2025-01143. Takes of Marine Mammals Incidental to Specified Activities; Taking Marine Mammals Incidental to a Marine Geophysical Survey in the Northwest Gulf of Mexico

Table 1—Species ¹ Likely Affected by the Specified Activities

Common name	Scientific name	Stock	ESA/MMPA status; strategic (Y/N) 2	Stock abundance (CV, N min , most recent abundance survey) 3	PBR	Annual M/SI 4	GOM population abundance 5
Odontoceti (toothed whales, dolphins, and porpoises)
Family Delphinidae
Atlantic spotted dolphin	Stenella frontalis	GOM	-/-; N	21,506 (0.26; 17,339; 2018)	166	6  36	7  12,240
Rough-toothed dolphin	Steno bredanensis	GOM	-/-; N	unk (n/a; unk; 2018)	undetermined	39	4,853
Bottlenose dolphin	Tursiops truncatus	GOM Western Coastal	-/-; N	20,759 (0.13; 18,585; 2018)	167	36	7  151,886
		Northern GOM Continental Shelf	-/-; N	63,280 (0.11; 57,917; 2018)	556	5  65
1  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/​).
2  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.
3  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.
4  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 M/SI often cannot be determined precisely and is in some cases presented as a minimum value or range. A CV associated with estimated mortality due to commercial fisheries is presented in some cases.
5  Model-predicted stock abundance for Atlantic spotted dolphins and bottlenose dolphins from the most recent GOM density models (Garrison et al., 2023). Stock abundance for rough-toothed dolphins was taken from Roberts et al. (2016) density models, as Garrison et al. (2023) did not create a model for this species.
6  M/SI is a minimum count and does not include projected mortality estimates for 2015-2019 due to the DWH oil spill.
7  This estimate includes both coastal and continental shelf bottlenose dolphins from other stocks.

As indicated above, all three species (with four managed stocks) 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 areas are included in table 2 of the IHA application.

A detailed description of the species likely to be affected by the marine 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 91340, November 19, 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 ( print page 5817) modeling, etc.). On October 24, 2024, NMFS published (89 FR 84872) the final Updated Technical Guidance, which includes updated thresholds and weighting functions to inform auditory injury estimates, and has replaced the 2018 Technical Guidance used previously (NMFS 2018). The updated hearing groups are presented below (table 2). The references, analysis, and methodology used in the development of the hearing groups are described in NMFS' 2024 Technical Guidance, which may be accessed at: https://www.fisheries.noaa.gov/​national/​marine-mammal-protection/​marine-mammal-acoustic-technical-guidance.

Table 2—Marine Mammal Hearing Groups

[NMFS, 2024]

Hearing group	Generalized hearing range *
Underwater:
Low-frequency (LF) cetaceans (baleen whales)	7 Hz to 36 * kHz.
High-frequency (HF) cetaceans (dolphins, toothed whales, beaked whales, bottlenose whales)	150 Hz to 160 kHz.
Very High-frequency (VHF) cetaceans (true porpoises, Kogia, river dolphins, Cephalorhynchid, Lagenorhynchus cruciger & L. australis)	200 Hz to 165 kHz.
Phocid pinnipeds (PW) (underwater) (true seals)	40 Hz to 90 kHz.
Otariid pinnipeds (OW) (underwater) (sea lions and fur seals)	60 Hz to 68 kHz.
* Represents the generalized hearing range for the entire group as a composite ( i.e., all species within the group), where individual species' hearing ranges may not be as broad. Generalized hearing range chosen based on ~65 dB threshold from composite audiogram, previous analysis in NMFS 2018, and/or data from Southall et al., 2007; Southall et al., 2019. Additionally, animals are able to detect very loud sounds above and below that “generalized” hearing range.

Potential Effects of Specified Activities on Marine Mammals and Their Habitat

The effects of underwater noise from UT'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 91340, November 19, 2024) included a discussion of the effects of anthropogenic noise on marine mammals and the potential effects of underwater noise from UT's survey activity on marine mammals and their habitat. That information and analysis is referenced in this final IHA determination and is not repeated here; please refer to the notice of proposed IHA (89 FR 91340, November 19, 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,” the negligible impact determinations, and impacts on subsistence uses.

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 will be by Level B harassment only, in the form of disruption of behavioral patterns for individual marine mammals resulting from exposure to sound from low energy seismic airguns. Based on the nature of the activity, i.e., use of a low energy 2-airgun array, auditory injury (Level A harassment) is neither anticipated nor authorized. As described previously, no serious injury or mortality is anticipated or authorized for this activity. 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 likely 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 auditory injury 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 decibels (dB) (referenced to 1 micropascal (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 ( print page 5818) thresholds are expected to include any likely takes by temporary threshold shift (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.

UT's planned survey includes the use of impulsive seismic sources ( e.g., GI-airguns) and therefore, the 160 dB re 1 μPa (rms) criteria is applicable for analysis of Level B harassment.

Level A harassment —NMFS' Updated Technical Guidance for Assessing the Effects of Anthropogenic Sound on Marine Mammal Hearing (NMFS, 2024 (2024 Updated Technical Guidance)) 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). NMFS defines auditory injury as “damage to the inner ear that can result in destruction of tissue . . . which may or may not result in permanent threshold shift (PTS)” (NMFS, 2024). 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, 2024). 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 (Au and Hastings, 2008; Finneran, 2016). UT's planned survey includes the use of impulsive seismic sources ( i.e., airguns).

UT previously used modeling based on NMFS' 2018 technical guidance in order to calculate their isopleths. Based on the outcome of these comparisons/analyses using the Updated 2024 Technical Guidance, the low-frequency and high-frequency (now very high-frequency) cetacean isopleths are slightly larger using the updated guidance, and the mid-frequency (now high-frequency) cetacean isopleth is the same as those calculated using the 2018 Technical Guidance. As NMFS is only authorizing takes of mid-frequency (now high-frequency) cetaceans, the isopleths based on the 2018 Technical Guidance will be used as the basis for take numbers and mitigation zones for this IHA.

The 2018 and 2024 thresholds are provided in the tables below. The references, analysis, and methodology used in the development of the thresholds are described in NMFS' 2018 Technical Guidance and in NMFS' 2024 Updated Technical Guidance, which may be accessed at: https://www.fisheries.noaa.gov/​national/​marine-mammal-protection/​marine-mammal-acoustic-technical-guidance.

Table 3—NMFS' 2018 Thresholds Identifying the Onset of Permanent Threshold Shift

[PTS]

Hearing group	PTS onset acoustic thresholds * (received level)
	Impulsive	Non-impulsive
Low-Frequency (LF) Cetaceans	Cell 1:Lpk,flat : 219 dB; LE,LF,24h : 183 dB	Cell 2:LE,LF,24h : 199 dB.
Mid-Frequency (MF) Cetaceans	Cell 3:Lpk,flat : 230 dB; LE,MF,24h : 185 dB	Cell 4:LE,MF,24h : 198 dB.
High-Frequency (HF) Cetaceans	Cell 5:Lpk,flat : 202 dB; LE,HF,24h : 155 dB	Cell 6:LE,HF,24h : 173 dB.
Phocid Pinnipeds (PW) (Underwater)	Cell 7:Lpk,flat : 218 dB; LE,PW,24h : 185 dB	Cell 8:LE,PW,24h : 201 dB.
Otariid Pinnipeds (OW) (Underwater)	Cell 9:Lpk,flat : 232 dB; LE,OW,24h : 203 dB	Cell 10:LE,OW,24h : 219 dB.
* Dual metric acoustic thresholds for impulsive sounds: Use whichever results in the largest isopleth for calculating PTS onset. If a non-impulsive sound has the potential of exceeding the peak sound pressure level thresholds associated with impulsive sounds, these thresholds should also be considered.
Note: Peak sound pressure ( Lpk ) has a reference value of 1 µPa, and cumulative sound exposure level ( LE ) has a reference value of 1µPa2 s. In this table, thresholds are abbreviated to reflect American National Standards Institute standards (ANSI, 2013). However, peak sound pressure is defined by ANSI as incorporating frequency weighting, which is not the intent for this Technical Guidance. Hence, the subscript “flat” is being included to indicate peak sound pressure should be flat weighted or unweighted within the generalized hearing range. The subscript associated with cumulative sound exposure level thresholds indicates the designated marine mammal auditory weighting function (LF, MF, and HF cetaceans, and PW and OW pinnipeds) and that the recommended accumulation period is 24 hours. The cumulative sound exposure level thresholds could be exceeded in a multitude of ways ( i.e., varying exposure levels and durations, duty cycle). When possible, it is valuable for action proponents to indicate the conditions under which these acoustic thresholds will be exceeded.

Table 4—NMFS' 2024 Thresholds Identifying the Onset of Auditory Injury

[AUD INJ]

Hearing group	AUD INJ onset acoustic thresholds * (received level)
	Impulsive	Non-impulsive
Low-Frequency (LF) Cetaceans	Cell 1:Lpk,flat : 222 dB; LE,LF,24h : 183 dB	Cell 2:LE,LF,24h : 197 dB.
High-Frequency (HF) Cetaceans	Cell 3:Lpk,flat : 230 dB; LE,HF,24h : 193 dB	Cell 4:LE,HF,24h : 201 dB.
Very High-Frequency (VHF) Cetaceans	Cell 5:Lpk,flat : 202 dB; LE,VHF,24h : 159 dB	Cell 6:LE,VHF,24h : 181 dB.
Phocid Pinnipeds (PW) (Underwater)	Cell 7:Lpk,flat : 223 dB; LE,PW,24h : 185 dB	Cell 8:LE,PW,24h : 195 dB.
Otariid Pinnipeds (OW) (Underwater)	Cell 9:Lpk,flat : 230 dB; LE,OW,24h : 185 dB	Cell 10:LE,OW,24h : 199 dB.
* Dual metric criteria for impulsive sounds: Use whichever criteria results in the larger isopleth for calculating AUD INJ onset. If a non-impulsive sound has the potential of exceeding the peak sound pressure level criteria associated with impulsive sounds, the PK SPL criteria are recommended for consideration for non-impulsive sources.
Note: Peak sound pressure level (Lp,0-pk) has a reference value of 1 µPa (underwater) and 20 µPa (in air), and weighted cumulative sound exposure level (LE,p) has a reference value of 1 µPa2s (underwater) and 20 µPa2s (in air). In this table, criteria are abbreviated to be more reflective of International Organization for Standardization standards (ISO 2017; ISO 2020). The subscript “flat” is being included to indicate peak sound pressure are flat weighted or unweighted within the generalized hearing range of marine mammals underwater ( i.e., 7 Hz to 165 kHz) or in air ( i.e., 42 Hz to 52 kHz). The subscript associated with cumulative sound exposure level criteria indicates the designated marine mammal auditory weighting function (LF, HF, and VHF cetaceans, and PW, OW, PA, and OA pinnipeds) and that the recommended accumulation period is 24 hours. The weighted cumulative sound exposure level criteria could be exceeded in a multitude of ways ( i.e., varying exposure levels and durations, duty cycle). When possible, it is valuable for action proponents to indicate the conditions under which these criteria will be exceeded.

( print page 5819)

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 initially 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 overestimation 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 qualitatively address the output where appropriate.

The planned survey will entail the use up to two 105 in³ airguns with a maximum total discharge of 210 in³ at a tow depth of 3-4 m. UT used modeling by Lamont-Doherty Earth Observatory (L-DEO), which determines the 160 dB_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 2-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).

The planned low-energy survey will acquire data with up to two 105-in³ GI guns, towed in-line, at a depth of 3-4 m. The shallow-water radii are obtained by scaling the empirically derived measurements from the GOM calibration survey to account for the differences in volume and tow depth between the calibration survey (6,600 in³ at 6 m tow depth) and the planned survey (210 in³ at 4 m tow depth). A simple scaling factor is calculated from the ratios of the isopleths calculated by the deep-water L-DEO model, which are essentially a measure of the energy radiated by the source array.

L-DEO's methodology is described in greater detail in UT's IHA application. The estimated distances to the Level B harassment isopleth for the planned airgun configuration are shown in table 5.

Table 5—Predicted Radial Distances from the R/V McCall Seismic Source to Isopleth Corresponding to Level B Harassment Threshold

Airgun configuration	Max tow depth (m)	Water depth (m)	Predicted distances (in m) to the Level B harassment threshold
2 105-in3 airguns	4	<100	1,750

Table 6—Modeled Radial Distance to Isopleths Corresponding to Level A Harassment Thresholds

[NMFS 2018]

	Mid frequency cetaceans
PTS SEL cum	0
PTS Peak	* 1.5
* The largest distance of the dual criteria (cumulative SEL (SEL cum ) or Peak) was used to estimate threshold distances and potential takes by Level A harassment.

Table 6 presents the modeled Level A harassment isopleths for the dolphin species considered here based on L-DEO modeling incorporated in the companion user spreadsheet, for the low-energy surveys with the shortest shot interval ( i.e., greatest potential to cause auditory injury or PTS based on accumulated sound energy) (NMFS 2018). Although NMFS' 2024 Updated Technical Guidance was finalized on October 24, 2024 (89 FR 84872), a similar survey using four 105-in³ GI guns showed no difference in the auditory injury (Level A harassment) isopleths for delphinid species in the same hearing group (89 FR 81429, October 8, 2024; 89 FR 101555, December 16, 2024), so the values based on the 2018 guidance were used here.

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, 2024). 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 2-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 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 farfield signature. Because the farfield signature does not take into account the large array effect ( print page 5820) near the source and is calculated as a point source, the farfield signature is not an appropriate measure of the sound source level for large arrays. See UT's application for further detail on acoustic modeling.

Auditory injury is unlikely to occur for the dolphin species considered here, given the very small modeled zones of injury for those species (all estimated zones are less than 10 m), 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 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 of the dolphin species considered here 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 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.

For the planned survey area in the NW GOM, UT determined that the best source of density data for marine mammal species that might be encountered in the project area was habitat-based density modeling conducted by Garrison et al., (2023). The Garrison et al., (2023) data provides abundance estimates for marine mammal species in the GOM within 40 km² hexagons (~3.9 km sides and ~7 km across from each side) on a monthly basis. To calculate expected densities specific to the survey area, UT calculated the mean of the predicted densities from the cells within the combined survey area (primary and alternate survey area) for each species and month. The highest mean monthly density was chosen for each species from the months of January to April ( i.e., the months within which the survey is expected to occur).

Rough-toothed dolphins were not modeled by Garrison et al., (2023) due to a lack of sightings, so habitat-based marine mammal density estimates from Roberts et al., (2016) were used. The Roberts et al., (2016) models consisted of 10 km x 10 km grid cells containing average annual densities for U.S. waters in the GOM. The combined survey area was used to select grid cells from the Roberts et al., (2016) dataset, and the mean of the selected grid cells for rough-toothed dolphins was calculated to estimate the annual average density of the species in the survey area. Estimated densities used and Level B harassment ensonified areas to inform take estimates are presented in table 7.

Table 7—Marine Mammal Densities and Total Ensonified Area of Activities in the Planned Survey Area

Species	Estimated density (#/km2 )	Level B ensonified area (km2 )
Atlantic spotted dolphin	b  0.0043	1,522
Bottlenose dolphin a	b  0.8596	1,522
Rough-toothed dolphin	c  0.0037	1,522
a  Bottlenose dolphin density estimate does not differentiate between coastal and shelf stocks.
b  Density calculated from Garrison et al., (2023).
c  Density calculated from Roberts et al., (2016).

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 B harassment, radial distances from the airgun array to the predicted isopleth corresponding to the Level B harassment threshold was 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 area expected to be ensonified on 1 day was determined by multiplying the number of line km possible in 1 day by two times the 160-dB radius plus adding endcaps to the start and beginning of the line. The daily ensonified area was then multiplied by the number of survey days (20 days). The highest mean monthly density for each species was then multiplied by the total ensonified area to calculate the estimated takes of each species.

No takes by Level A harassment are expected or authorized. Estimated exposures and authorized take numbers are shown in table 8.

Table 8—Authorized Take

Common name	Stock	Estimated Level B take	Authorized Level B take	Stock abundance 1	Percent of stock
Atlantic spotted dolphin	GOM	7	2  26	21,506	0.12
Bottlenose dolphin 3	GOM Western Coastal	1,309	1,309	20,759	6.31
	Northern GOM Continental Shelf			63,280	2.07
Rough-toothed dolphin	GOM	6	2  14	4,853	0.29
1  Stock abundance for Atlantic spotted dolphins and bottlenose dolphins was taken from Garrison et al., (2023). Stock abundance for rough-toothed dolphins was taken from Roberts et al., (2016), as Garrison et al., (2023) did not create a model for this species. ( print page 5821)
2  Authorized take increased to mean group size from Maze-Foley and Mullin (2006).
3  Estimated take for bottlenose dolphins is not apportioned to stock, as density information does not differentiate between coastal and shelf dolphins. However, based on the planned survey depths, we expect that most of the takes would be from the coastal stock, but some takes could be from the shelf stock. Percent of stock was calculated as if all takes authorized accrued to the single stock with the lowest population abundance.