The Norwegian spring-spawning herring – spawning, larval drift and larval retention

The Norwegian spring-spawning herring appear to spawn in areas where the larvae may be temporarily retained (retention areas). This article aims to shed light on the spawning and larval drift of this herring stock with special emphasis on the role of the retention areas. Year class strength seems to be positively correlated with high wind speed over the spawning fields in April and slow northerly larval drift. Most of the Norwegian continental shelf is covered by stratified water all year. In the retention areas, the vertical stratification is reduced during winter/spring compared with the surrounding waters. The windinduced turbulence generated by the high wind speeds in April will, therefore, reach deeper into the retention areas and thereby probably increase the predator–prey encounter rate for the first-feeding herring larvae and favour a high survival rate. The coincidence of slow northerly larval drift and good recruitment supports the conclusion that interannual variations in the degree of retention could be an important recruitment-regulating mechanism for the Norwegian spring-spawning herring.