Orcinus orca	NA	-, -, N	972
Short-Finned Pilot Whale	Globicephala melas	NA	-, -, N	6  264,907
Rough-toothed Dolphin	Steno bredanensis	NA	-, -, N	32,848
Bottlenose Dolphin	Tursiops truncatus	NA	-, -, N	418,151
Risso's Dolphin	Grampus griseus	NA	-, -, N	78,205
Common Dolphin	Delphinus delphis	NA	-, -, N	473,260
Striped Dolphin	Stenella coeruleoalba	NA	-, -, N	412,729
Pantropical Spotted Dolphin	Stenella attenuata	NA	-, -, N	321,740
Atlantic Spotted Dolphin	Stenella frontalis	NA	-, -, N	259,519
Spinner Dolphin	Stenella longirostris	NA	-, -, N	152,511
Clymene Dolphin	Stenella clymene	NA	-, -, N	181,209
Fraser's Dolphin	Lagenodelphis hosei	NA	-, -, N	19,585
Melon-headed Whale	Peponocephala electra	NA	-, -, N	64,114
Pygmy Killer Whale	Feresa attenuata	NA	-, -, N	9,001
False Killer Whale	Pseudorca crassidens	NA	-, -, N	12,682
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  Modeled abundance value from U.S Navy Atlantic Fleet Training and Testing Area Marine Mammal Density (AFTT) (Roberts et al., 2023) unless otherwise noted.
3  Abundance of minke whales (species unspecified) for the Southern Hemisphere (IWC 2024)
4  Abundance of blue whales (excluding pygmy blue whales) for Southern Hemisphere (IWC 2024)
5  Abundance of humpback whales on Antarctic feeding grounds (IWC 2024)
6  Pilot whale guild.
7  Estimate includes dwarf and pygmy sperm whales.
8  Beaked whale guild.

All 28 species in table 1 temporally and spatially co-occur with the activity to the degree that take is reasonably likely to occur. All species that could potentially occur in the planned survey area are listed in section 3 of the application.

A detailed description of the of the species likely to be affected by the geophysical survey, including brief introductions to the species and relevant stocks as well as available information regarding population trends and threats, and information regarding local occurrence, were provided in the Federal Register notice for the proposed IHA (89 FR 56158, July 8, 2024). Since that time, we are not aware of any changes in the status of these species and stocks; therefore, detailed descriptions are not provided here. Please refer to that Federal Register notice for these descriptions. Please also refer to NMFS' website ( https://www.fisheries.noaa.gov/​find-species) for generalized species accounts.

Marine Mammal Hearing

Hearing is the most important sensory modality for marine mammals underwater, and exposure to anthropogenic sound can have deleterious effects. To appropriately assess the potential effects of exposure to sound, it is necessary to understand the frequency ranges marine mammals are able to hear. Not all marine mammal species have equal hearing capabilities ( e.g., Richardson et al., 1995; Wartzok and Ketten, 1999; Au and Hastings, 2008). To reflect this, Southall et al. (2007, 2019) recommended that marine mammals be divided into hearing groups based on directly measured (behavioral or auditory evoked potential techniques) or estimated hearing ranges (behavioral response data, anatomical modeling, etc.). Subsequently, NMFS (2018) 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- ( print page 80217) 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 2.

Table 2—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).

For more detail concerning these groups and associated frequency ranges, please see NMFS (2018) for a review of available information.

Potential Effects of Specified Activities on Marine Mammals and Their Habitat

The effects of underwater noise from L-DEO's survey activities have the potential to result in behavioral harassment of marine mammals in the vicinity of the survey area. The notice of proposed IHA (89 FR 56158, July 8, 2024) included a discussion of the effects of anthropogenic noise on marine mammals and the potential effects of underwater noise from L-DEO on marine mammals and their habitat. That information and analysis is incorporated by reference into this final IHA determination and is not repeated here; please refer to the notice of proposed IHA (89 FR 56158, July 8, 2024).

Estimated Take of Marine Mammals

This section provides an estimate of the number of incidental takes authorized through the IHA, which will inform 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).

Anticipated takes would primarily be by Level B harassment, as the noise from use of the airgun array has the potential to result in disruption of behavioral patterns for individual marine mammals. There is also some potential for auditory injury (Level A harassment) to result for species of certain hearing groups (LF and HF) due to the size of the predicted auditory injury zones for those groups. Auditory injury is less likely to occur for mid-frequency species due to their relative lack of sensitivity to the frequencies at which the primary energy of an airgun signal is found as well as such species' general lower sensitivity to auditory injury as compared to high-frequency cetaceans. As discussed in further detail below, we do not expect auditory injury for mid-frequency cetaceans. No mortality or serious injury is anticipated as a result of these activities. Below we describe how the authorized 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 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 permanent threshold shift (PTS) of some degree (equated to Level A harassment). Acoustic thresholds used in this analysis were discussed in detail in the notice of proposed IHA (89 FR 56158, July 8, 2024) and not repeated here. Please see that notice for additional detail.

Ensonified Area

Here, we describe operational and environmental parameters of the activity that are used in estimating the area ensonified above the acoustic thresholds, including source levels and transmission loss coefficient.

When the Technical Guidance was published (NMFS, 2016), in recognition of the fact that ensonified area/volume could be more technically challenging to predict because of the duration component in the new thresholds, we developed a user spreadsheet that includes tools to help predict a simple isopleth that can be used in conjunction with marine mammal density or occurrence to help predict takes. We note that because of some of the assumptions included in the methods used for these tools, we anticipate that isopleths produced are typically going to be overestimates of some degree, which may result in some degree of overestimate of Level A harassment take. However, these tools offer the best way to predict appropriate isopleths when more sophisticated 3D modeling methods are not available, and NMFS continues to develop ways to quantitatively refine these tools and will ( print page 80218) qualitatively address the output where appropriate.

The planned survey would entail the use of a 36-airgun array with a total discharge volume of 6,600 in³ at a tow depth of 9 m to 12 m. L-DEO's model results are used to determine the 160 dB root mean square (rms) radius for the airgun source down to a maximum depth of 2,000 m. Received sound levels have been predicted by L-DEO's model (Diebold et al. 2010) as a function of distance from the 36-airgun array. This modeling approach uses ray tracing for the direct wave traveling from the array to the receiver and its associated source ghost (reflection at the air-water interface in the vicinity of the array), in a constant-velocity half-space (infinite homogeneous ocean layer, unbounded by a seafloor). In addition, propagation measurements of pulses from the 36-airgun array at a tow depth of 6 m have been reported in deep water (~1,600 m), intermediate water depth on the slope (~600-1,100 m), and shallow water (~50 m) in the Gulf of Mexico (Tolstoy et al. 2009; Diebold et al. 2010).

For deep and intermediate water cases, the field measurements cannot be used readily to derive the harassment isopleths, as at those sites the calibration hydrophone was located at a roughly constant depth of 350-550 m, which may not intersect all the sound pressure level (SPL) isopleths at their widest point from the sea surface down to the assumed maximum relevant water depth (~2000 m) for marine mammals. At short ranges, where the direct arrivals dominate and the effects of seafloor interactions are minimal, the data at the deep sites are suitable for comparison with modeled levels at the depth of the calibration hydrophone. At longer ranges, the comparison with the model—constructed from the maximum SPL through the entire water column at varying distances from the airgun array—is the most relevant.

In deep and intermediate water depths at short ranges, sound levels for direct arrivals recorded by the calibration hydrophone and L-DEO model results for the same array tow depth are in good alignment (see figures 12 and 14 in Diebold et al. 2010). Consequently, isopleths falling within this domain can be predicted reliably by the L-DEO model, although they may be imperfectly sampled by measurements recorded at a single depth. At greater distances, the calibration data show that seafloor-reflected and sub-seafloor-refracted arrivals dominate, whereas the direct arrivals become weak and/or incoherent (see figures 11, 12, and 16 in Diebold et al. 2010). Aside from local topography effects, the region around the critical distance is where the observed levels rise closest to the model curve. However, the observed sound levels are found to fall almost entirely below the model curve. Thus, analysis of the Gulf of Mexico calibration measurements demonstrates that although simple, the L-DEO model is a robust tool for conservatively estimating isopleths.

The planned high-energy survey would acquire data with the 36-airgun array at a tow depth of 9 to 12 m. For this survey, which occurs only in deep water (>1,000 m), we use the deep-water radii obtained from L-DEO model results down to a maximum water depth of 2,000 m for the 36-airgun array.

L-DEO's modeling methodology is described in greater detail in L-DEO's application. The estimated distances to the Level B harassment isopleth for the airgun configuration are shown in table 3.

Table 3—Predicted Radial Distances From the R/V Langseth Seismic Source to Isopleth Corresponding to Level B Harassment Threshold

Airgun configuration	Tow depth (m) 1	Water depth (m)	Predicted distances (in m) to the Level B harassment threshold
4 strings, 36 airguns, 6,600 in3	12	>1,000	2  6,733
1  Maximum tow depth was used for conservative distances.
2  Distance is based on L-DEO model results.

Table 4—Modeled Radial Distance to Isopleths Corresponding to Level A Harassment Thresholds

	Low frequency cetaceans	Mid frequency cetaceans	High frequency cetaceans
PTS SEL cum	426.9	0	1.3
PTS Peak	38.9	13.6	268.3
The largest distance (in bold ) of the dual criteria (cumulative sound exposure level (SEL cum ) or Peak) was used to estimate threshold distances and potential takes by Level A harassment.

Table 4 presents the modeled PTS isopleths for each cetacean hearing group based on L-DEO modeling incorporated in the companion user spreadsheet, for the high-energy surveys with the shortest shot interval ( i.e. greatest potential to cause PTS based on accumulated sound energy) (NMFS 2018).

Predicted distances to Level A harassment isopleths, which vary based on marine mammal hearing groups, were calculated based on modeling performed by L-DEO using the Nucleus software program and the NMFS user spreadsheet, described below. The acoustic thresholds for impulsive sounds contained in the NMFS Technical Guidance were presented as dual metric acoustic thresholds using both SEL_cum and peak sound pressure metrics (NMFS 2016). As dual metrics, NMFS considers onset of PTS (Level A harassment) to have occurred when either one of the two metrics is exceeded ( i.e., metric resulting in the largest isopleth). The SEL_cum metric considers both level and duration of exposure, as well as auditory weighting functions by marine mammal hearing group.

The SEL_cum for the 36-airgun array is derived from calculating the modified farfield signature. The farfield signature is often used as a theoretical representation of the source level. To compute the farfield signature, the source level is estimated at a large distance (right) below the array ( e.g., 9 km), and this level is back projected mathematically to a notional distance of ( print page 80219) 1 m from the array's geometrical center. However, it has been recognized that the source level from the theoretical farfield signature is never physically achieved at the source when the source is an array of multiple airguns separated in space (Tolstoy et al., 2009). Near the source (at short ranges, distances <1 km), the pulses of sound pressure from each individual airgun in the source array do not stack constructively as they do for the theoretical farfield signature. The pulses from the different airguns spread out in time such that the source levels observed or modeled are the result of the summation of pulses from a few airguns, not the full array (Tolstoy et al., 2009). At larger distances, away from the source array center, sound pressure of all the airguns in the array stack coherently, but not within one time sample, resulting in smaller source levels (a few dB) than the source level derived from the far-field signature. Because the far-field signature does not take into account the large array effect near the source and is calculated as a point source, the far-field signature is not an appropriate measure of the sound source level for large arrays. See L-DEO's application for further detail on acoustic modeling.

Auditory injury is unlikely to occur for mid-frequency cetaceans, given the very small modeled zones of injury for those species (all estimated zones are less than 15 m for mid-frequency cetaceans), in the context of distributed source dynamics.

In consideration of the received sound levels in the near-field as described above, we expect the potential for Level A harassment of mid-frequency cetaceans to be de minimis, even before the likely moderating effects of aversion and/or other compensatory behaviors ( e.g., Nachtigall et al., 2018) are considered. We do not anticipate that Level A harassment is a likely outcome for any mid-frequency cetacean and do not authorize any take by Level A harassment for these species.

The Level A and Level B harassment estimates are based on a consideration of the number of marine mammals that could be within the area around the operating airgun array where received levels of sound ≥160 dB referenced to 1 micropascal (re 1 µPa) rms are predicted to occur. The estimated numbers are based on the densities (numbers per unit area) of marine mammals expected to occur in the area in the absence of seismic surveys. To the extent that marine mammals tend to move away from seismic sources before the sound level reaches the criterion level and tend not to approach an operating airgun array, these estimates likely overestimate the numbers actually exposed to the specified level of sound.

Marine Mammal Occurrence

In this section we provide information about the occurrence of marine mammals, including density or other relevant information which will inform the take calculations.

Habitat-based stratified marine mammal densities for the North Atlantic are taken from the US Navy Atlantic Fleet Training and Testing Area Marine Mammal Density (Roberts et al., 2023; Mannocci et al., 2017), which represent the best available information regarding marine mammal densities in the region. This density information incorporates visual line-transect surveys of marine mammals for over 35 years, resulting in various studies that estimated the abundance, density, and distributions of marine mammal populations. The habitat-based density models consisted of 5 km x 5 km grid cells. As the AFTT model does not overlap the planned survey area, the average densities in the grid cells for the AFTT area that encompassed a similar-sized area as the planned survey area in the southeastern-most part of the AFTT area were used (between ~21.1° N-22.5° N and ~45.1° W-49.5° W). Even though these densities are for the western Atlantic Ocean, they are for an area of the Mid-Atlantic Ridge, which would be most representative of densities occurring at the Mid-Atlantic Ridge in the planned survey area. More information is available online at https://seamap.env.duke.edu/​models/​Duke/​AFTT/​.

Since there was no density data available for the actual survey area, L-DEO used OBIS sightings, available literature, and regional distribution maps of the actual survey area (or greater region) to determine which species would be expected to be encountered in the survey area. From the AFTT models, L-DEO excluded the following species, as they were not expected to occur in the survey area: seals, northern bottlenose whales, North Atlantic right whale (these had densities of zero) and harbor porpoise, white-beaked dolphin, and Atlantic white-sided dolphin (these species had non-zero densities). There were no additional species that might occur in the survey area that were not available in the AFTT model.

For most species, only annual densities were available. For some baleen whale species (fin, sei and humpback whale), monthly densities were available. For these species, the highest monthly densities were used. Densities for fin whales were near zero and the calculations did not result in any estimated takes. However, because this species could be encountered in the survey area, we authorize take of one individual.

Take Estimation

Here, we describe how the information provided above is synthesized to produce a quantitative estimate of the take that is reasonably likely to occur and authorized. In order to estimate the number of marine mammals predicted to be exposed to sound levels that would result in Level A or Level B harassment, radial distances from the airgun array to the predicted isopleth corresponding to the Level A harassment and Level B harassment thresholds are calculated, as described above. Those radial distances were then used to calculate the area(s) around the airgun array predicted to be ensonified to sound levels that exceed the harassment thresholds. The distance for the 160-dB Level B harassment threshold and PTS (Level A harassment) thresholds (based on L-DEO model results) was used to draw a buffer around the area expected to be ensonified ( i.e., the survey area). The ensonified areas were then increased by 25 percent to account for potential delays, which is equivalent to adding 25 percent to the planned line km to be surveyed. The density for each species was then multiplied by the daily ensonified areas (increased as described above) and then multiplied by the number of survey days (11.5) to estimate potential takes (see appendix B of L-DEO's application for more information).

L-DEO assumed that their estimates of marine mammal exposures above harassment thresholds equate to take and requested authorization of those takes. Those estimates in turn form the basis for our authorized take authorization numbers. For the species for which NMFS does not expect there to be a reasonable potential for take by Level A harassment to occur ( i.e., mid-frequency cetaceans), we have added L-DEO's estimated exposures above Level A harassment thresholds to their estimated exposures above the Level B harassment threshold to produce a total number of incidents of take by Level B harassment that is authorized. Estimated exposures and authorized take numbers are shown in table 5. ( print page 80220)

Table 5—Authorized Take

Species	Estimated take		Authorized take		Modeled abundance 1	Percent of abundance 2
	Level B	Level A	Level B	Level A
Humpback whale	39	2	39	2	4,990	0.82
Bryde's whale	4	0	4	0	536	0.75
Minke whale 3	23	1	23	1	13,784	0.17
Fin whale	0	0	1	0	11,672	0.01
Sei whale	11	1	11	1	19,530	0.06
Blue whale	1	0	1	0	191	0.52
Sperm whale	110	0	110	0	64,015	0.17
Beaked whales 4	106	0	106	0	65,069	0.16
Risso's dolphin	88	0	88	0	78,205	0.11
Rough-toothed dolphin	166	0	166	0	32,848	0.51
Bottlenose dolphin	1,229	2	1,231	0	418,151	0.30
Pantropical spotted dolphin	46	0	7  76	0	321,740	0.02
Atlantic spotted dolphin	435	1	436	0	259,519	0.17
Spinner dolphin	898	2	900	0	152,511	0.59
Striped dolphin	55	0	7  73	0	412,729	0.02
Clymene dolphin	1,038	2	1,040	0	181,209	0.57
Fraser's dolphin	110	0	110	0	19,585	0.56
Common dolphin	27	0	7  92	0	473,206	0.02
Short-finned pilot whale 5	1,301	2	1,303	0	264,907	0.49
Melon-headed whale	502	1	503	0	64,114	0.78
False killer whale	99	0	99	0	12,682	0.78
Pygmy killer whale	71	0	71	0	9,001	0.79
Killer whale	1	0	7  5	0	972	0.51
Kogia spp 6	122	5	122	5	26,043	0.49
1  Modeled abundance (Roberts et al. 2023) or North Atlantic abundance (NAMMCO 2023), where applicable.
2  Requested take authorization is expressed as percent of population for the AFTT Area only (Roberts et al. 2023).
3  Takes assigned equally between Common minke whales (11 Level B takes and 1 Level A take) and Antarctic minke whales (12 Level B takes).
4  Beaked whale guild. Includes Cuvier's beaked whale, Blaineville's beaked whale, and Gervais' beaked whale.
5  Takes based on density for Globicephala sp. All takes are assumed to be for short-finned pilot whales.
6  Kogia spp. Includes Pygmy sperm whale and Dwarf sperm whale.
7  Takes rounded to a mean group size (Weir 2011).