Ontogenetic allometry and body composition of the common bottlenose dolphin (Tursiops truncatus) from the U.S. mid‐Atlantic

Growth in common bottlenose dolphins (Tursiops truncatus) was investigated through examination of sex‐specific, ontogenetic changes in the mass of 38 discrete body compartments, utilizing stranded dolphins in good body condition (n = 145). Ontogenetic allometry and the body composition technique were used to quantitatively describe growth patterns. Although adult males were significantly larger than adult females in total body mass (TBM) and total length, overall patterns of growth were remarkably similar between sexes. The integument, locomotor muscle, and vertebral column together represented 50%–58% of TBM across all life history categories, although their relative contributions varied ontogenetically. Young dolphins invested the greatest percentage of TBM in integument, while locomotor muscle was the single largest body component in adults. In both sexes (1) most muscle groups displayed positive allometry, (2) most skeletal elements displayed negative allometric or isometric growth, (3) most abdominal viscera associated with digestion displayed positive allometry, and (4) the brain displayed negative allometric growth. Reproductive tissues exhibited the highest rates of growth in both sexes, and increased as a percentage of TBM with maturity. This study provides an integrated view of bottlenose dolphin growth and a quantitative baseline of body composition for future monitoring of this sentinel species of ecosystem health.     

Recovery rates of bottlenose dolphin (Tursiops truncatus) carcasses estimated from stranding and survival rate data

Recovery of cetacean carcasses provides data on levels of human‐caused mortality, but represents only a minimum count of impacts. Counts of stranded carcasses are negatively biased by factors that include at‐sea scavenging, sinking, drift away from land, stranding in locations where detection is unlikely, and natural removal from beaches due to wave and tidal action prior to detection. We estimate the fraction of carcasses recovered for a population of coastal bottlenose dolphins (Tursiops truncatus), using abundance and survival rate data to estimate annual deaths in the population. Observed stranding numbers are compared to expected deaths to estimate the fraction of carcasses recovered. For the California coastal population of bottlenose dolphins, we estimate the fraction of carcasses recovered to be 0.25 (95% CI = 0.20–0.33). During a 12 yr period, 327 animals (95% CI = 253–413) were expected to have died and been available for recovery, but only 83 carcasses attributed to this population were documented. Given the coastal habits of California coastal bottlenose dolphins, it is likely that carcass recovery rates of this population greatly exceed recovery rates of more pelagic dolphin species in the region.     

Retrospective analysis of bottlenose dolphin foraging: A legacy of anthropogenic ecosystem disturbance

We used stable isotope analysis to investigate the foraging ecology of coastal bottlenose dolphins (Tursiops truncatus) in relation to a series of anthropogenic disturbances. We first demonstrated that stable isotopes are a faithful indicator of habitat use by comparing muscle isotope values to behavioral foraging data from the same individuals. δ¹³C values increased, while δ³⁴S and δ¹⁵N values decreased with the percentage of feeding observations in seagrass habitat. We then utilized stable isotope values of muscle to assess temporal variation in foraging habitat from 1991 to 2010 and collagen from tooth crown tips to assess the time period 1944 to 2007. From 1991 to 2010, δ¹³C values of muscle decreased while δ³⁴S values increased indicating reduced utilization of seagrass habitat. From 1944 to 1989 δ¹³C values of the crown tip declined significantly, likely due to a reduction in the coverage of seagrass habitat and δ¹⁵N values significantly increased, a trend we attribute to nutrient loading from a rapidly increasing human population. Our results demonstrate the utility of using marine mammal foraging habits to retrospectively assess the extent to which anthropogenic disturbance impacts coastal food webs.     

Standardization and application of metrics to quantify human‐interaction behaviors by the bottlenose dolphin (Tursiops spp.)

The conditioning of dolphins to human‐interaction behaviors has been documented in several areas worldwide. However, the metrics used to report human‐interaction behaviors vary among studies, making comparison across study areas difficult. The purpose of this study was to develop standard metrics for reporting human‐interaction behaviors and utilize these metrics to quantify the prevalence of human‐interaction behaviors by common bottlenose dolphins (Tursiops truncatus) near Savannah, Georgia. The four metrics used were percentage of days with human‐interaction behaviors, percentage of sightings with human‐interaction behaviors, percentage of the catalog that interacted with humans, and spatial extent of human‐interaction behaviors. Human‐interaction behaviors were observed on 69.6% of days and 23.5% of sightings near Savannah. In addition, 20.1% of the animals in the catalog were observed interacting with humans. These rates are much higher than those found in other areas with known issues with human‐interaction behaviors. These behaviors were observed across an area of 272.6 km², which is larger than other reported areas. The four metrics used in this study proved to be a valuable way to report human‐interaction behaviors, and their use is recommended for future studies to allow for comparison among areas.     

High calf mortality in bottlenose dolphins in the Bay of Islands,New Zealand–a local unit in decline

Bottlenose dolphins (Tursiops truncatus) in the Bay of Islands, New Zealand, have been studied for almost two decades. Since 2003, fewer than 150 dolphins visited the bay during each season and the local unit has declined 7.5% annually from 1997 to 2006. The causes of decline are unclear but probably include mortality and emigration. Here, we used a long‐term database to estimate reproductive parameters of female bottlenose dolphins including recruitment rates. A total of 704 surveys were conducted in which 5,577 sightings of 408 individually identified dolphins were collected; of these 53 individuals were identified as reproductive females. The calving rate increased between periods (1997–1999 = 0.13, CL = 0.07–0.21; 2003–2005 = 0.25, CL = 0.16–0.35 calves/reproductive female/year). A 0.25 calving rate suggests that on average, a female gives birth only once every four years, which is consistent with the estimated calving interval (4.3 yr, SD = 1.45) but still is lower than values reported for other populations. Conversely, apparent mortality rates to age 1+ (range: 0.34–0.52) and 2+ (range: 0.15–0.59) were higher than values reported elsewhere. The high apparent calf mortality in conjunction with a decline in local abundance, highlight the vulnerability of bottlenose dolphins in the Bay of Islands. Long‐term studies are required to understand the causes of high calf mortality and the decline in local abundance. Meanwhile, management should focus on minimizing sources of anthropogenic disturbance and enforcing compliance with current legislation.     

Changes in the acoustic behavior of resident bottlenose dolphins near operating vessels

Maritime traffic is an issue of major ecological concern, and vessel noise may be an important source of disturbance for coastal cetaceans. In the Sado estuary, Portugal, core habitat areas of a small resident population of bottlenose dolphins (Tursiops truncatus) overlap with routes of intense maritime traffic, which presents an opportunity to assess vocal responses of these dolphins to specific vessel noise sources. Field recordings of dolphin vocalizations were made from April to November 2011, using a calibrated system. Dolphin behavior and group size were recorded, as well as the operating boat condition (no boats or specific boat type) in a 1,000 m radius. Spectral analyses of vocalizations allowed the categorization and quantitative analysis of echolocation click trains and social calls, including whistles. Mean overall call rates decreased significantly in the presence of operating vessels. Creaks (fast click trains) were significantly reduced in the presence of ferry boats. Significant differences were also observed in the whistles' minimum, maximum, and start frequencies. These changes in call emission rates and temporary shifts in whistles characteristics may be a vocal response to the proximity of operating vessels, facilitating communication in this busy, noisy estuary.     

Comparison of abundance and habitat usage for common bottlenose dolphins between sites exposed to differential anthropogenic stressors within the estuaries of southern Georgia,U.S.A

The health of common bottlenose dolphins (Tursiops truncatus) within southern Georgia estuaries is of particular concern due to high levels of anthropogenic contaminants in their tissues. Dolphins in this region have the highest polychlorinated biphenyl (PCB) concentrations recorded for any marine mammal and these concentrations correlate to distance from a Superfund point‐source in the Turtle/Brunswick River Estuary (TBRE). Currently, little is known about the population structure of dolphins in this region. This study identifies and compares baseline data on abundance, habitat use, site‐fidelity, and ranging patterns of dolphins across two adjacent field sites; Brunswick, including the TBRE, and Sapelo, including the Sapelo Island National Estuarine Research Reserve. Sapelo is relatively undeveloped and was selected for comparison to the more contaminated TBRE. Dolphin densities increased with tributary size in both sites but dolphin density and total abundance were significantly higher in Sapelo than in Brunswick. Anthropogenic stressors within the TBRE may be an important factor contributing to the differences in abundance, density, and habitat use observed in this study.     

Bottlenose dolphin (Tursiops truncatus) social structure in the Shannon Estuary,Ireland, is distinguished by age‐ and area‐related associations

Social and network analyses that incorporate information on individuals within a population enhance our knowledge of complex species. In this study, the social structure of wild bottlenose dolphins in the Shannon Estuary, Ireland, was analyzed by examining the dynamics of the whole population and then of individuals classed by sex, age, and area. One hundred and twenty‐one dolphins were identified during 522 sightings between 2012 and 2015. The mean half‐weight association index (HWI) of the population was 0.07 ± 0.02. The highest HWIs for individuals of known sex were for female‐male pairs. Mean within‐class HWI was significantly higher than mean between‐class HWI for both age and area classes. Ordinations and sociograms were used to visualize social networks. Permutation tests revealed nonrandom associations for the population overall and both between and within classes. Temporal analyses showed associations persisting for >1,000 d. The whole population's best fit model was for two levels of casual acquaintances. Movement analyses demonstrated the use of the inner estuary by only 25% of the population revealing a potential community division by area. The difference between mean HWI when socializing (0.09 ± 0.03) compared to foraging (0.06 ± 0.03) was significant. These results highlight the importance of localized research, reflecting the complexity found in bottlenose dolphin societies globally.     

Pulsed signal properties of free‐ranging bottlenose dolphins (Tursiops truncatus) in the central Mediterranean Sea

This study describes pulsed signals from bottlenose dolphins of the central Mediterranean Sea. Data were collected during 2011 and 2012 in 27 surveys in the Sicilian Channel, during which 163 animals were sighted. Based mainly on the pulse repetition rate, the signals were classified as Low‐frequency click (LF; single clicks without a regular pulse rate), Train click (TC; with a interclick interval of 80 ± 2 ms), Burst (with a interclick interval of 3.4 ± 0.2 ms), or Packed click (with a lower number of clicks per train and median interclick interval of 3.2 ± 0.0 ms). The measured parameters were peak sound pressure level (SPL_pk); signal duration; the 1st, 2nd, and 3rd peak of frequency; number of peaks frequency; bandwidth; centroid frequency; and the 10th, 25th, 75th, and 90th percentiles of the power spectrum distribution. Most of the parameters were significantly different among the groups, reflecting the different functions of these signals. LF clicks showed a lower peak frequency and percentiles and a longer duration and could be used to explore a wide area without a specific target focalization and with less resolution. The TC showed a higher SPL_pk, higher peak frequency, lower duration, and lower number of secondary peaks frequency, showing a better resolution to investigate a specific target.     

Progress report: Results of a long-range captive breeding program for the bottlenose dolphin,Tursiops truncatus and Tursiops truncatus gilli

There have been 36 bottlenose dolphins born in breeding colonies at Sea World, California, and Sea World, Florida, from 1978–1985. Significant features of this successful reproductive program are construction of a compatible breeding colony, early hormonal detection of pregnancy, pre- and postnatal association of inexperienced mothers with experienced females and their calves, and minimized handling of the mothers and calves until the calves are over 1 year of age. Inclusion in the breeding colonies of males old enough to be effective breeders is stressed. Females in the colonies have successfully bred from 8–9 years of age to 23–24 years of age. Calving intervals in the colonies vary from 2 to over 3 years. Calves are born all year round, with some peaking in calving activity in the spring and fall months. Serum progesterone levels greater than 6,000 pg/ml, maintained over a 4–6-week period, are considered indicative of pregnancy. Progesterone levels vary from less than 10,000 to over 50,000 pg/ml during gestation, averaging 25,000 pg/ml. The need for further study of variation in the pattern of progesterone levels during pregnancy is emphasized. A gestation period for Tursiops of 11.5 to 12 months is consistent with our cumulative progesterone data.     

REPRODUCTIVE SEASONALITY OF WESTERN ATLANTIC BOTTLENOSE DOLPHINS OFF NORTH CAROLINA, U.S.A.

We describe reproductive seasonality of bottlenose dolphins in North Carolina (NC), U.S.A., using strandings data from the entire coast of NC and sighting data from Beaufort, NC and by estimating dates of birth of known females. We found a strong peak of neonate strandings in the spring (April-May), and low levels of neonate strandings in the fall and winter. The distribution of neonate strandings was significantly different from a uniform distribution ( P < 0.001, K = 3.8). We found a unimodal distribution of 282 sightings of neonates with a diffuse peak in the summer. The temporal distribution of sightings of neonates departed significantly from a uniform distribution (P < 0.001, K = 5.1). Estimated birth dates of neonates from known females occurred in May ( n = 6) and June ( n = 4), with a single fall birth. These methods shed light on bottlenose reproductive patterns and underscore the value of using information from multiple types of data. Clarification of bottlenose dolphin reproductive patterns, such as the seasonality of birth, may enhance our understanding of the population structure of this species in the mid-Atlantic region.     

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.     

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.     

Isotopic niche width differentiation between common bottlenose dolphin ecotypes and sperm whales in the Gulf of California

World populations or stock distinction of Tursiops truncatus has been difficult to assess, because of large variations in morphology, habitat, feeding habits, and social structure among areas, which may reflect phylogenetic segregation or ecological plasticity. In the Gulf of California, Mexico, two common bottlenose dolphin ecotypes (inshore and offshore) have been reported. The offshore ecotype is frequently observed in association with sperm whales (Physeter macrocephalus) but the reason for this is still unknown. To explore the degree of resource partitioning/overlap between these species and stocks, we used skin stable isotope values (δ¹³C, δ¹⁵N) to estimate quantitative metrics of isotopic niche width (Bayesian standard ellipse areas, SEA_B) and estimated their diet composition using Bayesian isotopic mixing models. The inshore ecotype in different regions (north, central, and south) of the Gulf of California exhibited distinct δ¹⁵N values and SEA_B, suggesting a latitudinal gradient in nitrogen sources of coastal localities. The SEA_B of inshore and offshore bottlenose dolphin ecotypes was completely distinct, indicating resource partitioning. Associated offshore ecotype and sperm whales had overlapping SEA_B. The isotopic mixing model indicates that a considerable proportion of both species’ diet is large Humbolt squid. Our results suggest that resource partitioning and species association are two strategies that bottlenose dolphin ecotypes use in this zone.     

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.     

Selection of critical habitats for bottlenose dolphins (Tursiops truncatus) based on behavioral data,in relation to marine traffic in the Istanbul Strait,Turkey

Marine traffic is a significant source of disturbance to the bottlenose dolphin population in the Istanbul Strait, Turkey. To determine the importance of this threat, behavioral data together with sighting data of both dolphins and marine vessels were assessed for 2012. The current study suggests that the Istanbul Strait is used mostly as a foraging ground for bottlenose dolphins. Nonetheless, in the same area there is intense marine traffic as well as increase of industrial fishing activities in autumn. The findings of this study indicated that high‐speed ferries and high‐speed boats were the most significant source of disturbance. Moreover, increased densities of fishing vessels resulted in a drastic decline of dolphin sightings. This study highlights that vessel type, speed, distance, and density have a cumulative negative effect on dolphins. In order to mitigate the impacts of vessels, it is necessary to establish managed areas in the Istanbul Strait. Such proposed areas should limit speed and density of marine traffic and have specific restrictions on vessel routes. We propose three different seasonal managed areas according to their values as critical habitat for bottlenose dolphins in the strait.     

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.     

Subsurface behavior of bottlenose dolphins (Tursiops truncatus) interacting with fish trawl nets in northwestern Australia: Implications for bycatch mitigation

Most studies of delphinid‐trawler interactions have documented the surface behavior of dolphins feeding on discarded bycatch, but not their subsurface behavior around demersal trawl gear. Using video cameras mounted inside trawl nets, we recorded the subsurface behavior of common bottlenose dolphins (Tursiops truncatus) in a demersal fish trawl fishery in northwestern Australia. Footage from 36 trawls across the fishery was analyzed to determine the extent of dolphin‐gear interactions and the behavior of dolphins inside the nets. Interaction rates were high, with dolphins present inside and outside the nets during 29 and 34 trawls, respectively, and for up to 99% of the trawl duration. The proportion of foraging behaviors exhibited inside the nets was higher than the proportions of traveling and socializing behaviors. Twenty‐nine individuals were identified inside the net, seven of which returned repeatedly within and between trawls and fishing trips, but were observed primarily in the same localized areas in which they were first recorded. Our results suggest that entering trawl nets may be a frequently occurring, yet specialized behavior exhibited by a small subset of trawler‐associated dolphins. We propose that gear modifications, not spatial or temporal adjustments to fishing effort, have the greatest potential to reduce dolphin bycatch.     

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.     

Hip bone morphometrics of bottlenose dolphins (Tursiops truncatus) from Adriatic Sea: Sex determination and postnatal development

Bones are frequently the only cetacean samples available in the wild and may be useful for determination of sex, physical maturity, body length, and body mass. We analyzed the postnatal morphological development and morphometric characteristics of the cleaned and dried, paired hip bone (os coxae) from 131 bottlenose dolphins, Tursiops truncatus, (61 females, 70 males; total body length 117–322 cm) ranging in age from neonates to 28 yr, salvaged from Croatian waters of the eastern Adriatic Sea. Adult dolphins exhibited sexual dimorphism in their hip bone morphology and morphometrics: fully‐formed hip bones from males were significantly longer, heavier, and more robust than those from females. Moreover, the fully formed hip bone from males featured a prominent bony tuberosity protruding dorsolaterally from the caudal half that was significantly wider and higher than in females. Morphometric measurements were used to generate classification functions for reliable sex determination (97%). In addition, hip bone length in both sexes correlated tightly with total body length, allowing one measurement to be calculated from the other. Only a single ossification developed in all hip bones although soft tissue attachments and comparative review suggests homological equivalence with the tripartite terrestrial mammal hip bone.