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).