Impacts of deep-water spills on mesopelagic communities and implications for the wider pelagic food web

Mesopelagic fishes link lower trophic levels and higher predators, as well as production at the surface to the deep sea. Mesopelagic fish may be vulnerable to deep entrained oil plumes of the type seen in the 2010 Deepwater Horizon spill; even at low concentrations, exposure to polycyclic aromatic hydrocarbons (PAHs) may cause lethal and sublethal effects, such as endocrine disruption, growth inhibition, and genetic damage. A reduction in the abundance of mesopelagic fish could potentially shift predation pressure by large pelagics to epipelagic forage or other species, with potential fisheries consequences. We explored this hypothesis on the West Florida Shelf using an Atlantis model for the Gulf of Mexico. Atlantis is a 3-dimensional, spatially explicit marine and coastal modeling framework that incorporates multiple submodels integrating biophysical, chemical, ecological, and fisheries dynamics. We found that biomass, trophic niche width, and predation mortality exerted by mesopelagic predators showed small but varied responses across different contributions of mesopelagics to predator fish diet under oil impacts. We observed shifts in the diet of pelagic predators with increasing availability of mesopelagic fish prey, suggesting that pelagic fish predator populations are more vulnerable to oil exposure if they are tightly coupled to the mesopelagic food web. These results suggest that when measured at the population level, oil-spill impacts may be harder to detect due to the response of individual fish species.