Background. Recent research showed a particularly pronounced genetic diversity for the Atlantic herring, Clupea harengus, living in the transition zone from the Baltic Sea to the North Sea. Conserving genetic diversity is of high priority in fishery management. This requires, firstly, that we recognize life histories, not only of the major spawning components, but also of the smaller ones, and secondly, that we improve our knowledge on how genetic diversity is maintained also on smaller geographical scale. Materials and methods. Prevalence of the widely used tag parasite Anisakis sp. was applied to investigate whether the various spawning components, living in the Baltic Sea-North Sea transition zone, display any common features with respect to feeding migration patterns. Results. It is strongly suggested that spawning components in the transition zone migrate to feeding areas in Skagerrak or the North Sea. This implies that herring utilizing spawning grounds in the Limfjord and inner Danish waters engage in a northward feeding migration like their nearby neighbour, the Rügen Herring, instead of feeding locally or in the Baltic Sea. Conclusion. Firstly, these results support the validity of the present division of management areas, where the Western Baltic Sea is managed under ICES Division IIIa (Kattegat and Skagerrak), even when the newly discovered population complexity is considered. Secondly, it contributes to the debate on how population complexes can be sustained despite temporary mixing, where it support the idea that genetic divergence in herring population complexes can arise from colonization of unused spawning grounds and be sustained despite extensive temporal mixing by natal homing.

Anisakis, biological tag, parasite, fish, herring, Clupea harengus, migration, Western Baltic Sea