Australian Humpback Dolphins Research Articles

Attendance patterns of provisioned Australian humpback dolphins (Sousa sahulensis) in Tin Can Bay, Australia – Further indication of male bonding and alliance

Tin Can Bay, Queensland, is one of four free-ranging dolphin feed provisioning sites in Australia and the only one featuring Australian humpback dolphins (Sousa sahulensis). The site attracts the same 5–10 dolphins a year, most of which return regularly, and some which have been in attendance since the 1990s. Historical records were obtained for the period March 2010 to March 2021 covering dolphin attendance to the provisioning station including dolphin name, arrival time, and departure time. Tourist numbers were also obtained which included the total number and how many of that total directly took part in provisioning. Attendance of dolphins to the provisioning site has increased over the years both in frequency and total numbers. There was evidence of group cohesion between four particular dolphins (one mother-offspring pair and one male pair). The strongest pairing was seen between two males, indicating bond formation in adult male humpback dolphins, which may be indicative of an alliance. Specific events were also looked at in detail including cyclones, flooding COVID-19 restrictions, which reduced dolphin and/or human attendance.

Estimated reproductive parameters for a vulnerable Australian humpback dolphin population

AbstractUnderstanding reproductive output and success is integral to the conservation and management of threatened species and populations. Little is known about the reproductive parameters of Australian humpback dolphins (Sousa sahulensis), a species subject to cumulative pressures from anthropogenic threats due to their coastal distribution and strong site fidelity. This study assesses several reproductive parameters, including crude birth, recruitment, fecundity, and calf survival rates, in addition to interbirth intervals of Australian humpback dolphins inhabiting the near‐urban embayment of Moreton Bay, Queensland. Between 2014 and 2020, 106 adult females were photographically identified during 222 boat surveys. Of these, 75.5% (n = 80) were observed with calves. Birth seasonality was apparent and coincided with austral autumn and winter months. Interbirth intervals ranged between 1 and >6 years, with an average of 3.1 (SD = 1.1) years if offspring survived. Findings indicate declining fecundity rates (p < .05, M = 0.12, SD = 0.02), relatively low recruitment rates (M = 0.04, SD = 0.01) and calf survival rates to 1 year of age (M = 0.63, SD = 0.15). This study provides a useful baseline that can inform viability assessments and conservation measures, for both this population and others throughout the species range.

Incidence of shark-inflicted bite injuries on Australian snubfin (Orcaella heinsohni) and Australian humpback (Sousa sahulensis) dolphins in coastal waters off east Queensland, Australia.

The ecology and evolution of prey populations are influenced by predation and predation risk. Our understanding of predator-prey relationships between sharks and dolphins is incomplete due to the difficulties in observing predatory events directly. Shark-inflicted wounds are often seen on dolphin bodies, which can provide an indirect measure of predation pressure. We used photographs of Australian humpback and snubfin dolphins from north, central, and south Queensland to assess the incidence of shark-inflicted bite injuries and to examine interspecific differences in bite injuries and their relationship with group sizes, habitat features, and geographical locations characteristic of where these individuals occurred. The incidence of shark-inflicted scarring did not differ between species (χ 2 = 0.133, df = 1, p = .715), with 33.3% of snubfin and 24.1% of humpback dolphins showing evidence of shark bites when data were pooled across all three study sites. Generalized additive models indicated that dolphins closer to the coast, with greater photographic coverage, and in north Queensland were more likely to have a shark-inflicted bite injury. The similar incidence of shark-inflicted wounds found on snubfin and humpback dolphins suggests both are subject to comparable predation pressure from sharks in the study region. Results highlight the importance that habitat features such as distance to the coast and geographical location could have in predation risk of dolphins from sharks, as well as the importance of considering photographic coverage when assessing the incidence of shark-inflicted bites on dolphins or other marine animals. This study serves as a baseline for future studies on shark-dolphin interactions in Queensland and into how predation may influence dolphin habitat usage, group living, and behavior.

Multiple social benefits drive the formation of mixed-species groups of Australian humpback and Indo-Pacific bottlenose dolphins

Mixed-species groups are common amongst diverse taxa including fishes, birds, and mammals. Antipredator, foraging, and social benefits have been proposed as functional explanations for mixed-species group formation. Amongst delphinids, mixed-species groups are widespread, but little is known about their function. To investigate the potential benefits of delphinid mixed-species groups, we compared the number of individuals, the age composition, and the behaviour of single- and mixed-species sightings of Australian humpback (Sousa sahulensis) and Indo-Pacific bottlenose dolphins (Tursiops aduncus) observed around the North West Cape, Western Australia. We found no difference in the number of individuals or the age composition of humpback dolphins present in single- and mixed-species sightings, whereas bottlenose dolphins were present in larger numbers in single-species sightings than in mixed-species sightings due to a higher number of adults. Socialising was the initial observed behavioural state of 36.1% of mixed-species sightings, compared to only 5.1% and 10.3% of humpback and bottlenose dolphin single-species sightings, respectively. Furthermore, both species travelled and foraged less frequently when in mixed-species groups. Of 93 mixed-species groups observed during a focal follow of ≥ 10 min, 32 (34.4%) involved aggressive and/or sexual behaviours typically initiated by bottlenose dolphins towards humpback dolphins while the remaining 61 (65.6%) involved only neutral and affiliative behaviours. The results of this study suggest that the observed mixed-species groups provide multiple social benefits, particularly those pertaining to socio-sexual behaviours and the development and care of young.Significance statementNumerous species of dolphin are known to form mixed-species groups. Few studies have, however, investigated the antipredator, foraging, and/or social benefits that these species may gain by grouping with other species. Investigating these benefits is key to understanding the impacts of mixed-species groups on the species and individuals involved. We evaluated the potential functions of mixed-species groups of two sympatric, inshore dolphin species — the Australian humpback dolphin and the Indo-Pacific bottlenose dolphin. We found that both species engage in socialising behaviour more frequently when in mixed-species groups and that they engage in a diverse variety of interspecific behavioural interactions. Our results not only indicate that social benefits are the key drivers of these mixed-species groups, but they also highlight the complexity of social interactions between dolphin species.

Habitat partitioning, co-occurrence patterns, and mixed-species group formation in sympatric delphinids

Numerous species have been reported to form mixed-species groups, however, little is known about the interplay between niche partitioning and mixed-species group formation. Furthermore, it is often unclear whether species come together by chance due to overlapping habitat preferences, by shared attraction to resources, or by attraction between them. We assessed habitat partitioning, co-occurrence patterns, and mixed-species group formation of sympatric Australian humpback (Sousa sahulensis) and Indo-Pacific bottlenose dolphins (Tursiops aduncus) around the North West Cape, Western Australia, with a joint species distribution model and temporal analyses of sighting data. Australian humpback dolphins preferred shallower and more nearshore waters than Indo-Pacific bottlenose dolphins, yet these species co-occurred more often than expected by chance given shared responses to environmental variables. Indo-Pacific bottlenose dolphins were sighted more often than Australian humpback dolphins during the afternoon, however, we did not find any temporal patterns in the occurrence of mixed-species groups. We propose that the positive association in the species’ occurrence indicates the active formation of mixed-species groups. By evaluating habitat partitioning and co-occurrence patterns, this study provides direction for future work which should proceed to investigate the benefits that these species may gain from grouping with each other.

Aerial abundance estimates for two sympatric dolphin species at a regional scale using distance sampling and density surface modeling

Monitoring wildlife populations over scales relevant to management is critical to supporting conservation decision-making in the face of data deficiency, particularly for rare species occurring across large geographic ranges. The Pilbara region of Western Australia is home to two sympatric and morphologically similar species of coastal dolphins—the Indo-pacific bottlenose dolphin (Tursiops aduncus) and Australian humpback dolphin (Sousa sahulensis)—both of which are believed to be declining in numbers and facing increasing pressures from the combined impacts of environmental change and extensive industrial activities. The aim of this study was to develop spatially explicit models of bottlenose and humpback dolphin abundance in Pilbara waters that could inform decisions about coastal development at a regional scale. Aerial line transect surveys were flown from a fixed-wing aircraft in the austral winters of 2015, 2016, and 2017 across a total area of 33,420 km2. Spatio-temporal patterns in dolphin density were quantified using a density surface modeling (DSM) approach, accounting for imperfect detection as well as both perception and availability bias. We estimated the abundance of bottlenose dolphins at 3,713 (95% CI = 2,679–5,146; average density of 0.189 ± 0.046 SD individuals per km2) in 2015, 2,638 (95% CI = 1,670–4,168; 0.159 ± 0.135 individuals per km2) in 2016 and 1,635 (95% CI = 1,031–2,593; 0.101 ± 0.103 individuals per km2) in 2017. Too few humpback dolphins were detected in 2015 to model abundance, but their estimated abundance was 1,546 (95% CI = 942–2,537; 0.097 ± 0.03 individuals per km2) and 2,690 (95% CI = 1,792–4,038; 0.169 ± 0.064 individuals per km2) in 2016 and 2017, respectively. Dolphin densities were greatest in nearshore waters, with hotspots in Exmouth Gulf, the Dampier Archipelago, and Great Sandy Islands. Our results provide a benchmark on which future risk assessments can be based to better understand the overlap between pressures and important dolphin habitats in tropical northwestern Australia.

Using Unoccupied Aerial Vehicles to estimate availability and group size error for aerial surveys of coastal dolphins

AbstractAerial surveys are frequently used to estimate the abundance of marine mammals, but their accuracy is dependent upon obtaining a measure of the availability of animals to visual detection. Existing methods for characterizing availability have limitations and do not necessarily reflect true availability. Here, we present a method of using small, vessel‐launched, multi‐rotor Unoccupied Aerial Vehicles (UAVs, or drones) to collect video of dolphins to characterize availability and investigate error surrounding group size estimates. We collected over 20 h of aerial video of dive‐surfacing behaviour across 32 encounters with Australian humpback dolphins Sousa sahulensis off north‐western Australia. Mean surfacing and dive periods were 7.85 sec (se = 0.26) and 39.27 sec (se = 1.31) respectively. Dolphin encounters were split into 56 focal follows of consistent group composition to which example approaches to estimating availability were applied. Non‐instantaneous availability estimates, assuming a 7 sec observation window, ranged between 0.22 and 0.88, with a mean availability of 0.46 (CV = 0.34). Availability tended to increase with increasing group size. We found a downward bias in group size estimation, with true group size typically one individual more than would have been estimated by a human observer during a standard aerial survey. The variability of availability estimates between focal follows highlights the importance of sampling across a variety of group sizes, compositions and environmental conditions. Through data re‐sampling exercises, we explored the influence of sample size on availability estimates and their precision, with results providing an indication of target sample sizes to minimize bias in future research. We show that UAVs can provide an effective and relatively inexpensive method of characterizing dolphin availability with several advantages over existing approaches. The example estimates obtained for humpback dolphins are within the range of values obtained for other shallow‐water, small cetaceans, and will directly inform a government‐run program of aerial surveys in the region.

Prevalence of Skin Lesions and Injuries in Australian Humpback Dolphins (Sousa sahulensis) and Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) in Moreton Bay, Queensland

Coastal dolphin populations are highly vulnerable due to their proximity to major urban centres and exposure to cumulative threats from anthropogenic activities. As bioindicators of environmental condition, it is crucial to understand and monitor the health of these coastal dolphin populations. Visual assessments of skin lesions on dolphins can provide useful insights into the health of these populations and exposure to environmental stressors. We examined the prevalence of skin lesions in Australian humpback dolphins (Sousa sahulensis) and Indo-Pacific bottlenose dolphins (Tursiops aduncus) of different age classes inhabiting the near-urban embayment of Moreton Bay, Queensland. The prevalence and extent of nontraumatic and traumatic skin lesions on individual dolphins were assessed using photographs taken during 103 boat-based surveys completed between 2014 and 2016. A total of 15 primary skin lesion categories were identified from 126 humpback and 100 bottlenose dolphins. Differences in the prevalence of skin lesions were evident between age classes and species. Nontraumatic skin lesions were prevalent in 48.4% of the humpback and 61.0% of the bottlenose dolphins. Comparatively, traumatic lesions were evident in almost all humpback (92.3%) and bottlenose (99.0%) dolphins. Anthropogenic-related injuries from entanglement in fishing gear and vessel strikes were substantial and significantly differed between species (p < 0.05). Injuries from fishing and vessel activities affected 11.0% of humpback dolphins and 30.0% of bottlenose dolphins, suggesting that these activities pose a major threat to these populations. Findings from this study provide an important baseline to inform ongoing health monitoring and conservation efforts of these vulnerable dolphin populations inhabiting a near-urban embayment.

Isotopic niche overlap between sympatric Australian snubfin and humpback dolphins.

Ecological niche theory predicts the coexistence of closely related species is promoted by resource partitioning in space and time. Australian snubfin (Orcaella heinsohni) and humpback (Sousa sahulensis) dolphins live in sympatry throughout most of their range in northern Australian waters. We compared stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) in their skin to investigate resource partitioning between these ecologically similar species. Skin samples were collected from live Australian snubfin (n = 31) and humpback dolphins (n = 23) along the east coast of Queensland in 2014–2015. Both species had similar δ13C and δ15N values and high (>50%) isotopic niche space overlap, suggesting that they feed at similar trophic levels, have substantial dietary overlap, and rely on similar basal food resources. Despite similarities, snubfin dolphins were more likely to have a larger δ15N value than humpback dolphins, indicating they may forage on a wider diversity of prey. Humpback dolphins were more likely to have a larger δ13C range suggesting they may forage on a wider range of habitats. Overall, results suggest that subtle differences in habitat use and prey selection are likely the principal resource partitioning mechanisms enabling the coexistence of Australian snubfin and humpback dolphins.

Dolphin Distribution and Habitat Suitability in North Western Australia: Applications and Implications of a Broad-Scale, Non-targeted Dataset

Understanding species’ distribution patterns and the environmental and ecological interactions that drive them is fundamental for biodiversity conservation. Data deficiency exists in areas that are difficult to access, or where resources are limited. We use a broad-scale, non-targeted dataset to describe dolphin distribution and habitat suitability in remote north Western Australia, where there is a paucity of data to adequately inform species management. From 1,169 opportunistic dolphin sightings obtained from 10 dugong aerial surveys conducted over a four-year period, there were 661 Indo-Pacific bottlenose dolphin (Tursiops aduncus), 191 Australian humpback dolphin (Sousa sahulensis), nine Australian snubfin dolphin (Orcaella heinsohni), 16 Stenella sp., one killer whale (Orcinus orca), one false killer whale (Pseudorca crassidens), and 290 unidentified dolphin species sightings. Maximum Entropy (MaxEnt) habitat suitability models identified shallow intertidal areas around mainland coast, islands and shoals as important areas for humpback dolphins. In contrast, bottlenose dolphins are more likely to occur further offshore and at greater depths, suggesting niche partitioning between these two sympatric species. Bottlenose dolphin response to sea surface temperature is markedly different between seasons (positive in May; negative in October) and probably influenced by the Leeuwin Current, a prominent oceanographic feature. Our findings support broad marine spatial planning, impact assessment and the design of future surveys, which would benefit from the collection of high-resolution digital images for species identification verification. A substantial proportion of data were removed due to uncertainties resulting from non-targeted observations and this is likely to have reduced model performance. We highlight the importance of considering climatic and seasonal fluctuations in interpreting distribution patterns and species interactions in assuming habitat suitability.

Coastal dolphins and marine megafauna in Exmouth Gulf, Western Australia: informing conservation management actions in an area under increasing human pressure

Context Exmouth Gulf is adjacent to the Ningaloo Marine Park, a UNESCO-listed area in Western Australia. The gulf remains largely unprotected, and is under increasing anthropogenic pressure from proposed industrial activities that pose threats to marine megafauna inhabiting the gulf. Threatened and near threatened species, such as the Australian humpback dolphin (Sousa sahulensis) and Indo-Pacific bottlenose dolphin (Tursiops aduncus), reside in the gulf; however, detailed information on their ecology and behaviour is lacking. Aims The aim was to (1) provide baseline data on the distribution, encounter rate, group size and behaviour of coastal dolphins over an area where current industrial developments are proposed, and (2) report on the occurrence of other marine megafauna within this area. Methods Boat-based photo-identification surveys were conducted on the western coastline of Exmouth Gulf along pre-determined line transects (150 km2) over austral autumn/winter 2021. Key results Across 809.35 km of surveyed waters (181 h), a total of 93 bottlenose dolphin, 15 humpback dolphin, and six interspecific dolphin groups were sighted. Bottlenose dolphin groups were encountered at a rate of 0.077/km, humpback dolphin groups at 0.015/km and interspecific dolphin groups at 0.005/km. Dolphins were predominantly recorded in shallow (mean 10 m) and warm (mean 21°C) waters, and were commonly travelling and foraging. In total, 199 individual bottlenose dolphins and 48 humpback dolphins were photo-identified (excluding calves). There were 30 bottlenose dolphin calves (including three newborns) and four humpback dolphin calves (including two newborns) identified. Other marine megafauna group sightings included humpback whales (Megaptera novaeangliae; n = 32), southern right whales (Eubalaena australis, n = 1), dugongs (Dugong dugon, n = 25), turtles (n = 54), sea snakes (n = 27), manta rays (Mobula alfredi, n = 13) and sharks (n = 2). Conclusions The presence of threatened marine species feeding, socialising, and resting highlights the importance of these waters for the identified species. Implications The information provided is applicable for the spatial management and conservation efforts of these species, and aids in informing environmental impact assessments of individual and cumulative pressures.

Feasibility of Using Small UAVs to Derive Morphometric Measurements of Australian Snubfin (Orcaella heinsohni) and Humpback (Sousa sahulensis) Dolphins

Analysis of animal morphometrics can provide vital information regarding population dynamics, structure, and body condition of cetaceans. Unmanned aerial vehicles (UAVs) have become the primary tool to collect morphometric measurements on whales, whereas on free ranging small dolphins, have not yet been applied. This study assesses the feasibility of obtaining reliable body morphometrics from Australian snubfin (Orcaella heinsohni) and humpback dolphins (Sousa sahulensis) using images collected from UAVs. Specifically, using a dolphin replica of known size, we tested the effect of the altitude of the UAV and the position of the animal within the image frame on the accuracy of length estimates. Using linear mixed models, we further assessed the precision of the total length estimates of humpback and snubfin dolphins. The precision of length estimates on the replica increased by ~2% when images were sampled at 45–60 m compared with 15–30 m. However, the precision of total length estimates on dolphins was significantly influenced only by the degree of arch and edge certainty. Overall, we obtained total length estimates with a precision of ~3% and consistent with published data. This study demonstrates the reliability of using UAV based images to obtain morphometrics of small dolphin species, such as snubfin and humpback dolphins.

Identifying priority habitat for conservation and management of Australian humpback dolphins within a marine protected area

Increasing human activity along the coast has amplified the extinction risk of inshore delphinids. Informed selection and prioritisation of areas for the conservation of inshore delphinids requires a comprehensive understanding of their distribution and habitat use. In this study, we applied an ensemble species distribution modelling approach, combining results of six modelling algorithms to identify areas of high probability of occurrence of the globally Vulnerable Australian humpback dolphin in northern Ningaloo Marine Park (NMP), north-western Australia. Model outputs were based on sighting data collected during systematic, boat-based surveys between 2013 and 2015, and in relation to various ecogeographic variables. Water depth and distance to coast were identified as the most important variables influencing dolphin presence, with dolphins showing a preference for shallow waters (5–15 m) less than 2 km from the coast. Areas of high probability (> 0.6) of dolphin occurrence were primarily (90%) in multiple use areas where extractive human activities are permitted, and were poorly represented in sanctuary (no-take) zones. This spatial mismatch emphasises the need to reassess for future spatial planning and marine park management plan reviews for NMP. Shallow, coastal waters identified here should be considered priority areas for the conservation of this Vulnerable species.

Vulnerability of threatened Australian humpback dolphins to flooding and port development within the southern Great Barrier Reef coastal region

In this study, we used a 10-year (2007–2016) mark-recapture dataset to investigate the potential effects of flooding and port development on the population dynamics of Australian humpback dolphins (Sousa sahulensis), inhabiting the Fitzroy River and Port Curtis, within the southern Great Barrier Reef region. A Multisite Capture-Recapture model was used to quantify population size and demographic parameters for both sexes and sites. Flood occurrence and intensity (both sites), and port development (Port Curtis) were included as explanatory variables. Abundance estimates indicated that about 77 adult dolphins were present in both sites, of which 69% were females. Most females (69%) were resident with a yearly recruitment close to zero for most years. Most males and unsexed (68%) individuals showed little evidence of long-term residency. The abundances of males and unsexed individuals varied between 15 and 20 dolphins in the Fitzroy River and 19–26 in Port Curtis, but the accuracy was too low to assess changes. Female abundances started at 56 in both sites and declined to about 32 per site in 2011, coinciding with port development construction activities and a concurrent major flood. In Port Curtis, the number of females returned to their original levels once the port development was completed in 2013. In the Fitzroy River, the declining trend continued and reached the lowest estimated abundance of 29 in 2016. As port developments and floods are expected to increase along the Queensland coastal region over coming decades, the results of this study highlight increasing concerns about the vulnerability and long-term sustainability of inshore dolphins in the GBR.

Geographic and temporal variation in persistent pollutants in Australian humpback and snubfin dolphins

Abstract We investigated between 2014 and 2016 the concentrations of PCBs, DDTs and HCB and trace elements in blubber and skin samples of Australian humpback and snubfin dolphins collected from the Fitzroy River estuary and Port Curtis, Queensland, Australia. The concentrations of PCBs, DDTs and HCB from this study were compared with results from samples collected in 2009 and 2010 from the same areas. Our results showed that the concentrations of PCBs, DDTs and HCB have increased over time. A large proportion (~68%) of the sampled population accumulated organochlorines contaminants above thresholds over which immunosuppression and reproductive anomalies are known to occur. Results also suggest a general enrichment of some trace elements above natural levels in humpback and snubfin dolphins from the Fitzroy River estuary and Port Curtis. Extensive flooding in adjacent catchments is the most likely primary factor responsible for increased distribution and bioavailability of these contaminants into coastal waters. Land-based pollution from the adjacent catchments is an ongoing threat to coastal systems and particularly for top predators such as dolphins in the Great Barrier Reef. Climate change models predict increasing frequency and intensity of extreme rainfall events that will impact coastal Queensland and the Great Barrier Reef. Reduction of land-based persistent pollutants into coastal waters of the Great Barrier Reef will be critical for the health and survival of humpback and snubfin dolphins in this region.

Social dynamics and sexual segregation of Australian humpback dolphins (Sousa sahulensis) in Moreton Bay, Queensland

AbstractUnderstanding the social organization of coastal dolphins, particularly those that reside adjacent to major cities and urbanized regions, is important for understanding the vulnerability of populations to human impacts. We investigated the sexual segregation and social dynamics in a population of Australian humpback dolphins (Sousa sahulensis) inhabiting the near‐urban embayment of Moreton Bay, Queensland. Over 160 vessel‐based surveys, 148 adults were photographically identified. This included 116 assumed females and 32 assumed males. The population was sexually segregated (SSAS = 0.52, p < .001) and individuals preferred to associate with others of the same sex occupying the same spatial area (p < .025). Association indices between paired males were almost double those of females (, SD = 0.04 and , SD = 0.02, respectively). However, females were more gregarious than males and maintained a larger network of associates based on preferred companionships. The social network was also modular with five clusters identified, and a limited number of individuals had a disproportionate influence on connectivity between clusters. This highly fragmented social network implies that the population may be vulnerable to social disruptions from human impacts.

Challenges of collecting blow from small cetaceans

AbstractWe trialed the collection of blow samples using a waterproof electric multirotor (quadcopter) drone from two free‐ranging dolphin species, the abundant and approachable bottlenose dolphin (Tursiops aduncus) and the less common and boat shy humpback dolphin (Sousa sahulensis). This drone was fast, maneuverable, and quiet compared to other drones commonly used in studies of cetaceans and relative to their hearing thresholds. We were successful in collecting blow samples from four individual dolphins (three bottlenose dolphins and one humpback dolphin) in two groups. The success of obtaining samples was dependent on the individual dolphin's activity. We were successful in sampling when dolphins were resting and socializing but found that socializing dolphins were not predictable in their surfacing and direction and therefore do not recommend drone sampling socializing dolphins. The suitability and preference of the sampling technique over biopsy sampling is highly dependent on the dolphin activity. We also attempted to extract DNA from the blow samples with the aim of assessing the feasibility of using blow sampling by drone for population genetic studies. We were unsuccessful in extracting DNA and recommend that others attempting to sample dolphin blow with a drone should prioritize collecting a larger volume of blow that may yield adequate concentrations of DNA to be amplified. Blow sample volume could potentially be increased by sampling with more absorbent materials.

Assortative interactions revealed in a fission–fusion society of Australian humpback dolphins

Abstract Understanding individual interactions within a community or population provides valuable insight into its social system, ecology, and, ultimately, resilience against external stimuli. Here, we used photo-identification data, generalized affiliation indices, and social network analyses to investigate dyadic relationships, assortative interactions, and social clustering in the Australian humpback dolphin (Sousa sahulensis). Boat-based surveys were conducted between May 2013 and October 2015 around the North West Cape, Western Australia. Our results indicated a fission–fusion society, characterized by nonrandom dyadic relationships. Assortative interactions were identified both within and between sexes and were higher among members of the same sex, indicating same-sex preferred affiliations and sexual segregation. Assortative interactions by geographic locations were also identified, but with no evidence of distinct social communities or clusters or affiliations based on residency patterns. We noted high residency among females. Models of temporal patterns of association demonstrated variable levels of stability, including stable (preferred companionships) and fluid (casual acquaintances) associations. We also demonstrated some social avoidance. Our results point to greater social complexity than previously recognized for humpback dolphins and, along with knowledge of population size and habitat use, provide the necessary baseline upon which to assess the influence of increasing human activities on this endemic, Vulnerable species.

Long-term trends in habitat use and site fidelity by Australian humpback dolphins Sousa sahulensis in a near-urban embayment

Long-term trends in habitat use and site fidelity by Australian humpback dolphins Sousa sahulensis in a near-urban embayment

Records of Australian humpback dolphins (Sousa sahulensis) from an offshore island group in Western Australia

We report records of humpback dolphins at the Montebello Islands (20°26’S, 115°33′E) approximately 80 km from the mainland coast of Western Australia. This island group is a marine protected area (MPA), with approximately half of the area a sanctuary (no take) zone managed solely for nature conservation that excludes activities such as fishing and resource extraction, activities known to impact dolphins elsewhere. The habitat varies but includes fringing coral reef and shallow, sheltered sandy lagoons mostly < 10 m water depth and not exceeding 20 m. Several opportunistic sightings and one targeted five-day survey have resulted in 28 dolphins being catalogued in the MPA through photo-identification of natural marks on the dorsal fin and are considered to be recognisable through either distinctive or subtle marks. The species was initially reported in the MPA in June 2015 and was present in summer, autumn and winter of 2017. Six calves have been observed in the MPA, with one a neonate. Given the recent listing of the species as Vulnerable on the IUCN Redlist, and increasing pressure on near-shore coastal habitat, recognising the presence of this species at offshore islands such as these, and understanding their status and connectivity is a priority for the conservation and management of the species in Western Australia.