Disentangling the effects of fishing and temperature to explain increasing fish species richness in the North Sea

Climate change and fishing have impacted marine species richness ( R ) at global and local scales. It has previously been shown that R of the fish community in the North Sea has increased since the early 1980’s. Over the same period, ocean temperature has increased, and fishing mortality has decreased in the North Sea. Because these are confounded over time, either trend could plausibly explain the increase in R . Therefore, a logic-based approach was used to disentangle the effects of temperature and fishing on R , using spatio-temporal models fitted to survey data. To investigate the effect of temperature on R , fish species were subset by thermal affinity, as either Lusitanian (warm) or Boreal (cold) species. To investigate the effect of fishing mortality on R , species were subset by management category as either quota (assumed to be targeted) or non-quota species. Trends in these subsets were plotted separately to investigate which subsets of the fish community have contributed to the overall R increase. Over three decades, fish R increased by an average of 2.5 species per haul. These increases were predominantly of Lusitanian non-quota species (1.9). A small increase was observed in quota species (0.6); however, this increase was driven by quota-Lusitanian species (0.4). Our results suggest that temperature rather than fishing mortality was the driver of R increase in the North Sea since 1991 and highlight the importance of long-term monitoring in detecting ecological responses to climate change at the community level.