Warming impacts on fish species composition in the Kattegat-Belt Sea

Sea temperatures have been rising in the waters near Denmark during the past 1-2 decades and are expected to affect marine populations, species, communities and foodwebs. Here we investigate whether and how the species richness and composition of the marine fish community in the Kattegat and Belt Sea have been influenced by these changes. We hypothesize that the recent warming has led to an increase in species richness of the local community and that this increase is due to immigration of species from warmer areas. We use spring and fall survey data collected by DTU Aqua during the years 1994-2013 and bottom temperature data (observed and model outputs from a regional phyical oceanographic model) to test these ideas. Species richness has increased significantly during the time period in both seasons of the year, and the increases
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were significantly correlated with bottom temperatures. Analysis of the southern range limits of all species captured in the surveys shows that the mean southern latitudinal limit of the fish community has been decreasing and is also corrrelated with bottom temperatures; these patterns are consistent with immigration of fish from southerly zoogeographic regions. Warm-adapted species also increased their distributional area (i. e., probability of occurrence) in the sampling region and some cold-adapted species became rarer and more narrowly distributed. Similar results were generally obtained using both observed and modelled temperature data. The changes reported in this study contribute to a growing documentation of how climate variability and changes are affecting marine communities and biodiversity