The role of fishing
The Northeast Atlantic region was found to have the highest fisheries exploitation rates, whereas Aleutian Islands and Barents Sea had the lowest rates (Figure S2.7). This finding is consistent with previous work on the footprint of bottom trawling, where around 2% of the total area was trawled in the Aleutian Islands and 45% in the North Sea (Amoroso et al. 2018). The exploitation rates in the North Sea showed the most pronounced temporal decline in catch per biomass (i.e., from 0.4 in the 1980s to 0.2 year-1 in recent years), supporting previous studies documenting a strong reduction of fishing pressure on the demersal community (Couce et al. 2020). All North American regions had exploitation rates <0.06 year-1. These lower rates, compared with the Northeast Atlantic region, may be due to a different fisheries management strategy (Battista et al. 2018) and/or because fewer demersal species are commercially exploited.
We found a strong negative relationship between demersal community biomass and a log10-transformed fishing exploitation rate. This implies that small increases at low exploitation rate (based on the untransformed data) may cause large declines in demersal community biomass. We expect that this non-linear effect is caused by the decline of large and long-lived individuals that have accumulated biomass over their lifetime. Additionally, fishing had a weak but negative relationship with the mean trophic level of the community, which in turn had a negative relationship with biomass (see Figure S2.2 for hypothesized mechanisms). This indirect effect of fishing channeled through mean trophic level is thus positive on demersal community biomass but is weaker than the direct negative effect of fishing. The sensitivity of the demersal fish community to fishing highlights that reducing fishing mortality is an effective way of reducing the impacts of climate change on the fish community, sensu Brander (2007). Our results further stress the need for future exploitation rate scenarios in line with the Shared Socioeconomic Pathways for making climate change projections of fish biomass (Hamon et al.2021).