Common Dolphins Research Articles

Observed trends in scavenging by common bottlenose dolphins (Tursiops truncatus truncatus) in for‐hire fisheries in the eastern U.S. Gulf of Mexico

AbstractInteractions between bottlenose dolphin and recreational rod and reel fisheries are a complex issue for resource managers in the United States, which may impact anglers' catch and lead to dangerous situations for scavenging or depredating dolphins. To examine this issue, we analyzed data collected by observers on for‐hire fishing vessels off the eastern U.S. Gulf Coast from 2009 to 2020. A generalized additive model indicated number of anglers, number of discarded fish, proximity to shore, prior scavenging events, type of released fish, and area fished were all significant predictors of scavenging by dolphins. The Florida panhandle had the highest odds of scavenging, while scavenging in the Big Bend and Tampa Bay has increased through time. The Florida panhandle is a well‐known area for illegal feeding of dolphins, suggesting human behavior may be influencing fisheries interactions. Model outputs indicate that dolphins are primarily cuing on fishing activity rather than number of discards, but are more likely to scavenge discards from the family Lutjanidae (snappers), which comprised 40% of observed discards but 80% of scavenging events. This study highlights factors influencing the frequency of dolphin scavenging events, guiding managers and scientists on additional studies and mitigation measures needed to address this issue.

A First Study on Distribution Characteristics of Common Dolphin in Korean Waters: A Study Using Data Collected during the Past 20 Years

The common dolphin (Delphinus delphis) is the second-most bycaught species in Korean waters. To provide key information about their habitat boundaries and hotspots for spatial conservation and management, the spatial use of this species was examined using data obtained from sighting and bycatch surveys of cetaceans in the past 20 years. The 95% minimum convex polygon and 95% density contour of fixed-kernel analysis suggested that the boundary of the home range of common dolphins is limited to the coastal region (Busan–Sokcho) of the East Sea/Sea of Japan. From 50% density contours drawn by kernel density estimation, it was suggested that their hotspots are around the coast of Ulsan–Pohang, Doghae, and Sokcho within the home range. Common dolphins were not observed in the Yellow Sea. Hence, shallow waters in the geographic area of the coastal region of the Yellow Sea are likely not a suitable habitat for common dolphins in this region.

Inferring odontocete life history traits in dentine using a multiproxy approach (δ15 N, δ44/42 Ca and trace elements).

Understanding the interactions between marine mammals and their environment is critical for ecological and conservation purposes. Odontocetes offer a continuous record of their life history from birth as recorded in annual increments of their tooth dentine. Because dentine is not remodeled and contains collagen, nitrogen stable isotope compositions (δ15 N) reflect nursing and weaning events, life history traits that would otherwise be impossible to retrieve in such elusive marine animals. Yet, capturing the magnitude and temporal changes in these events is constrained by tooth size and sampling resolution. Moreover, historical and fossil specimens undergo collagen decay, hence the need to develop the measurements of other proxies. Here, we present a multiproxy approach to investigate the use of Ca isotope compositions (δ44/42 Ca) in relation to δ15 N and laser ablation profiles for different trace metal (Ba, Mg, Sr, Zn) concentrations across the dentine of a single individual of the common bottlenose dolphin Tursiops truncatus. To help interpret the dentine data, we provide milk elemental compositions and δ44/42 Ca values for two odontocete individuals. We discuss the observed changes in δ44/42 Ca across the dentine as potential markers of birth, weaning interval, incidental ingestion of seawater, trophic level and physiology. Incidental ingestion of seawater during nursing induces a positive offset in δ44/42 Ca values recorded in the early formed dentine. Life history parameters of individual marine mammals are extremely difficult to retrieve due to limitations in observing specimens in the wild and the methodology presented here offers new ecological and paleoecological perspectives.

Short-beaked common dolphins of Dilek Peninsula National Park, South Aegean Sea: Preliminary results on group structure, spatial distribution, behavioural patterns, whistle characteristics and behavioural context

Basic knowledge about populations of short‐beaked common dolphins (Delphinus delphis) in the eastern part of the Mediterranean Sea is still lacking. Classified as ‘Endangered’ in an IUCN assessment released in 2022, it is necessary to gain baseline knowledge to understand the species’ population status and develop impactful conservation measures. The current study conducted boat surveys, employing visual and acoustic techniques between January 2019 and July 2022 in the Dilek Peninsula National Park, central Aegean Sea. Results revealed the continuous coastal presence of common dolphins, with subadults recorded during every sighting in low group sizes. Travelling‐related activities made up 58% of observed behaviour while resting comprised just 1%. Whistles were produced in the range of 1.83–48kHz with a peak frequency of 10.4kHz and median duration of 0.54s. Concave whistles were the most frequently recorded whistle type. Whistles were found to be generated when dolphins were interacting with boats and diving/travelling fast, forming 53% and 31% of whistle production respectively. The continuous presence of common dolphins, particularly subadults, emphasises the importance of the Dilek Peninsula. Whistle parameters show similarities with other common dolphin populations of the Mediterranean and Black Sea, albeit with shorter durations. The high whistle presence during boat interactions requires investigation to assess whether whistles carry avoidance or predatory messages, or whether there are potential consequences for the species’ energy budget. The current study presents preliminary information on the behavioural context of acoustic patterns. Further research is essential to understand the click and whistle characteristics within the biological, environmental and anthropogenic variables of their surroundings.

Gut bacterial communities in Atlantic bottlenose dolphins (Tursiops truncatus) throughout a disease-driven (Morbillivirus) unusual mortality event.

Gut microbiomes are important determinants of animal health. In sentinel marine mammals where animal and ocean health are connected, microbiome impacts can scale to ecosystem-level importance. Mass mortality events affect cetacean populations worldwide, yet little is known about the contributory role of their gut bacterial communities to disease susceptibility and progression. Here, we characterized bacterial communities from fecal samples of common bottlenose dolphins, Tursiops truncatus, across an unusual mortality event (UME) caused by dolphin Morbillivirus (DMV). 16S rRNA gene sequence analysis revealed similar diversity and structure of bacterial communities in individuals stranding before, during, and after the 2013-2015 Mid-Atlantic Bottlenose Dolphin UME and these trends held in a subset of dolphins tested by PCR for DMV infection. Fine-scale shifts related to the UME were not common (10 of 968 bacterial taxa) though potential biomarkers for health monitoring were identified within the complex bacterial communities. Accordingly, acute DMV infection was not associated with a distinct gut bacterial community signature in T. truncatus. However, temporal stratification of DMV-positive dolphins did reveal changes in bacterial community composition between early and late outbreak periods, suggesting that gut community disruptions may be amplified by the indirect effects of accumulating health burdens associated with chronic morbidity.

Bycatch of short-beaked common dolphin (Delphinus delphis) in the pair bottom trawl fishery of the Bay of Biscay and its mitigation with an active acoustic deterrent device (pinger)

Bycatch of common dolphin (Delphinus delphis) in commercial trawl fisheries in the Bay of Biscay (NE Atlantic) is of concern and its mitigation a priority. Active acoustic deterrent devices (pingers) attached to fishing gear seem to be promising for bycatch mitigation, as they have demonstrated to effectively reduce cetacean bycatch in some set-net fisheries. However, the low occurrence of common dolphin bycatch in many trawl fisheries, coupled with the extensive amount of time needed to monitor them, makes it difficult to prove the effectiveness of pingers. Remote electronic monitoring (REM) systems in fisheries can substantially increase onboard observation, providing access to extensive databases to comprehensively address bycatch mitigation studies. In this study, the effectiveness of DDD®03H Dolphin Dissuasive Device (hereinafter DDD pingers) to reduce common dolphin bycatch was evaluated in a demersal pair trawler in FAO Division 27.8.c. In 195 fishing days, one of the vessels in the pair operated with a set of DDD pingers whereas the other operated without them, and the bycatch of common dolphin was monitored through the REM system. In total, 660 fishing hauls were conducted of which 223 hauls had the DDDs attached. The results showed that the DDDs reduced common dolphin bycatch by more than 90%, with both bycatch frequency and the number of individuals bycaught per haul being significantly lower. The results also showed that common dolphin bycatch in this fishery is related to factors such as the fishing zone and depth, whereas the type of net deployed, time of day and haul duration were found to not significantly affect the bycatch of this species.

Establishment and characterization of a novel kidney cell line derived from the common bottlenose dolphin.

Common bottlenose dolphin (Tursiops truncatus) is a well-known cetacean species that inhabits temperate and tropical seas worldwide. Limited supply and poor quality of samples hinder the investigation of the effects of various pathogens and environmental pollutants on this cetacean species. Cultured cells are useful for experimental studies; however, no cell lines derived from cetaceans are generally available. Therefore, in this study, we established a novel kidney cell line, TK-ST, derived from T. truncatus. Primary cells exhibited the morphological characteristics of epithelial and fibroblast cells, but their immortalization and passaging resulted in a predominantly epithelial cell morphology. TK-ST was immortalized using the large T SV40 antigen and human telomerase reverse transcriptase and exhibited long-term stable cell growth. TK-ST cells are generally cultured in Dulbecco's modified Eagle's medium with 10% fetal bovine serum at 37°C and 5% CO2 but can also be cultured in 5-20% fetal bovine serum and several other classical media commonly used for common animal cell culture. TK-ST cells were found to be susceptible to several viruses, including the dolphin morbillivirus (most important virus in cetaceans), and exhibited cytopathic effects, facilitating the replication of the dolphin morbillivirus. Furthermore, mRNA expression levels of cytokine genes were increased in TK-ST cells after stimulation with lipopolysaccharides and poly(I:C). Therefore, the novel TK-ST cell line derived in this study can potentially be used for further in vitro studies on cetaceans.

Species distribution modeling of deep‐diving cetaceans

AbstractSpecies distribution models (SDMs) have been developed and extensively validated for diverse cetaceans within the California Current Ecosystem off the West Coast of the United States. These studies have recognized the challenges associated with developing robust models for deep‐diving cetaceans—sperm whales and beaked whales—thus limiting the accuracy of predictions for management and ecological understanding. In this study, we explore whether additional biologically relevant predictor variables can improve models for deep‐divers. These variables are related to the oxygen minimum layer and phytoplankton and micronekton biomass and could influence prey availability for cetacean top predators. We found that the addition of these variables improved the performance of SDMs for sperm whales, as well as for some more common baleen whale and dolphin species, but that the accuracy of deep‐diver models was nevertheless poor. The sightings data sets for deep‐diving cetaceans have small sample sizes compared to other cetaceans, and sightings are distributed nearly randomly across the study area and model domain. These factors hinder the development of useful environmentally driven models of spatial distribution.

Development of an integrated indicator to assess chemical contamination in different marine species: The case of mercury on the French Atlantic continental shelf

Good Environmental Status (GES) for Descriptor 8 (D8) of the Marine Strategy Framework Directive (MSFD) is considered to be achieved when concentrations of contaminants are at levels not giving rise to pollution effects. This study proposes a framework to assess GES in marine waters adjacent to France, including four groups of species (bivalves, fish, birds and mammals) living on the continental shelf and covering different dimensions of the marine environment. This framework is applied to mercury (Hg) in the three marine regions along the French Atlantic coast and includes two assessment types: i) an absolute assessment by comparing contamination levels with environmental thresholds, and ii) a relative assessment by comparing contamination levels over time, performed for bivalves and mammals that had long time-series available. Mercury concentrations were higher than environmental thresholds for bivalves and fish in all the three studied regions. Plus, they significantly increased since the 2000s for most bivalve stations and for the common dolphin Delphinus delphis. Our results therefore indicate that Hg concentrations have increased in marine waters and have reached levels possibly giving rise to pollution effects in biota from the three marine regions. The present study also highlighted the complementarity of monitoring Hg concentrations in each group of species and each type of assessment, making it possible to propose a conceptual framework for assessing the environmental pressure of bioaccumulated and biomagnified contaminants over the continental shelf.

Whistle characterization of long-beaked common dolphin (Delphinus delphis bairdii) in La Paz Bay, Gulf of California.

Long-beaked common dolphin (Delphinus delphis bairdii) distribution is limited to the Eastern North Pacific Ocean. Its whistle repertoire is poorly investigated, with no studies in the Gulf of California. The aim of the present study is to characterize the whistles of this species and compare their parameters with different populations. Acoustic monitoring was conducted in La Paz Bay, Gulf of California. Recordings were inspected in spectrogram view in Raven Pro, selecting good quality whistles (n = 270). In the software Luscinia, contours were manually traced to obtain whistle frequencies and duration. Number of steps, inflection points and contour type were visually determined. We calculated the descriptive statistics of the selected whistle parameters and we compared the results with a dolphins population from the Eastern Pacific Ocean. Permutational multivariate analysis of variance (PERMANOVA) was performed to test the intraspecific variation of the whistle parameters among groups. In the present study the mean values (±SD) of the whistle parameters were: maximum frequency = 14.13 ±3.71 kHz, minimum frequency = 8.44 ±2.58 kHz and duration = 0.44 ±0.31 s. Whistles with the upsweep contour were the most common ones (34.44%). The coefficient of variation (CV) values for modulation parameters were high (>100%), in accordance with other studies on dolphins. Whistle parameters showed significant differences among groups. Finally, ending and maximum frequencies, duration and inflection points of the whistles recorded in the present study were lower compared with the parameters of the long-beaked common dolphins from the Eastern Pacific Ocean. This study provides the first whistle characterization of long-beaked common dolphin from the Gulf of California and it will help future passive acoustic monitoring applications in the study area.

Trophic ecology of common bottlenose dolphins in a pelagic insular environment inferred by stable isotopes

The common bottlenose dolphin (Tursiops truncatus) is a top marine predator widely dispersed in coastal and pelagic habitats and with a generalist feeding behavior. Yet, information on the trophic ecology of animals inhabiting pelagic environments is still scarce. Using carbon (δ13C: 13C/12C) and nitrogen (δ15N: 15N/14N) stable isotope ratios, we identified and quantified the main groups of prey assimilated by bottlenose dolphins inhabiting an oceanic habitat (Madeira Island, East Atlantic). Bottlenose dolphins assimilated pelagic, schooling fish (such as blue jack mackerel, Trachurus picturatus) and mesopelagic and demersal squids, which reinforces the pelagic dietary composition of insular/oceanic dolphins. Also, intra-seasonal differences were found in their stable isotope ratios, which suggest intraspecific variability in the feeding behavior among individuals living in the same area. Sex was not the main factor contributing to these differences, suggesting the lack of trophic niche segregation between adult males and females in this offshore environment. Nonetheless, further studies including different life stages and information on the ecophysiological requirements are necessary to disclose the factors responsible for the observed variability. This study showed that insular dolphins fed primarily on economically important pelagic prey, highlighting the need of developing management strategies that integrate conservation in fisheries plans.

The fitness consequences of human‐wildlife interactions on foraging common bottlenose dolphins (Tursiops truncatus) in Sarasota Bay, Florida

AbstractHuman‐wildlife interactions (HI) are becoming more prevalent with increasing human population. These interactions could have important eco‐evolutionary consequences that become apparent only after observing populations for multiple generations. Here, we analyzed 28 years (1993–2020) of data from the world's longest‐running study of a wild dolphin population to assess the fitness consequences of HI on common bottlenose dolphins (Tursiops truncatus) in Sarasota Bay, Florida. We investigated how human‐related foraging activities such as depredation, begging, and patrolling mediate reproductive output and reproductive success of conditioned (HI) and unconditioned (non‐HI) females. The analysis of 84 females and their 286 calves born during 1993–2020 found a confluence of effects on individual fitness. Reproductive output of females engaging in moderate levels of human‐related foraging was 94% greater than that of non‐HI females. However, high frequencies of human‐related foraging had a negative effect on female reproductive success by increasing the risk of calf death up to nine times when compared to non‐HI females, resulting in 31% less calf survival. These findings provide evidence that human‐wildlife interactions have considerable potential to catalyze population‐level changes by altering individual fitness, and demonstrate the value of comprehensive, long‐term data to better understand the ecological and evolutionary implications of human‐wildlife conflict.

Evidence of signature whistles produced by Indian Ocean bottlenose dolphins (Tursiops aduncus) in Mozambique

ABSTRACT Individually distinctive signature whistles are used by common bottlenose dolphins (Tursiops truncatus) during social interactions and to facilitate group cohesion. There is evidence from a few populations that Indian Ocean bottlenose dolphins (T. aduncus) also use signature whistles. We investigated this from a coastal resident population of T. aduncus in Mozambique. Video-audio data were collected during recreational swim-with activities over 12 years, where potential signature whistles were identified from 28 individuals. Of the 405 whistles documented, 75% were produced in SIGID bouts. Visual assessment of individual signature whistle contours demonstrated temporal stability for up to 8 years. Overall, most signature whistle types had upsweep frequency modulation and were emitted as multiloop whistles. Comparing all whistle contours to each other indicated low discrimination between individuals, with contours frequently categorised together. However, sex differences in the frequency characteristics of whistles were identified, with females whistling at lower frequencies than males. Our results indicate either a shared whistle repertoire or identity encoding with subtle contour features, requiring acute auditory perception and discrimination to decipher caller identity. More widespread geographic investigation into signature whistle use may demonstrate variation in acoustic communication systems for bottlenose dolphins, which are thus far not well understood.

Distribution of short and long-beaked common dolphin morphs (Delphinus spp.) based on skull’s rostrum index analysis along the Peruvian and Chilean coast

Distribution of short and long-beaked common dolphin morphs (Delphinus spp.) based on skull’s rostrum index analysis along the Peruvian and Chilean coast

Evaluating dolphin damage in trammel net fisheries in the Valencia region: Insights to improve management

Interactions between dolphins and trammel net fisheries have been described in the Mediterranean Sea as having negative consequences for both cetaceans and fishers. There is a need for studies that evaluate the economic costs caused by dolphins feeding from entangled fishes in trammel nets since most cetacean species involved in these interactions are endangered. This study aims to evaluate the economic impact of the interaction between dolphins and trammel nets fisheries in the coastal waters of the Valencian region in the western Mediterranean Sea. We conducted at-port visual inspections of nets from April 2018 to March 2019, which included 1,849 fishing operations with trammel nets, and we recorded damages to the net and catch and the value of the fishery. Thirty-two interactions were detected during the year of the study. The only observed species interacting was the common bottlenose dolphin. No bycatch of dolphins was reported. The interactions have been impacted seasonally and most of them took place in February and March. There were no significant differences in catch when comparing sets with and without depredation or damage in nets due to depredation by dolphins. Also on average, no differences were found in the value of the catch when comparing sets with or without damage caused by dolphins. The main cost to the fisher was the cost of repairing the nets. The estimated financial loss per vessel and year was €556 (95%CI: €303–€809). This represents less than 1% of the total yearly income of each vessel in our area. We suggest implementing tools to economically compensate artisanal vessels for any damage caused by dolphins in order to avoid increasing the hostile feeling of fishers towards dolphins and to apply conservation measures to protect this cetacean species.

Assessment of the presence of UV filters and UV stabilizers in stranded dolphin blubber

The presence of ultraviolet filters (UVFs) and stabilizers (UVSs) was evaluated for the first time in the common bottlenose dolphin (Tursiops truncatus). UVFs and UVSs are compounds of growing concern because their effects on the environment are not completely known. UVFs and UVSs are added to personal care products (PCPs), such as cosmetics and products related to sun care and once released to the aquatic ecosystem, marine organisms can bioaccumulate these substances.This work aimed to determine the presence of 12 UVFs and UVSs in cetacean blubber samples to assess the pollution to which these animals of the highest trophic chain levels are exposed due to human activity. Analytical determinations were carried out using a method based on microwave-assisted extraction combined with ultrahigh-performance liquid chromatography and tandem mass spectrometry detection. The developed method was successfully applied to determine the target compounds in the blubber tissues of five necropsied common bottlenose dolphins.Three of the 12 studied compounds, namely 2-ethylhexyl 2-cyano-3,3-diphenylprop-2-enoate (octocrilene, OC), 2-hydroxy-4-methoxybenzophenone (benzophenone 3, BP3) and 3-methylbutyl (E)-3-(4methoxyphenyl) prop-2-enoate (IMC), were detected in several samples. Of the identified compounds, OC was present in all the samples and at the highest concentration within the range from 52.61 ± 18.59 to 108.0 ± 11.32 ng·g−1.

Cranial variation between coastal and offshore bottlenose dolphins, Tursiops truncatus (Cetacea: Delphinidae) in Ecuador and the Mediterranean: a three-dimensional geometric morphometric study

Abstract Skull shape analysis provides useful information on wildlife ecology and potential local adaptations. Common bottlenose dolphins (Tursiops truncatus) often differentiate between coastal and offshore populations worldwide, and skull shape analyses can be particularly useful in this context. Here we quantify skull shape variation between coastal populations from the Gulf of Guayaquil (Ecuador) and the Mediterranean Sea, compared to offshore specimens from multiple oceans. We analysed skull shape differences using 3D models from museum specimens through geometric morphometrics (3DGM). Two complementary landmark approaches included single-point semi-landmarks in homologous features, as well as pseudo-landmarks placed automatically. Results show skull shape distinction between both coastal populations and offshore specimens. Offshore specimens showed little differentiation between distinct locations. Skull shape patterns mostly diverged in the shape and length of rostrum, as well as the shape of the ascending processes of the maxilla, pterygoids, and occipital bones. However, both coastal populations differed in the patterns and direction of change of those features and were also morphologically distinct. Our results are consistent with local data on site fidelity and social structure in the coastal populations. Skull shape changes suggest divergent feeding and sound production patterns are potential drivers, probably specific to the local environment of each community.

Group dynamics and activity patterns of two dolphin species, the Common Dolphin (Delphinus delphis) and Dusky Dolphin (Lagenorhynchus obscurus) in direct sympatry

Abstract Co-occurrence of species with similar ecological requirements could bring individual advantages, but when resources are scarce, co-occurring species would segregate to reduce competition and thus enable coexistence. We studied co-occurrence, group dynamics, and activity budgets, along with the seasonal and diel variation, of two sympatric species: the Common Dolphin (Delphinus delphis) and Dusky Dolphin (Lagenorhynchus obscurus), in Northern Patagonia, Argentina. Dolphin groups were recorded by nautical surveys between 2006 and 2018, and classified the groups by their constituent species (single- or multispecies groups), group size, age/sex composition, predominant activity, season, and time block. Frequencies were arranged in contingency tables and analyzed by log-linear models. Activity bout duration also was recorded by focal-group follows and compared using nonparametric tests. Single- and multispecies groups’ occurrence varied throughout the year, with a period of high co-occurrence in spring and summer, when both species and most of multispecies groups were present, and a period of low co-occurrence, when mostly common dolphins remained in the area. Common dolphins occur in groups of more than a hundred while dusky dolphins do so in groups of 50 animals at most. Feeding time budget was higher in Dusky Dolphin single- and multispecies groups. Common dolphins showed a seasonal and diurnal pattern in activity budget. During the period of high co-occurrence, dusky dolphins’ group size showed a diurnal pattern, and both species increased their feeding time budgets, a putative indication of more favorable foraging conditions. During this period, dusky dolphins associate occasionally in multispecies groups, increasing feeding time budget and feeding bouts duration, suggesting that the association between the two species is beneficial.

Stable isotope ecology and interspecific dietary overlap among dolphins in the Northeast Atlantic

Dolphins are mobile apex marine predators. Over the past three decades, warm-water adapted dolphin species (short-beaked common and striped) have expanded their ranges northward and become increasingly abundant in British waters. Meanwhile, cold-water adapted dolphins (white-beaked and Atlantic white-sided) abundance trends are decreasing, with evidence of the distribution of white-beaked dolphins shifting from southern to northern British waters. These trends are particularly evident in Scottish waters and ocean warming may be a contributing factor. This mobility increases the likelihood of interspecific dietary overlap for prey among dolphin species previously separated by latitude and thermal gradients. Foraging success is critical to both individual animal health and overall population resilience. However, the degree of dietary overlap and plasticity among these species in the Northeast Atlantic is unknown. Here, we characterise recent (2015-2021) interspecific isotopic niche and niche overlap among six small and medium-sized delphinid species co-occurring in Scottish waters, using skin stable isotope composition (δ13C and δ15N), combined with stomach content records and prey δ13C and δ15N compiled from the literature. Cold-water adapted white-beaked dolphin have a smaller core isotopic niche and lower dietary plasticity than the generalist short-beaked common dolphin. Striped dolphin isotopic niche displayed no interspecific overlap, however short-beaked common dolphin isotopic niche overlapped with white-beaked dolphin by 30% and Atlantic white-sided dolphin by 7%. Increasing abundance of short-beaked common dolphin in British waters could create competition for cold-water adapted dolphin species as a significant portion of their diets comprise the same size Gadiformes and high energy density pelagic schooling fish. These priority prey species are also a valuable component of the local and global fishing industry. Competition for prey from both ecological and anthropogenic sources should be considered when assessing cumulative stressors acting on cold-water adapted dolphin populations with projected decline in available habitat as ocean temperatures continue to rise.

Response of cetaceans to fluctuations of pelagic fish stocks and environmental conditions within the Celtic Sea ecosystem

Inshore waters off the south coast of Ireland are an important foraging area for a range of cetacean species. Some of the main prey species of these cetaceans are herring and sprat, two economically valuable fish species in the region. The Celtic Sea herring stock suffered a marked decline in 2013. The present study aimed to investigate potential changes in the ecosystem associated to the herring decline and to determine the potential impacts on predators. Here we analyzed sightings information of common dolphins, fin, minke, humpback and unidentified whale species, acoustic data of herring and sprat, and a range of environmental variables mainly derived from satellites. Firstly, we characterized spatio-temporal patterns in the relative abundance of predator and prey species, and environmental variables, and compared periods before and since the herring decline. Since the 2013 herring decline, (i) the herring stock has mainly concentrated in south-eastern coastal waters and southern offshore Irish waters, (ii) sprat density has increased, (iii) chlorophyll concentration has decreased, (iv) sea surface temperature has risen, and (v) the euphotic layer has extended deeper. Secondly, we modelled the effects of prey density and environmental conditions on the relative abundance and distribution of cetaceans, as well as the effects of environmental conditions on prey density, between 2005-2018 by applying Hurdle Generalized Additive Models. The models for herring and sprat support the idea that these species have different environmental relationships, for example herring tended to be found in shallower waters than was the case for sprat. The presence and relative abundance of common dolphins were significantly affected by both environmental conditions and herring density, whereas whale species presence and relative abundance were found to be correlated with sea surface temperature and prey density. The model results suggest differences in prey choice among whale species. Understanding the dynamic relationships between predators, prey and the environment is important to inform an ecosystem-based approach to fisheries management.