The effects of periodic marine inflow into the Baltic Sea on the migration patterns of Western Baltic spring-spawning herring

Western Balticspring-spawning herring(WBSSH, Clupeaharengus L.) perform seasonal migrations between feeding grounds in the Skagerrak and Kattegat and their spawning sites in the Western BalticSea. The Sound, connecting the Kattegat to the Western BalticSea, is an important aggregation and transition zone for this herringstock during its spawning migration. We analysed data from the German autumn acoustic surveys of the years 1993 –2009. These data revealed at least two different distribution patterns of herringin autumn: herringgenerally aggregated in the Sound, but in some years the majority of herringwere detected further south, being outside of the Sound by the time of the survey. We tested whether observed annual differences in the herringmigration can be explained by either stock characteristics (age and size) or hydrographical variables (salinity and oxygen concentration). Our results suggest that rather than being related to stock characteristics, the distribution pattern of herringwas related to environmental conditions, i.e. to marine inflow events into the BalticSea. Barotropic inflow events in late summer and early autumn seem to prevent deoxygenationin the Sound and thereby favour the prolonged aggregation of herringin the Sound.