Conserving a Sea of Shadow and Substance: Should there be a Moratorium on the Harvest of Twilight Zone Fish?

The ocean’s mesopelagic zone or “twilight zone” (200-1000m) has been understudied relative to other areas of the ocean, such as the surface waters, but mesopelagic fish are now thought to be highly abundant. Through diel vertical migrations (DVM) to consume prey in surface waters at night, these fish may contribute to ocean carbon sequestration, a valuable regulating ecosystem service. Apex predators from the surface waters are known to consume mesopelagic fish, establishing an important ecological connection. As overharvesting continues to deplete surface fisheries, especially on the high seas, some fishing interests have begun exploring the potential harvest of mesopelagic fish to supply fishmeal and fish oil markets. Off the US West Coast, where the mesopelagic extends into the exclusive economic zone (EEZ), the regional Pacific Fishery Management Council established in 2016 a moratorium on the harvest of certain mesopelagic fish families. This study adapted a bioeconomic decision model originally designed for the Pacific sardine fishery to examine the tradeoffs between the values gained from a hypothetical mesopelagic fishery with the potential values lost from declines in predators of mesopelagic fish facing a reduced prey source. Biological parameters from ecological relationships were obtained from a recent Ecopath model of the California Current system. Economic values comprised the net price of mesopelagic fish (in a fishmeal end use), the net price of commercially harvested predators, and estimates of the nonmarket demand for non-commercial predators. From an economic perspective, when considering only the potential lost values associated with commercial predators, a moratorium on mesopelagic fish would not be justified. When the lost values of noncommercial predators were also considered, a moratorium would be justified. The economic rationale for a moratorium is sensitive to the scale of the non-market values attributed to noncommercial predators as well as other parameters describing ecological relationships.