Small Commercial Fish Biomass Limits the Catch Potential in the High Seas

Abstract

The High Seas, lying beyond the boundaries of nations' Exclusive Economic Zones, cover most of the ocean surface and host half of marine primary production. Yet, a tiny fraction of global wild fish catch comes from the High Seas, despite intensifying industrial fishing efforts. The paradoxically small fish catch could reflect economic barriers to exploiting the High Seas - such as the difficulty and cost of fishing in remote ocean parts - or ecological features resulting in a small biomass of commercial fish (10g–100 kg) relative to primary production. We use the coupled biological-economic model BOATS to estimate contributing factors, comparing observed catches with simulations where: (a) fishing cost depends on distance from shore and seafloor depth; (b) catchability depends on seafloor depth or vertical habitat extent; (c) regions with micronutrient limitation have reduced biomass production; (d) the trophic transfer of energy from primary production to demersal food webs depends on water depth; and (e) High Seas biomass migrates to coastal regions. The dominant factor is ecological: commercial fish communities receive much primary production in shallow waters but less in deep waters, limiting exploitable biomass in High Seas. Other factors play a secondary role, with migrations having a potentially large but uncertain role, while economic factors have smaller effects. We estimate the High Seas hosted 25$\%$ of a global 2.8 $\pm$ 0.7 Gt biomass in the early 20th century, changing to 47% of a global 1.5 $\pm$ 0.4 Gt of commercial fish biomass during the early 21st century. Our results stress the limited potential of High Seas to provide food through wild capture fisheries.