Prey preferences and trophic relationships of sympatric Atlantic spotted (Stenella frontalis) and common bottlenose dolphins (Tursiops truncatus) in the northeastern Gulf of Mexico

Investigating resource use and partitioning by sympatric marine predators is critical to elucidating their ecological role within communities and understanding their biological interactions with fisheries. Here we used stable carbon and nitrogen isotope analysis in skin tissue samples collected using remote biopsy sampling to assess the trophic interactions of the two most abundant small cetaceans in the northeastern Gulf of Mexico (GoMex): common bottlenose ( Tursiops truncatus ) and Atlantic spotted dolphins ( Stenella frontalis ). Results indicated that the isotopic niche of the two species overlaps significantly (97.6 %), but spatial variation of isotopic values was significant, with individuals sampled closer to shore exhibiting higher δ 13 C values. Bayesian mass-balance mixing models indicated that demersal piscivorous fish species had the highest relative contribution to the diet of both Atlantic spotted dolphins (50.8 ± 6.0 %) and bottlenose dolphins (41.5 ± 5.8 %). Lower trophic level demersal fish contributed the second most to the diet of both species ( S. frontalis = 30.8 ± 10.1 %; T. truncatus = 37.6 ± 13.0 %). Despite the lack of other complementary diet data (e.g., stomach contents) that limits the interpretation of isotope data, this work improves our understanding of the trophic interactions of small cetaceans in the northeastern GoMex and their potential biological interaction with fisheries. • Stable isotope analysis key to diet research on dolphins from the continental shelf. • Atlantic spotted and bottlenose dolphins overlap highly in dietary niche. • Demersal fish contribute the most to the diet of dolphins. • Potential prey may overlap with fisheries on the shelf of the Gulf of Mexico.