Characterising injuries and pathologies of common dolphin Delphinus delphis mortalities in the South Australian Sardine Fishery

Mortality of dolphins in fishing operations is often under-estimated, as shown by studies of beach-washed carcasses. Linking evidence obtained during necropsies with fishing method is fundamental to understanding the extent of mortality and the manner in which animals die. The South Australian Sardine Fishery (SASF) has operated a purse-seine industry since 1991. This study characterised injuries, pathological changes and life history of 49 dead dolphins collected from SASF during 2006-2019. Histology examination was conducted on 25 animals. Neonates, calves and juveniles accounted for 63% of the sample. Of mature females (n = 14), 11 were pregnant or lactating, with cryptic mortality estimated to be 20% of dolphins studied. Body condition was robust in 48 dolphins. Net marks were seen on 82%, mostly on the head, trunk and peduncle. Broken/missing teeth were noted in 63%. All dolphins had subdermal haemorrhage (moderate to severe in 96%), particularly around the head. Deep haemorrhage was common, including around occipital and flipper condyles, and organs. Copious fluid was present in the thoracic (pleural) and abdominal (ascites) cavities of half of the dolphins. Within the lungs, watery fluid and froth were observed in 100 and 39%, respectively. Recent bone fractures were documented in 43% of dolphins, mostly associated with haemorrhage. Severe blunt trauma appeared to be the primary cause of death, and 10 dolphins also had other significant pathologies. Visceral organ congestion and mild cardiomyopathy were observed. Stomachs contained prey remains in 75% of cases. The results of this study may help identify unreported purse-seine mortalities washed up in South Australia and elsewhere.

Age- and sex-specific survivorship of the Southern Hemisphere long-finned pilot whale (Globicephala melas edwardii)

Biodiversity loss is a major global challenge of the 21st century. Ultimately, extinctions of species are determined by birth and death rates; thus, conservation management of at-risk species is dependent on robust demographic data. In this study, data gathered from 381 (227 females, 154 males) long-finned pilot whales (Globicephala melas edwardii) that died in 14 stranding events on the New Zealand coast between 2006 and 2017 were used to construct the first age- and sex-specific life tables for the subspecies. Survivorship curves were fitted to these data using (1) a traditional maximum likelihood approach, and (2) Siler's competing-risk model. Life table construction and subsequent survival curves revealed distinct differences in the age- and sex-specific survival rates, with females outliving males. Both sexes revealed slightly elevated rates of mortality among the youngest age-classes (<2 years) with postweaning mortality rates decreasing and remaining relatively low until the average life expectancy is reached; 11.3 years for males and 14.7 years for females. Overall (total) mortality is estimated to be 8.8% and 6.8% per annum for males and females, respectively. The mortality curve resembles that of other large mammals, with high calf mortality, lower postweaning mortality, and an exponentially increasing risk of senescent mortality. An accelerated mortality rate was observed in mature females, in contrast to the closely related short-finned pilot whale (G. macrorhynchus), which selects for an extension to the postreproductive life span. The reason for the observed differences in the mortality rate acceleration and postreproductive life span between the two pilot whale species have not been established and warrant further investigation. Obtaining robust information on the life history of long-lived species is challenging, but essential to improve our understanding of population dynamics and help predict how future pressures may impact populations. This study illustrates how demographic data from cetacean stranding events can improve knowledge of species survival rates, thus providing essential information for conservation management.

Australian long-finned pilot whales (Globicephala melas) emit stereotypical, variable, biphonic, multi-component, and sequenced vocalisations, similar to those recorded in the northern hemisphere

While in the northern hemisphere, many studies have been conducted on the vocal repertoire of long-finned pilot whales (Globicephala melas), no such study has been conducted in the southern hemisphere. Presented here, is the first study on the vocalisations of long-finned pilot whales along the southern coast of mainland Australia. Multiple measures were taken of 2028 vocalisations recorded over five years in several locations. These vocalisations included tonal sounds with and without overtones, sounds of burst-pulse character, graded sounds, biphonations, and calls of multiple components. Vocalisations were further categorised based on spectrographic features into 18 contour classes. Altogether, vocalisations ranged from approximately 200 Hz to 25 kHz in fundamental frequency and from 0.03 s to 2.07 s in duration. These measures compared well with those from northern hemisphere pilot whales. Some call types were almost identical to northern hemisphere vocalisations, even though the geographic ranges of the two populations are far apart. Other call types were unique to Australia. Striking similarities with calls of short-finned pilot whales (Globicephala macrorhynchus) and sometimes sympatric killer whales (Orcinus orca) were also found. Theories for call convergence and divergence are discussed.

Skeletal and Cardiac Rhabdomyolysis in a Live-Stranded Neonatal Bryde's Whale With Fetal Distress

The main objective of wildlife forensic investigation is to recognize pathologic changes and cause of death. Even though it may not always be possible to determine the specific illness and/or etiology, the description and subsequent interpretation of the injuries provide an invaluable understanding of pathology in cetacean post-mortem investigations. Although pathological studies have been previously reported in various cetacean species, such descriptions of the infraorder Mysticeti remain rare. A live-stranded neonatal Bryde's whale (Balaenoptera edeni) which subsequently died soon after the stranding, was assessed by physical exam, blood examination, gross necropsy evaluation, histopathology, and immunohistochemistry. It presented with elevated serum levels of creatine kinase, cardiac troponin I, urea, and creatinine. Microscopically, we observed keratin spicules (squamous epithelial cells) and areas of atelectasis in the lungs. Acute degeneration in the myocytes and cardiomyocytes were comparable to the findings previously described in cases of capture myopathy in live-stranded cetaceans. Immunohistochemistry biomarkers such as myoglobin, fibrinogen, and troponin were analyzed. Skeletal and myocardial damage has been documented in several cetacean species. However, this is the first reported case of skeletal and cardiac rhabdomyolysis associated with live-stranding in a newborn Bryde's whale that suffered from fetal distress.

Cetacean biodiversity, spatial and temporal trends based on stranding records (1920-2016), Victoria, Australia

Cetacean stranding records can provide vital information on species richness and diversity through space and time. Here we collate stranding records from Victoria, Australia and assess them for temporal, spatial and demographic trends. Between 1920 and 2016, 424 stranding events involving 907 individuals were recorded across 31 Cetacea species from seven families, including five new species records for the state. Seven of these events were mass strandings, and six mother and calf strandings were recorded. Importantly, 48% of the species recorded are recognised as data deficient on the IUCN Red List. The most commonly recorded taxa were Tursiops spp. (n = 146) and Delphinus delphis (common dolphins, n = 81), with the greatest taxonomic richness (n = 24) and highest incidence of stranding events documented within the Otways mesoscale bioregion. We found no seasonal stranding patterns anywhere in the state. While our findings improve understanding of the spatial and temporal patterns of cetacean diversity within Victoria, we suggest greater effort to collect demographic data at stranding events in order to better study state-wide patterns through time. We conclude with guidelines for minimum data collection standards for future strandings to maximise information capture from each event.

Neonatal line may develop after birth in the Indo-Pacific bottlenose dolphin (Tursiops aduncus)

Studies using teeth to estimate age in marine mammals presume that the neonatal line (NNL) develops at birth. This study of Indo-Pacific bottlenose dolphins (Tursiops aduncus (Ehrenberg, 1833)) is the first to investigate when the NNL appears in odontocete dentine. Two to four teeth were prepared by decalcification, thin-sectioning, and staining for 103 dolphins, including 7 dolphins of known age. Tooth length, prenatal and postnatal dentine and NNL widths were measured. Developmental class (foetus, young neonate, older neonate, <1-year-old calf, 1-year-old calf) was assigned using carcass external features. NNL presence or absence was categorised for individual dolphins. The NNL was absent in a near-term foetus and all except one young neonate and fully formed in 50% of older neonates, whose estimated ages were 1 week to 2 months. It was absent in a known-age dolphin aged 4–7 weeks. NNL width was greater in dolphins less than 1 year old compared with those that were 1 year old. Factors that trigger NNL development are unknown. The present study suggests that the NNL may not be related to birth per se in dolphins, as has been clearly demonstrated in humans. Physiological processes, driven by diet, and behavioural changes during the first few months of postnatal life may be important drivers for NNL formation in odontocetes.

A survey of the sperm whale (Physeter catodon) commensal microbiome

Background

Mammalian commensal microbiota play important roles in the health of its host. In comparison to terrestrial mammals, commensal microbiota of marine mammals is mainly focused on the composition and function of skin and gut microbiota, with less attention paid to the health impact of bacteria and viruses. Previous studies on sperm whales (Physeter catodon) have affirmed their important phylogenetic position; however, studies on their commensal microbiota have not been published, due to difficulty in sample collection.

Methods

Here, we sequenced the metagenomes of blood, muscle and fecal samples from a stranded sperm whale using the BGISEQ-500 platform. We compared the diversity and abundance of microbiomes from three different tissues and tried to search pathogenic bacterial and virulence genes probably related to the health of the sperm whale. We also performed 16S rDNA sequencing of the fecal sample to compare to published gut metagenome data from other marine mammals.

Results

Our results demonstrated notable differences in species richness and abundance in the three samples. Extensive bacteria, including Enterococcus faecium, Fusobacterium nucleatum, Pseudomonas aeruginosa, Streptococcus anginosus, Streptococcus pneumoniae, and Streptococcus suis, and five toxigenic Clostridium species usually associated with infection, were found in the three samples. We also found the taxa composition of sperm whale gut microbiota was similar to that of other whales, suggesting co-evolution with its host. This study is the first report of the sperm whale gut microbiome, and provides a foundation for the pathogen detection and health assessment of the sperm whale.

Abundance estimates and habitat preferences of bottlenose dolphins reveal the importance of two gulfs in South Australia

Informed conservation management of marine mammals requires an understanding of population size and habitat preferences. In Australia, such data are needed for the assessment and mitigation of anthropogenic impacts, including fisheries interactions, coastal zone developments, oil and gas exploration and mining activities. Here, we present large-scale estimates of abundance, density and habitat preferences of southern Australian bottlenose dolphins (Tursiops sp.) over an area of 42,438km² within two gulfs of South Australia. Using double-observer platform aerial surveys over four strata and mark-recapture distance sampling analyses, we estimated 3,493 (CV = 0.21; 95%CI = 2,327-5,244) dolphins in summer/autumn, and 3,213 (CV = 0.20; 95%CI = 2,151-4,801) in winter/spring of 2011. Bottlenose dolphin abundance and density was higher in gulf waters across both seasons (0.09-0.24 dolphins/km²) compared to adjacent shelf waters (0.004-0.04 dolphins/km²). The high densities of bottlenose dolphins in the two gulfs highlight the importance of these gulfs as a habitat for the species. Habitat modelling associated bottlenose dolphins with shallow waters, flat seafloor topography, and higher sea surface temperatures (SSTs) in summer/autumn and lower SSTs in winter/spring. Spatial predictions showed high dolphin densities in northern and coastal gulf sections. Distributional data should inform management strategies, marine park planning and environmental assessments of potential anthropogenic threats to this protected species.

Pathology of the skeleton of Indo-Pacific bottlenose dolphins Tursiops aduncus: a comparison of adjacent gulfs in South Australia

Studies of skeletal pathology of marine mammals can contribute to conservation measures, yet few have focused on causative factors. Museums hold vast collections of skeletons relevant to this knowledge gap. Indo-Pacific bottlenose dolphin Tursiops aduncus carcasses (n = 162) were collected from Gulf St Vincent (GSV) and Spencer Gulf (SG), South Australia (SA), between 1988 and 2013, and post-mortem examinations were carried out. After preparing skeletons, their gross pathology was classified into 4 categories: lytic lesions, degenerative lesions, fractures and malformations. Comparisons were made between gulfs based on pathology type, relative age, period of collection and cause of death (anthropogenic vs. non-anthropogenic). Pathology prevalence (76%) was higher than reported in other studies but observed pathologies were similar. More than one pathology was observed in 88% of dolphins that had pathology. Vertebrae were often the site of pathology. Prevalence of lesions were: fractures (82%), lytic lesions (75%) including spondylo-osteomyelitis and osteomyelitis, degenerative lesions (57%) including spondylosis deformans, and malformations (17%). Prevalence of pathology increased with dolphin relative age, and GSV dolphins had more pathology than those in SG. In SG, anthropogenic cases had more pathology than non-anthropogenic cases, and dolphins collected after 2000 had more degenerative lesions than those collected before 2000. There were more malformations in SG dolphins than those from GSV, although this was not statistically significant. In one anomalous case, an individual was found with 5 detached transverse processes on 4 lumbar vertebrae. Heavy metal toxicity reported for T. aduncus from SG may be related to the malformations reported in this study.

Modelling Dolphin Distribution to Inform Future Spatial Conservation Decisions in a Marine Protected Area

As marine predators experience increasing anthropogenic pressures, there is an urgent need to understand their distribution and their drivers to inform spatial conservation planning. We used an ensemble modelling approach to investigate the spatio-temporal distribution of southern Australian bottlenose dolphins (Tursiops cf. australis) in relation to a variety of ecogeographical and anthropogenic variables in Coffin Bay, Thorny Passage Marine Park, South Australia. Further, we evaluated the overlap between current spatial management measures and important dolphin habitat. Dolphins showed no distinct seasonal shifts in distribution patterns. Models of the entire study area indicate that zones of high probability of dolphin occurrence were located mainly within the inner area of Coffin Bay. In the inner area, zones with high probability of dolphin occurrence were associated with shallow waters (2-4 m and 7-10 m) and located within 1,000 m from land and 2,500 m from oyster farms. The multi-modal response curve of depth in the models likely shows how the different dolphin communities in Coffin Bay occupy different embayments characterized by distinct depth patterns. The majority of areas of high (>0.6) probability of dolphin occurrence are outside sanctuary zones where multiple human activities are allowed. The inner area of Coffin Bay is an important area of year-round habitat suitability for dolphins. Our results can inform future spatial conservation decisions and improve protection of important dolphin habitat.

Morbillivirus-associated unusual mortality event in South Australian bottlenose dolphins is largest reported for the Southern Hemisphere

Cases of morbillivirus have been recorded in the Southern Hemisphere but have not been linked to significant marine mammal mortality. Post-mortems were conducted on 58 carcasses (44 Indo-Pacific bottlenose dolphins, two common bottlenose dolphins, 12 short-beaked common dolphins) from South Australia during 2005-2013, including an unusual mortality event (UME) in St Vincent Gulf Bioregion (SVG) during 2013. Diagnostic pathology, circumstance of death, body condition, age and stomach contents were documented for Indo-Pacific bottlenose dolphins. At least 50 dolphins died during the UME, 41 were Indo-Pacific bottlenose dolphins and most were young. The UME lasted about seven months and had two peaks, the first being the largest. Effect on the population is unknown. Diagnostic testing for morbillivirus was conducted on 57 carcasses, with evidence for infection in all species during 2011-2013. All tested UME bottlenose dolphins were positive for cetacean morbillivirus (CeMV), and the pathology included interstitial pneumonia, lymphoid depletion and syncytia. Concurrent pathologies, including lung parasite and fungal infections, and severe cutaneous bruising were observed in many dolphins. The event coincided with elevated water temperatures, a diatom bloom and significant fish die-offs. We conclude that the cause for the UME was multifactorial and that CeMV was a major contributor.

Atypical residency of short-beaked common dolphins (Delphinus delphis) to a shallow, urbanized embayment in south-eastern Australia

Short-beaked common dolphins (Delphinus delphis) are typically considered highly mobile, offshore delphinids. This study assessed the residency of a small community of short-beaked common dolphins in the shallow, urbanized Port Phillip Bay, south-eastern Australia. The ability to identify common dolphins by their dorsal fin markings and coloration using photo-identification was also investigated. Systematic and non-systematic boat surveys were undertaken between 2007 and 2014. Results showed that 13 adult common dolphins and their offspring inhabit Port Phillip Bay, of which 10 adults exhibit residency to the bay. The majority of these adults are reproductively active females, suggesting that female philopatry may occur in the community. Systematic surveys conducted between 2012 and 2014 revealed that the dolphins were found in a median water depth of 16 m and median distance of 2.2 km from the coast. The shallow, urbanized habitat of this resident common dolphin community is atypical for this species. As a result, these common dolphins face threats usually associated with inshore bottlenose dolphin communities. We suggest that the Port Phillip Bay common dolphin community is considered and managed separate to those outside the embayment and offshore to ensure the community's long-term viability and residency in the bay.

Forensic Veterinary Pathology

Sharp-force injuries are injuries caused by a mechanical force using sharp objects against the skin. Sharp-force injuries are mainly classified as stab, incised, chop, and therapeutic wounds and are less frequent than blunt-force injuries in animals. The analysis of the edges of the wound is crucial, especially if more than one type of lesion is involved. It may be difficult to differentiate between sharp trauma and blunt trauma, because lacerations can resemble incised wounds. The accurate documentation and examination of these injuries may indicate the instrument involved, the relationship between the animal and the perpetrator, and the force of the stab. Situations in which this type of trauma occurs may involve social violence, accidents, hunting, veterinary medical management, and religious rituals. The causes of death related to this type of trauma include hypovolemic shock, pneumothorax, or asphyxiation due to aspiration of blood. Necropsy findings should provide objective and unbiased information about the cause and manner of death to aid the investigation and further judgment of a possible crime.

Cetacean strandings in South Australia (1881–2008)

Long-term monitoring of cetacean strandings is essential for good management. This study updates previous summaries for South Australia by adding up to 20 years of comprehensive data, including results of necropsy examinations. A total of 1078 records were examined. Thirty-one species were recorded: 9 (7% of records) mysticetes, 22 (88%) odontocetes and the rest (5%) unidentified. The number of species new to South Australia did not reach an asymptote, with potential for at least five additional species. Small cetaceans were more frequently recorded after 1990, possibly due to increased reporting effort. Stranding records increased markedly after 1970. Records for all species occurred year-round. Beaked whales stranded primarily during January–April, baleen whales during July–January and common dolphins during February–May. Geographic hotspots were identified and related to upwelling and reporting effort. A necropsy program since 1990 resulted in 315 of 856 records being assigned to a circumstance of death, with anthropogenic circumstances accounting for 42% of these. Known Entanglement (21%, 66 of 315) and Probable Entanglement (12%, 37 of 315) were the most recorded anthropogenic circumstances of death. Future research correlating strandings with oceanographic/climatic conditions may help to explain the documented patterns but first the effects of reporting effort need to be accounted for.

Behaviour of free-ranging short-beaked common dolphins (Delphinus delphis) in Gulf St Vincent, South Australia

Common dolphins are subject to large-scale fishing activity and tourism operations in South Australia; however, there is a paucity of data on this species. Understanding the behaviour of a population can contribute greatly to our knowledge of a species and how to manage potential population-level threats. This paper describes the behaviour of short-beaked common dolphins (Delphinus delphis) in Australian waters for the first time. Data were collected from 109 independent dolphin groups during boat-based surveys conducted in Gulf St Vincent, South Australia, between September 2005 and May 2008. Activity budgets were used to assess behaviour of common dolphins in relation to diel patterns, season, water depth, sea surface temperature (SST), group size and composition. Foraging (33.9%) and resting (2.8%) were the most and least frequently observed behaviours, respectively. Travelling (33.0%), socialising (20.2%) and milling (10.1%) accounted for the remainder of the activity budget. Diurnal differences were detected, with foraging (59.5%) and socialising (31.8%) groups most frequently observed from 10.00 to 11.59 hours. Behaviour did not vary seasonally or with water depth, SST, group size or composition. Behaviour varied significantly between single- and multispecies aggregations. Foraging was more frequent in multispecies aggregations, as 78.4% of all foraging behaviour observed for common dolphins occurred in the presence of other species. Multispecies aggregations were most frequently observed with flesh-footed shearwaters (Puffinus carneipes), which were present during 29.4% of common dolphin encounters. Behaviour varied significantly during aggregations with shearwaters, as 62.2% of foraging groups occurred in the presence of shearwaters. Resting, milling or socialising was rarely observed in the presence of any other species, indicating that the primary mechanism for aggregations is likely prey-related.

Impacts of climate change on Australian marine mammals

Increasing evidence suggests that climate change is negatively affecting marine ecosystems and biota. However, little is known of how climate change will impact marine mammals. This review aims to identify the effects of climatic variations on Australian marine mammals and determine their potential responses to climate change. Shifts in distributions and reproductive success have been associated with climatic factors, while stranding events, drowning of seal pups, exposure to altered water conditions and disease in several marine mammal species have followed extreme weather events. Climate change may produce distributional shifts as the ranges of warm-water species expand or shift southwards, and those of cold-water species contract. Reductions in the extent of key habitats, changes in breeding success, a greater incidence of strandings in dugongs and cetaceans, and increased exposure of coastal species to pollutants and pathogens are likely. The capacity of Australian marine mammals to adapt to climate change is poorly understood, though there is evidence that several species may be able to modify their physiology or behaviour in response to warming temperatures. To increase the resilience of marine mammals, it is necessary to address non-climatic threats, such as ensuring that key habitats are protected in Australia.

Cetaceans registered on the coast of Ilhéus (Bahia), northeastern Brazil

Stranded cetaceans have scientific value and may confirm the occurrence of some species or indicate their geographical distribution in an area. The collection of biological material can also contribute to improve the knowledge of the species or populations inhabiting certain areas. This study aimed to record live or dead cetaceans found ashore along the coast of Ilhéus, Bahia State, northeastern Brazil. Data were collected through a campaign called "SOS strandings, whales, porpoises and dolphins." Between 1997 and 1999 three hundred posters and five hundred folders were distributed to beach huts, shops, fishing associations and settlements, the city's municipal government, the Brazilian Environmental Agency (IBAMA) and fire stations. During this campaign, which lasted until 2007 it was possible to record 38 cetaceans of ten species on the coast of Ilhéus: Physeter macrocephalus, Megaptera novaeangliae, Globicephala macrorhynchus, Orcinus orca, Peponocephala electra, Stenella clymene, Feresa attenuata, Ziphius cavirostris, Steno bredanensis and Sotalia guianensis. The greatest numbers of records were observed between 2000 and 2003, which was the period after the distribution of banners and posters. The majority of the carcasses were recovered near the city downtown area (<10 km) and there was not a relationship between the state of the carcasses and the distance from that area.

The use of carcasses for the analysis of cetacean population genetic structure: a comparative study in two dolphin species

Advances in molecular techniques have enabled the study of genetic diversity and population structure in many different contexts. Studies that assess the genetic structure of cetacean populations often use biopsy samples from free-ranging individuals and tissue samples from stranded animals or individuals that became entangled in fishery or aquaculture equipment. This leads to the question of how representative the location of a stranded or entangled animal is with respect to its natural range, and whether similar results would be obtained when comparing carcass samples with samples from free-ranging individuals in studies of population structure. Here we use tissue samples from carcasses of dolphins that stranded or died as a result of bycatch in South Australia to investigate spatial population structure in two species: coastal bottlenose (Tursiops sp.) and short-beaked common dolphins (Delphinus delphis). We compare these results with those previously obtained from biopsy sampled free-ranging dolphins in the same area to test whether carcass samples yield similar patterns of genetic variability and population structure. Data from dolphin carcasses were gathered using seven microsatellite markers and a fragment of the mitochondrial DNA control region. Analyses based on carcass samples alone failed to detect genetic structure in Tursiops sp., a species previously shown to exhibit restricted dispersal and moderate genetic differentiation across a small spatial scale in this region. However, genetic structure was correctly inferred in D. delphis, a species previously shown to have reduced genetic structure over a similar geographic area. We propose that in the absence of corroborating data, and when population structure is assessed over relatively small spatial scales, the sole use of carcasses may lead to an underestimate of genetic differentiation. This can lead to a failure in identifying management units for conservation. Therefore, this risk should be carefully assessed when planning population genetic studies of cetaceans.

Niche differentiation of bottlenose dolphin species in South Australia revealed by stable isotopes and stomach contents

Context Animal species with overlapping ranges are common worldwide, but how these species coexist is less obvious. Dolphins are protected in Australia and yet little information is available on their resource use which is essential for successful management and conservation. Aim The aim of this study was to determine the degree of overlap in diet and feeding ecology of the two Tursiops spp. that have overlapping ranges in South Australia. Methods Stomach content (91 Tursiops sp. and 14 T. truncatus) and stable isotope (δ13C, δ15N) analyses (39 Tursiops sp. and 14 T. truncatus) were conducted. Carcasses of dolphins were opportunistically collected between 1974 and 2005. Diet was quantified by frequency of occurrence (FOO) and numerical abundance (NA) of prey. Key results Delta13C from teeth revealed distinct differences in primary source of carbon, corresponding to coastal (Tursiops sp. n = 39,  = –12.24, s.d. = 1.32) and offshore habitats (T. truncatus n = 14,  = –14.21, s.d. = 0.55). Differences in δ15N revealed Tursiops sp. ( = 11.66, s.d. = 0.58) feeds at a lower trophic level than T. truncatus ( = 14.29, s.d. = 0.88). Stomach content analyses for Tursiops sp. corroborated stable isotope results. There was a significant difference between the diets of Tursiops sp. from north Spencer Gulf and south Spencer Gulf (ANOSIM R = 0.249, P = 0.001). Prey were generally demersal in habit with the most important from the cephalopod families Octopodidae, Sepiidae and Loliginidae and fish families Carangidae, Clupeidae, Terapontidae and Apogonidae. For T. truncatus there were insufficient stomach contents for assessment. Conclusions South Australian bottlenose dolphin species exhibit distinct niche differentiation with clear evidence of regional variation in the diet of Tursiops sp. Implications The between and within species diet differentiation demonstrated, highlights the importance of regional management. Such results are internationally significant as coastal and offshore forms of Tursiops spp. occur worldwide.