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International audience; The non-essential metal mercury (Hg) can have deleterious effects on health of organisms, and tends to bioaccumulate with age in long-lived organisms and to biomagnify along food chains. Because sharks are fished for human consumption and their Hg levels are frequently above the maximum Hg concentration recommended for fish consumption, understanding the drivers of Hg concentration is of considerable interest. Total Hg concentrations were analysed in muscle tissues of 14 shark and 2 batoid species (n = 339 individuals) sampled across multiple habitats (coastal, oceanic and bathyal) in the southwestern Indian Ocean. Stable isotope ratios of carbon (δ13C) and nitrogen (δ15N) were analysed to assess whether relative trophic position and foraging habitats affected Hg concentrations. Hg concentrations increased with δ15N and body length, highlighting the mechanisms of bioaccumulation and biomagnification in relation with the trophic position and size of the individuals. Habitats where elasmobranchs were collected also affected their Hg concentrations. Bathyal sharks had high Hg concentrations that were almost similar to those of oceanic species, despite their lower relative trophic position. Higher bioavailability of Hg due to its enhanced methylation in deeper waters was considered as the most likely explanation for this result. These results highlight that multiple factors contribute to mercury accumulation in elasmobranchs.

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.

Acquiring and storing energy is vital to sharks of all age-classes. Viviparous shark embryos receive endogenous maternal energy reserves to sustain the first weeks after birth. Then, in order to maintain body condition, sharks must start foraging. Our goal was to understand whether maternal energy investments vary between blacktip reef sharks (Carcharhinus melanopterus) from two populations and to what extent body condition and the initiation of foraging might be affected by presumably variable maternal investments. A total of 546 young sharks were captured at St. Joseph atoll (Seychelles) and Moorea (French Polynesia) between 2014 and 2018, and indices of body condition and percentage of stomachs containing prey were measured. Maternal investment was found to be site-specific, with significantly larger, heavier, and better conditioned individuals in Moorea. Despite these advantages, as time progressed, Moorea sharks exhibited significant decreases in body condition and were slower to initiate foraging. We suggest that the young sharks’ foraging success is independent of the quality of maternal energy resources, and that other factors, such as prey availability, prey quality, and/or anthropogenic stressors are likely responsible for the observed differences across sites. Insights into intraspecific variations in early life-stages may further support site-specific management strategies for young sharks from nearshore habitats.

International audience; In the French West Indies (Caribbean), the insecticide Chlordecone (CLD) has been extensively used to reducebanana weevil (Cosmopolites sordidus) infestations in banana plantations. Previous studies have shown high CLDconcentrations in freshwater and coastal communities of the region. CLD concentrations, however, have not yetbeen assessed in marine top predators. We investigated CLD concentrations in cetacean blubber tissues fromGuadeloupe, including Physeter macrocephalus, Lagenodelphis hosei, Stenella attenuata and Pseudorca crassidens.Chlordecone was detected in all blubber samples analysed, with the exception of four P. macrocephalus.Concentrations (range: 1 to 329 ng·g−1 of lipid weight) were, however, lower than those found in species fromfresh and brackish water. Ecological factors (open ocean habitat), CLD kinetics, and cetacean metabolism (highor specific enzymatic activity) might explain low concentrations found in cetacean blubber. Future analyses thatinclude internal organ sampling would help to confirm CLD levels observed in this study.

International audience; During a mark–recapture survey from November 2014 until April 2017, 333 neonatal and juve-nile blacktip reef sharks Carcharhinus melanopterus and 302 neonatal and juvenile sicklefinlemon sharks Negaprion acutidens were tagged and measured at the uninhabited and isolatedSt. Joseph Atoll (Republic of Seychelles). Both species demonstrated seasonal reproductive syn-chronicity and relatively large sizes at birth. Despite the extended times at liberty > 2.5 years,the majority of recaptures were found in close proximity to the initial tagging location(< 500 m). Annual growth rates of C. melanopterus (n = 24) and N. acutidens (n = 62) rangedfrom 6.6 to 31.7 cm year −1 (mean ± SE; 16.2 ± 1.2 cm year −1 ) and 0.2 to 32.2 cm year −1(11.8 ± 1 cm year −1 ), respectively and are to date the most variable ever recorded in wild juve-nile sharks. High abundances of both species coupled with long-term and repeated recapturesare indicative of a habitat where juveniles can reside for their first years of life. However, largevariability in annual growth rates in both species may suggest high intra and interspecific com-petition induced by a possibly resource limited, isolated habitat.

International audience; A novel image analysis-based technique applied to unmanned aerial vehicle (UAV) survey data isdescribed to detect and locate individual free-ranging sharks within aggregations. The methodallows rapid collection of data and quantification of fine-scale swimming and collective patternsof sharks. We demonstrate the usefulness of this technique in a small-scale case study exploringthe shoaling tendencies of blacktip reef sharks Carcharhinus melanopterus in a large lagoonwithin Moorea, French Polynesia. Using our approach, we found that C. melanopterus displayedincreased alignment with shoal companions when distributed over a sandflat where they areregularly fed for ecotourism purposes as compared with when they shoaled in a deeper adjacentchannel. Our case study highlights the potential of a relatively low-cost method that combinesUAV survey data and image analysis to detect differences in shoaling patterns of free-rangingsharks in shallow habitats. This approach offers an alternative to current techniques commonlyused in controlled settings that require time-consuming post-processing effort.

International audience; Unmanned aerial vehicles (UAVs) are being increasingly used in studies of marinefauna. Here, we tested the use of a UAV (DJI Phantom II®) to assess fine-scale variation in densitiesof 2 elasmobranchs (blacktip reef sharks Carcharhinus melanopterus and pink whiprays Himanturafai) on reef systems off Moorea (French Polynesia). We flew parallel transects designed to samplereef habitats (fringing, channel and sandflat habitats) across 2 survey blocks. Block 1 included ashark and ray provisioning site with potentially higher elasmobranch densities, whereas Block 2most likely had lower densities with no provisioning activities. Across 10 survey days in July 2014,we flew 3 transects (400 m) within each survey block (n = 60 total transect passes). As expected,densities (animals ha−1) were significantly higher in Block 1 than in Block 2, particularly where provisioningactivities occur. Differences between habitats surveyed were also found. Our study providesthe first direct estimates of shark and ray densities in coral-reef ecosystems and demonstratesthat UAVs can produce important fishery-independent data for elasmobranchs, particularly inshallow-water habitats.

International audience; Short-beaked common dolphins (Delphinus delphis) and Atlantic spotted dolphins (Stenella frontalis) are the two most abundant cetacean species in the oceanic waters of Madeira and the Azores. They are of similar size, occur in similar habitats and are regularly observed in mixed-species groups to forage together. Genetic analyses suggested that, within each species, dolphins ranging around both archipelagos belong to the same panmictic population. We tested the hypotheses that (1) within each species, individuals from the two archipelagos belong to a single ecological stock; (2) between species, common and spotted dolphins have distinct trophic niches; using fatty acid (FA) and stable isotope (SI) analyses. Fatty acids and stable isotopes were analysed from 86 blubber and 150 skin samples of free-ranging dolphins, respectively. Sex-related differences were not significant, except for common dolphin FA profiles. In S. frontalis, FA and SI differences between archipelagos suggested that individuals belonged to different ecological stocks, despite the existence of gene flow between the two archipelagos. In D. delphis, differences were more pronounced, but it was not possible to distinguish between stock structure and a seasonal effect, due to differential sampling periods in the Azores and Madeira. Inter-specific comparisons were restricted to the Azores where all samples were collected during summer. Differences in FA proportions, noticeably for FA of dietary origin, as well as in nitrogen SI profiles, confirmed that both species feed on distinct resources. This study emphasizes the need for an integrated approach including both genetic and biochemical analyses for stock assessment, especially in wide-ranging marine top predators.

International audience; Delphinid populations show highly variable patterns of genetic diversity and population structure. Previous studies indicate that habitat discontinuities and geographic isolation are major drivers of population division in cetaceans. Spinner dolphins (Stenella longirostris) are distributed in all tropical oceans, but they are particularly common around islands and atolls. This species occurs in shallow waters at daytime to rest and socialise, and feeds on offshore mesopelagic prey overnight. Here we investigated the genetic population structure of spinner dolphins in the Southwest (SW) Indian Ocean along a west-east geographical gradient, from eastern Africa to the Mascarene archipelago. We combined analyses of 12 microsatellite loci, mtDNA control region sequences, and sighting data to assess genetic differentiation and characterise habitat preferences of these populations. Significant genetic structure among the three sampled sites (Zanzibar, Mayotte and La Réunion) was observed using both types of molecular markers. Overall, our results indicate that geographic isolation, and potentially other factors such as shallow water habitats to rest and socialise, may be important drivers of the genetic population structure of insular spinner dolphins in this region.