Spatial feeding patterns of herring (Clupea harengus L.), sprat (Sprattus sprattus L.), and the three-spined stickleback (Gasterosteus aculeatus L.) in the Gulf of Finland, Baltic Sea

The pelagic-fish fauna in the Gulf of Finland, Baltic Sea was sampled by trawling and hydroacoustics in September 2002. Spatial and size/age-dependent patterns in the diets of herring (Clupea harengus), sprat (Sprattus sprattus), and the three-spined stickleback (Gasterosteus aculeatus) were explored. At night the fish concentrated at thermocline depth but at dawn they scattered over a larger depth range. All three fish species fed on mesozooplankton but nektobenthos, for example, was scarce. In the eastern Gulf of Finland, where there is a strong freshwater inflow, the cladoceran Bosmina longispina was the dominant prey item, but it was also abundant in the diets of young-of-the-year (total length <10 cm) clupeids in the western Gulf of Finland. In these more saline western areas, calanoid copepods, especially Eurytemora affinis, were the most important prey for large (≥10 cm) clupeids. The large clupeids in particular, also fed on Temora longicornis. The diet of three-spined stickleback overlapped with that of the clupeids. However, Cercopagis pengoi, a recent arrival to the area, was much more abundant in the diet of stickleback than in the diet of clupeids.