Hidden in Plain Sight: Decades of Industrial-Scale Fishing in the Ocean's Twilight Zone

There is a common misconception among ocean scientists and policy makers that mesopelagic (200-1000 m) food webs are an unexploited "final frontier" of living marine resources. It is true that there are currently no substantial fisheries for the small-bodied fish species that are typically caught in the nets used in research studies. However, midwater ecosystems contain a rich biodiversity of larger fishes, including mesopredators, that largely evade scientific sampling and are absent from biogeochemical and socio-economic models of the open ocean. Drawing on studies from around the world and decades of detailed catch data from a longline fishery, we demonstrate that this "dark web" of mid-trophic fish species has already been exploited by industrial-scale fisheries in plain sight for decades. The ongoing effects of these activities on a suite of critical ecosystem services, from regulating the oceanic carbon cycle to sustaining food webs, remain unquantified and largely ignored by ocean scientists and policy makers. From a policy perspective, we recommend leveraging the Biodiversity Beyond National Jurisdiction agreement and promoting cooperation among Regional Fisheries Management Organizations to standardize data collection and strengthen environmental impact assessments under the United Nations Convention on the Law of the Sea, which could facilitate sustainable use of mesopelagic resources while balancing food security and carbon storage in a changing climate.