Foraging patterns and trophic interactions of sympatric populations of young-of-the-year bluefin and skipjack tunas in the East Atlantic (Gulf of Cadiz)

Abstract

Foraging patterns and trophic interactions of young-of-the-year Atlantic bluefin (Thunnus thynnus, ABFT) and skipjack (Katsuwonus pelamis, SKJ) tunas co-occurring in coastal waters off the Gulf of Cadiz (GoC, East Atlantic Ocean) were investigated using stomach content and stable isotope analyses. Both species fed primarily on epipelagic fishes such as European anchovy (Engraulis encrasicholus), sardine (Sardina pilchardus) and Atlantic chub mackerel (Scomber colias). Significant relationships were found between the fork length (SFL) of the tunas and their body condition (Fulton’s index, K); this suggests that the available food sources covered their nutritional requirements and therefore the GoC represents a suitable nursery habitat for young ABFT and SKJ. In SKJ, a positive correlation was found between δ15N values of muscle and liver and SFL. Differences in liver δ15N values suggest that ABFT were recently foraging at higher trophic levels than SKJ. Isotopic niche areas estimated by standard ellipse areas (SEAC) and kernel utilization density (KUD) analyses showed a broader niche for SKJ compared to ABFT, indicating a more diverse diet in the former species. There was no significant niche overlap between both tuna species, suggesting trophic niche partitioning and reduced competition for the resources.