Trophic niche segregation between the sympatric tunas Thunnus alalunga and Katsuwonus pelamis in the Gulf of Cadiz (East Atlantic)

Abstract

This study presents information about trophic relationships between juvenile albacore (ALB, Thunnus alalunga) and skipjack tuna (SKJ, Katsuwonus pelamis) in the Gulf of Cadiz (East Atlantic Ocean). Data on stomach contents (SCA) and stable isotope ratios of carbon and nitrogen (SIA) were analyzed to ascertain foraging patterns and trophic niche breadths and overlapping. SCA showed that European anchovy (Engraulis encrasicolus) and Atlantic chub mackerel (Scomber colias) were the most important prey species in both tuna species. A PERMANOVA analysis of the SCA data revealed interspecific differences in the diet (p < 0.05). δ15N and δ13C values from ALB muscle and liver tissues were higher than those from SKJ samples. As with SCA, the PERMANOVA analysis of isotopic data indicated significant interspecific differences in both muscle and liver samples (p < 0.01). Isotopic trophic niche values estimated by Bayesian standard ellipse corrected areas (SEAC) showed low (muscle) or no (liver) overlap between ALB and SKJ. Taken together, the present results suggest that, although juveniles of the two species fed for an extended period of time in the same area and shared some prey species, there is evidence of food resource partitioning, thus preventing trophic competition.