Future change in global fisheries biomass and harvest due to changes in temperature and primary production: A study with a simple global model

The overharvesting of living marine resources and the global-scale environmental alterations due to climate change are expected to have significant negative impacts on fish biomass over the 21st century. We use the BiOeconomic mArine Trophic Size-spectrum (BOATS) model, together with future scenarios from global climate models to examine the interactive effects of ocean warming and net primary production changes on global fish biomass, with and without economic harvest. In an unharvested world, we find that a reduction of net primary production and a rise in temperature have important negative impacts, reducing biomass by approximately 30\% under the RCP8.5 scenario by year 2100. In BOATS, the biomass decrease is dominated by a large increase of fish mortality due to the increase in temperature, which shrinks the standing stock of biomass despite more rapid growth rates. However, under harvest, climate change plays a less important role. With harvest, net primary production becomes less important in limiting fish growth since there is more food available per fish. Moreover, harvest replaces natural fish mortality and limits the ability of temperature to affect mortality rates, thereby mitigating the effects of warming. The negative impact of climate warming is entirely eliminated in a future scenario in which rapid increases in fishing technology and weak management lead to a catastrophic decrease of fish biomass. It is important to point out that BOATS does not attempt to resolve the effects of ocean acidification or deoxygenation, or the impacts that harvesting and climate change can have on ecosystem structure and habitat quality, all of which may exacerbate future fish biomass decreases. Nonetheless, our results suggest that the future of global fisheries will be mostly determined by fisheries management strategies, with the effects of climate change on net primary production and metabolic rates playing a secondary role.