Navigation mechanisms of herring during feeding migration: the role of ecological gradients on an oceanic scale

The feeding migration of Norwegian spring-spawning herring was studied in relation to prey abundance and environmental factors that may affect their feeding migration. Temperature, salinity, chlorophyll a, nitrate concentration, abundance of Calanus finmarchicus, zooplankton biomass, acoustic data on herring and trawl samples were collected during four basin-scale surveys in the Norwegian Sea from April to August 1995. Herring abundance was positively associated with the overwintering population of C. finmarchicus. We suggest that spatial gradients and temporal dynamics of the seasonal ascent of the C. finmarchicus overwintering generation provide stimuli for and affect the feeding migration of herring. The clockwise migration pattern of herring, observed during the 1990s, can be explained by delayed ascent and development of C. finmarchicus toward the west and north in the Norwegian Sea. We further suggest that herring leave a specific area before their zooplankton prey is depleted. The first generation of C. finmarchicus likely has a minor influence in directing the herring feeding migration due to the low abundance of older stages available as prey. The feeding migration was constantly directed toward colder water, and temperature probably has a secondary effect on herring distribution.