Long-term trophic ecology of juvenile Atlantic bluefin tuna in the western Mediterranean Sea

Abstract

Global marine ecosystems confront increasing anthropogenic pressures, such as overfishing, pollution, habitat degradation, and climate change, leading to declining fish populations. Atlantic bluefin tuna (ABFT; Thunnus thynnus), an ecologically important apex predator, has historically suffered from overexploitation. Conservation efforts have been implemented to mitigate its decline, leading to a recovery of its stock in recent years. In the western Mediterranean Sea, concerns have emerged among purse seine fishers regarding the potential impact of the resurgence of ABFT on sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus) populations, crucial prey for ABFT and the fishing industry, and currently overexploited or with low population biomass levels. This study investigates the trophic ecology of juvenile ABFT in the Gulf of Valencia, western Mediterranean Sea, across three periods: 1989, 2012–2014, and 2018–2019. Through a combination of stomach content analysis and stable isotope analysis, which complement each other, we aim at achieving a comprehensive understanding of the trophic ecology. ABFT's dietary preferences are revealed as flexible and opportunistic, encompassing a diversity of prey species, including fish, cephalopods, and crustaceans. Variations in the composition of the diet of juvenile ABFT can be attributed to fluctuations in the availability of prey, mainly due to a synergistic effect of overfishing and climate change. Likely in response to declining populations of sardine and anchovy, juvenile ABFT have changed their dietary habits, decreasing their consumption of these species and increasing their ingestion of horse mackerels (Trachurus spp.), another pelagic predatory fish present in the western Mediterranean Sea. Our data suggest that concerns of fishers regarding the consumption of sardine and anchovy by the increasing ABFT population are not supported, at least for juvenile individuals. Understanding the trophic relationships and feeding dynamics of ABFT has wider implications, and can provide insights into ecosystem resilience, predator-prey interactions, and adaptative responses to changing environments.