Dolphin Bait‐Balling Behaviors in Relation to Prey Ball Escape Behaviors

Delphinids frequently coordinate behaviors to contain or capture schooling fishes, but we know little about how these behaviors relate to prey escape behaviors, and corresponding costs and benefits. In this study, we described prey ball escape behaviors and investigated how prey ball behaviors related to dusky dolphin, Lagenorhynchus obscurus, prey herding and capturing behaviors using above‐water and underwater video methods. Prey balls exhibited horizontal and vertical movements, both of which would have increased feeding costs for dolphins compared with feeding on stationary prey at the surface. The only prey ball behavior that we observed to precede escape was ‘funneling’ or the brief formation of a prey ball shape where the height was at least twice the width. Funneling was observed most often for large prey balls, immediately before they descended. When prey balls ascended, there was an insignificant trend for dolphins to do a greater proportion of herding passes that did not include a prey capture (p = 0.094), and dolphins did a larger proportion of prey‐capture attempts on the sides of prey balls (p = 0.017) and a smaller proportion through the edges of prey balls (p = 0.037). When prey balls ascended, dolphins also did a smaller proportion of attempts with a dorsal body orientation (p = 0.037); there were insignificant trends for them to do a smaller proportion with a side orientation (p = 0.100) but a larger proportion with a ventral body orientation (p = 0.081). Thus, funneling is an effective behavior by which large prey balls are able to vertically escape, and dolphins may counter vertical prey escape behaviors by doing herding passes that do not include a prey‐attempt, and changing location or body orientation of prey‐capture behaviors.     

Diel and lunar variation in diving behavior of rough-toothed dolphins (Steno bredanensis) off Kauaʻi,Hawaiʻi

Observational studies describe rough-toothed dolphins (Steno bredanensis) actively foraging during the day on epipelagic species. Using data from depth-transmitting satellite tags deployed on nine individuals off Kauaʻi, we investigated diving behavior and the effects of lunar phase and solar light levels on vertical movements. Overall, tagged rough-toothed dolphins primarily used near-surface waters, spending between 83.6% and 93.7% of their time in the top 30 m of the water column. When diving, grand mean, median, and maximum dive depths were 76.9 m, 67.5 m, and 399.5 m, although individuals were in water with depths from approximately 700–1,450 m. Dive rates varied by time of day, being lowest during the day and at dawn and highest at dusk and night. Dives were deepest (M = 133.7 m, SD = 52.6 m, median = 106.5 m) and longest (M = 4.0 min, SD = 0.4 min, median = 4.0 min) at dusk, suggesting dolphins were taking advantage of prey rising to the surface in response to reduced light levels. Lunar phase indirectly affected diving, with deeper and longer dives occurring with increasing illumination. The variations in dive behavior across solar and lunar cycles indicate diving patterns shift based on the distribution of prey.     

Sequential foraging of dusky dolphins with an inspection of their prey distribution

The aim of this work was to analyze the sequential foraging behavior of dusky dolphins (Lagenorhynchus obscurus). Foraging sequences were defined when more than two feeding bouts occur with a traveling bout between them. We hypothesized that traveling costs of searching for prey patches were related to the time spent feeding on a patch. In addition, the distribution and seasonal variation of anchovy schools were studied in the area to better understand dolphins' behavior. We observed dolphins from a research vessel from 2001 to 2007, and recorded their location and behavior. Anchovy data were collected during two hydro‐acoustic surveys. Dusky dolphin behaviors varied seasonally; they spent a greater proportion of time traveling and feeding in the warm season (Kruskal‐Wallis: H = 172.07, P < 0.01). During the cold season dolphin groups were more likely to exhibit diving behavior and less surface feeding. We found a positive correlation between searching and foraging time (r = 0.88, P = 0.019), suggesting that the costs associated with searching were compensated by an increase in the energy intake during the foraging bout. There was an association between dusky dolphin and anchovy distribution, in that they co‐varied spatially and seasonally.     

HABITAT USE PATTERNS AND RANGES OF THE BOTTLENOSE DOLPHIN IN THE GULF OF CALIFORNIA, MEXICO

Abstract: I studied behavior and range patterns of individual bottlenose dolphins during 1984 in the mid-eastern Gulf of California, Mexico. Dolphin sighting rate was significantly higher in areas close to estuary mouths, 0.306 sightings per hour compared with 0.155 sightings per hour in areas distant from estuary mouths. Dolphins used these estuarine areas to feed; 61% of all behavior observed near estuaries was feeding as compared with 23% elsewhere. Traveling comprised 61% of all behavior observed in areas distant from estuary mouths. Estuaries are sites of large concentrations of nutrients which support great numbers of filter-feeding zooplankton and fish. Bottlenose dolphins may specialize on esmarine prey, or they may feed in estuarine areas simply because of the abundance of potential prey that these systems support. In either case, data on relative numbers, distribution patterns, behavior and diet indicate that this is a general trend in habitat use for many coastal populations of this species in the Pacific and Atlantic.
Ranges of a few individuals spanned a minimum of 65 km of coastline, and animals were not permanent residents of a monitored bay. In contrast, dolphins off the coast of Sarasota, Florida, have been reported to be year-round residents with smaller ranges. This difference in degree of site fidelity may be related to habitat differences. The west coast of Florida is dotted with numerous and large estuarine systems which may host permanent prey populations and support resident groups of dolphins. The Gulf of California coastline contains few estuaries; most are small and perhaps support prey resources which are ephemeral, requiring dolphins to range over larger distances in search of food.     

Diving for food: a switch of foraging strategy of dusky dolphins in Argentina

During winter, dusky dolphins (Lagenorhynchus obscurus) were observed in coordinated diving apparently in a feeding activity, contrasting with the surface feeding observed during summer. The aim of this work consisted in analyzing the diving activity as an alternative foraging strategy in Argentine dusky dolphins, based on sequential analysis. The study area was Golfo Nuevo, located in Northern Patagonia, Argentina. Random transects were surveyed by a research boat from 2001 to 2007. During behavioral sampling, group members were observed continuously and the predominant activity was recorded at 2-min intervals. Six predominant activities were identified. Each 2-min interval was classified according to the activity at the previous interval (preceding activity), the activity at the interval (following activity), and the season (cold or warm). Z scores were calculated and then used to construct sequence diagrams. An association between diving and milling behavior was observed. This could be another foraging tactics different to the surface foraging sequences and this could be related with the distribution or abundance of preys.     

BEHAVIOR AND ECOLOGY OF THE ATLANTIC WHITE-SIDED DOLPHIN (LAGENORHYNCHUS ACUTUS) IN COASTAL NEW ENGLAND WATERS

Atlantic white-sided dolphins ( Lagenorhynchus acutus ) are among the most abundant, and least studied, cetaceans in coastal New England. Between April and October 1984 through 1997 we sighted 1,231 groups of Atlantic white-sided dolphins, primarily on Stellwagen Bank and Jeffreys Ledge (two shallow glacial deposits along the coasts of Massachusetts, New Hampshire, and Maine). Mean group size was 52 (≥90.9), and was significantly larger from August through October (71.9 ± 111.4) than April through June (35.0 ± 45.4). Calf sightings were uncommon until June and July, after which they were present in over 50% of groups. Combined with observations of apparent newborn calves, this confirms that early summer is an important calving period. The presence of calves did not, however, solely account for the increase in group size. Boat interaction (bow- and stern-wake riding) was the most commonly recorded behavior (47.4% of sightings), followed by traveling (31.4%), interactions with other cetacean species (27.6%), social interaction (15.5%), and feeding (9.5%). While feeding was uncommon, one observation of apparently coordinated "ball" feeding was seen with sand lance ( Ammodytes spp.) as the visible prey. Aerial behavior showed a positive correlation with group size, although it was often impossible to tell whether the same dolphins were leaping repeatedly. Eighty-eight dolphins were photo-identified using either unusual body pigment or a distinctive dorsal fin. While several individuals were reidentified between years and between areas, no reidentifications were made within a year in the same area. Unusually pigmented individuals were much more likely to be reidentified than those with distinctive dorsal fins, most likely due to higher visibility. We suggest that Atlantic white-sided dolphins are generally using the study area as transients in what appears to be a large home range.     

异色瓢虫幼虫的食物搜索行为

本试验采用Nakamuta(1982)装置研究两种光照条件下异色瓢虫幼虫的食物搜索行为,结果表明：(1)摄食刺激均能激发搜索行为由广域型转换为地域集中型;(2)摄食时间越长,地域集中型搜索时间(GUT)值越大;(3)摄食的最后一个食饵大小决定GUT的长短;(4)光照对搜索行为有影响;(5)饥饿度对GUT的长短有一定影响。     

Activity budget of inshore Indo‐Pacific bottlenose dolphins (Tursiops aduncus): A critical evaluation of methods and comparison among other populations

Activity budgets are widely used to compare behavior patterns but sampling methods vary, rendering comparisons difficult. The two main methods used are instantaneous and continuous sampling. Their comparability was examined by applying them to data obtained from bottlenose dolphins in the Port River estuary, South Australia. They gave comparable results for activity budgets, but instantaneous sampling did not detect most of the behavioral events. Individual differences in behavior and/or follow duration influenced results. Variability in activity definitions and categories among studies makes comparative analysis difficult. Comparison of the Port River dolphin's activity budget with other inshore populations indicated the former spent more time feeding and resting, and less time traveling. The greater feeding time seemed to be due to small prey size rather than reduced abundance or unpredictable distribution. The reduced traveling time, possibly the result of low predation pressure and/or evenly distributed prey, gave them more time to rest. They traveled mostly at 2.5 kn or less, consistent with studies from other shallow areas. Most feeding was individual, probably on demersal species. Surface feeding incorporated physical barriers rather than cooperative behavior. Activity durations ranged from 2 s to 2.9 h, with mean durations varying from 7.8 to 22.9 min.     

ECOLOGY, BEHAVIOR AND SOCIAL ORGANIZATION OF THE BOTTLENOSE DOLPHIN: A REVIEW

The authors review the literature on bottlenose dolphin ecology, behavior and social organization, focusing on data collected on free-ranging animals. Most bottlenose dolphins studied to date have had definable home ranges, and behavioral, morphological and biochemical information indicates discrete stocks in some areas. Bottlenose dolphins appear to form relatively permanent social groups based on sex and age. Mother—calf bonds are long-lasting. Movement patterns are extremely variable from location to location but are relatively predictable at any given site. Food resources are one of the most important factors affecting movements. Bottlenose dolphin behavior is very flexible, and these dolphins are generally active day and night. Feeding peaks in the morning and afternoon have been observed at several sites. Social behavior is an important component of daily activities. Sharks are the most significant predator on bottlenose dolphins in most areas, but captive and wild studies show that dolphins and sharks frequently live in harmony as well. Human activities may be helpful, harmful or neutral to bottlenose dolphins, but interactions with humans are frequent for these coastal cetaceans.     

Diving and haulout behavior of crabeater seals in the Weddell Sea,Antarctica, during March 1986

Summary Time-depth recorders were used to study the diving and haulout behavior of six crabeater seals in the marginal. ice edge zone of the Weddell Sea during March 1986. Haulout patterns revealed the seals' clear preference for diving during darkness and hauling out onto sea ice during daylight. Seals did not necessarily haul out every day; individual seals hauled out on 80–100% of days during the study period. Four general dive types were identified: 1) traveling dives, 2) foraging dives, 3) crepuscular foraging dives, and 4) exploratory dives. Nearly continual diving occurred for extended periods (about 16 h) nightly, with one individual diving up to 44 h without interruption. Foraging dives occurring during crepuscular periods were deeper than those made during the darkest hours. The authors suggest that the distinct diel pattern of dive timing and depth may be related to possible predator avoidance behavior by the seals' principal prey, Antarctic Krill.     

COMPARATIVE FEEDING ECOLOGY OF SPINNER DOLPHINS (STENELLA LONGIROSTRIS) AND FRASER'S DOLPHINS (LAGENODELPHIS HOSEI) IN THE SULU SEA

Fraser's and spinner dolphins are known to feed on mesopelagic prey, bur their diets and feeding ecologies have not been compared in areas where they are observed together. In this study we examined the stomach contents of both species caught incidentally in a driftnet fishery for tuna in the eastern Sulu Sea. Importance of prey items was determined using the percent occurrence, percent number, and volumetric methods. Mesopelagic fishes, particularly myctophids (mainly Ceratoscopelus warmingi, Diaphus spp. and Myctophum asperum ), were the most important component in the diet of spinner dolphins, whereas in Eraser's dolphins, mesopelagic cephalopods ( Abraliopsis, Onychoteuthis, Histioteuthis , and Chiroteuthis ), and crustaceans ( Notostomos elegans, Acanthephyra quadrispinosa , and Acanthephyra carinata ) appeared to be equally important as the myctophid species. Fraser's dolphins appeared to feed preferentially on larger prey and had more diversified prey (Shannon's diversity index = 1.2) than spinner dolphins (diversity index = 0.9). Vertical distributions of the prey items summarized from published literature indicate that spinner dolphins forage in the upper 200 m and probably occasionally to as deep as 400 m, whereas Fraser's dolphins have a wider vertical foraging range, from near the surface to probably as deep as 600 m.     

Quantitative analysis of bottlenose dolphin movement patterns and their relationship with foraging

1. Broad-scale telemetry studies have greatly improved our understanding of the ranging patterns and habitat-use of many large vertebrates. However, there often remains considerable uncertainty over the function of different areas or the factors influencing habitat selection. Further insights into these processes can be obtained through analyses of finer scale movement patterns. For example, search behaviour may be modified in response to prey distribution and abundance. 2. In this study, quantitative analysis techniques are applied to the movements of bottlenose dolphins, recorded from land using a theodolite, to increase our understanding of their foraging strategies. Movements were modelled as a correlated random walk (CRW) and a biased random walk (BRW) to identify movement types and using a first-passage time (FPT) approach, which quantifies the time allocated to different areas and identifies the location and spatial scale of intensive search effort. 3. Only a quarter of the tracks were classed as CRW movement. Turning angle and directionality appeared to be key factors in determining the type of movement adopted. A high degree of overlap in search effort between separate movement paths indicated that there were small key sites (0.3 km radius) within the study area (4 km(2)). Foraging behaviour occurred mainly within these intensive search areas, indicating that they were feeding sites. 4. This approach provides a quantitative method of identifying important foraging areas and their spatial scale. Such techniques could be applied to movement paths for a variety of species derived from telemetry studies and increase our understanding of their foraging strategies.     

Seasonal Movements,Aggregations and Diving Behavior of Atlantic Bluefin Tuna (Thunnus thynnus) Revealed with Archival Tags

Electronic tags were used to examine the seasonal movements, aggregations and diving behaviors of Atlantic bluefin tuna (Thunnus thynnus) to better understand their migration ecology and oceanic habitat utilization. Implantable archival tags (n = 561) were deployed in bluefin tuna from 1996 to 2005 and 106 tags were recovered. Movement paths of the fish were reconstructed using light level and sea-surface-temperature-based geolocation estimates. To quantify habitat utilization we employed a weighted kernel estimation technique that removed the biases of deployment location and track length. Throughout the North Atlantic, high residence times (167±33 days) were identified in four spatially confined regions on a seasonal scale. Within each region, bluefin tuna experienced distinct temperature regimes and displayed different diving behaviors. The mean diving depths within the high-use areas were significantly shallower and the dive frequency and the variance in internal temperature significantly higher than during transit movements between the high-use areas. Residence time in the more northern latitude high-use areas was significantly correlated with levels of primary productivity. The regions of aggregation are associated with areas of abundant prey and potentially represent critical foraging habitats that have seasonally abundant prey. Throughout the North Atlantic mean diving depth was significantly correlated with the depth of the thermocline, and dive behavior changed in relation to the stratification of the water column. In this study, with numerous multi-year tracks, there appear to be repeatable patterns of clear aggregation areas that potentially are changing with environmental conditions. The high concentrations of bluefin tuna in predictable locations indicate that Atlantic bluefin tuna are vulnerable to concentrated fishing efforts in the regions of foraging aggregations.     

INTERACTIONS BETWEEN THE INDIAN RIVER LAGOON BLUE CRAB FISHERY AND THE BOTTLENOSE DOLPHIN, TURSIOPS TRUNCATUS

Anecdotal reports from blue crab fishermen in the Indian River Lagoon (IRL), Florida suggested that bottlenose dolphins ( Tursiops truncatus ) followed their boats, and stole bait fish from crab pots soon after they were deployed. To investigate these reports, we made biweekly observations from IRL commercial crab boats from January 1998 to January 1999 (670 h). Only 2.8% of the 18,891 crab pots surveyed revealed evidence of dolphin/crab pot interaction. Dolphin interactions included: (1) begging at boats, (2) feeding on discarded bait fish, (3) engaging in crab pot tipping behavior, and (4) dolphin mortality from crab pot float line entanglement. Overall, 16.6% of the 1,296 dolphins sighted interacted with fishing boats. Seasonal trends were evident, with fishery interactions peaking in the summer. Crab pot interactions ranged from 0% to 36% of the traps checked daily. Different methods of securing the bait-well door and the role of trap locality were tested using a replicated experimental design. Results showed significant differences in successful bait removal by dolphins, among the degrees of door security ( P < 0.001) and between trap location ( P < 0.01). Thus, increased door security may help to reduce the negative impacts to the fishery and dolphins involved.     

异色瓢虫雌成虫的食饵搜索行为

本试验采用Nakamuta（1982）装置研究两种光照条件下异色瓢虫雌成虫的食饵搜索行为,结果表明：1、五种刺激均能激发搜索行为由广域型转换为地域集中型;2、摄食时间越长,GUT值越大;3、摄食的最后一个食饵大小决定GUT的长短;4、光照对搜索行为影响不大．     

At-Sea Associations in Foraging Little Penguins

Prey distribution, patch size, and the presence of conspecifics are important factors influencing a predator’s feeding tactics, including the decision to feed individually or socially. Little is known about group behaviour in seabirds as they spend most of their lives in the marine environment where it is difficult to observe their foraging activities. In this study, we report on at-sea foraging associations of little penguins (Eudyptula minor) during the breeding season. Individuals could be categorised as (1) not associating; (2) associating when departing from and/or returning to the colony; or (3) at sea when travelling, diving or performing synchronised dives. Out of 84 separate foraging tracks, 58 (69.0%) involved associations with conspecifics. Furthermore, in a total of 39 (46.4%), individuals were found to dive during association and in 32 (38.1%), individuals were found to exhibit synchronous diving. These behaviours suggest little penguins forage in groups, could synchronise their underwater movements and potentially cooperate to concentrate their small schooling prey.     

Diving pattern and performance in relation to foraging ecology in the gentoo penguin, Pygoscelis papua

We present data on diving pattern and performance (dive depth, duration, frequency and organization during the foraging trip) in gentoo penguins Pygoscelis papua , obtained using time-depth recorders ( n = 9 birds, 99 foraging trips). These data are used to estimate various parameters of foraging activity, e.g. foraging range, prey capture rates, and are compared in relation to breeding chronology. Foraging trip duration was 6 h and 10 h, and trip frequency 1.0/day and 0.96/day, during the brooding and creche periods, respectively. Birds spent on average 52%of each foraging trip diving. Dive depth and duration were highly bimodal: shallow dives (< 21 m) averaged 4 m and 0.23 min, and deep dives (> 30 m) 80 m and 2.5 min, respectively. Birds spent on average 71%and 25%of total diving time in deep and shallow dives, respectively. For deep dives, dive duration exceeded the subsequent surface interval, but shallow dives were followed by surface intervals 2–3 times dive duration. We suggest that most shallow dives are searching/exploratory dives and most deep dives are feeding dives. Deep dives showed clear diel patterns averaging 40 m at dawn and dusk and 80–90 m at midday. Estimated foraging ranges were 2.3 km and 4.1 km during the brood and creche period, respectively. Foraging trip duration increased by 4 h between the brood and creche periods but total time spent in deep dives (i.e. time spent feeding) was the same (3 h). Of 99 foraging trips, 56%consisted of only one dive bout and 44%of 2–4 bouts delimited by extended surface intervals > 10 min. We suggest that this pattern of diving activity reflects variation in spatial distribution of prey rather than the effect of physiological constraints on diving ability.     

Assessment of scale-dependent foraging behaviour in southern elephant seals incorporating the vertical dimension: a development of the First Passage Time method

1. Identifying the spatial scales at which top marine predators forage is important for understanding oceanic ecosystems. Several methods quantify how individuals concentrate their search effort along a given path. Among these, First-Passage Time (FPT) analysis is particularly useful to identify transitions in movement patterns (e.g. between searching and feeding). This method has mainly been applied to terrestrial animals or flying seabirds that have little or no vertical component to their foraging, so we examined the differences between classic FPT and a modification of this approach using the time spent at the bottom of a dive for characterizing the foraging activity of a diving predator: the southern elephant seal. 2. Satellite relayed data loggers were deployed on 20 individuals during three successive summers at the Kerguelen Islands, providing a total of 72 978 dives from eight juvenile males and nine adult females. 3. Spatial scales identified using the time spent at the bottom of a dive ( = 68.2 +/- 42.1 km) were smaller than those obtained by the classic FPT analysis ( = 104.7 +/- 67.3 km). Moreover, foraging areas identified using the new approach clearly overlapped areas where individuals increased their body condition, indicating that it accurately reflected the foraging activity of the seals. 4. These results suggest that incorporating the vertical dimension into FPT provides a different result to the surface path alone. Close to the Antarctic continent, within the pack-ice, sinuosity of the path could be explained by a high sea-ice concentration (restricting elephant seal movements), and was not necessarily related to foraging activity. 5. Our approach distinguished between actual foraging activity and changes in behaviour induced by the physical environment like sea ice, and could be applied to other diving predators. Inclusion of diving parameters appears to be essential to identify the spatial scale of foraging areas of diving animals.     

The effect of time and digestion constraints in Common Eiders while feeding and diving over Blue Mussel beds

1. To maintain energy intake Common Eiders ( Somateria mollissima ) in winter should compensate for reduced day-length by increasing both the proportion of time spent feeding and diving efficiency, defined as the proportional duration of a dive bout within a dive cycle. Common Eiders swallowed Blue Mussel ( Mytilus edulis ) whole with their shells where any behavioural compensation in relation to short days may be limited by digestive processes.
2. Based on time budget studies conducted from mid-December to the end of April in the Gulf of St. Lawrence, Québec, Canada, diving and feeding efficiency were compared for three seasonal periods varying in day-length (from 557 to 890 min).
3. Results showed that eiders were compensating for short days by feeding 56% of the time in mid-winter compared with 33% in spring. However, diving efficiency remained constant through the season and apparently no compensation occurred at this level of their foraging behaviour. Despite this, the daily rate of prey ingestion was much higher in mid-winter than in spring.
4. Ingestion rate values for mid-winter individuals approached or even exceeded the rate at which prey items are defecated and it was concluded that shell crushing performed by the muscular gizzard is physiologically more demanding during that period. On this basis, gizzard mass should be larger in winter when ingestion rates are higher. Data presented support that hypothesis and suggest that compensation in relation to short days can be both behavioural and physiological.     

Fueling phocids: Divergent exploitation of primary energy sources and parallel ontogenetic diet switches among three species of subarctic seals

Determining how marine predators partition resources is hindered by the difficulty in obtaining information on diet and distribution. Stable isotopes (SI) of carbon (¹³C/¹²C, δ¹³C) and nitrogen (¹⁵N/¹⁴N, δ¹⁵N) provide a two‐dimensional estimate of the dietary space of consumers; an animal's isotopic composition is directly influenced by what they consume and where they feed. Harp (Pagophilus groenlandicus) and hooded (Cystophora cristata) seals are abundant phocid species found in the North Atlantic. We measured and contrasted SI values between seals sampled at nearshore and offshore sites to test for effects of sampling location, sex, age‐class, and body size to gain insight into how these species partition space and prey resources. In addition we contrasted previously published results for gray seals (Halichoerus grypus). Isotope values differed significantly by age class and location in harp and hooded seals. We found significant differences in SI values (mean δ¹³C and δ¹⁵N ± SE) between all species. Hooded seals, a continental shelf‐edge, deep‐diving species, exhibited low SI values (juveniles: ?20.9‰ ± 0.03‰, 13.36‰ ± 0.05‰; adults: ?20.41‰ ± 0.03‰, 14.81‰ ± 0.04‰) characteristic of feeding on meso‐ to bathypelagic prey. Harp seals, which dive to moderate depths primarily on the shelf had intermediate SI values (juveniles: ?20.53‰ ± 0.01‰, 13.91‰ ± 0.01‰; adults: ?20.13‰ ± 0.01‰, 14.96‰ ± 0.01‰) characteristic of feeding on epipelagic prey, whereas gray seals, which feed on or near the sea floor in shallow shelf waters, had high SI values (juveniles: ?19.74‰ ± 0.04‰, 17.51‰ ± 0.05‰; adults: ?18.86‰ ± 0.01‰, 17.23‰ ± 0.02‰) characteristic of feeding on demersal prey. In all species, δ¹³C values increased with body size and age in the same manner, indicating that seals exploit or forage in deeper habitats as they get larger and older. We hypothesize that the consistent ontogenetic shift in foraging niche, despite large differences between species in their diving behavior, geographic range and habitat use, not only reflects increased access to different prey due to increased diving capacity, but a progressive adjustment to balance energy budgets by reducing foraging costs.