Fish, seabirds and trophic cascades in the Baltic Sea

Abstract

In the relatively simple Baltic Sea ecosystem, zooplankton-feeding sprat Sprattus sprattus is a major food source for breeding seabirds and piscivorous fish, and an important resource for commercial fisheries. Large-scale and long-term ecosystem changes resulting mainly from over fishing and recruitment failure of cod Gadus morhua, which is the main fish predator of sprat, have affected natural-history patterns in a piscivorous seabird, the common guillemot Uria aalge, in a complex way. As the sprat stock increased, leading to lower energy content of fish, common guillemot chick body mass at fledging decreased. However, chick fledging body mass recovered in recent years as the sprat stock diminished, which brought about corresponding increases in sprat weight-at-age and energy content. The cod and sprat fishery affect the common guillemots in the Baltic Sea, but the effects differ depending on the management strategy.