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.     

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.     

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.     

Blubber development in bottlenose dolphins (Tursiops truncatus)

Blubber, the lipid-rich hypodermis of cetaceans, functions in thermoregulation, buoyancy control, streamlining, metabolic energy storage, and locomotion. This study investigated the development of this specialized hypodermis in bottlenose dolphins (Tursiops truncatus) across an ontogenetic series, including fetuses, neonates, juveniles, subadults, and adults. Blubber samples were collected at the level of the mid-thorax, from robust specimens (n = 25) that stranded along the coasts of North Carolina and Virginia. Blubber was dissected from the carcass and its mass, and the depth and lipid content at the sample site, were measured. Samples were prepared using standard histological methods, viewed by light microscopy, and digital images of blubber captured. Images were analyzed through the depth of the blubber for morphological and structural features including adipocyte size, shape, and numbers, and extracellular, structural fiber densities. From fetus to adult, blubber mass and depth increased proportionally with body mass and length. Blubber lipid content increased dramatically with increasing fetal length. Adult and juvenile blubber had significantly higher blubber lipid content than fetuses, and this increase was reflected in mean adipocyte size, which increased significantly across all robust life history categories. In juvenile, subadult, and adult dolphins, this increase in cell size was not uniform across the depth of the blubber, with the largest increases observed in the middle and deep blubber regions. Through-depth counts of adipocytes were similar in all life history categories. These results suggest that blubber depth is increased during postnatal growth by increasing cell size rather than cell number. In emaciated adults (n = 2), lipid mobilization, as evidenced by a decrease in adipocyte size, was localized to the middle and deep blubber region. Thus, in terms of both lipid accumulation and depletion, the middle and deep blubber appear to be the most metabolically dynamic. The superficial blubber likely serves a structural role important in streamlining the animal. This study demonstrates that blubber is not a homogeneous tissue through its depth, and that it displays life history-dependent changes in its morphology and lipid content.     

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.     

Field assessment of C‐POD performance in detecting echolocation click trains of bottlenose dolphins (Tursiops truncatus)

We evaluated the performance of dolphin echolocation detectors (C‐PODs) in the New River, North Carolina, by ground‐truthing echolocation detections with digital acoustic recordings. We deployed C‐PODs at three sites for a total of 204 monitoring hours. We also performed detection range trials at two sites where water depths ranged from 1.0 to 4.5 m. We used Detection Positive Minutes (DPMs), minutes of C‐POD recordings that contained at least one echolocation click train, to indicate the presence of at least one dolphin. The C‐PODs performed well in detecting dolphin click trains, although all units performed conservatively by failing to detect some echolocation events and therefore underestimated the true occurrence of dolphins. C‐PODs reported only a small number of false detections, as indicated by low false positive rates ranging between 1% and 4% for individual units. Overall, C‐PODs performed with a high accuracy (72%–91%) and detected echolocation at a distance of at least 933 m. We conclude that C‐PODs hold considerable promise in future monitoring studies of this species, but recommend a careful study design especially in complex, coastal environments.     

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.     

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.     

Evoked potential audiometry of 13 Pacific bottlenose dolphins (Tursiops truncatus gilli)

Evoked potential audiograms were measured in 13 Pacific bottlenose dolphins (Tursiops truncatus gilli) to determine the variability in hearing sensitivity and range of hearing. The auditory evoked potential system used a transducer embedded in a suction cup to deliver sinusoidal amplitude modulated tones to each dolphin through the pan region of the lower right jaw. Evoked potentials were recorded noninvasively using surface electrodes, and hearing thresholds were estimated by tracking the amplitude of the envelope following response, an evoked potential that is phase‐locked to the stimulus modulation rate. Frequencies tested ranged from 10 to 180 kHz in each animal. Variability in the range of hearing and age‐related reductions in hearing sensitivity and range of hearing were consistent with those observed in Atlantic bottlenose dolphins. Comparison of audiograms to a captive population of Atlantic bottlenose dolphins demonstrated that the Pacific bottlenose dolphins tested in this study had significantly lower thresholds at frequencies of 40 and 60–115 kHz. Differences in thresholds between the groups are unlikely to be due to methodological factors.     

Ontogenetic diet changes in bottlenose dolphins (Tursiops truncatus) reflected through stable isotopes

The ability of stable isotope analysis to provide insight into ontogenetic dietary changes was examined using bottlenose dolphin tooth and skin samples. Teeth were subsampled to compare tissue produced early in life (outer tooth) to that produced later in life (inner tooth). Outer tooth had significantly higher δ¹⁵N values than the corresponding inner sample from the same tooth (n= 60, P= 0.0041), indicating that there was a temporal shift to a lower δ¹⁵N diet. There were no significant δ¹³C differences. Higher δ¹⁵N values in young have previously been attributed to the period of suckling. Analysis of skin tissue from stranded animals of different developmental stages similarly indicated that the δ¹⁵N values were significantly higher in young animals. Further comparisons indicated that the primary influence for this difference was animals with lengths less than or equal to the largest neonatal dolphin. This difference likely reflects an ontogenetic dietary shift from a sole reliance on milk to a combination of milk and prey species during the first year of life.     

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.     

Ecology and conservation of common bottlenose dolphins Tursiops truncatus in the Mediterranean Sea

• 1 Bottlenose dolphins Tursiops truncatus are amongst the best‐known cetaceans. In the Mediterranean Sea, however, modern field studies of cetaceans did not start until the late 1980s. Bottlenose dolphins have been studied only in relatively small portions of the basin, and wide areas remain largely unexplored.
• 2 This paper reviews the ecology, behaviour, interactions with fisheries and conservation status of Mediterranean bottlenose dolphins, and identifies threats likely to have affected them in historical and recent times.
• 3 Whilst intentional killing was probably the most important cause of mortality until the 1960s, important ongoing threats include incidental mortality in fishing gear and the reduced availability of key prey caused by overfishing and environmental degradation throughout the region. Additional potential or likely threats include the toxic effects of xenobiotic chemicals, epizootic outbreaks, direct disturbance from boating and shipping, noise, and the consequences of climate change.
• 4 The flexible social organization and opportunistic diet and behaviour of bottlenose dolphins may allow them to withstand at least some of the effects of overfishing and habitat degradation. However, dolphin abundance is thought to have declined considerably in the region and management measures are needed to prevent further decline.
• 5 Management strategies that could benefit bottlenose dolphins, such as sustainable fishing, curbing marine pollution and protecting biodiversity, are already embedded in legislation and treaties. Compliance with those existing commitments and obligations should be given high priority.

     

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.     

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.     

Anthropogenic injuries disrupt social associations of common bottlenose dolphins (Tursiops truncatus) in Sarasota Bay,Florida

Social connectivity is important for measuring the fitness of common bottlenose dolphins (Tursiops truncatus). While interactions in fission-fusion societies vary between individuals, studies show that repeated interactions enhance reproduction and foraging success. Injuries that potentially remove an individual from its association network may disrupt these interactions. Using data from the long-term resident dolphin community in Sarasota Bay, Florida, we investigated how anthropogenic injuries affect the dolphins' social associations by examining the differences before and after injury to individuals. We examined group size, strength, eigenvector centrality, clustering coefficient, and number of triangles and analyzed whether the animal's sex, age class, type of injury, or human intervention affected these values. We found that while group size did not change, injured dolphins had fewer preferred associates (HWI > 0.14) and were found in more fluid groups immediately after injury, but started returning to normal association levels after 2 years. This initial decrease in connectivity was not related to the age, sex, type of injury, or intervention. Despite the fluidity in individual associations, the strongest bonds remained stable, those between mothers and calves and those between male alliance partners. These findings provide some of the first information relating injuries and social networks for animals.     

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.     

Changes in interspecies association patterns of Atlantic bottlenose dolphins,Tursiops truncatus,and Atlantic spotted dolphins,Stenella frontalis,after demographic changes related to environmental disturbance

Animal populations can be affected by environmental disturbances in many ways including demographic and behavioral changes. This can affect interspecies associations for regularly interacting sympatric species, like bottlenose and spotted dolphins in the Bahamas (observed since 1985 and interspecies associations analyzed since 1993). After two hurricanes in 2004 each species lost roughly 30% of their respective communities resulting in differing social structure and behavioral changes. During mixed species encounters (MSE) group sizes for spotted dolphins ( = 14.1 ± 9.2) were significantly larger than bottlenose dolphins ( = 6.0 ± 7.3; F = 11.74, df = 1, P < 0.001), however, t‐tests revealed no differences between aggressive vs. affiliative encounters. Sexual/aggressive behavior regularly seen previously was not observed posthurricanes and aggressive encounters were greatly reduced. Generally results were similar to prehurricane data including high resightings of spotted dolphins with male alliances prevalent (including new juvenile alliances seen only posthurricane), and individualized bottlenose participation with few male alliances. However temporal associations varied compared to prehurricane. Interspecies association and behavior patterns were altered and likely affected by the changes in intraspecies association patterns following the hurricanes. However both species still participated in MSE, suggesting this is an important component of their ability to coexist as sympatric species.     

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.     

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.