Population structure of island-associated dolphins: Evidence from photo-identification of common bottlenose dolphins (Tursiops truncatus) in the main Hawaiian Islands

Management agencies often use geopolitical boundaries as proxies for biological boundaries. In Hawaiian waters a single stock is recognized of common bottlenose dolphins, Tursiops truncatus , a species that is found both in open water and near-shore among the main Hawaiian Islands. To assess population structure, we photo-identified 336 distinctive individuals from the main Hawaiian Islands, from 2000 to 2006. Their generally shallow-water distribution, and numerous within-year and between-year resightings within island areas suggest that individuals are resident to the islands, rather than part of an offshore population moving through the area. Comparisons of identifications obtained from Kaua'i/Ni'ihau, O'ahu, the "4-island area," and the island of Hawai'i showed no evidence of movements among these island groups, although movements from Kaua'i to Ni'ihau and among the "4-islands" were documented. A Bayesian analysis examining the probability of missing movements among island groups, given our sample sizes for different areas, indicates that interisland movement rates are less than 1% per year with 95% probability. Our results suggest the existence of multiple demographically independent populations of island-associated common bottlenose dolphins around the main Hawaiian islands.     

Integrating multiple data sources to assess the distribution and abundance of bottlenose dolphins Tursiops truncatus in Scottish waters

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Genetic differentiation among coastal and offshore common bottlenose dolphins,Tursiops truncatus,in the eastern North Pacific Ocean

Common bottlenose dolphins (Tursiops truncatus) are found worldwide in temperate and tropical regions, often occurring as distinct coastal and offshore ecotypes. Along the west coast of the United States, two stocks are recognized for management based on morphological and photo‐identification studies: a California coastal stock, estimated at 450–500 individuals, and a California/Oregon/Washington offshore stock of about 1,000 animals. This study is the first to analyze genetic differentiation between these stocks. We examined both the hypervariable portion of the mitochondrial DNA (mtDNA) control region and fifteen microsatellite markers for coastal (n = 64) and offshore (n = 69) dolphins. Significant genetic differentiation was found between the two stocks for mtDNA (Φ_ST = 0.30, P < 0.001; F_ST = 0.14, P < 0.001) and microsatellite loci (F_ST = 0.19, P < 0.001). Coastal dolphins had less genetic diversity than offshore dolphins. Further substructuring within the offshore stock was not detected. The level of genetic differentiation between the coastal and offshore dolphins is consistent with long‐term separation and reinforces recognizing them as separate stocks. These findings are particularly important for management of the smaller, less genetically diverse, coastal stock that is vulnerable to a variety of anthropogenic threats.     

Fine-scale genetic population structure in a mobile marine mammal: inshore bottlenose dolphins in Moreton Bay, Australia

Highly mobile marine species in areas with no obvious geographic barriers are expected to show low levels of genetic differentiation. However, small‐scale variation in habitat may lead to resource polymorphisms and drive local differentiation by adaptive divergence. Using nuclear microsatellite genotyping at 20 loci, and mitochondrial control region sequencing, we investigated fine‐scale population structuring of inshore bottlenose dolphins (Tursiops aduncus) inhabiting a range of habitats in and around Moreton Bay, Australia. Bayesian structure analysis identified two genetic clusters within Moreton Bay, with evidence of admixture between them (F_ST = 0.05, P = 0.001). There was only weak isolation by distance but one cluster of dolphins was more likely to be found in shallow southern areas and the other in the deeper waters of the central northern bay. In further analysis removing admixed individuals, southern dolphins appeared genetically restricted with lower levels of variation (AR = 3.252, π = 0.003) and high mean relatedness (r = 0.239) between individuals. In contrast, northern dolphins were more diverse (AR = 4.850, π = 0.009) and were mixing with a group of dolphins outside the bay (microsatellite‐based STRUCTURE analysis), which appears to have historically been distinct from the bay dolphins (mtDNA Φ_ST = 0.272, P < 0.001). This study demonstrates the ability of genetic techniques to expose fine‐scale patterns of population structure and explore their origins and mechanisms. A complex variety of inter‐related factors including local habitat variation, differential resource use, social behaviour and learning, and anthropogenic disturbances are likely to have played a role in driving fine‐scale population structure among bottlenose dolphins in Moreton Bay.     

Site fidelity,residency, and abundance of bottlenose dolphins (Tursiops sp.) in Adelaide's coastal waters,South Australia

Little is known about the ecology and behavior of southern Australian bottlenose dolphins (Tursiops sp.). This hinders assessment of their conservation status and informed decision‐making concerning their management. We used boat‐based surveys and photo‐identification data to investigate site fidelity, residency patterns, and the abundance of southern Australian bottlenose dolphins in Adelaide's coastal waters. Sighting rates and site fidelity varied amongst individuals, and agglomerative hierarchical cluster analysis led to the categorization of individuals into one of three groups: occasional visitors, seasonal residents, or year‐round residents. Lagged identification rates indicated that these dolphins used the study area regularly from year to year following a model of emigration and reimmigration. Abundance estimates obtained from multisample closed capture‐recapture models ranged from 95 individuals (SE ± 45.20) in winter 2013 to 239 (SE ± 54.91) in summer 2014. The varying levels of site fidelity and residency, and the relatively high number of dolphins found throughout the study area highlights the Adelaide metropolitan coast as an important habitat for bottlenose dolphins. As these dolphins also appear to spend considerable time outside the study area, future research, conservation, and management efforts on this population must take into account anthropogenic activities within Adelaide's coastal waters and their adjacencies.     

Remarkably low genetic diversity and strong population structure in common bottlenose dolphins (Tursiops truncatus) from coastal waters of the Southwestern Atlantic Ocean

Knowledge about the ecology of bottlenose dolphins in the Southwestern Atlantic Ocean is scarce. Increased by-catch rates over the last decade in coastal waters of southern Brazil have raised concerns about the decline in abundance of local dolphin communities. Lack of relevant data, including information on population structure and connectivity, have hampered an assessment of the conservation status of bottlenose dolphin communities in this region. Here we combined analyses of 16 microsatellite loci and mitochondrial DNA (mtDNA) control region sequences to investigate genetic diversity, structure and connectivity in 124 biopsy samples collected over six communities of photographically identified coastal bottlenose dolphins in southern Brazil, Uruguay and central Argentina. Levels of nuclear genetic diversity were remarkably low (mean values of allelic diversity and heterozygosity across all loci were 3.6 and 0.21, respectively), a result that possibly reflects the small size of local dolphin communities. On a broad geographical scale, strong and significant genetic differentiation was found between bottlenose dolphins from southern Brazil–Uruguay (SB–U) and Bahía San Antonio (BSA), Argentina (AMOVA mtDNA Φ_ST = 0.43; nuclear F_ST = 0.46), with negligible contemporary gene flow detected based on Bayesian estimates. On a finer scale, moderate but significant differentiation (AMOVA mtDNA Φ_ST = 0.29; nuclear F_ST = 0.13) and asymmetric gene flow was detected between five neighbouring communities in SB–U. Based on the results we propose that BSA and SB–U represent two distinct evolutionarily significant units, and that communities from SB–U comprise five distinct Management Units (MUs). Under this scenario, conservation efforts should prioritize the areas in southern Brazil where dolphins from three MUs overlap in their home ranges and where by-catch rates are reportedly higher.     

Occurrence of false killer whales (Pseudorca crassidens) and their association with common bottlenose dolphins (Tursiops truncatus) off northeastern New Zealand

On a global scale, false killer whales (Pseudorca crassidens) remain one of the lesser‐known delphinids. The occurrence, site fidelity, association patterns, and presence/absence of foraging in waters off northeastern New Zealand are examined from records collected between 1995 and 2012. The species was rarely encountered; however, of the 61 distinctive, photo‐identified individuals, 88.5% were resighted, with resightings up to 7 yr after initial identification, and movements as far as 650 km documented. Group sizes ranged from 20 to ca. 150. Results indicate that all individuals are linked in a single social network. Most observations were recorded in shallow (<100 m) nearshore waters. Occurrence in these continental shelf waters is likely seasonal, coinciding with the shoreward flooding of a warm current. During 91.5% of encounters, close interspecific associations with common bottlenose dolphins (Tursiops truncatus) were observed. Photo‐identification reveals repeat inter‐ and intraspecific associations among individuals with 34.2% of common bottlenose dolphins resighted together with false killer whales over 1,832 d. While foraging was observed during 39.5% of mixed‐species encounters, results suggest that social and antipredatory factors may also play a role in the formation of these mixed‐species groups.     

Staying close to home? Genetic differentiation of rough-toothed dolphins near oceanic islands in the central Pacific Ocean

Rough-toothed dolphins have a worldwide tropical and subtropical distribution, yet little is known about the population structure and social organization of this typically open-ocean species. Although it has been assumed that pelagic dolphins range widely due to the lack of apparent barriers and unpredictable prey distribution, recent evidence suggests rough-toothed dolphins exhibit fidelity to some oceanic islands. Using the most comprehensively extensive dataset for this species to date, we assess the isolation and interchange of rough-toothed dolphins at the regional and oceanic scale within the central Pacific Ocean. Using mtDNA and microsatellite genotyping (nDNA), we analyzed samples of insular communities from the main Hawaiian (Kaua‘i n = 93, O‘ahu n = 9, Hawai‘i n = 57), French Polynesian (n = 70) and Samoan (n = 16) archipelagos, and pelagic samples off the Northwestern Hawaiian Islands (n = 18). An overall AMOVA indicated strong genetic differentiation among islands (mtDNA F_ST = 0.265; p < 0.001; nDNA F_ST = 0.038; p < 0.001), as well as among archipelagos (mtDNA F_ST = 0.299; p < 0.001; nDNA F_ST = 0.055; p < 0.001). Shared haplotypes (n = 4) between the archipelagos may be a product of a relatively recent divergence and/or periodic exchange from poorly understood pelagic populations. Analyses using STRUCTURE and GENELAND identified four separate management units among archipelagos and within the Hawaiian Islands. These results confirm the presence of multiple insular populations within the Pacific and island-specific genetic isolation among populations attached to islands in each archipelago. Insular populations seem most prevalent where oceanographic conditions indicate high local productivity or a discontinuity with surrounding oligotrophic areas. Our findings have important implications for a little studied species that faces increasing anthropogenic threats around oceanic islands.     

Restricted dispersal in a continuously distributed marine species: common bottlenose dolphins Tursiops truncatus in coastal waters of the western North Atlantic

The marine environment provides an opportunity to examine population structure in species with high dispersal capabilities and often no obvious barriers to genetic exchange. In coastal waters of the western North Atlantic, common bottlenose dolphins, Tursiops truncatus, are a highly mobile species with a continuous distribution from New York to Florida. We examine if the highly mobile nature coupled with no obvious geographic barriers to movement in this region result in a large panmictic population. Mitochondrial control region sequences and 18 microsatellite loci indicate dolphins are partitioning the habitat both latitudinally and longitudinally. A minimum of five genetically differentiated populations were identified among 404 samples collected in the range of New Jersey to northern Florida using both genetic marker types, some inhabiting nearshore coastal waters and others utilizing inshore estuarine waters. The genetic results reject the hypothesis of a single stock of coastal bottlenose dolphins put forth after the 1987–1988 epizootic that caused a large‐scale die‐off of dolphins and suggest instead the disease vector was transferred from one population to the next as a result of seasonal migratory movements of some populations. These coastal Atlantic populations also differ significantly from bottlenose dolphin samples collected in coastal waters of the northern Gulf of Mexico, implying a long‐term barrier to movement between the two basins.     

Temporal–spatial distribution of an island‐based offshore population of common bottlenose dolphins (Tursiops truncatus) in the equatorial Atlantic

A little‐studied common bottlenose dolphin (Tursiops truncatus) population inhabits the offshore waters surrounding Saint Paul's Rocks, a Brazilian marine protected area in the equatorial Atlantic Ocean. Five field expeditions (May 2011–May 2013) were conducted to characterize the habitat use, population size, and site fidelity of this population. Three different survey methods were employed: line‐transect surveys, land‐based surveys, and photo‐identification surveys. A population size of 23 individuals (19–28, CI 95%), which were present on most sampling days (>90% of surveys), was estimated. The maximum resighting interval of photo‐identified animals was 9 yr and 3 mo for five distinct individuals, based on data from nonsystematic efforts that have been ongoing since 2004. The dolphins exhibited strong site fidelity, as the minimum convex polygon (MCP, 95%) method revealed that they restricted their movements to a 0.5 km² area across seasons and a 0.99 km² area across years (95% kernel). The dolphins preferred shallow waters close to the archipelago (<1.2 km from the islands), especially on the eastern and southeastern sides, where oceanographic models have revealed persistent upwelling that may result from underwater currents and where food may be more predictably available.     

Rolling stones and stable homes: social structure,habitat diversity and population genetics of the Hawaiian spinner dolphin (Stenella longirostris)

Spinner dolphins (Stenella longirostris) exhibit different social behaviours at two regions in the Hawaiian Archipelago: off the high volcanic islands in the SE archipelago they form dynamic groups with ever‐changing membership, but in the low carbonate atolls in the NW archipelago they form long‐term stable groups. To determine whether these environmental and social differences influence population genetic structure, we surveyed spinner dolphins throughout the Hawaiian Archipelago with mtDNA control region sequences and 10 microsatellite loci (n = 505). F‐statistics, Bayesian cluster analyses, and assignment tests revealed population genetic separations between most islands, with less genetic structuring among the NW atolls than among the SE high islands. The populations with the most stable social structure (Midway and Kure Atolls) have the highest gene flow between populations (mtDNA Φ_ST < 0.001, P = 0.357; microsatellite F_ST = ?0.001; P = 0.597), and a population with dynamic groups and fluid social structure (the Kona Coast of the island of Hawai’i) has the lowest gene flow (mtDNA 0.042 < Φ_ST < 0.236, P < 0.05; microsatellite 0.016 < F_ST < 0.040, P < 0.001). We suggest that gene flow, dispersal, and social structure are influenced by the availability of habitat and resources at each island. Genetic comparisons to a South Pacific location (n = 16) indicate that Hawaiian populations are genetically depauperate and isolated from other Pacific locations (mtDNA 0.216 < F_ST < 0.643, P < 0.001; microsatellite 0.058 < F_ST < 0.090, P < 0.001); this isolation may also influence social and genetic structure within Hawai’i. Our results illustrate that genetic and social structure are flexible traits that can vary between even closely‐related populations.     

Decline in local abundance of bottlenose dolphins (Tursiops truncatus) in the Bay of Islands,New Zealand

Regional populations of bottlenose dolphins (Tursiops truncatus) around New Zealand are genetically isolated from each other and the species was recently classified as nationally endangered based on relatively small population sizes and reports of high calf mortality. Here, we estimate the abundance and trends in one of these regional populations, the Bay of Islands, using a photo‐identification database collected from 1997 to 1999 and from 2003 to 2006, containing a total of 3,841 records of 317 individual dolphins. Estimates of abundance obtained with the robust design fluctuated widely but showed a significant decline in the number of dolphins present in the bay over time (7.5% annual rate of decline). Temporary emigration was random and fluctuated considerably (γ  =  0.18, SE = 0.07 to γ  =  0.84, SE = 0.06). Apparent survival was estimated at 0.928 (CI = 0.911–0.942). Seasonal estimates (26 seasons) obtained in POPAN also showed a significant decline in abundance (5.8% annual rate of decline). Despite the decline observed in local abundance, dolphins continue to be found regularly in the Bay of Islands, suggesting that fewer dolphins use the bay on regular basis. Consequently, it seems that a change in habitat use, mortality and possibly low recruitment could underlie the apparent local decline.     

Habitat and resource partitioning among Indo‐Pacific bottlenose dolphins in Moreton Bay,Australia

Investigating resource partitioning among mobile marine predators such as cetaceans is challenging. Here we integrate multiple methodologies (analyses of habitat use, stable isotopes and trace elements) to assess ecological niche partitioning amongst two genetically divergent sympatric subpopulations (North and South) of Indo‐Pacific bottlenose dolphins (Tursiops aduncus) in Moreton Bay, Australia. Comparisons of the mean locations (latitude, longitude) and environmental variables (distance from sandbanks, distance from shore and water depth) observed at sightings of biopsy‐sampled individuals indicated that the North subpopulation occurred in the northwestern bay in significantly deeper water than the South subpopulation, which was found in southeastern nearshore waters and closer to sandbanks. Ratios of stable carbon and nitrogen isotopes in skin samples suggested that North dolphins foraged on higher trophic level prey in relatively more pelagic, offshore habitats, while South dolphins foraged on lower trophic prey in more nearshore, demersal and/or benthic habitats. Habitat partitioning was also reflected in higher blubber concentrations of most of the 13 measured trace elements, in particular lead, in the coastal South compared to the more pelagic North dolphins. These findings indicate that genetic subpopulations of bottlenose dolphins in Moreton Bay are adapted to different niches.     

High gene flow in oceanic bottlenose dolphins (Tursiops truncatus) of the North Atlantic

Despite the openness of the oceanic environment, limited dispersal and tight social structure often induce genetic structuring in marine organisms, even in large animals such as cetaceans. In the bottlenose dolphin, mitochondrial and nuclear DNA analyses have revealed the existence of genetic differentiation between pelagic (or offshore) and coastal (or nearshore) ecotypes in the western North Atlantic, as well as between coastal populations. Because previous studies concentrated on continental margins, we analysed the population structure of bottlenose dolphins in two of the most isolated archipelagos of the North Atlantic: the Azores and Madeira. We analysed 112 samples collected on live animals in the two archipelagos, and nine samples collected on stranded animals in Madeira and mainland Portugal. Genetic analyses consisted in molecular sexing, sequencing of part of the mitochondrial hyper-variable region, and screening of ten microsatellite loci. We predicted that: (1) there is at least one pelagic and one or more coastal populations in each archipelago; (2) populations are differentiated between and possibly within archipelagos. Contrary to these predictions, results indicated a lack of population structure in the study area. In addition, comparison with published sequences revealed that the samples from the Azores and Madeira were not significantly differentiated from samples of the pelagic population of the western North Atlantic. Thus, bottlenose dolphins occurring in the pelagic waters of the North Atlantic belong to a large oceanic population, which should be regarded as a single conservation unit. Unlike what is known for coastal populations, oceanic bottlenose dolphins are able to maintain high levels of gene flow.     

Defining bottlenose dolphin (Tursiops truncatus) stocks based on environmental,physical, and behavioral characteristics

The population structure of bottlenose dolphins, Tursiops truncatus, along the U.S. Atlantic coast has recently been redefined from one homogenous population into five coastal stocks. Local studies indicate even finer structure, primarily based on isolation of dolphins inhabiting estuaries. We identified population structuring of non‐estuarine coastal bottlenose dolphins during a study in New Jersey, the northern range along the Atlantic Coast. Using photo‐identification and distribution survey results, an analysis identified two major clusters of individuals significantly separated by five variables (distance from shoreline, group size, occurrence of the barnacle Xenobalanus globicipitis, avoidance behavior, and individual coloration). Sightings assigned to cluster 1 occurred in nearshore shallow waters (0–1.9 km, x?= 3.5 m), and those assigned to cluster 2 occurred further offshore in deeper waters (1.9–6 km, x?= 9.5 m). Only eight of 194 individuals (4%) were identified in both regions. Collectively, this suggests an occurrence of two stocks that are spatially, physically, and behaviorally distinguishable over a small distance. These results indicate that complexity in Tursiops population structure is not limited to latitudinal gradients or barriers created by estuarine habitats, but also by partitioning of habitat as a function of distance from shore and depth over small distances.     

Genetic divergence between two phenotypically distinct bottlenose dolphin ecotypes suggests separate evolutionary trajectories

Due to their worldwide distribution and occupancy of different types of environments, bottlenose dolphins display considerable morphological variation. Despite limited understanding about the taxonomic identity of such forms and connectivity among them at global scale, coastal (or inshore) and offshore (or oceanic) ecotypes have been widely recognized in several ocean regions. In the Southwest Atlantic Ocean (SWA), however, there are scarce records of bottlenose dolphins differing in external morphology according to habitat preferences that resemble the coastal‐offshore pattern observed elsewhere. The main aim of this study was to analyze the genetic variability, and test for population structure between coastal (n = 127) and offshore (n = 45) bottlenose dolphins sampled in the SWA to assess whether their external morphological distinction is consistent with genetic differentiation. We used a combination of mtDNA control region sequences and microsatellite genotypes to infer population structure and levels of genetic diversity. Our results from both molecular marker types were congruent and revealed strong levels of structuring (microsatellites F_ST = 0.385, p < .001; mtDNA F_ST =  0.183, p < .001; Φ_ST = 0.385, p < .001) and much lower genetic diversity in the coastal than the offshore ecotype, supporting patterns found in previous studies elsewhere. Despite the opportunity for gene flow in potential “contact zones”, we found minimal current and historical connectivity between ecotypes, suggesting they are following discrete evolutionary trajectories. Based on our molecular findings, which seem to be consistent with morphological differentiations recently described for bottlenose dolphins in our study area, we recommend recognizing the offshore bottlenose dolphin ecotype as an additional Evolutionarily Significant Unit (ESU) in the SWA. Implications of these results for the conservation of bottlenose dolphins in SWA are also discussed.     

Habitat‐driven population structure of bottlenose dolphins,Tursiops truncatus,in the North‐East Atlantic

Despite no obvious barrier to gene flow, historical environmental processes and ecological specializations can lead to genetic differentiation in highly mobile animals. Ecotypes emerged in several large mammal species as a result of niche specializations and/or social organization. In the North‐West Atlantic, two distinct bottlenose dolphin (Tursiops truncatus) ecotypes (i.e. ‘coastal’ and ‘pelagic’) have been identified. Here, we investigated the genetic population structure of North‐East Atlantic (NEA) bottlenose dolphins on a large scale through the analysis of 381 biopsy‐sampled or stranded animals using 25 microsatellites and a 682‐bp portion of the mitochondrial control region. We shed light on the likely origin of stranded animals using a carcass drift prediction model. We showed, for the first time, that coastal and pelagic bottlenose dolphins were highly differentiated in the NEA. Finer‐scale population structure was found within the two groups. We suggest that distinct founding events followed by parallel adaptation may have occurred independently from a large Atlantic pelagic population in the two sides of the basin. Divergence could be maintained by philopatry possibly as a result of foraging specializations and social organization. As coastal environments are under increasing anthropogenic pressures, small and isolated populations might be at risk and require appropriate conservation policies to preserve their habitats. While genetics can be a powerful first step to delineate ecotypes in protected and difficult to access taxa, ecotype distinction should be further documented through diet studies and the examination of cranial skull features associated with feeding.     

Site fidelity and association patterns in a deep-water dolphin: Rough-toothed dolphins (Steno bredanensis) in the Hawaiian Archipelago

In the Pacific, rough-toothed dolphins ( Steno bredanensis ) are typically found in the open ocean and in deep waters around oceanic islands. We examined habitat use, site fidelity, movements, and association patterns of this species in the main Hawaiian Islands. Sighting rates were highest in depths >1,500 m. There were frequent within- and between-year resightings off the island of Hawai'i, indicating a small population size with high site fidelity. Resighting rates were lower off Kaua'i/Ni'ihau, indicating a larger population size, but with some site fidelity. Two individuals were documented moving from Kaua'i to Hawai'i, a distance of 480 km, but were not seen to associate with dolphins off Hawai'i. Observed movements were consistent with at most 2% dispersal per year between these two areas. Differences in group sizes, habitat use, and behavior imply that movements among the islands may be limited. Little is known about the diet of rough-toothed dolphins in Hawai'i, but they are thought to feed primarily on near-surface species. High fidelity to deep-water areas off the island of Hawai'i likely reflects an increase in the predictability of prey associated with upwelling due to the island mass effect, wind stress curl and cyclonic eddies that form off the island.     

Genetic and family structure in a group of 165 common bottlenose dolphins caught off the Japanese coast

The biological and genetic structure of common bottlenose dolphins (Tursiops truncatus) that migrate seasonally near Japan remains largely unknown. We investigated the genetic and family structure in a group of 165 common bottlenose dolphins caught off the coast of Japan using mitochondrial DNA (mtDNA) and 20 microsatellite DNA markers. Phylogenetic analysis of the mtDNA control region sequences suggested that the dolphins were related more closely to oceanic types from Chinese waters than other geographic regions. The information on sex, sexual maturation and age together with the genetic markers revealed a strong likelihood for 37 familial relationships related mostly to maternity and an under‐representation of juvenile female offspring. The maternal dolphins had a similar offspring‐birth interval as the coastal types from North Atlantic Ocean, but a slightly younger first‐progeny age. The sex bias in the captured group was particularly marked towards an over‐representation of males among the young and immature dolphins, whereas the mature adults had an equal number of males and females. These results should be useful for future comparative biological, genetic and evolutionary investigations of bottlenose dolphins from the North Pacific Ocean with those from other regions.     

Blood mercury concentrations in common bottlenose dolphins from the Indian River Lagoon,Florida: Patterns of social distribution

Social network analysis has been shown to be effective in studying the social structure of cetacean populations. Common bottlenose dolphins (Tursiops truncatus) inhabiting the Indian River Lagoon (IRL), Florida, have among the highest concentrations of total mercury (THg) in blood reported worldwide. The purpose of this study was to examine the relationship between THg concentrations in IRL dolphins and their social affiliations. Whole blood samples from 98 dolphins with photo‐identification sighting histories were collected between 2003–2007 and 2010–2012. Dolphins were categorized into approximate tertiles of low (mean 199.7 μg/L), medium (mean 366.8 μg/L), and high (mean 990.5 μg/L) THg exposure. Social associations between individual dolphins were defined by the proportion of sightings documented with another known individual. Social network measures of individuals and associations between dyads were examined to determine differences among THg categories. Strong social affiliations of individuals within the highest category of THg were found (P = 0.04), suggesting shared exposures among dolphins foraging in specific areas of the estuary. Network measures of strength and affinity were significantly higher in the highest exposure category. This report used social network analysis as a novel way to examine patterns of exposure to an environmental contaminant in a cetacean population.