Isotopic Niche Analysis and Trophic Ecology of Sea Turtles Foraging in the Coastal Waters of Nigeria

Sea turtles occupy diverse trophic roles across their life stages, and stable isotope analysis provides an integrative measure of long-term dietary ecology that complements direct gut content analysis. This study applied carbon and nitrogen stable isotope analysis to characterise the trophic ecology and isotopic niche of olive ridley (Lepidochelys olivacea) and leatherback (Dermochelys coriacea) turtles foraging in Nigerian coastal waters. Blood plasma, red blood cells, and skin biopsy tissue were collected from 74 live-captured individuals at foraging grounds off Lagos and Ondo States using laparotomy-free biopsy sampling. Delta carbon-13 and delta nitrogen-15 values were determined using isotope ratio mass spectrometry. Bayesian isotopic niche analysis using SIBER was conducted to compare niche breadth between species and life stages. Isotopic mixing model MixSIAR was used to estimate proportional dietary contributions of potential prey items. Leatherback turtles showed more depleted delta carbon-13 values (mean negative 16.8 per mil) consistent with oceanic prey sources, while olive ridley values (mean negative 14.1 per mil) indicated nearshore neritic foraging. Delta nitrogen-15 indicated that leatherbacks fed at a significantly higher trophic level. Isotopic niche overlap was low (12%), suggesting resource partitioning between species. Life-stage comparisons revealed ontogenetic dietary shifts in olive ridley with subadults showing broader niche breadths than adults. Spatial isotopic mapping identified the Bight of Benin shelf as a key olive ridley foraging area. The study provides the first stable isotope dietary characterisation of sea turtles in Nigerian waters, informing bycatch risk area mapping. Keywords: stable isotopes, sea turtle trophic ecology, isotopic niche, SIBER, Nigeria.