ABSTRACT Many large marine predators forage on mesopelagic fish stocks, including commercially valuable tunas. The mesopelagic is under increasing interest for commercial exploitation, given its large biomass with potential to supply fishmeal for aquaculture feed or fish oil. However, the implications of large‐scale mesopelagic fisheries on tunas and other predators may be substantial. Here, we adapt a bioeconomic model with a new predator–prey dynamic to evaluate the effects of potential commercial‐scale mesopelagic fisheries on bigeye tuna ( Thunnus obesus ). Specifically, we use a novel diet share model term and the latest stable isotope food web data to operationalize predator–prey interactions. Model results highlight the importance of accounting for the predator–prey interactions in management of mesopelagic fisheries and demonstrate the sensitivity of equilibrium economic and ecological conditions for the tuna stock under different price and cost scenarios. Overall, this work suggests that a new mesopelagic fishery could be economically viable in and of itself, but may have significant negative impacts on existing tuna fisheries by reducing forage availability.