Stable isotope ratios indicate trophic niche overlap in three sympatric delphinid species in the Eastern Ionian Sea

Abstract Studying the sympatric ecology of closely related marine top predator species is of interest both from an ecological and conservation standpoint, because it provides insights into ecosystem functioning, how such species avoid competitive exclusion, and how human stressors may impact these patterns. In this study, we examine the isotopic niche of three sympatric dolphin species that inhabit the eastern Ionian Sea: the common dolphin ( Delphinus delphis ), the striped dolphin ( Stenella coeruleoalba ), and the common bottlenose dolphin ( Tursiops truncatus ). We used carbon, nitrogen, and sulfur isotope ratios to construct three‐dimensional isotope niches and calculate niche sizes and their overlaps among the three species to study interspecific competition for resources. Common dolphin niche overlapped with that of the striped dolphin by 77%, suggesting potential resource competition between these two species. The niche overlap between striped/common dolphins and bottlenose dolphins was much lower (47%). Large differences in isotopic ratios were found between the bottlenose dolphins of the Ionian Sea and those of a largely isolated population in the adjacent semienclosed Gulf of Ambracia, further corroborating the previously documented isolation between the two populations.