Understanding how key environmental factors affect fish communities is necessary for sound environmental management. Accordingly, we studied fish species–environment relationships in Finnish boreal rivers. A self-organizing map (SOM) analysis showed strong relationships between the occurrence of 18 fish species and 10 environmental variables (variance explained: 43.4%). In our logistic regression models run for each fish species, the predictive power varied from poor (AUC = 0.67–0.68) to excellent (AUC = 0.96), indicating that the responses were species-specific. The overall best predictors of the species occurrence were the local variables water depth and water temperature, and the regional variables stream size, altitude and annual mean temperature. Our results on fish assemblage types (clusters), identified by SOM, and co-occurrence of fish species, discovered by a hierarchical cluster analysis, reveal insights into the structure of the river fish assemblages. Moreover, our results suggest that the stream size contributes to thermal ranges in which each species is able to occur, which has relevance to predicting the impacts of climate change on riverine fish.

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