Do cormorant colonies affect local fish communities in the Baltic Sea?

Abstract

The effects that cormorants have on fish communities have been debated, yet few studies have used the necessary controls and replications to make valid and general assessments. Using both a paired design and time series analysis, we studied associations between colony size of the great cormorant ( Phalacrocorax carbo sinesis ) in the Baltic Sea and local fish community composition. Overall, colony size showed few associations with local fish communities. Most evident was a negative association between colony size and perch ( Perca fluviatilis ) abundances in some areas. But for other species, results were nonsignificant or variable among areas. The Baltic Sea cormorant population has increased over the last decades, and our results suggest that management actions to reduce local cormorant colonies may have a positive effect on local perch abundance. Otherwise, our results predict colony reductions to either have variable or weak effects on adjacent local fish communities.