Feeding of Atlantic bluefin tuna (Thunnus thynnus) around the Canary Islands assessed from stomach content and stable isotope analyses

Abstract

The Canary archipelago, which is part of the Atlantic biogeographical region of the Macaronesia, provides an appropriate habitat for tropical and temperate water large pelagics. Atlantic bluefin tuna (ABFT, Thunnus thynnus) occur all year round in the Canary Islands. Life history traits of ABFT, specifically foraging patterns, have not been studied thus far in this region. We investigated the ABFT trophic biology, combining stomach content and stable isotope analyses. A high proportion (~77%) of the stomachs contained prey, denoting active foraging. The diet primarily consisted of fishes, among which the snipefish, Macroramphosus sp., was the major prey. Cephalopods and crustaceans were less important diet components. Besides natural prey, a well-preserved specimen of Cory's shearwater, Calonectris borealis, was found in one stomach sampled in 2018. Moreover, 16.7% of the stomachs contained plastic debris. Inter-annual isotopic differences were detected in liver tissue samples (PERMANOVA, p < 0.05), reflecting dietary shift in 2018, where δ15N values were lower than in 2016 and 2017. Isotopic niche width estimations from stable isotope Bayesian ellipse in R (SIBER) and kernel utilization density (KUD) suggest a more diverse diet in 2017. Standard ellipse corrected area (SEAC) and Bayesian standard ellipse area (SEAB) values from muscle and liver data indicate that the diet of ABFT in the Canary Islands is more euryphagous than in the Strait of Gibraltar (East Atlantic) but more stenophagous than it is in the Gulf of Saint Lawrence (West Atlantic). The present results indicate that the Canary archipelago represents a forging ground for ABFT in spring.