Spatiotemporal trends in abundance and richness of elasmobranchs in Frégate Island, Seychelles

Abstract Elasmobranch diversity has been poorly investigated within the Seychelles archipelago. The present study aimed to describe the spatial-temporal changes in the species richness and abundance of elasmobranchs on Frégate Island, Seychelles. Baited Remote Underwater Video Systems (BRUVS) were performed between May 2019 and December 2022. A total of 18 elasmobranch species were observed, comprising nine sharks and nine rays. Most species face extinction risks according to IUCN criteria; 50% are categorized as Vulnerable, 28% as Endangered, and 11% as Critically Endangered. Higher relative abundances were detected for the grey reef shark ( Carcharhinus amblyrhynchos ), followed by the nurse shark ( Nebrius ferrugineus ), short-horned pygmy devil ray ( Mobula kuhlii ), whitetip reef shark ( Triaenodon obesus ), and the critically endangered white-spotted guitarfish ( Rhyncobatus australiae ). While month and year did not affect elasmobranch relative abundance (MaxN), elevated temperatures (~ 30 ºC) significantly reduced elasmobranch MaxN. Moreover, habitat substrate was the best predictor of elasmobranch MaxN. Sharks were often encountered on rock and rubble, while batoids were more likely found on rock and sand. The presence of juvenile tiger sharks was consistent in all sampled years, but with low MaxN values. Our findings highlight the significant conservation concerns for elasmobranch species in the region and emphasize the urgent need for protective measures within the Seychelles Archipelago.