Abstract

It is widely recognized that knowledge of population structure and connectivity is vital for sustainable use of the world's marine fisheries resources. The harvest of each individual population unit should ideally be managed to account for its specific level of productivity directly. Whereas most emphasis has been placed on identifying discrete population units, much less effort has been given to the mechanisms behind stock separation. Evidence is accumulating to indicate that philopatric behaviors (stock-specific behaviors) are important, in this regard, not only in other kinds of species, but also in marine free-spawning fish species. Recent studies on herring and cod in the waters bounded by Norway, Sweden, and Denmark (Skagerrak–Kattegat) indicate that philopatric behavior is a key factor in shaping stock structure. Under such circumstances, an area depleted of its stock components can only slowly be repopulated by fish from adjacent areas. Repopulation, for example, may depend on adults successfully aggregating and reproducing at new locations, with new migratory patterns that are dependent on new behavior. This can be a very time-consuming process, and the slow formation of new viable populations units results in a bleak forecast for population recovery.