Passive acoustic monitoring (PAM) is widely used to monitor small cetaceans and assess changes in relative density, but factors affecting reliability and statistical power remain unclear. In Denmark, 6 marine protected areas called Special Areas of Conservation (SACs) designated within the distributional range of the declining Belt Sea harbour porpoise Phocoena phocoena population have been monitored using PAM since 2012, providing data to assess the effectiveness of this programme. The aims of this study were to (1) investigate temporal changes in the relative density of harbour porpoises, (2) determine the statistical power to detect the observed changes, and (3) assess the factors influencing the power obtained to assist with the future design of PAM studies. In each SAC, acoustic data loggers were deployed at 5 stations for 3-4 approximately 1 yr long monitoring periods. The relative density of harbour porpoises was either stable or increasing over time in each SAC, which is in strong contrast to observed population-level decline. There was sufficient power (>80% with alpha = 0.2) to detect the observed change in 4 of the 6 SACs. The current setup allows detection of a 30% change in relative harbour porpoise density between 2 monitoring periods across all SACs. To detect a 20% change, however, a 40% increase in the number of PAM stations, or 111% increase in monitoring period length would be required. In conclusion, the number of stations, monitoring period, and variation in mean animal density have a strong influence on power and should be considered when designing PAM studies.

Abstract Grey seals are divided into two subspecies, Halichoerus grypus grypus in the Baltic Sea and H. grypus atlantica in the North Atlantic Ocean. Historically, intense hunting caused local extinctions of grey seals across their range and promoted geographical isolation of the subspecies. However, recent population recovery and recolonization have renewed their overlap in a contact zone in Danish and Swedish waters. Here, using whole-genome sequencing data from 119 individuals, we investigate the demographic history of grey seals, the divergence of the subspecies, and genomic signatures of local adaptation and potential hybridization in the contact zone. We estimate that divergence began approximately 10,000 years ago, with gene flow ceasing 2000 years ago. We detect peaks of high genetic differentiation near genes with putative functions in osmo- and thermoregulation, consistent with salinity and temperature differences between the Baltic Sea and the North Atlantic. As evidence of the geographic isolation breaking up, we report a hybrid individual in the southwest Baltic contact zone, with Baltic maternal and Atlantic paternal ancestry, and identify a migrant of Baltic origin in the North Sea. Our study illustrates how local environmental variation and long-term hunting pressure have contributed to divergence between mammal subspecies over a short evolutionary timescale, with recent population recovery, recolonization and hybridization reshaping gene flow dynamics. Broadly, our work emphasizes the importance of studying evolutionary processes amid anthropogenic influences, including both disturbances and conservation successes, where demographic fluctuations, range shifts, altered gene flow, and adaptation to changing environments interact in complex, potentially consequential ways.

The hooded seal (Cystophora cristata) is an Arctic phocid seal occasionally observed far outside its native range, including the European Atlantic shelf seas. This study presents the movements and diving behaviour of two female hooded seal pups after rehabilitation and release in the German North Sea. We tracked the animals using satellite telemetry to evaluate post-release survival and movement behaviour. Within one month, one seal travelled through the North Sea to the coast of United Kingdom, and then further northward to the Norwegian west coast, suggesting a successful return to Arctic waters. In contrast, the second seal remained in the North Sea and exhibited shallower and shorter dives over time, with extensive surface resting. She was ultimately found in poor condition in the United Kingdom and placed in permanent captivity. These cases highlight the species’ high dispersal capacity, yet also underline the challenges vagrant individuals face in suboptimal non-native habitats. The contrasting outcomes suggest that rehabilitation and release of such Arctic vagrants should be assessed case-by-case, considering the animal’s age, health, and timing of the release in relation to the expected prey availability. This study emphasizes the importance of satellite telemetry in evaluating post-release success and contributes to the broader discussion on the viability and ethics of rehabilitating extralimital marine mammals.

Seal haulout behavior is of interest in relation to energetics and abundance estimation. To assess year-round and diel haulout patterns and estimate correction factors, data were obtained from 100 seals (66 harbor and 34 gray seals) tagged with satellite transmitters in Kattegat and the western Baltic Sea during 2004-2017. Kattegat harbor seals had highest haulout probability during the night throughout the year, except for adults shifting to daytime preference during the mating season. Western Baltic harbor seals had higher haulout probabilities during daytime, peaking in March-May, with lowest probabilities in December-February. We found no clear differences in haulout behavior among age classes in Kattegat, but pups of the year hauled out more than other age classes in autumn in the western Baltic. Harbor seal haulout probabilities were similar to other parts of their range during autumn, winter, and spring, but lower during summer. Our data on harbor seal haulout probabilities in both June and August, months during which abundance counts are made, suggest that previous abundance estimates were too low. Baltic gray seals mostly hauled out at night throughout the year. Juveniles had highest haulout probabilities in March-April, while pups of the year peaked during molt (May-June). Our data highlight that haulout behavior can be highly variable, even between neighboring areas and should not be geographically extrapolated, and that it is essential to include recent and local knowledge of haulout behavior to adjust the estimates of abundance based on aerial counts.

Chronic anthropogenic activity is an ongoing threat to the marine environment and presents unique challenges in shallow, coastal habitats where harbor porpoises reside. Harbor porpoises have high metabolic needs and are highly susceptible to negative environmental factors; however, no long-term studies have been conducted on wild populations to quantify the negative impacts of vessel presence on their behavior. To evaluate this impact on harbor porpoises in the Little Belt, Denmark, data from a static hydrophone array were used to distinguish porpoise click train events and quantify buzzes (vocalizations with an inter-click interval < 10 ms), used as an indication of foraging and social behavior, and Automated Identification System data were collected to identify vessels within a 3000 m radius of the study site. Using generalized additive models, we showed that harbor porpoises were most active during the night in summer months but most often overlapped with vessels during the day in the summer. Generalized linear models showed that harbor porpoise buzz rate was significantly reduced in the presence of vessels, particularly during daylight hours in the summer when vessel traffic peaked. These results suggest that porpoise foraging and socializing are negatively affected by vessel presence. Furthermore, high porpoise density, which is likely driven by prey availability as found in other studies, may prevent animals from moving to less trafficked areas. Mitigation measures to reduce impacts should be considered to help reduce the ongoing decline in the studied population.

Quantifying and mitigating transboundary effects of anthropogenic activity is a key challenge in environmental management, particularly for wide-ranging species such as large predators, fish and migratory birds, relying on habitats across multiple national jurisdictions. This challenge is especially complex in marine ecosystems, where the movement of species and impacts across borders is largely unobserved. Central-place foragers, such as pinnipeds and seabirds, exemplify this complexity: abundance is typically assessed on local (regional or national) scales on land, yet at-sea movements and drivers of abundance occur on broader transboundary scales. Resolving this mismatch is critical to effective conservation, especially in areas such as the Northwest European Shelf (NWES), which features globally important predator populations (including two pinniped species) alongside growing anthropogenic pressures and a mosaic of national maritime borders. We model an unprecedented GPS dataset from 236 grey (Halichoerus grypus) and 606 harbour seals (Phoca vitulina) tracked in waters of seven countries across the NWES (United Kingdom, Ireland, France, Belgium, Netherlands, Germany and Denmark). Using regional habitat association models, we generate at-sea distribution estimates for both species at 5 km resolution, scaled to haulout counts, producing country-specific and NWES-wide density maps. Analysis of the extent to which seals making foraging trips from one country occupy the Exclusive Economic Zones (EEZs) of other countries revealed substantial transboundary overlap, particularly for grey seals, and harbour seals in the southern North Sea. A case study apportioning grey seal density within three adjacent offshore marine protected areas in different EEZs revealed that, where total density in a given area is required, overlooking transboundary distribution can underrepresent numbers by an order of magnitude. Synthesis and applications. This study provides the first comprehensive, regionally scalable distribution estimates for pinnipeds across the NWES and its constituent countries. The modelling framework is adaptable to other central-place and migratory species, supporting transboundary biodiversity assessments and international conservation policy. We discuss common limitations and misconceptions of species distribution estimates, highlight priorities for future work and underscore the need for transboundary efforts to manage wide-ranging species, providing a foundation for future ecological modelling and decision-making across shared ecosystems. Quantifier et att & eacute;nuer les effets transfrontaliers des activit & eacute;s anthropiques est un d & eacute;fi majeur dans la gestion environnementale, en particulier pour les esp & egrave;ces & agrave; large r & eacute;partition telles que les grands pr & eacute;dateurs, les poissons et les oiseaux migrateurs, qui d & eacute;pendent d'habitats r & eacute;partis sur plusieurs juridictions nationales. Ce d & eacute;fi est particuli & egrave;rement complexe dans les & eacute;cosyst & egrave;mes marins, o & ugrave; les d & eacute;placements des esp & egrave;ces et les impacts transfrontaliers sont largement inobserv & eacute;s. Les esp & egrave;ces & agrave; places centrales, telles que les pinnip & egrave;des et les oiseaux marins, illustrent bien cette complexit & eacute;: leur abondance est g & eacute;n & eacute;ralement & eacute;valu & eacute;e & agrave; l'& eacute;chelle locale (r & eacute;gionale ou nationale) sur terre, alors que leurs mouvements en mer et les facteurs qui d & eacute;terminent leur abondance se produisent & agrave; une & eacute;chelle transfrontali & egrave;re plus large. Il est essentiel de r & eacute;soudre ce d & eacute;calage pour assurer une conservation efficace, en particulier dans des zones telles que le plateau nord-ouest europ & eacute;en (NWES), qui abrite des populations de pr & eacute;dateurs d'importance mondiale (dont deux esp & egrave;ces de pinnip & egrave;des) et qui est soumise & agrave; des pressions anthropiques croissantes et & agrave; une mosa & iuml;que de fronti & egrave;res maritimes nationales. Nous avons mod & eacute;lis & eacute; un jeu de donn & eacute;es GPS sans pr & eacute;c & eacute;dent provenant de 236 phoques gris (Halichoerus grypus) et 606 phoques veaux-marins (Phoca vitulina) suivis dans les eaux de sept pays du NWES (Royaume-Uni, Irlande, France, Belgique, Pays-Bas, Allemagne et Danemark). & Agrave; partir de mod & egrave;les r & eacute;gionaux d'association d'habitats, nous avons g & eacute;n & eacute;r & eacute; des estimations de r & eacute;partitions en mer des deux esp & egrave;ces avec une r & eacute;solution de 5 km, mises & agrave; l'& eacute;chelle en fonction du nombre d'animaux recens & eacute;s sur les zones de repos terrestres, afin de produire des cartes de densit & eacute; sp & eacute;cifiques & agrave; chaque pays et & agrave; l'ensemble de la r & eacute;gion NWES. L'analyse de la r & eacute;partition des zones de chasse des phoques partant d'un pays et se superposant aux Zones & Eacute;conomiques Exclusives (ZEEs) d'autres pays a r & eacute;v & eacute;l & eacute; un chevauchement transfrontalier important, en particulier pour les phoques gris et les phoques veaux-marins dans le sud de la mer du Nord. Une & eacute;tude de cas portant sur la r & eacute;partition de la densit & eacute; des phoques gris dans trois zones marines prot & eacute;g & eacute;es adjacentes situ & eacute;es dans diff & eacute;rentes ZEE a r & eacute;v & eacute;l & eacute; que, lorsqu'il est n & eacute;cessaire de d & eacute;terminer la densit & eacute; totale dans une zone donn & eacute;e, le fait de n & eacute;gliger la r & eacute;partition transfrontali & egrave;re peut entra & icirc;ner une sous-estimation des chiffres d'un ordre de grandeur. Synth & egrave;se et applications: Cette & eacute;tude fournit les premi & egrave;res estimations compl & egrave;tes et adaptables & agrave; l'& eacute;chelle r & eacute;gionale de l Le cadre de mod & eacute;lisation est adaptable & agrave; d'autres esp & egrave;ces & agrave; place centrales et migratrices, permettant les & eacute;valuations transfrontali & egrave;res de la biodiversit & eacute; et les politiques internationales de conservation. Nous discutons des limites et des id & eacute;es fausses courantes concernant les estimations de la r & eacute;partition des esp & egrave;ces, soulignons les priorit & eacute;s pour les travaux futurs et insistons sur la n & eacute;cessit & eacute; d'efforts transfrontaliers pour g & eacute;rer les esp & egrave;ces & agrave; large r & eacute;partition, fournissant ainsi une base pour la mod & eacute;lisation & eacute;cologique et la prise de d & eacute;cision futures dans les & eacute;cosyst & egrave;mes partag & eacute;s.

We present temporal trends in juvenile harbour porpoises from Danish Waters collected during 1980-2019, and investigate effect concentrations for both juveniles and adults for a range of organochlorines (OCs), brominated flame retardants (BFRs), per- and polyfluoroalkyl substances (PFASs), organotins and total mercury (THg). We found declining concentrations for ∑PCBs, ∑DDTs, ∑HCHs, HCB, Trans-nonachlor, ∑PBDEs, TBT and ∑BTs over the three periods 1987-1990, 1996-1999 and 2010-2019. Concentrations above effect thresholds were detected for ∑PCBs, ∑DDTs and THg. All adult males were above PCB effect thresholds for reproductive and immune system (9000 ng/g lw) effects during the first two periods, declining to 50% of effect levels in the last period. For ∑DDTs, the highest concentrations were also detected among adults with all animals above effect threshold for immune system (1430 ng/g lw). while During the two first periods, all juveniles were also above the threshold, this proportion declined to 50% in the last period. Most animals were within the THg Low or No risk group (<64,00 ng/g ww) and no animals had concentrations above the Severe risk threshold (>126,000 ng/g ww). Concentrations of other OCs and BFRs and PFASs were below effect thresholds and only the first two groups decreased significantly over the study periods. Given our study's high concentrations of particularly PCBs and DDTs bordering the Baltic Proper, our results strongly indicates that these contaminants played a major role in the decimation of the Baltic Proper harbour porpoise population during 1960s-1980,s prior to national bans and implementation of the Stockholm Convention.

Over the past two decades, changes in sea temperature and sea-ice conditions have driven profound ecological shifts along the coast of East Greenland. One of the most conspicuous developments has been the recent and widespread occurrence of boreal baleen whale species in coastal waters. Fin whales, humpback whales, and minke whales now regularly feed along the East Greenland coast, at least as far north as 70°N and likely beyond, whereas historically, these species were only rarely observed in the region. Systematic surveys conducted in 2015 and 2024 indicate regional abundances of approximately 4,000 humpback whales, 6,000 fin whales, and 4,500 minke whales in the region. Capelin have been identified in stomach contents of minke whales from East Greenland, and both fin whales and humpback whales are likely to feed on capelin and krill in the region. The combined prey consumption of these baleen whales during their approximately four-month seasonal residency in East Greenland is estimated at about approximately 1 million tonnes annually, a substantial proportion of which may consist of capelin and krill. At the same time, recent observations suggest a summer redistribution of capelin from Icelandic waters toward East Greenland, consistent with rising sea temperatures around Iceland and along the East Greenland shelf. Oceanographic records show a clear change point in East Greenland sea surface temperatures in the late 1990s, followed by an average increase during the summer months of approximately 1 °C. The summer sea-ice regime shifted markedly after 2000, reducing the extent of pack ice along the coast. Together, these physical changes have facilitated access of boreal baleen whales to coastal East Greenland waters and may have enabled exploitation of newly established capelin resources. These developments indicate a rapid borealization of the East Greenland marine ecosystem, with potentially major implications for pelagic food webs and predator–prey dynamics in the region.

This comprehensive species-specific chapter covers all aspects of the mammalian biology, including paleontology, physiology, genetics, reproduction and development, ecology, habitat, diet, mortality, and behavior. The economic significance and management of mammals and future challenges for research and conservation are addressed as well. The chapter includes a distribution map, a photograph of the animal, and a list of key literature.

The Baltic Sea region is impacted by pollution, noise, offshore construction, shipping, fisheries, eutrophication and climate change. Seals are important top predators here, but little is known about their at-sea distribution. Using satellite telemetry data from 90 harbour seals Phoca vitulina and 65 grey seals Halichoerus grypus together with haulout counts, we modelled habitat suitability and relative population densities across the region. We used generalized additive mixed models (GAMMs), incorporating a range of static and dynamic environmental variables. Habitat suitability of both species was positively related to proximity to haulouts, explaining 25-30% of the total deviance. Other important variables, explaining 2.5-6% of the deviance in habitat suitability, were increased wave exposure and colder sea surface temperature for harbour seals and wave exposure and higher sea bottom oxygen saturation for grey seals. Harbour seals showed a preference for more coastal habitats than grey seals. The analyses revealed an imbalance between habitat suitability and actual distribution, particularly for grey seals, where some areas with high suitability had low seal densities. This likely reflects the dramatic population fluctuations of seals in the area over recent centuries related to hunting and pollution. Overlap of suitable habitats between the species in the southern and western Baltic indicates interspecific competition that will likely increase as grey seals recolonise their previous range. Our study constitutes the first comprehensive analysis of seal distribution in the Baltic area and is a valuable tool for managers and ecologists assessing pressures and ecosystem interactions in an area where anthropogenic activities are increasing.

In the North Sea, offshore oil and gas (O&G) platforms must be totally removed through decommissioning at the end of their productive life. However, the role of O&G platforms in marine ecosystems, especially for fish assemblages, is not well enough defined yet. Here, we document the association between an O&G platform in the North Sea and the fish assemblages along a distance gradient of 1-600 m from the platform. Scientific angling provided data on fish biodiversity, abundance, and body size. In addition, acoustic data on fish density and target strength were collected to explore spatial and diel fish distributions. The angling data comprised 1217 fish from 11 species, with the highest fish abundance, and all species occurring, within 20 m from the platform. Platform proximity was positively associated with fish biodiversity, and total fish abundance, as well as the specific abundances of Atlantic cod Gadus morhua, dab Limanda limanda, whiting Merlangius merlangus, and mackerel Scomber scombrus. Body sizes of Atlantic cod, whiting, and mackerel were also positively associated with the platform. Absent non-native or invasive species provided no support for stepping stone scenarios. This study highlights the attraction of a variety of fish species towards O&G platforms in the North Sea. Potential local loss of marine biodiversity following full platform removal should be considered in future discussions on platform decommissioning policies.

Wildlife populations globally have experienced widespread historical declines due to anthropogenic and environmental impacts, yet for some species, contemporary management and conservation programmes have enabled recent recovery. The impacts of decline and recovery on genomic diversity and, vice versa, the genetic factors that contribute to conservation success or failure are rich areas for inquiry, with implications for shaping how we manage species into the future. To comprehensively characterise these processes in natural systems requires range-wide sampling and international collaboration, particularly for species with wide dispersal capabilities, broad geographic distributions, and complex regional metapopulation dynamics. Here, we present the first range- and genome-wide population genomic analysis of grey seals based on 3812 nuclear SNPs genotyped in 188 samples from 17 localities. Our analyses support the existence of three main grey seal populations centred in the NW Atlantic, NE Atlantic and Baltic Sea, and point to the existence of previously unrecognised substructure within the NE Atlantic. We detected remarkably low levels of genetic diversity in the NW Atlantic population, and demographic analyses revealed a turbulent history of NE Atlantic and Baltic Sea grey seals, with bottlenecks in the Middle Ages and the 20th century due to hunting and habitat alterations. We found some localities deviated from isolation by distance patterns, likely reflecting wide-scale metapopulation dynamics associated with recolonisation and recovery in regions where they were historically extirpated. We identify at least six grey seal genetic populations and reveal marked genetic effects of past declines and recent recovery across the species' range.

The North Sea, home to many different species including the threatened Atlantic cod, remains one of the most important fishing grounds in the world, but it is also a hotbed for oil and gas exploration. Numerous oil and gas platforms are reaching the end of their lifespan in the North Sea and are expected to be decommissioned and removed from the seabed. However, increasing evidence suggests that offshore platforms may function as artificial reefs and even benefit marine biodiversity.

The effectiveness and power of passive acoustic monitoring (PAM) of wildlife in ecosystems depends on our ability to discern between species' vocal repertoires. For dolphin echolocation clicks there is heavy overlap within the bioacoustic niche, which presents challenges in signal processing. Furthermore, because echolocation is a highly directional signal, it is imperative to identify a robust categorizing characteristic that is consistent across different methods, animal position, and behavior. We applied a computationally simple method to identify instances of frequency peaks and notches within the click bandwidth; a spectral characteristic associated with white-beaked dolphins. In studies with visual species confirmation 75-90 % of events contained spectral banding in white-beaked dolphin echolocation and burst pulses. A pilot study between May-August 2021 placed 4 autonomous acoustic recorders in the Skagerrak and was reviewed for white-beaked dolphin using freeware PAMGuard and R. A decision tree process automatically reviewed the PAMGuard output to identify instances of possible dolphin activity. Those timeframes were manually audited to find true white-beaked dolphin events (total 23). We split data into testing and training datasets and used a nonlinear least squares model to fit a spectral template within the bandwidth where spectral peaks and notches are most prevalent. We tested our modelled template against the mean spectrum of every event in the testing dataset, and against noise timeframes which contained no white-beaked dolphin signals as a control. Using a Pearson Correlation test, we found that 87 % of the true white-beaked dolphin events and 3 % of the noise events were significantly correlated with the modelled template, aligning with studies using visual species confirmation. Our results show that using basic machine learning techniques can be used to reliably categorize white-beaked dolphin signals in PAM data from other odontocetes in Scandinavian waters. Categorization methods such as these which focus on robust signal characteristics are key for applying autonomous PAM data to broader ecology studies.

Echolocating animals rely on hearing to forage and navigate while moving through complex acoustic scenes. However, most auditory data from toothed whales come from stationary animals, limiting our understanding of how they receive and process sound during natural biosonar behavior. Here, we used suction-cup biologging tags with external electrodes to record auditory brainstem responses (ABRs) from four freely swimming, trained bottlenose dolphins and a wild harbor porpoise during prey capture. ABRs were measured in response to each individual’s outgoing clicks and returning echoes during target approach. We found that biosonar adjustments reduced ABR amplitudes for outgoing clicks, while amplitudes for returning echoes remained generally constant. Notably, faint echoes elicited response amplitudes similar to those for higher-level self-produced clicks with reduced amplitudes to their own clicks as click rate increased. These findings indicate that auditory responsivity dynamically changes during target approach and suggest an additional gain control mechanism for minimizing sensations of self-generated clicks. Tag-based ABRs close a major gap between stationary lab studies and natural behavior, offering direct insight into sensory reception under real-world conditions. This method is broadly applicable to other toothed whale species and provides a novel tool to study hearing in free-ranging animals.

Recent hearing studies on humpback whales (Megaptera novaeangliae) and common minke whales (Balaenoptera acutorostrata) have advanced our understanding of baleen whale hearing. The studies adapted physiological and behavioral approaches used in human clinical hearing assessments to address the unique challenges of working with baleen whales. Behavioral observation audiometry (BOA), historically used to test hearing in small children, was modified to assess the minimum response level from a population of humpback whales. Individuals (or a group of individuals) were exposed to a single sound, which differs from human applications in which a single individual is presented multiple sounds at varying levels. The modification has value provided that enough individual exposures are achieved. Auditory evoked potential methods, which have largely replaced BOA for infant hearing tests, were modified to test hearing in the minke whale. Modifications included the use of chirps to mitigate attenuation of evoked responses due to the whale’s large body mass, and repetitive tone pips presented at optimal rates for this species. Both methods have potential for use in additional baleen whale hearing studies, but application will be limited to species and populations that are available, approachable, and demonstrate the necessary behavioral and physical traits.

Marine ecosystems across the globe are under pressure requiring evaluations of whether marine protected areas (MPAs) remain effective in safeguarding marine life and critical habitats. Our objectives were to 1) estimate shifts in seasonal habitat suitability over a 25-year period (1997-2022) for the harbour porpoise (Phocoena phocoena) across the North Sea-Baltic Sea transition zone, and 2) quantify any spatiotemporal changes in the overlap between suitable habitats and an existing network of 62 MPAs (an area covering 16,404 km2) dedicated to harbour porpoise protection. Habitat suitability was estimated using the maximum entropy (MaxEnt) algorithm, based on satellite tag data of 111 individuals and a range of dynamic and static environmental conditions as predictor variables. Model output revealed a northward shift of medium and high suitable habitats during winter (Nov-Apr) over the study period. Changes in habitat suitability during summer (May-Oct) were also evident though without clear directional shifts. A maximum of 14.5 % of the MPAs (n = 9; area of 1096 km2) were found to overlap with highly suitable habitat during the summers of 2005-2012, with a minimum of 6.4 % (n = 4 MPAs; area of 194 km2) during the winters of 2013-2022. These results clearly indicate a decline in the spatiotemporal overlap between habitat suitability during winter and the location of MPAs dedicated to harbour porpoise protection in the North Sea-Baltic Sea transition zone. The model output may inform effective design of forthcoming MPAs under the EU Biodiversity Strategy for 2030.

Determining the frequency range of hearing and hearing sensitivity of baleen whales has long been a goal of marine mammal bioacousticians. Two recent investigations into baleen whale hearing, one on the common minke whale (Balaenoptera acutorostrata) and one on the humpback whale (Megaptera novaeangliae), have significantly advanced our understanding of baleen whale hearing. Thestudies used evoked potential audiometry, a direct measure of hearing, and modified behavioral observation audiometry, an indirect measure, to determine the frequency range of hearing and hearing sensitivity. Results from both studies suggest that these baleen whale species hear at higher frequencies than previously predicted based on vocalization frequencies and anatomical modeling. In the case of the minke whale, the frequency of best hearing sensitivity might reflect adaptations for predator detection or sound localization. In humpbacks, the received signal-to-noise ratios at which behavioral responses were observed were similar to critical ratios experimentally measured in multiple odontocete species. Additional research utilizing these techniques, and matched to species and populations where animals are available and show the behavioral and physiological traits suitable for their implementation (e.g., small size, predictable patterns of occurrence and behavior), will likely provide further information on baleen whale hearing.