Niche overlap and diet composition of three sympatric coastal dolphin species in the southwest Atlantic Ocean

Sympatric species are expected to differ in ecological requirements to minimize niche overlap and avoid competition. Here we assess the trophic interactions among three coexisting dolphin species from southern Brazil: the franciscana dolphin (Pontoporia blainvillei), the Guiana dolphin (Sotalia guianensis), and the Lahille's bottlenose dolphin (Tursiops truncatus gephyreus). We evaluated temporal variation in carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope values of bone collagen to examine potential dietary shifts resulting from increased fishing activity over the past three decades. We estimated the degree of niche overlap among these species and the contribution of potential prey sources to their diet. δ¹⁵N values were consistent among species and across years, while δ¹³C values increased for Guiana dolphins and decreased for bottlenose dolphins, suggesting changes in diet and/or foraging habitats through time. The similar δ¹³C and δ¹⁵N values and the high niche overlap between Guiana and bottlenose dolphins indicate that these species are primarily feeding on demersal prey. The franciscana diet is primarily composed of pelagic prey, resulting in a lower niche overlap in comparison with the other dolphin species. Our study provides further information about the foraging ecology of this unique dolphin community in southern Brazil with implications for its management and conservation.     

Differences in acoustic signals from Delphinids in the western North Atlantic and northern Gulf of Mexico

Whistle characteristics were quantitatively compared between both geographically separated and neighboring populations of Atlantic spotted dolphins (Stenella frontalis), bottlenose dolphins (Tursiops truncatus), and pilot whales (Globicephala spp.) in U.S. waters to evaluate if intraspecific acoustic differences exist between groups. We compared nine whistle characteristics between continental shelf and offshore Atlantic spotted dolphins in the western North Atlantic and between northern Gulf of Mexico and western North Atlantic bottlenose dolphins and pilot whales using discriminant analysis. Offshore Atlantic spotted dolphin whistles were significantly different (Hotelling's T², P= 0.0003) from continental shelf whistles in high frequency, bandwidth, duration, number of steps, and number of inflection points. Atlantic bottlenose dolphin whistles were significantly different (Hotelling's T², P < 0.0001) from those in the Gulf of Mexico in duration, number of steps, and number of inflection points. There was no significant difference between pilot whale whistles in the two basins. The whistle differences indicate acoustic divergence between groups in different areas that may arise from geographic isolation or habitat separation between neighboring but genetically distinct populations of dolphins. This study supports the premise that acoustic differences can be a tool to evaluate the ecological separation between marine mammal groups in field studies.     

Distribution and abundance of Indo‐Pacific humpback dolphins in Leizhou Bay,China

Abstract

The Indo‐Pacific humpback dolphin (Sousa chinensis) is under severe threat in Chinese waters. Only four small populations of this dolphin are known to survive in the East China Sea and South China Sea. We conducted exploratory surveys from June to September 2005, and found a population of humpback dolphins in Leizhou Bay. Thirty‐eight individuals were identified and catalogued, of which 26 were photographed and re‐identified on more than one occasion, and nine were sighted on five or more occasions each. Our preliminary estimate of the size of the Leizhou population is about 237 individuals, second only to the Pearl River estuary population. We suggest that Leizhou Bay has the potential to serve as a “humpback dolphin sanctuary” in Chinese waters.     

At the Heart of the Industrial Boom: Australian Snubfin Dolphins in the Capricorn Coast,Queensland, Need Urgent Conservation Action

The recent industrial boom along the Australian coastline has increased concerns about the long term conservation of snubfin dolphins along the Queensland coast. National assessment of the conservation status and management of the Australian snubfin dolphin is currently hindered by the lack of adequate biological and ecological information throughout most of its range. In response to the issue of determining the conservation status of species with broad ranges, the IUCN has provided a framework for assessing the threatened status of regional populations. In this study we assessed the conservation status of a small geographically isolated population of snubfin dolphins living in the Fitzroy River region, Queensland, Australia, against the IUCN criteria for regional populations. A review of all available sightings data and stranding information indicates that this is the southernmost resident population of snubfin dolphins in Australian waters. The Fitzroy River snubfin dolphin population is composed of less than 100 individuals, with a representative range and core area of less than 400 and 300 km² respectively. The area most often used by snubfin dolphins within the representative range and core area was estimated to be about 292 and 191 km², respectively. A decrease in representative range, core area and preferred habitat between 14 and 25% is projected to occur if a planned industrial port development were to occur. These results are robust to uncertainty and considering the low level of formal protection and future threats, a classification of this subpopulation under the IUCN Red List as “Endangered” is appropriate.     

厦门湾中华白海豚潜在生态廊道识别及人类活动干扰评估

生态廊道具有维持或恢复生态连通性的功能,对于连接生物栖息地、保护物种多样性具有重要意义。现有的生态廊道研究主要集中于陆地,而海洋生态系统具有水体广泛连通、缺乏直观的景观斑块等特点,导致海洋生态廊道的研究成为长期以来的科学难题。以栖息地位于厦门湾的国家一级保护动物中华白海豚(Sousa chinensis)为对象,尝试基于物种分布模型和最小成本路径分析法建立海洋生态廊道的识别方法。研究采用物种分布模型识别厦门湾内中华白海豚的适宜生境分布区和节点,并利用模型产出的生境适宜性结果生成海洋中的阻力表面,模拟计算节点与节点间在阻力表面上的最小成本路径,从而生成物种扩散网络。研究结果显示,厦门湾中华白海豚的分布主要受到航道距离、到岸线距离和叶绿素浓度三项因素的影响,主要适宜生境位于西海域至九龙江口和大嶝海域。潜在的核心生态廊道面积93.19km²,次级生态廊道面积170.41km²,九龙江口-鼓浪屿南侧-黄厝-大小嶝岛沿线可能是厦门湾中华白海豚的主要迁移路线。在此基础上,从用海空间重叠和桥梁影响两方面开展了人类活动对廊道的干扰评估。评估结果显示旅游活动和...     

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.     

Breathing synchrony in franciscana (Pontoporia blainvillei) and Guiana dolphins (Sotalia guianensis) in Southern Brazil

Synchronous breathing may be a useful proxy for studying other, and perhaps more complex, aspects of cetacean behavior. Here we describe a study conducted in Babitonga Bay, southern Brazil, where we investigated the synchrony of breathing in two small populations of franciscana (Pontoporia blainvillei) and Guiana dolphins (Sotalia guianensis). The bay is affected by different sources of anthropogenic disturbances, such as boat activity and point‐source pollution. We assumed breathing synchrony to be the inverse of the time between breathing surfacing displays of dolphins within a swimming pair, which we refer to as lag. The relationship between lag and anthropogenic and animal‐related variables was evaluated with generalized additive models. For franciscana dolphins, breathing synchrony was only positively related to the proximity between animals. Breathing synchrony in Guiana dolphins was positively related to both the proximity between animals and to group size, and varied significantly depending on the research boat used. Proximal characteristics (i.e., of individuals or of the group) of these dolphin species seem to be more related to the synchronization in breathing than are the environmental variables assessed here. Results presented expand the current knowledge of these two dolphin species and provide general insights into the breathing synchrony for cetaceans.     

Spatiotemporal patterns of marine mammal distribution in coastal waters of Galicia, NW Spain

The spatial and seasonal distribution of cetaceans and possible links with environmental conditions were studied at the Galician continental shelf. Data were collected between February–August 2001 and June–September 2003 during opportunistic surveys onboard fishing boats. Seven species of cetaceans were identified from 250 sightings of 6,846 individuals. The common dolphin (Delphinus delphis) was by far the most frequently sighted and the most widely distributed species. Spatiotemporal trends in cetacean distribution and abundance, and their relationships with environmental parameters (sea depth, SST and chlorophyll-a) were quantified using generalised additive models (GAMs). Results for all cetaceans were essentially the same as for common dolphins alone. Modelling results indicated that the number of common dolphin sightings per unit effort was higher further south. The number of individual common dolphins seen per sighting of this species (i.e. group size) was however higher in the north and west of the study area, higher later in the year and higher in 2001 than in 2003. In contrast, the number of common dolphin calves seen (per sighting of this species) was higher in the south. Models including environmental variables indicated larger common dolphin group sizes in deeper waters and at higher chlorophyll concentrations (i.e. in more productive areas). There was also a positive relationship between survey effort and group size, which is probably an artefact of the tendency of the survey platforms (fishing boats) to spend most time in areas of high fish abundance. Numbers of common dolphin calves per sighting were found to be higher in shallower waters. The results are consistent with common dolphins foraging mainly in deeper waters of the Galician continental shelf, while more southern inshore waters may represent a nursery area.     

A long‐term study of bottlenose dolphins (Tursiops aduncus) in an Australian industrial estuary: Increased sightings associated with environmental improvements

Delphinids are long‐lived, have delayed maturity and low reproductive rates which necessitate long‐term monitoring programs to detect changes in abundance. Between 1990 and 2013, an observational study of bottlenose dolphins (Tursiops aduncus) was conducted in the Port River estuary (Adelaide, Australia). The estuary has received pollution from industry, sewage plants and storm water. In recent years, pollution entering the system has reduced and the establishment of the Adelaide Dolphin Sanctuary (ADS) increased dolphin protection from harassment and deliberate attacks. Nevertheless, the estuary remains a busy port. Over a 24 yr period, we conducted boat‐based surveys (n = 735) for dolphin groups (n = 3,634) along a predetermined route in the Inner and Outer Estuary of Adelaide's Port River estuary. It is our conjecture that major infrastructure changes and pollution abatement have yielded a more favorable marine environment for dolphins, resulting in an estimated 6% annual increase in sightings, from a near absence of sightings in the 1980s. Increased dolphin numbers were likely the result of improved water quality, augmented by surveillance and education arising from the proclamation of the ADS. This study highlights the importance of long‐term monitoring and has implications for dolphin conservation in heavily impacted urban areas and their protection via protected areas.     

Ultrastructure of enamel and dentine in extant dolphins (Cetacea: Delphinoidea and Inioidea)

Longitudinal and cross sections of teeth from 17 species of the Recent dolphins (Delphinoidea and Inioidea) were examined under scanning electron microscope to study the arrangement and ultrastructure of dental tissues with reference to phylogenetic and functional constraints. For most species, enamel had a simple bi-layered structure of radial enamel and an outer layer of prismless enamel. The outer prismless layer varied from 5 to 30 % of enamel thickness. The enamel of Burmeister’s porpoise (Phocoena spinipinnis) was entirely prismless. The prisms had an open sheath; tubules and tuft-like structures were common at the enamel-dentine junction. Cetacean dentine was characterized by irregularly distributed dentinal tubules in a relatively homogenous dentinal matrix. Radial enamel was observed in all Delphinoidea and in the franciscana (Pontoporia blainvillei), whereas the Amazon river dolphin (Inia geoffrensis) had prisms organized in Hunter–Schreger bands. HSB in enamel are regarded as a device for resisting propagation of cracks. These may occur due to increased functional demands, possibly related to the hardness of the species diet. Simplification in tooth shape and reduced biomechanical demands plausibly explain the primitive radial organization among delphinoids and Pontoporia. The HSB structure in the Amazon river dolphin, similar to those of extinct archaeocetes, seems to have secondary functional implications. However, the distribution of HSB in more-basal odontocetes is too poorly known to judge whether the HSB of Inia are a retained plesiomorphic feature or convergence.     

Prey selection by the common dolphin: Fulfilling high energy requirements with high quality food

Which characteristics define the prey species constituting the diet of a given predator? Answering this question would help predict a predator's diet and improve our understanding of how an ecosystem functions. The aim of this study was to test if the diet of common dolphins, Delphinus delphis, in the oceanic Bay of Biscay reflected prey availability or a selection shaped by prey energy densities (ED). To do this, the community of potential prey species, described both in terms of relative abundance and energy densities, was compared to the common dolphin diet in this area. This analysis of a predator's diet and its prey field revealed that the common dolphin selected its diet on the basis of prey energy densities (significant values of Chesson's index for ED > 5 kJ g^− 1). High-energy prey were positively selected in the diet [e.g. Notoscopelus kroeyeri, ED = 7.9 kJ g^− 1, 9% of relative abundance in the environment (%Ne); 62% of relative abundance in the diet (%Nd)] and low-energy prey disregarded (Xenodermichthys copei, ED = 2.1 kJ g⁻¹, 20%Ne, 0%Nd). These results supported the hypothesis that common dolphins selected high energy density prey species to meet their energetically expensive life style and disregard prey organisms of poor energy content even when abundant in the environment.     

Evidence for Distinct Coastal and Offshore Communities of Bottlenose Dolphins in the North East Atlantic

Bottlenose dolphin stock structure in the northeast Atlantic remains poorly understood. However, fine scale photo-id data have shown that populations can comprise multiple overlapping social communities. These social communities form structural elements of bottlenose dolphin (Tursiops truncatus) populations, reflecting specific ecological and behavioural adaptations to local habitats. We investigated the social structure of bottlenose dolphins in the waters of northwest Ireland and present evidence for distinct inshore and offshore social communities. Individuals of the inshore community had a coastal distribution restricted to waters within 3 km from shore. These animals exhibited a cohesive, fission-fusion social organisation, with repeated resightings within the research area, within a larger coastal home range. The offshore community comprised one or more distinct groups, found significantly further offshore (>4 km) than the inshore animals. In addition, dorsal fin scarring patterns differed significantly between inshore and offshore communities with individuals of the offshore community having more distinctly marked dorsal fins. Specifically, almost half of the individuals in the offshore community (48%) had characteristic stereotyped damage to the tip of the dorsal fin, rarely recorded in the inshore community (7%). We propose that this characteristic is likely due to interactions with pelagic fisheries. Social segregation and scarring differences found here indicate that the distinct communities are likely to be spatially and behaviourally segregated. Together with recent genetic evidence of distinct offshore and coastal population structures, this provides evidence for bottlenose dolphin inshore/offshore community differentiation in the northeast Atlantic. We recommend that social communities should be considered as fundamental units for the management and conservation of bottlenose dolphins and their habitat specialisations.     

Behavioural Effects of Tourism on Oceanic Common Dolphins,Delphinus sp., in New Zealand: The Effects of Markov Analysis Variations and Current Tour Operator Compliance with Regulations

Common dolphins, Delphinus sp., are one of the marine mammal species tourism operations in New Zealand focus on. While effects of cetacean-watching activities have previously been examined in coastal regions in New Zealand, this study is the first to investigate effects of commercial tourism and recreational vessels on common dolphins in an open oceanic habitat. Observations from both an independent research vessel and aboard commercial tour vessels operating off the central and east coast Bay of Plenty, North Island, New Zealand were used to assess dolphin behaviour and record the level of compliance by permitted commercial tour operators and private recreational vessels with New Zealand regulations. Dolphin behaviour was assessed using two different approaches to Markov chain analysis in order to examine variation of responses of dolphins to vessels. Results showed that, regardless of the variance in Markov methods, dolphin foraging behaviour was significantly altered by boat interactions. Dolphins spent less time foraging during interactions and took significantly longer to return to foraging once disrupted by vessel presence. This research raises concerns about the potential disruption to feeding, a biologically critical behaviour. This may be particularly important in an open oceanic habitat, where prey resources are typically widely dispersed and unpredictable in abundance. Furthermore, because tourism in this region focuses on common dolphins transiting between adjacent coastal locations, the potential for cumulative effects could exacerbate the local effects demonstrated in this study. While the overall level of compliance by commercial operators was relatively high, non-compliance to the regulations was observed with time restriction, number or speed of vessels interacting with dolphins not being respected. Additionally, prohibited swimming with calves did occur. The effects shown in this study should be carefully considered within conservation management plans, in order to reduce the risk of detrimental effects on common dolphins within the region.     

Site fidelity and association patterns of Indo-Pacific humpback dolphins off the east coast of Zhanjiang,China

This study examined the population of Indo-Pacific humpback dolphins, Sousa chinensis, inhabiting the waters off the east coast of Zhanjiang, China. A total of 116 dolphins were identified during 147 boat-based surveys, completed between June 2005 and June 2007. Abundance estimates indicated that a small population of 268 dolphins (95% CI = 189–413) inhabited this coastal area. The sighting frequencies of identified dolphins varied between one and five when the 2006 and 2007 data were organized into six occasions. Twenty-three percent (27 individuals) of the cataloged dolphins were identified in more than one calendar year. The two animals (ZJ001, ZJ011) with the highest degree of site fidelity were present in the study area for a period of 23 months. The most frequently (15 times) sighted dolphin (ZJ046) was recorded in nine (56%) of the 16 months surveyed. This evidence indicates the interannual site fidelity of S. chinensis to the area off the eastern coast of Zhanjiang City. Range sizes of eight identified individuals calculated by the minimum convex polygon varied from 2.07 to 331.20 km². Associations between 34 individuals sighted at least three times and more than three times were measured by the half-weight index and SOCPROG program. The majority of these dolphins (80.57%) were not seen together during surveys and the mean association rate between dyads was low at 0.05. Temporal analyses for all the individuals were conducted using a lagged association rate. The results indicated a non-random social structure made by constant companions.     

Clicking in Shallow Rivers: Short-Range Echolocation of Irrawaddy and Ganges River Dolphins in a Shallow,Acoustically Complex Habitat

Toothed whales (Cetacea, odontoceti) use biosonar to navigate their environment and to find and catch prey. All studied toothed whale species have evolved highly directional, high-amplitude ultrasonic clicks suited for long-range echolocation of prey in open water. Little is known about the biosonar signals of toothed whale species inhabiting freshwater habitats such as endangered river dolphins. To address the evolutionary pressures shaping the echolocation signal parameters of non-marine toothed whales, we investigated the biosonar source parameters of Ganges river dolphins (Platanista gangetica gangetica) and Irrawaddy dolphins (Orcaella brevirostris) within the river systems of the Sundarban mangrove forest. Both Ganges and Irrawaddy dolphins produced echolocation clicks with a high repetition rate and low source level compared to marine species. Irrawaddy dolphins, inhabiting coastal and riverine habitats, produced a mean source level of 195 dB (max 203 dB) re 1 µPa_pp whereas Ganges river dolphins, living exclusively upriver, produced a mean source level of 184 dB (max 191) re 1 µPa_pp. These source levels are 1–2 orders of magnitude lower than those of similar sized marine delphinids and may reflect an adaptation to a shallow, acoustically complex freshwater habitat with high reverberation and acoustic clutter. The centroid frequency of Ganges river dolphin clicks are an octave lower than predicted from scaling, but with an estimated beamwidth comparable to that of porpoises. The unique bony maxillary crests found in the Platanista forehead may help achieve a higher directionality than expected using clicks nearly an octave lower than similar sized odontocetes.     

Feeding ecology of the highly threatened common bottlenose dolphin of the Gulf of Ambracia,Greece, through stable isotope analysis

The Gulf of Ambracia, in northwestern Greece, hosts a highly threatened community of about 150 common bottlenose dolphins (Tursiops truncatus). Until now, information on their feeding habits was derived exclusively from fish scale samples collected during surface-feeding events targeting small schooling epipelagic fish. The aim of this study was to determine the diet of bottlenose dolphins living in the Gulf of Ambracia through the application of Bayesian isotopic mixing models. Skin biopsy samples of 16 dolphins were analyzed and no difference related to sex or age-class was found in δ¹³C and δ¹⁵N values. Results suggested that the dolphin diet was mainly based on Trachurus trachurus, species belonging to the family Sparidae: Diplodus annularis, Lithognathus mormyrus, and Sepia officinalis, which represented together about 42% ± 15% of the biomass ingested, followed by species belonging to the order Clupeiformes (Engraulis encrasicolus, Sardinella aurita, and Sardina pilchardus) and the genus Gobius (37% ± 17%). A better understanding of the feeding habits of these dolphins sheds light on the feeding ecology of this highly threatened population by, for instance, evidencing interactions with artisanal fisheries sharing the same target species, and is key for identifying adequate management measures consistent with an ecosystem-based approach.     

Unique seasonal forage bases within a local population of bottlenose dolphin (Tursiops truncatus)

Using photo‐identification data, bottlenose dolphin (Tursiops truncatus) populations can be differentiated based on their use of particular estuaries or coastal habitats. Questions remain, however, about the validity of such fine‐scale population partitioning and whether the resulting assemblages utilize unique forage bases. To address the issue of forage base use, stable isotopes of carbon (δ¹³C), nitrogen (δ¹⁵N) and sulfur (δ³⁴S) were analyzed from skin tissues (n= 74) of bottlenose dolphins sampled seasonally along the coast and in three estuaries near Charleston, South Carolina. Autumn values of δ³⁴S, δ¹⁵N, and δ¹³C and summer values of δ³⁴S indicated that dolphins sampled from these four assemblages utilized unique forage bases, despite limited sample sizes. Likewise, autumn and spring differences in δ¹⁵N and δ¹³C values were evident in the North Edisto River, and in δ³⁴S from dolphins sampled from all three estuarine assemblages; no seasonal differences were identified in the coastal assemblage. Results demonstrate the importance of considering spatial and temporal variation in forage base when developing local management plans for bottlenose dolphin and highlight the discriminatory power of δ³⁴S for estuarine and coastal marine mammals. These results also suggest that stable isotopes could be developed as a complementary tool for photo‐identification based partitioning of bottlenose dolphin populations.     

Fine‐scale habitat use in Indo‐Pacific humpback dolphins,Sousa chinensis,may be more influenced by fish rather than vessels in the Pearl River Estuary,China

Shifts in habitat use and distribution patterns in dolphins are often concerns that can result from habitat degradation. We investigated how potential changes to a habitat from human activity may alter dolphin distributions within Lingding Bay in the Pearl River Estuary, China, by studying the relationship between fish choruses, vessel presence and Indo‐Pacific humpback dolphin (Sousa chinensis) detection rates. Analyses revealed temporal and spatial variation within fish choruses, vessel presence and dolphin detection rates. After accounting for any temporal autocorrelation, correlations between fish choruses and dolphin detection rates were also found; however, no relationship between fish choruses and vessel presence or dolphin detection rates and vessel presence were observed. Furthermore, fewer dolphins were detected at sites where fish activity was less intense. Thus fish activity, rather than vessels, may be a key factor influencing the distribution of the dolphins within the estuary. These findings emphasize the risk of potential shifts in habitat use for Indo‐Pacific humpback dolphins due to detrimental changes to prey availability and dolphin feeding grounds from human activity, such as overfishing and coastal developments, within the estuary. This is a critical conservation issue for this dolphin population that is facing intense anthropogenic pressure.     

A comparison of pectoral fin contact behaviour for three distinct dolphin populations

Tactile exchanges involving the pectoral fin have been documented in a variety of dolphin species. Several functions (e.g., social, hygienic) have been offered as possible explanations for when and why dolphins exchange pectoral fin contacts. In this study, we compared pectoral fin contact between dolphin dyads from three distinct dolphin populations: two groups of wild dolphins; Atlantic spotted dolphins (Stenella frontalis) from The Bahamas and Indo-Pacific bottlenose dolphins (Tursiops aduncus) from around Mikura Island, Japan; and one group of captive bottlenose dolphins (Tursiops truncatus) residing at the Roatan Institute for Marine Sciences, Anthony's Key Resort. A number of similarities were observed between the captive and wild groups, including; rates of pectoral fin contact, which dolphin initiated contact, posture preference, and same-sex rubbing partner preference. Unlike their wild counterparts, however, dolphins in the captive study group engaged in petting and rubbing at equal rates, females were more likely to contact males, males assumed the various rubbing roles more frequently than females, and calves and juveniles were more likely to be involved in pectoral fin contact exchanges. These results suggest that some aspects of pectoral fin contact behaviour might be common to many dolphin species, whereas other aspects could be species specific, or could be the result of differing environmental and social conditions.     

Diet and foraging opportunism of the Guiana dolphin (Sotalia guianensis) in the Abrolhos Bank,Brazil

Diet analysis allows exploring how coastal dolphins interact with the environment and their role in the marine food webs. We studied the diet and feeding ecology of the Guiana dolphin, Sotalia guianensis, through analysis of stomach content from 42 animals stranded on the eastern coast of Brazil. A total of 1,336 semidigested prey items (fish, otoliths, cephalopod beaks, and crustaceans) were identified. Teleost fish comprised the most frequent food item (92% of the total), followed by cephalopods, and crustaceans. Prey belonged to 34 taxa and richness in individual stomachs varied from 1 to 15 prey taxa. Prey were generally small, but showed a significant trend to increase in size with dolphin length. The main prey of Guiana dolphins were demersal, estuarine, and sound-making fish such as catfish and sciaenids. No sex-related differences in diet were found. Diet composition varied seasonally and occurrence of prey items was coherent with breeding or high abundance periods of some fish species and squids. Our study shows the importance of demersal prey from estuarine and soft-sediment habitats to Guiana dolphin in the Abrolhos Bank and reveals that feeding habits are generalist and opportunistic, with diet reflecting the seasonal abundance and availability of prey.