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International audience; Ontogenetic niche shifts are widespread. However, individual differences in size at birth, morphology, sex, and personalitiescan cause variability in behavior. As such, inherent inter-individual differences within populations may lead to context-dependent changes in behavior with animal body size, which is of concern for understanding population dynamics andoptimizing ecological monitoring. Using stable carbon and nitrogen isotope values from concurrently sampled tissues, wequantified the direction and magnitude of intraspecific variation in trophic shifts among three shark species, and how thesechanged with body size: spurdogs (Squalus spp.) in deep-sea habitats off La Réunion, bull sharks (Carcharhinus leucas) inestuarine habitats of the Florida Everglades, and blacktip reef sharks (Carcharhinus melanopterus) in coral reef ecosystemsof Moorea, French Polynesia. Intraspecific variation in trophic shifts was limited among spurdogs, and decreased with bodysize, while bull sharks exhibited greater individual differences in trophic shifts, but also decreased in variability throughontogeny. In contrast, blacktip reef sharks exhibited increased intraspecific variation in trophic interactions with body size.Variability in trophic interactions and ontogenetic shifts are known to be associated with changes in energetic requirements,but can vary with ecological context. Our results suggest that environmental stability may affect variability within popula-tions, and ecosystems with greater spatial and/or temporal variability in environmental conditions, and those with morediverse food webs may facilitate greater individual differences in trophic interactions, and thus ontogenetic trophic shifts. Inlight of concerns over environmental disturbance, elucidating the contexts that promote or dampen phenotypic variability isinvaluable for predicting population- and community-level responses to environmental changes.

The bottlenose dolphin ( Tursiops truncatus ) is an upper trophic level predator and the most common cetacean species found in nearshore waters of southern Florida, including the Lower Florida Keys (LFK) and the Florida Coastal Everglades (FCE). The objective of this study was to assess contamination levels of total mercury (T-Hg) in skin and persistent organic pollutants (PCBs, PBDEs, DDXs, HCHs, HCB, Σ PCDD/Fs and Σ DL-PCBs) in blubber samples of bottlenose dolphins from LFK (n = 27) and FCE (n = 24). PCBs were the major class of compounds found in bottlenose dolphin blubber and were higher in individuals from LFK (Σ 6 PCBs LFK males: 13,421 ± 7730 ng g −1 lipids, Σ 6 PCBs LFK females: 9683 ± 19,007 ng g −1 lipids) than from FCE (Σ 6 PCBs FCE males: 5638 ng g −1 ± 3627 lipids, Σ 6 PCBs FCE females: 1427 ± 908 ng g −1 lipids). These levels were lower than previously published data from the southeastern USA. The Σ DL-PCBs were the most prevalent pollutants of dioxin and dioxin like compounds (Σ DL-PCBs LFK: 739 ng g −1 lipids, Σ DL-PCBs FCE: 183 ng g −1 lipids) since PCDD/F concentrations were low for both locations (mean 0.1 ng g −1 lipids for LFK and FCE dolphins). The toxicity equivalences of PCDD/Fs and DL-PCBs expressed as TEQ in LFK and FCE dolphins is mainly expressed by DL-PCBs (81% LFK - 65% FCE). T-Hg concentrations in skin were significantly higher in FCE (FCE median 9314 ng g −1 dw) compared to LFK dolphins (LFK median 2941 ng g −1 dw). These concentrations are the highest recorded in bottlenose dolphins in the southeastern USA, and may be explained, at least partially, by the biogeochemistry of the Everglades and mangrove sedimentary habitats that create favourable conditions for the retention of mercury and make it available at high concentrations for aquatic predators.

International audience; Sharks are a major component of the top predator guild in oceanic ecosystems, but their trophic relationships remain poorly understood. We examined chemical tracers of diet and habitat (δ15N and δ13C, respectively) and total mercury (Hg) concentrations in muscle tissue of seven pelagic sharks: blue shark (Prionace glauca), short-fin mako shark (Isurus oxyrinchus), oceanic whitetip shark (Carcharhinus longimanus), scalloped hammerhead shark (Sphyrna lewini), pelagic thresher shark (Alopias pelagicus), crocodile shark (Pseudocarcharias kamoharai) and silky shark (Carcharhinus falciformis), from the data poor south-western tropical Indian Ocean. Minimal interspecific variation in mean δ15N values and a large degree of isotopic niche overlap - driven by high intraspecific variation in δ15N values - was observed among pelagic sharks. Similarly, δ13C values of sharks overlapped considerably for all species with the exception of P. glauca, which had more 13C-depleted values indicating possibly longer residence times in purely pelagic waters. Geographic variation in δ13C, δ15N and Hg were observed for P. glauca and I. oxyrinchus. Mean Hg levels were similar among species with the exception of P. kamoharai which had significantly higher Hg concentrations likely related to mesopelagic feeding. Hg concentrations increased with body size in I. oxyrinchus, P. glauca and C. longimanus. Values of δ15N and δ13C varied with size only in P. glauca, suggesting ontogenetic shifts in diets or habitats. Together, isotopic data indicate that – with few exceptions - variance within species in trophic interactions or foraging habitats is greater than differentiation among pelagic sharks in the south-western Indian Ocean. Therefore, it is possible that this group exhibits some level of trophic redundancy, but further studies of diets and fine-scale habitat use are needed to fully test this hypothesis.

International audience; Understanding trophic interactions is critical for elucidating ecological roles of marine predators. We used behavioural observations and stable isotope mixing models to investigate the feeding ecology of Indo-Pacific bottlenose dolphins (Tursiops aduncus) in the lagoon of Mayotte (East Africa). We identified prey during 77 % of 54 observed feeding events, observed in both rainy (61 % of events) and dry (39 %) seasons. Caranx melampygus and Gnathanodon speciosus were involved in 67 % of these events, with Tylosurus crocodilus (20 %) and Mugil cephalus (13 %) also consumed. Mixing models, based on δ13C and δ15N values of skin and blubber (n = 30 samples for both tissues), suggest that behavioural observations are representative of general feeding patterns. Indeed, C. melampygus and T. crocodilus (G. speciosus could not be included in models) were estimated to contribute most to dolphin diets, with mean estimated contributions of 44.6 % (±18.9) and 48.1 % (±19.1) for skin and 73.7 % (±14.9) and 16.9 % (±12.4) for blubber, respectively. Our results highlight the value of two independent methods (stable isotopes and behavioural observations) to assess prey preferences of free-ranging dolphins.