Resources partitioning by seabirds and their relationship with other consumers at and around a small tropical archipelago

Abstract Seabirds occupy a range of trophic levels in marine foodwebs, and as top predators, they potentially may compete with large fish for prey. Here, we examine trophic segregation among seabird species breeding on São Pedro and São Paulo Archipelago (SPSPA), in the tropical Atlantic Ocean, as well as the trophic relationships between the seabirds and pelagic fish. We used stable isotopes (δ15N, δ13C) combined with a conventional dietary analysis of brown booby (Sula leucogaster), brown noddy (Anous stolidus), and black noddy (Anous minutus) and also analysed stable isotopes in the muscles of tropical two-wing flying fish (Exocoetus volitans), blackfin tuna (Thunnus atlanticus), and common dolphinfish (Coryphaena hippurus). Brown boobies shared a similar trophic level with large predatory fish probably due to their intense consumption of flying fish, but seabirds are most likely not competing with blackfin tuna and common dolphinfish, despite relying on the same prey. Food resources seem to be abundant around SPSPA, minimizing competition. Despite relying mostly on two-wing flying fish, the three seabirds studied had non-overlapping trophic niches during the breeding period, with segregation apparently occurring by prey size. Sex-related trophic differences were not found, but age-related trophic segregation was observed in brown boobies and brown noddies. Overall, although seabirds and marine pelagic fish rely on the same prey species, the overlap in their trophic niches was limited, most likely due to an overabundant food resource, i.e. flying fish, a common prey of large predators in most pelagic tropical oceans.