Community-level modelling of pelagic fish assemblages in the Baltic Sea reveals temporal shifts and effects of environmental drivers

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Abstract The Baltic Sea is characterised by strong environmental gradients that shape its biodiversity and community structure. Despite a well-established body of work on individual fish species’ responses to these gradients, basin-wide, multi-species assessments of pelagic fish community patterns remain scarce. Using 17 years of standardised autumn data (2007–2023) from the Baltic International Acoustic Survey (BIAS)—a fishery-independent survey primarily targeting herring and sprat that consistently samples the broader pelagic fish assemblage—we fitted species distribution models for 45 fish taxa. Predicted occurrence probabilities were used to estimate species richness and to delineate regions of common profile (RCPs), characterising the spatial structure and temporal dynamics of the clupeid-associated pelagic fish community across most of the Baltic basin. Salinity emerged as the dominant environmental driver, influencing 31 species. Seven distinct RCPs broadly tracked the southwest–northeast salinity gradient, from the saline Kattegat to the brackish northern basins, while 10 species clusters reflected corresponding ecological affinities. Modelled species richness peaked in the Kattegat and increased over the study period, particularly in the Gulf of Finland, Åland Sea, and Bothnian Sea. Between 2007 and 2023, RCP spatial extents shifted: marine-associated communities retracted westward while brackish and low-salinity assemblages expanded northward and eastward, consistent with projected climate-driven salinity reductions. Our findings provide a spatially explicit, basin-wide quantification of how environmental gradients structure the pelagic fish community and document ongoing reorganisation that has direct implications for biodiversity assessment and ecosystem-based management in the Baltic Sea.

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