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

Western Baltic spring-spawning herring (WBSSH, Clupea harengus L.) perform seasonal migrations between feeding grounds in the Skagerrak and Kattegat and their spawning sites in the Western Baltic Sea. The Sound, connecting the Kattegat to the Western Baltic Sea, is an important aggregation and transition zone for this herring stock 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 herring in autumn: herring generally aggregated in the Sound, but in some years the majority of herring were detected further south, being outside of the Sound by the time of the survey. We tested whether observed annual differences in the herring migration 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 herring was related to environmental conditions, i.e. to marine inflow events into the Baltic Sea. Barotropic inflow events in late summer and early autumn seem to prevent deoxygenation in the Sound and thereby favour the prolonged aggregation of herring in the Sound.