THE CHALLENGE OF THE HERRING IN THE NORWEGIAN SEA - MAKING OPTIMAL COLLECTIVE SPATIAL DECISIONS

Norwegian spring-spawning herring, Clupea harengus harengus L., are lo ng-lived multiple spawners subject to strong variation in recruitment success. They tend to adopt low-risk, preferred-conservative strategie s, yet they display considerable plasticity in migratory behaviour and associated spatial dynamics. Although their migration patterns have l ong been investigated, few studies have analysed the factors and mecha nisms governing spatial dynamics. In this study an ecological and evol utionary 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 beha viour can be explained by an interplay of a few key factors. Despite s patial and temporal variations, the Norwegian Sea has consistent and p redictable features, such as the distribution of water masses and timi ng of seasonal plankton production. Herring may locate favourable habi tats by using a combination of predictive orientation mechanisms, base d upon genetic factors and learning, and of reactive mechanisms, such as memory-based state-location comparisons and orientation to gradient s in the sea. Changes in herring distribution and density occur on mic ro-, meso- and macroscale. After reviewing the available information o n school density, school size, school size adjustments, synchronized b ehaviour patterns and swimming speed of both individual schools and sc hool clusters, the authors attempt to form a link across spatio-tempor al scales to explain patterns in distribution. Existing theory seems i nadequate to explain the dynamic behaviour exhibited by large herring schools, though it must in some way reflect optimal decisions by indiv idual fish. It is suggested that the appropriate resolution for the an alysis of herring spatial dynamics in meso- and macroscale could be th e school unit, and that we have to analyse how individual fish behavio urs bring about these dynamics.