Comparing demersal fish assemblages between periods of contrasting climate and fishing pressure

Fish communities are dynamic and their structure is known to change over time. Traditionally, these changes were considered to be fisheries-induced, but recent analyses also suggest that global warming could affect the distribution, abundance, and assemblage composition of marine fish. However, disentangling the effects of fisheries and those resulting from climate change is difficult, because both potential drivers act simultaneously. In our study, we distinguished between the effects of fisheries and climate change on the fish assemblage of the southern North Sea by comparing survey catch data for that region during four unique periods throughout the past century, characterized by (i) low fishing pressure during a cold period (1902-1908), (ii) low fishing pressure during a warm period (1950-1956), (iii) high fishing pressure during a cold period (1978-1984), and (iv) high fishing pressure during a warm period (2002-2008). Our analysis indicates that the demersal fish community in the southern North Sea has changed in response to changes in both climate and fishing pressure. Our results suggest both a relatively higher richness of Lusitanian (warm-favouring) species compared with boreal (cool-favouring) species, and a lower mean body size of the fish community during times of warming, independent of fishing pressure.