Passive acoustic localization and tracking provide valuable information for characterizing the behavior of marine mammals. When using time-difference-of-arrival (TDOA) localization on a wide-baseline array, certain factors such as stereotyped vocalizations from multiple callers and sparse array configuration, can make the call association problem challenging. A "track-before-detect" localization and tracking framework is presented in which this challenge is addressed by taking a hard-association approach and performing the tracking in the TDOA domain. The framework is demonstrated on low-frequency stereotyped calls produced by Rice's whales (Balaenoptera ricei) recorded along the shelf break offshore of northwest Florida. Using the framework, a total of 398 whale tracks were extracted between May and August 2021. A median inter-call interval of 5.8 min was estimated from the calls identified in each set of TDOA tracks. Analysis of 1731 h from 368 two-dimensional tracks showed that whales primarily swam along, rather than across, the continental shelf break, with a mean horizontal swim speed of 0.9 m/s (standard deviation ± 0.6 m/s). By providing quantitative estimates of Rice's whale behavioral parameters, these results illustrate the utility of the developed passive acoustic localization and tracking framework.

Understanding abundance and trends of beaked whales in the heavily industrialized Gulf of America (formerly Gulf of Mexico), is critical for management but challenging with visual-based distance-sampling due to their elusive surface behavior. Acoustic-based distance-sampling methods rely on accurate modeling of detection probability as a function of distance from a recorder, requiring population-specific diving and acoustic behavior parameters, which is currently lacking for Gulf populations. To address this, we used passive acoustic tracking with two 4-channel High-Frequency Acoustic Recording Packages (HARPs) deployed off Louisiana (~1100 m depth) in 2021. Echolocation clicks detected on both recorders were localized in 3D to characterize acoustic and diving behavior. These data informed a Monte Carlo cue-based simulation to estimate the probability of detection by a near-seafloor single-sensor HARP. A trial-based approach also estimated detection probability as a function of range to a single-channel sensor deployed at the site. Results show species-specific differences. Goose-beaked whales (Ziphius cavirostris), were detected for longer periods during foraging dives (n = 24 dives, mean: 20.5 min; range: 7-42) compared with Blainville's (Mesoplodon densirostris, n = 2 dives, 13.6 min; 11-16) and Gervais' (Mesoplodon europaeus, n = 24 dives, 12.7 min; 7-19) beaked whales. Maximum dive depths also differed, with some goose-beaked whales foraging at or near the seafloor. Descent and ascent rates were similar within species but differed among them (1.34/1.40 m/s for goose-beaked and 1.15/1.19 m/s for Gervais' beaked whales). Source level and broadband directivity index were estimated at 225 dBpp re 1 μPa-1m and 26 dB for goose-beaked whales, and 218 dBpp re 1 μPa-1m and 20 dB for Gervais' beaked whales. Estimates were not possible for Blainville's beaked whales due to limited data. In both the Monte Carlo simulation and trial-based approach, detection probability declined sharply with ranges, reflecting the highly directional beam of beaked whale echolocation clicks.

Understanding the foraging ecology of endangered marine mammals provides important information for their conservation yet remains challenging due to the elusive, underwater nature of their feeding habits. Here, we used environmental DNA (eDNA) metabarcoding with two complementary 12S rRNA markers to characterize potential prey communities available to the critically endangered Rice's whale (Balaenoptera ricei) in its core habitat in the northeastern Gulf of America (formerly Gulf of Mexico). Water samples (N = 21) collected during a 2019 survey within Rice's whale feeding areas detected 99 unique fish species across 62 families, exceeding the diversity recorded by concurrent trawl surveys. The combined metabarcoding approach revealed 74 fish species not recorded in trawls, while 16 trawl-caught species went undetected by eDNA. Notably, eDNA yielded higher detection rates for several potential prey taxa previously identified through stable isotope analysis and trawl surveys, resulting in an updated list of top potential prey. These findings suggest that key prey species may be more prevalent in Rice's whale habitat than previously documented. To support these analyses, existing reference databases were expanded by sequencing the 12S rRNA gene from 15 regional fish species, using new primers developed for this study. Our study demonstrates the value of eDNA as a complementary tool for monitoring the prey community of this critically endangered cetacean, while highlighting the need for continued development of reference databases to maximize the ecological insights gained from marine metabarcoding applications.

Marine mammals have high potential for dispersal, yet behavioral or environmental constraints can limit gene flow. This is true for the endangered sperm whale, Physeter macrocephalus, which has a global distribution and long-distance migrations. While previous studies revealed mitochondrial population structure with weak nuclear structure globally, genomic approaches examining this pattern have been limited. Understanding connectivity is critical for the management of this species due to population declines relative to pre-whaling numbers and increased recent anthropogenic stressors. We investigated connectivity between two regions, the U.S. Gulf of Mexico and the western North Atlantic Ocean, using reduced representation genomic and mitochondrial control region sequencing of 73 sperm whales. Relatedness decreased with geographic distance, likely due to the presence of social groups and familial structure. Nuclear markers showed no population structure (FST = 0.001-0.008), while mitochondrial structure was high (FST = 0.36-0.65), consistent with male-biased dispersal and female philopatry. Female-only analyses showed higher differentiation for mitochondrial but not nuclear markers; male-only analyses revealed no structure. Across all samples, genetic diversity (nuclear: 0.0014; mitochondrial: 0.0017) and effective population size (Ne = 460) were low. Given this low diversity and evidence for partitioning of genetic variation, we recommend managers treat these two regions as distinct to preserve existing variation and promote resilience of this species. These results illustrate that despite the increased power of a genomic approach, it is essential to consider the biology of the species at hand and leverage both mitochondrial and nuclear markers to understand the genetic structure of threatened species.

Gulf of Mexico (GOM) is a marine ecosystem with high species diversity and provides critical ecosystem services. Nektonic communities support both fisheries and higher trophic level consumers. With potential declines in marine predator populations in the GOM over the past decade, understanding the spatial and temporal dynamics of nekton is critical for ecosystem-based management. We used hierarchical generalized additive models (HGAMs) to investigate the relationship between environmental conditions and the distribution of acoustically detected zooplankton and nekton in the northeastern GOM. We hypothesized that salinity, depth, and distance to bathymetric features are associated with the vertical and horizontal distribution of these organisms. Acoustic data were collected from May to August 2018-2019 and November to December 2018. Pelagic cephalopods exhibited ubiquitous distributions and low relative backscatter, while zooplankton and micronekton had consistently high relative backscatter. Macronekton showed significantly higher backscatter, with the maximum backscatter being up to 2600 times greater than pelagic cephalopods. Macronekton also exhibited significant variation in depth distribution, with increasing backscatter below 200 m, and showed different vertical movements in response to the diel cycle. Each acoustic class responded uniquely to environmental conditions, underscoring the complexity of zooplankton and nekton distributions in the GOM. We offer recommendations for future studies, particularly for incorporating additional environmental and seasonal variables. Overall, our research improves understanding of how zooplankton and nekton in the northern GOM respond to environmental variability, informing ecosystem dynamics and pelagic food web structure.

Vessel strikes are a substantial source of mortality for large whales worldwide and may pose conservation threats for small populations. Model-based estimates of mortality rates, which inform management strategies to reduce vessel strike mortality, typically assume a reduced likelihood that a whale-vessel collision will be lethal to the whale at slower vessel speeds. In this study, we reviewed and updated available data on observed whale-vessel interactions in U.S. waters and developed a new model characterizing the probability that an interaction will be lethal to the whale as a function of vessel speed, length (as a proxy for mass), and whale taxon. We found a significant effect of vessel size class on the probability of lethality. In addition, decreasing vessel speeds reduced the likelihood of a lethal outcome for all vessel size classes, but this effect was strongest for vessels less than 108m in length. The probability that a strike by a very large ocean-going vessel will be lethal exceeded 0.80 at all speeds above 5 knots. Whale taxon also affected both the likelihood of a lethal strike and the effect of vessel speed. Humpback whales (Megaptera novaeangliae) had significantly lower rates of lethal strikes compared to other large whales. This difference may be associated with data limitations, differing behavioral responses between species, varying vessel types between regions or differences in body composition and blubber thickness. The model is consistent with biophysical models that demonstrate a high rate of strike lethality for large vessels with high masses. Vessel speed restrictions are one of the primary approaches to reduce the risk of vessel strikes to whales in the face of continued industrialization of the oceans, and the model presented here will help better inform management efforts.

The Rice's whale (Balaenoptera ricei) is a critically endangered baleen whale species that resides year-round in the Gulf of America (formerly the U.S. Gulf of Mexico). The majority of whale sightings occur where the seafloor varies between 100 and 400 m in depth in a restricted region of the northeastern Gulf of America; however, historical whaling records suggest the species may have inhabited other areas and recent acoustic and visual observation data have detected their presence in the northwestern Gulf of America and southwestern Gulf of Mexico. A better understanding of their distribution and movements could aid in this species' recovery. In recent decades, non-invasive genetic techniques have been developed to detect the presence or absence of species through the use of environmental DNA (eDNA). We developed a quantitative polymerase chain reaction (qPCR) assay targeting the mitochondrial DNA control region to detect eDNA shed by Rice's whales. A rigorous validation process was completed to determine the assay's sensitivity and specificity. The final assay detects low copy numbers of the target eDNA, is highly specific to Rice's whale, and was successfully used to detect the whale's presence in "flukeprint" samples (n = 23). In addition, we tested eDNA collection methods such as filter pore size and the ability to detect Rice's whale eDNA at different time intervals after a whale surfaced and were able to detect eDNA as long as 10 min later. We also used eDNA methods to detect Rice's whale DNA obtained from Acousonde tags after being attached to a whale. Taken together, data from these sampling events will help to refine and improve eDNA collection techniques for many marine mammal species. Future research combining the newly developed eDNA assay with traditional survey methods could improve our understanding of the distribution and habitat use of this endangered and rare species.

Vessel strikes are a critical threat to endangered North Atlantic right whales (Eubalaena glacialis), significantly contributing to their elevated mortality. Accurate estimates of these mortality rates are essential for developing effective management strategies to aid in the species’ recovery. This study enhances existing vessel strike models by incorporating detailed regional data on vessel traffic characteristics as well as whale distribution and behavior. Our model assesses the spatial and temporal variability in vessel strike risk along the U.S. east coast apportioned into three vessel length classes (26–65 feet, 65–350 feet, > 350 feet). By including regional right whale depth distributions and parameterizing potential whale avoidance based on factors such as descent rate, bottom depth, and vessel speed and size, the model provides a refined estimation of mortality risk. We also address the underrepresentation of smaller vessel activity via a correction factor, offering a more accurate annual mortality risk estimate for each vessel size class. These findings highlight that vessels > 350 feet in length pose the greatest risk to right whales. Simulations of reduced vessel speeds indicate that speed measures can mitigate mortality rates; however, residual risk remains even at speeds of 10 knots or less suggesting limitations to this mitigation approach.

The recent steep decline of the endangered North Atlantic right whale Eubalaena glacialis can be attributed to high mortality combined with low reproduction. While the former is a clear result of anthropogenic activity, the latter involves more complexity. Evidence suggests that both short-term fluctuations in prey availability and long-term decline in health are responsible for depressed right whale calving rates. To facilitate an assessment of extinction risk, we developed a multistate capture-recapture model that estimated the probability of calving using extensive sightings data from 1990-2019. The model estimated sub-lethal effects of severe injury on calving probability and modeled temporal variability in calving as related to indices of prey availability (Calanus spp. biomass) and an apparent regime shift. The average annual probability of calving for known-breeding females, given average prey conditions, decreased from 0.217 [95% CI: 0.162, 0.281] to 0.142 [95% CI: 0.067, 0.252] after the 2010 regime shift. The model indicated strong evidence of a relationship between calving probability and the prey index from the eastern Gulf of Maine, although this relationship effectively disappeared after 2010; moderate evidence for a relationship with prey from the southwest Gulf of St. Lawrence remained. Weak evidence of reduced calving probability due to severe injury resulted from low sample sizes, given increased mortality for individuals observed with severe injuries. The regime effect is hypothesized to be capturing a long-term decline in health due to a combination of decreasing habitat quality resulting from climate change and potentially chronic sublethal injuries (e.g. entanglements). Our reproduction model provides demographic parameter estimates that can be used in population projections for North Atlantic right whales, although uncertainty remains in the mechanisms responsible for recent declines in calving.

The Rice's whale is among the world's most endangered whales. It has a small population size, low genetic diversity, and is exposed to several anthropogenic threats. In this study, we compiled photographs taken from whale sightings during vessel-based research surveys conducted by the U.S. National Marine Fisheries Service, Southeast Fisheries Science Center between 2004 and 2019 and used photo-ID techniques to develop an identification catalog. Thirty-one whales were individually identified based on dorsal fin attributes and body marks. On the dorsal fin, lacerations, nicks and notches were the most commonly available attributes used for identification and matching. Cookiecutter shark bite scars were widely present on the body of the whales and also served for identification and matching. Of the 31 whales, 28 were sighted multiple times with time between sightings ranging from seven days to more than 15 years. Individual genotyping and sexing were available for 25 cataloged whales. Genotyping confirmed that whales identified via photographs were genetically unique and sexing resulted in 14 females and 11 males. Here we also present insights into rarely recorded presumed mother and calf pairs, with three female whales identified as presumed mothers. Finally, we document dorsal fin disfigurements, body deformities and confirm the identity of a whale mortality. Our study reveals the need for the long-term monitoring of Rice's whale individuals, especially presumed mothers and calves, and to further investigate potential human threats to this population using photo-identification techniques.

Sperm whales Physeter macrocephalus are highly sexually dimorphic, with adult males having larger bodies, more powerful echolocation clicks, and slower echolocation clicking rates compared to females. This study introduces methods for estimating sperm whale population densities in the Gulf of Mexico (GoMex) by accounting for the population demographics using passive acoustic monitoring and reveals that ignoring the differences between demographic segments can introduce bias in density estimates. Weekly densities were estimated per 3 demographic segments: social groups consisting of adult females and their offspring, mid-size animals, and adult males. Analysis revealed that the GoMex sperm whale population is primarily composed of social groups, which account for 92 to 98% of the overall population. Mid-size animals and adult males made up a small proportion of the population and were only intermittently present. Our 7 yr GoMex density estimates, including the 2010 Deepwater Horizon (DWH) oil spill period and subsequent years, revealed demographic-specific trends. Declines found at 2 north-central GoMex sites, coupled with increases at a southeastern site, may indicate population movements and potential impacts from the 2010 DWH oil spill and elevated noise levels from anthropogenic activities in the north-central GoMex.

AbstractShortly after the Deepwater Horizon oil spill began in April 2010, a widely spaced passive acoustic monitoring array was deployed in the northeastern Gulf of Mexico to document the impacts of this unprecedentedly large and deep offshore oil spill on oceanic marine mammals. The array was subsequently maintained for over a decade. Here we document decadal density declines for seven of eight monitored species groups, including sperm whales (up to 31%), beaked whales (up to 83%), and small delphinids (up to 43%). Declines were observed both within and outside of the surface oil footprint. Though not conclusively linked to the oil spill, the broad spatial and temporal scale of these declines observed for disparate marine mammal species is consistent with Deepwater Horizon impacts. These declines have exceeded and outlasted post-spill damage assessment predictions, suggesting that the offshore ecosystem impacts of Deepwater Horizon may have been larger than previously thought.

Abstract Understanding the causes of mortality for a declining species is essential for developing effective conservation and management strategies, particularly when anthropogenic activities are the primary threat. Using a competing hazards framework allows for robust estimation of the cause‐specific variation in risk that may exist across multiple dimensions, such as time and individual. Here, we estimated cause‐specific rates of severe injury and mortality for North Atlantic right whales (Eubalaena glacialis), a critically endangered species that is currently in peril due to human‐caused interactions. We developed a multistate capture–recapture model that leveraged 30 years of intensive survey effort yielding sightings of individuals with injury assessments and necropsies of carcass recoveries. We examined variation in the hazard rates of severe injury and mortality due to entanglements in fishing gear and vessel strikes as explained by temporal patterns and the age and reproductive status of the individual. We found strong evidence for increased rates of severe entanglement injuries after 2013 and for females with calves, with consequently higher marginal mortality. The model results also suggested that despite vessel strikes causing a lower average rate of severe injuries, the higher mortality rate conditional on injury results in significant total mortality risk, particularly for females resting from a recent calving event. Large uncertainty in the estimation of carcass recovery rate for vessel strike deaths permeated into the apportionment of mortality causes. The increased rates of North Atlantic right whale mortality in the last decade, particularly for reproducing females, has been responsible for the severe decline in the species. By apportioning the human‐caused threats using a quantitative approach with estimation of relevant uncertainty, this work can guide development of conservation and management strategies to facilitate species recovery. Our approach is relevant to other monitored populations where cause‐specific injuries from multiple threats can be observed in live and dead individuals.

The Critically Endangered North Atlantic right whale Eubalaena glacialis entered a population decline around 2011. To save this species without closing the ocean to human activities requires detailed information about its intra-annual density patterns that can be used to assess and mitigate human-caused risks. Using 2.9 million km of visual line-transect survey effort from the US Atlantic and Canadian Maritimes conducted in 2003-2020 by 11 institutions, we modeled the absolute density (ind. km-2) of the species using spatial, temporal, and environmental covariates at a monthly time step. We accounted for detectability differences between survey platforms, teams, and conditions, and corrected all data for perception and availability biases, accounting for platform differences, whale dive behavior, group composition, and group size. We produced maps of predicted density and evaluated our results using independently collected passive acoustic monitoring (PAM) data. Densities correlated positively (r = 0.46, ρ = 0.58, τ = 0.46) with acoustic detection rates obtained at 492 stationary PAM recorders deployed across the study area (mean recorder duration = 138 d). This is the first study to quantify the concurrence of visual and acoustic observations of the species in US waters. We summarized predictions into mean monthly density and uncertainty maps for the 2003-2009 and 2010-2020 eras, based on the significant changes in the species’ spatial distribution that began around 2010. The results quantify the striking distribution shifts and provide effort- and bias-corrected density surfaces to inform risk assessments, estimations of take, and marine spatial planning.

The newly recognized Rice’s whale Balaenoptera ricei is among the most endangered large whale species in the world and primarily occupies a region near the continental shelf break in the northeastern Gulf of Mexico (GoMex). We analyzed visual line-transect survey data collected throughout the northern GoMex from 2003-2019 and developed spatially explicit density maps using a density surface modeling approach to examine relationships between Rice’s whale density and bathymetric and oceanographic features. We identified water depth, surface chl a concentration, bottom temperature, and bottom salinity as key parameters that define the Rice’s whale habitat. This is consistent with upwelling of cold, high-salinity water along the continental shelf break and seasonal input of high-productivity surface water originating from coastal sources. The dominant circulation patterns in the GoMex, including the presence of Loop Current eddies, lead to increased productivity and likely play a role in maintaining high densities of forage species needed to support Rice’s whales. Extrapolation of the model suggests additional regions in Mexican waters of GoMex that may be suitable for Rice’s whales. This study informs the designation of critical habitat as defined by the US Endangered Species Act and will assist in marine spatial planning activities to avoid additional anthropogenic impacts to Rice’s whales associated with the development of wind energy and aquaculture.

Rice's whales (Balaenoptera ricei) are one of the most endangered marine mammal species in the world. Their known distribution is restricted to the Gulf of Mexico (GoMx) and basic knowledge of their ecology is limited. In their core distribution area along the northeastern GoMx shelf break (Rosel & Garrison, 2021), their abundance was estimated at 51 individuals, 95% CI [20, 130], based on line transect surveys conducted during 2017 and 2018 (Garrison et al., 2020). Most Rice's whale sightings and acoustic detections during the last 30 years occur in this area off the northwestern coast of Florida (Rice et al., 2014; Rosel et al., 2021; Širović et al., 2014; Soldevilla et al., 2017; Soldevilla, Ternus, et al., 2022). While visual sightings are rare (e.g., Rosel et al., 2021), recent passive acoustic detections during one year of recordings (Soldevilla, Debich, et al., 2022) establish that they routinely occur along the shelf break of the northwestern GoMx off Louisiana as well. Currently, Rice's whales are only known to occur within U.S. waters of the northern GoMx, although whaling records (Reeves et al., 2011) suggest they were distributed more broadly across the GoMx historically. Understanding their range and distribution is important for evaluating the impacts of human activities, including climate change, that threaten their long-term survival. Considering the high levels of anthropogenic activity throughout the GoMx (e.g., oil and gas exploration and extraction, fisheries, shipping, and oil spills), a comprehensive knowledge of the current distribution of Rice's whales is needed to understand the risk of these activities to the whales and to develop effective recovery and conservation strategies for this endangered species (Rosel et al., 2016). Long-term autonomous passive acoustic monitoring (PAM) is a highly effective method for establishing the distribution of rare whale species, particularly in areas where they occur infrequently or were sighted by whalers historically (e.g., Mellinger et al., 2011; Munger et al., 2008). Rice's whales produce highly stereotyped call types, including long-moans, tonal-sequence calls, and pulsed downsweep sequences, which are readily identifiable in autonomous recordings (Rice et al., 2014; Širović et al., 2014; Soldevilla, Ternus, et al., 2022). Variants of the long-moan calls that have been described include one detected only in the northeastern GoMx and six detected primarily in the northwestern GoMx (Soldevilla, Debich, et al., 2022; Soldevilla, Ternus, et al., 2022). The northeastern long-moan call type is a long-duration frequency-downswept call with an average initial frequency of 150 Hz, center frequency of 107 Hz, and duration of 22 s (Rice et al., 2014). Northwestern long-moan call types also begin with a 2–3 s tone at 150 Hz, but rather than a continuous downsweep to a long lower frequency tonal tail, they have a more abrupt transition between the 150 Hz tone starting segment and a lower frequency tonal tail (Soldevilla, Debich, et al., 2022). The six northwestern variants each have a stereotyped frequency-modulation pattern to the transition component that distinguishes them. Additionally, frequently reported calls consisting of only the 150 Hz tone were proposed to be partial northwestern long-moan calls in which the transition and tail components are masked by increased low-frequency (below 125 Hz) noise levels common in the western Gulf (Soldevilla, Debich, et al., 2022). As a component of several projects monitoring cetaceans in oceanic GoMx waters, we deployed long-term passive acoustic recorders offshore of Louisiana and Texas, and in Mexican waters of the southern GoMx (Figure 1). Between September 7, 2019, and August 29, 2020, autonomous passive acoustic recorders were deployed to monitor for Rice's whales at two shelf-break sites (Table 1), including the previously studied Flower Garden West site (WF), offshore of the Flower Garden Banks located off Louisiana (Soldevilla, Debich, et al., 2022), and a new site offshore of Corpus Christi, Texas (CC) near where a Rice's whale was sighted in 2017 (Rosel et al., 2021). These recorders yielded continuous recordings over 356 days at each site. Autonomous passive acoustic recorders were also deployed to monitor for cetaceans from September 7, 2020, to July 19, 2022, at a new site in Mexican continental slope waters northeast of Tampico, near the Mexican Ridges (MR). These recorders yielded continuous acoustic recordings over 680 days (Table 1). Our objective in this study was to find whether Rice's whales occur in Mexican waters of the western GoMx and to evaluate how frequently they occur at all three western Gulf sites. With this aim, we conducted manual spectrogram reviews and ran automated long-moan call and downsweep-sequence detectors on the combined four instrument-years of acoustic recordings. The acoustic recorders used in this study were High-frequency Acoustic Recording Packages (HARPs), which were moored to the seafloor and consisted of a calibrated hydrophone tethered ~10 m above a packaged data logger, batteries, flotation, acoustic release, and ballast weight system (Wiggins & Hildebrand, 2007). To improve processing efficiency, recordings originally sampled at 200 kHz were decimated to a sample rate of 2 kHz, yielding an effective frequency bandwidth from 10 to 1,000 Hz. This bandwidth is sufficient for recording Rice's whale calls, which fall within 60–160 Hz. In recordings from sites CC and WF, Rice's whale calls were manually detected in long-term spectral averages (LTSA's) by trained acoustic analysts (S.J., I.C.) and verified by a Rice's whale acoustic expert (A.J.D.) following previously established methods (Soldevilla, Debich, et al., 2022). In recordings at site MR, Rice's whale calls were automatically detected using long-moan call and downsweep sequence spectrogram correlation detectors (Soldevilla, Debich, et al., 2022). Detector thresholds were set to minimize missed calls at the cost of higher false positives. However, the GoMx soundscape is heavily influenced by anthropogenic noise from seismic airgun surveys and shipping (Wiggins et al., 2016), resulting in high numbers of false positive detections. To ensure only true calls were retained for further analysis, all call detections were manually validated by an experienced acoustic analyst (A.J.D.) and false detections were removed. Rice's whale calls were frequently detected at site CC, providing the first evidence of their regular occurrence in waters offshore of Texas. A total of 1,694 long-moan calls were detected in the one year of recordings at CC, and 4,323 long-moan calls were detected at WF (Table 2). Of these, 1,102 and 2,115 calls from CC and WF, respectively, could be definitively identified as western long-moan calls (including the 150 Hz tone, transition, and tail), while only the 150 Hz tone could be identified for the remaining 592 (34% of total) and 2,208 (51% of total) calls at each of the sites, respectively (Table 3). Western long-moan calls and 150 Hz tones cooccurred on 77% of days and 88% of days at CC and WF, respectively, adding further support that the 150 Hz tones are partial western long-moan calls. Compared to previous recordings from 2016 to 2017, there were more than twice as many calls detected at WF in 2019–2020 recordings, and 150 Hz tones were more common, representing 51% of total calls at WF compared to 20% of calls in 2016 to 2017 (Soldevilla, Debich, et al., 2022). Rice's whale long-moan calls were present throughout the year at sites CC and WF (Figure 2), with calls detected on 24.7% of days (4.5% of hours) at CC and 33.4% of days (10.1% of hours) at WF (Table 2). While call detections occurred year-round, with no strong seasonal cycle, there was a peak in call detections from November to January at site WF and from June to August at sites WF and CC during the September 2019 to August 2020 deployment year (Figure 2, Table 4). Winter and summer peaks in call detections were also seen during the previous 2016 to 2017 deployment at WF, when call detections were highest in December and August (Soldevilla, Debich, et al., 2022). It is notable that Rice's whale calls were present at WF on one third of all days in 2019–2020, which was twice as often as in 2016–2017 (Soldevilla, Debich, et al., 2022). This highlights both their persistence at this site over multiple years, as well as the variability among years. The interannual variability may be due to the slight 5 km north shift in site location, differences in detectability across years, or a response to oceanographic conditions. Rice's whale calls were also detected at site MR, providing the first evidence of their occurrence in Mexican waters, and demonstrating that Rice's whales have a transboundary range including both U.S. and Mexican waters of the GoMx. In each of the two deployment years, a total of 226 and 353 Rice's whale western long-moan calls were detected, respectively (Table 3). Although the majority (99%) of these calls consisted of the 150 Hz tone only, six calls from the MR02 deployment definitively exhibited both the 150 Hz tone and a transition or tail portion of the call. These six western long-moan variants were detected over three separate days (October 1, 2021, October 3, 2021, and December 27, 2021), and the five from October were each followed by a 150-Hz-tone-only call. Rice's whale calls were detected sporadically throughout the year at site MR (Figure 2), with calls detected on 14.9% of days (1.9% of hours) across the 2 years (Table 2). A peak in call detections occurred from August 2021 to January 2022 (Figure 2, Table 4). Nevertheless, seasonality is not evident at this site as there was no concordant peak in detections during August 2020 to January 2021. The MR HARP site occurs in deeper water (1,200 m) than Rice's whales have typically been found in; however, it is only 28 km from the 400 m isobath and Rice's whale calls have been detected as far as 75 km away (Rice et al., 2014). With only a single acoustic sensor on the HARP, it cannot be determined whether the whales occur in deeper waters or if the whales producing the calls detected at MR were located in the typical 100–400 m water depths. The high proportion of call detections at this site containing only the 150 Hz tone suggests the whales may be farther away. The discovery that Rice's whales regularly occur in waters off Texas and in Mexican waters of the western GoMx has numerous implications for the management and conservation of this critically endangered species. The Gulf's extensive industrialization poses multiple threats to Rice's whales. These threats include injury or mortality due to vessel strikes, oil and gas exposure, entanglement in fishery gear, and exposure to marine debris, and habitat degradation due to noise from shipping traffic and seismic airgun surveys (Rosel et al., 2021). The levels of these threats are highest in the northwestern GoMx offshore of Texas and Louisiana, where numerous major shipping ports and high levels of oil and gas exploration and extraction occur near Rice's whale habitat (Garrison et al., 2023; Soldevilla, Debich, et al., 2022). Moreover, new aquaculture and wind energy industries have areas under consideration for development on the nearby GoMx shelf (Farmer et al., 2022, 2023), and oil and gas leasing opportunities are being considered along the northwestern and northcentral shelf-break where Rice's whales are found. Further, effects of warming waters due to climate change on the GoMx ecosystem may impact their distribution and population health. Until now, efforts for the management and conservation planning for the recovery of Rice's whales have been limited to U.S. management agencies as there was only evidence for their occurrence in U.S. waters. Similar industries are active in Mexican waters of the GoMx (Benitez et al., 2014). Two major shipping ports, Puerto Industrial Altamira and Puerto de Tampico, occur within 125 km of the MR HARP site, oil and gas exploration and extraction occur within Campeche Bay to the south (Murawski et al., 2020), and a gas pipeline installation is planned near the 200 m isobath from Tuxpan to Tabasco. This suggests whales in Mexican waters are similarly at risk of injury or mortality from vessel strike and oil and gas exposure, as well as habitat degradation from shipping and seismic airgun surveys, and impacts of climate change. This new evidence demonstrating Rice's whales' transboundary distribution underscores the imperative for a collaborative approach to management actions to recover Rice's whales across U.S. and Mexican environmental and marine resource agencies. Furthermore, these results have implications for the population monitoring of these endangered whales. To discern whether the population is declining, steady, or recovering, it is crucial to determine the full extent of their distribution throughout the GoMx and to conduct regular abundance monitoring throughout their entire range. The findings presented here, in combination with those of Soldevilla, Debich, et al. (2022), support the hypothesis that the Rice's whale habitat might encompass all 100–400 m depth waters encircling the entire GoMx along the shelf break (Garrison et al., 2023). The latest population estimate indicates there are fewer than 100 individuals in the northeastern core distribution area (Garrison et al., 2020), and this small population size is considered dangerously small, posing a significant extinction risk (Rosel et al., 2016). The IUCN categorizes Rice's whales as Critically Endangered due to this small population size (Rosel et al., 2016). The new acoustic recordings in the western GoMx support that appropriate habitat is present and being utilized outside the northeastern GoMx. This could imply the population size is somewhat larger than the current estimate of 51 individuals, which would be welcome news. Alternatively, it could be that whales simply have a broader distribution than just the northeastern GoMx. Differences in Rice's whale call types recorded in the eastern GoMx and those recorded in the western GoMx provide some support for the former. Further research to determine the full extent of the Rice's whale range, to understand their spatial density throughout U.S. and Mexican waters and how it may be changing over time, and to estimate a GoMx-wide population abundance is needed to better assess and mitigate the risks these whales face. The authors would like to thank the many people who participated in the collection and data processing of this data set. Ashley Cook, Bruce Thayre, Eva Hildalgo-Pla, Gania Figueroa, John Hurwitz, Kieran Lenssen, Kristen Rosier, Natalie Posdaljian, Ryan Pierson, Sean Wiggins, and Vanessa ZoBell assisted with building, deploying, and recovering of HARPs. Erin O'Neill, Diego Majewski, and Shelby Bloom processed the acoustic data, and Macey Kadifa assisted with archiving. Captain Tad Berkey and the crew of the R/V Pelican made the fieldwork possible. Keith Mullin, Patricia Rosel, Timothy Rowell, Mridula Srinivasan, and two anonymous reviewers provided thoughtful suggestions that improved this manuscript. Funding for this study was provided by NOAA's Southeast Fisheries Science Center's Ecological-based Fisheries Management program, NOAA Fisheries' Ocean Acoustics Program, NOAA Fisheries' International Science Program, and as part of the “Assessing long-term trends and processes driving variability in cetacean density throughout the Gulf of Mexico using passive acoustic monitoring and habitat modeling” project under federal funding opportunity Grant No. NOAA-NOS-NCCOS-2019-2005608 from the National Oceanic and Atmospheric Administration's RESTORE Science Program through the Gulf Coast Restoration Trust Fund to the NOAA Southeast Fisheries Science Center. Ship-time aboard the R/V Pelican for HARP deployment and servicing cruises in August 2020 and August 2021 were funded under the Office of Naval Research Task Force Ocean (Robert Headrick). The authors thank Graciela Alvarez and Rosa Vazquez of the Economic Section, U.S. Embassy, Mexico City and Gabriella David of the U.S. Department of State for their assistance with obtaining permits. HARP deployment fieldwork conducted in Mexican waters was permitted under diplomatic agreements CTC/01659/2020 and OPM/0428/2022 and under SEMARNAT permits SGPA/DGVS/02269/20, SGPA/DGVS/01801/21, and SGPA/DGVS/03614/22. The scientific results and conclusions, as well as any views or opinions expressed herein, are those of the author(s) and do not necessarily reflect those of NOAA or the Department of Commerce. Melissa Soldevilla: Conceptualization; formal analysis; funding acquisition; investigation; methodology; project administration; software; visualization; writing – original draft; writing – review and editing. Amanda Debich: Data curation; investigation; validation; writing – review and editing. Itzel Perez-Carballo: Investigation; writing – review and editing. Sierra Jarriel: Investigation; writing – review and editing. Kaitlin Frasier: Funding acquisition; methodology; project administration; writing – review and editing. Lance Garrison: Conceptualization; funding acquisition; writing – review and editing. Adolfo Gracia: Funding acquisition; writing – review and editing. John A. Hildebrand: Conceptualization; funding acquisition; methodology; resources; writing – review and editing. Patricia Rosel: Funding acquisition; writing – review and editing. Arturo Serrano: Funding acquisition; writing – review and editing.

Determining the drivers of prey selection in marine predators is critical when investigating ecosystem structure and function. The newly recognized Rice’s whale ( Balaenoptera ricei ) is one of the most critically endangered large whales in the world and endemic to the industrialized Gulf of Mexico. Here, we investigated the drivers of resource selection by Rice’s whales in relation to prey availability and energy density. Bayesian stable isotope (δ 13 C, δ 15 N) mixing models suggest that Rice’s whales feed primarily on a schooling fish, Ariomma bondi (66.8% relative contribution). Prey selection using the Chesson’s index revealed that active prey selection was found to be positive for three out of the four potential prey identified in the mixing model. A low degree of overlap between prey availability and diet inferred from the mixing model (Pianka Index: 0.333) suggests that prey abundance is not the primary driver of prey selection. Energy density data suggest that prey selection may be primarily driven by the energy content . Results from this study indicate that Rice’s whales are selective predators consuming schooling prey with the highest energy content. Environmental changes in the region have the potential to influence prey species that would make them less available to Rice’s whales.

Aim: Understanding the distribution of marine organisms is essential for effective management of highly mobile marine predators that face a variety of anthropogenic threats. Recent work has largely focused on modelling the distribution and abundance of marine mammals in relation to a suite of environmental variables. However, biotic interactions can largely drive distributions of these predators. We aim to identify how biotic and abiotic variables influence the distribution and abundance of a particular marine predator, the bottlenose dolphin (Tursiops truncatus), using multiple modelling approaches and conducting an extensive literature review.Location: Western North Atlantic continental shelf.Methods: We combined widespread marine mammal and fish and invertebrate surveys in an ensemble modelling approach to assess the relative importance and capacity of the environment and other marine species to predict the distribution of both coastal and offshore bottlenose dolphin ecotypes. We corroborate the modelled results with a systematic literature review on the prey of dolphins throughout the region to help explain patterns driven by prey availability, as well as reveal new ones that may not necessarily be a predator-prey relationship.Results: We find that coastal bottlenose dolphin distributions are associated with one family of fishes, the Sciaenidae, or drum family, and predictions slightly improve when using only fish versus only environmental variables. The literature review suggests that this tight coupling is likely a predator-prey relationship. Comparatively, offshore dolphin distributions are more strongly related to environmental variables, and predictions are better for environmental-only models. As revealed by the literature review, this may be due to a mismatch between the animals caught in the fish and invertebrate surveys and the predominant prey of offshore dolphins, notably squid.Main Conclusions: Incorporating prey species into distribution models, especially for coastal bottlenose dolphins, can help inform ecological relationships and predict marine predator distributions.

This study quantifies the abundance and spatial distribution of the cetacean community occupying continental shelf edge and inner continental slope waters along the US southeast Atlantic coast. A shipboard visual line-transect survey was conducted between June and August of 2004 that included effort in waters >50m deep encompassing the shelf break and inner continental slope off the US east coast between 28°N and 38°N latitude. The abundance of nine cetacean taxa was estimated using line-transect distance analysis and an independent observer approach to correct for visibility bias. Canonical correspondence analysis was used to examine the spatial distribution of the cetaceans encountered during the survey as a function of surface temperature, surface salinity, surface fluorescence, bottom depth, and bottom slope. The abundance estimates for most species were much higher than those from a study of the area conducted in 1998. This is primarily due to increased coverage of the shelf-break region and correction for visibility bias. The multivariate analysis indicated four distinct groups of cetaceans that partitioned habitat as a function of salinity, depth, and a latitudinal gradient. These groups were associated with specific water masses and hydrographic features including mid-Atlantic shelf waters (Group I), the shelf break (Group II), mid-Atlantic slope waters (Group III), and south Atlantic slope water (Group IV). Areas where water masses converge such as the continental shelf break along the mid-Atlantic and near Cape Hatteras, North Carolina are therefore areas of both high diversity and density of cetaceans.