Using stable isotope analysis to assess trophic relationships between Atlantic bluefin tuna (Thunnus thynnus) and striped dolphin (Stenella coeruleoalba) in the Strait of Gibraltar

Abstract Stable isotope analysis ( δ 13 C and δ 15 N from liver and muscle) was used to assess trophic relationships between Atlantic bluefin tuna (ABFT) ( Thunnus thynnus ) and striped dolphin (SC) ( Stenella coeruleoalba ) in the Strait of Gibraltar (SoG). δ 15 N values from ABFT muscle and liver tissues were significantly different from those of dolphin samples, but no for δ 13 C values. Diet estimation by MixSIAR models from muscle and liver revealed that ABFT fed mainly on squids ( Todaropsis eblanae and Illex coindetii ). The shrimp Pasiphaea sp. was estimated to be the most important prey-species in the diet of SC. Trophic positions estimated from muscle and liver isotopic data suggested that ABFT occupy a higher trophic level than SC. Estimations of isotopic niche, as measured by the standard ellipse area, indicated that ABFT show a broader trophic niche than SC; furthermore, SEAc did not show trophic overlap between both predators. The results of this study suggest that resource partitioning occurs between ABFT and SC in the SoG ecosystem.