Long-term series demonstrate small-scale differences in trends within fish assemblages explained by climate variability

Over 30 years (1985-2018) of environmental monitoring data in the Belgian part of the North Sea allowed us to study common and individual species trends within three spatially distinct demersal and benthopelagic fish assemblages in relation to climate-related variability. A combination of multivariate (dynamic factor analysis) and univariate modelling techniques (Trendspotter and Spearman rank correlations) showed that the observed trends in fish densities in two coastal (mud and fine sand) assemblages were best explained by sea surface temperature (SST)-related variables and the Atlantic multidecadal oscillation winter index (AMO(winter)). Higher fish densities seem to be linked with extreme cold winters in the year before (e.g. 1985, 1996, 2010), while warmer years may be linked to lower fish densities (fine sand assemblage) a few years later. Trends for the more offshore coarse sand fish assemblage were rather related to the North-Atlantic Oscillation index (NAO) (next to AMO(winter)), coupled to the circulation and inflow of warmer saline waters through the English Channel. The temporal decrease in densities in this assemblage may partially reflect climate and subsequent biological regime shifts, as described for the broader North Sea region. At the species level, both positive, negative and varying trends in densities were noted, dependent on the fish assemblage. Overall, densities of most Lusitanean (warm water) species increased over the last decade, positively correlated with most SST-related variables and AMO(winter)-values, indicating a northward expansion of their habitat range, which corroborates the findings of other studies. On the contrary, several Boreal (cold water) species were negatively correlated with SST and AMO(winter), with more or less stable trends in density between 1985 and 2000 followed by a decrease in the last decade, potentially explained by a relocation to deeper waters to cope with increasing seawater temperatures. For plaice Pleuronectes platessa, a commercially important species, a linear and significant decrease in length was observed over the 34-year period, most probably related to a combination of climatic influences and fishing induced selection. The current study shows that environmental, climate related drivers are important to explain part of the biological variability and should be taken into account when investigating the direct impact of human activities on the marine, demersal and benthopelagic ecosystem.

Automated summary

The study of environmental monitoring data in the Belgian part of the North Sea over the past 30 years has revealed that climate-related factors have an impact on fish densities and species trends. Sea surface temperature and the Atlantic multidecadal oscillation winter index were found to be related to fish densities in coastal assemblages, while the North-Atlantic Oscillation index was related to fish densities in an offshore assemblage. The densities of warm water species from the Lusitanean region increased, positively correlated with sea surface temperature and the winter index, indicating a northward expansion of their habitat range. In contrast, cold water species from the Boreal region showed a negative correlation with sea surface temperature and the winter index, with stable densities between 1985 and 2000 followed by a decrease in the last decade. The study also observed a significant decrease in the length of the commercially important plaice species, likely influenced by a combination of climate and fishing-induced selection. This study highlights the importance of environmental and climate-related drivers in understanding the changes in marine ecosystems.