Evaluating the importance of mesopelagic prey to three top teleost predators in the northwest Atlantic Ocean

Abstract The ocean’s twilight zone is a vast area of the global ocean that lies between the sunlit surface waters and perpetually dark midnight zones, covering depths from ∼200 to 1000 m. Recent work in the twilight (or mesopelagic) zone has revealed unexpected biomass and diversity that may not only challenge scientific understanding of marine systems but also provide a new and largely untapped resource for fisheries harvest. A key knowledge gap in our understanding of the mesopelagic is how its food webs support foraging activity by commercially valuable, highly migratory top predators. Here, we use compound-specific stable isotope analyses to trace the flow of carbon through pelagic ecosystems in the northwest Atlantic to three predators: bigeye tuna (Thunnus obesus), swordfish (Xiphias gladius), and yellowfin tuna (Thunnus albacares). Temperate mesopelagic-associated carbon was estimated as both a direct and an indirect source of predator carbon, alongside temperate epipelagic and mixed epi-mesopelagic tropical carbon, via Bayesian mixing models. The contribution of temperate mesopelagic carbon to individual predators ranged from 5% to 94%, with means of 62%, 46%, and 28% for bigeye tuna, yellowfin tuna, and swordfish, respectively. We also found that carbon sources of predators shifted seasonally as they moved between temperate and tropical waters by contrasting tissues (liver, muscle) and season of sampling (summer, fall). These results inform our understanding of the adaptive value of deep diving behaviors in large marine predators and provide key estimates of food web linkages to inform multi-species fisheries management of both mesopelagic prey and migratory predators.