Feeding and migration of Norwegian spring spawning herring in the Norwegian Sea
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After spawning on the shelf off Møre, Western Norway, the spawning stock of Norwegian spring spawning herring migrates to the Norwegian Sea for feeding. Simultaneously, the adolescent part of the stock leaves the wintering areas in fjords and coastal areas of northern Norway and starts feeding in the Norwegian Sea. During two cruises in April and June 1994, a transect from coastal water over the Norwegian continental shelf across the frontal zone into Atlantic water in the central Norwegian Sea was covered. Distribution and development of major prey species of the herring, Calanus finmarchicus and C. hyperboreus, were described in relation to hydrography and phytoplankton spring bloom dynamics. In April, all year-classes of herring were found in Atlantic water where zooplankton biomass was high in the upper 50 m of the water column. C. finmarchicus occurred in the overwintering stages CV and adult females, and feeding conditions were probably good. The older year-classes of herring showed a westerly movement along the transect from April to June, and the oldest herring were found farthest to the west. The herring did not cross the Polar front into Arctic water masses in June, although feeding conditions, judged from zooplankton biomass distributions, seemed far better than in Atlantic water. The herring in June was distributed in Atlantic water masses with low biomass of small stages of the new generation of C. finmarchicus, and feeding conditions were probably not optimal. The young year-classes of herring, which were found in Atlantic water in April, occurred over the shelf in June. There the zooplankton biomass was intermediate, but the new generation of C. finmarchicus had developed into late copepodite stages and adults and served as potential large particle food items for the herring.