The ocean’s mesopelagic zone, extending from ~200 to 1000 m depth, harbors significant biomass and biodiversity that is increasingly recognized as a potential resource for fisheries. This zone plays a crucial role in supporting the foraging activity of commercially valuable, highly migratory predators such as bigeye tuna (Thunnus obesus), yellowfin tuna (Thunnus albacares), and swordfish (Xiphias gladius). Using compound-specific stable isotope analyses, we traced the flow of carbon through the pelagic ecosystem of the northwest Atlantic to estimate the contribution of mesopelagic carbon to predator diets. Results reveal that temperate mesopelagic food webs contributed approx. 50-60% of mean predator dietary carbon.
Simultaneously, the increasing interest in mesopelagic fisheries - driven by its large biomass, which could be used for aquaculture feed or fish oil - raises concerns regarding their impacts on top predators. We adapted a bioeconomic model, incorporating predator-prey dynamics informed by the stable isotope data, to quantify trade-offs between fisheries on mesopelagic and bigeye tuna stocks. Model results demonstrate the sensitivity of equilibrium economic and ecological conditions for the tuna stock under different price and cost scenarios, emphasizing the need for careful management. These findings underscore the importance of understanding mesopelagic food web linkages and predator-prey interactions for informed, multi-species fisheries management in a rapidly changing ocean.