The challenge of the herring in the Norwegian sea: Making optimal collective spatial decisions

Norwegian spring-spawning herring, Clupea harengus harengus L., are long-lived multiple spawners subject to strong variation in recruitment success. They tend to adopt low-risk, preferred-conservative strategies, yet they display considerable plasticity in migratory behaviour and associated spatial dynamics. Although their migration patterns have long been investigated, few studies have analysed the factors and mechanisms governing spatial dynamics. In this study an ecological and evolutionary perspective is adopted, emphasizing proximate mechanisms that restrict the extent to which herring can localize an optimal habitat. The starting point is the assertion that the herring's migratory behaviour can be explained by an interplay of a few key factors. Despite spatial and temporal variations, the Norwegian Sea has consistent and predictable features, such as the distribution of water masses and timing of seasonal plankton production. Herring may locate favourable habitats by using a combination of predictive orientation mechanisms, based upon genetic factors and leaming, and of reactive mechanisms, such as memorybased state-location comparisons and orientation to gradients in the sea. Changes in herring distribution and density occur on micro-, meso- and macroscale. After reviewing the available information on school density, school size, school size adjustments, synchronized behaviour patterns and swimming speed of both individual schools and school clusters, the authors attempt to form a link across spatio-temporal scales to explain patterns in distribution. Existing theory seems inadequate to explain the dynamic behaviour exhibited by large herring schools, though it must in some way reflect optimal decisions by individual fish. It is suggested that the appropriate resolution for the analysis of herring spatial dynamics in meso- and macroscale could be the school unit, and that we have to analyse how individual fish behaviours bring about these dynamics.