The food of Norwegian spring-spawning herring in the western Norwegian Sea in relation to the annual cycle of zooplankton

The food of the Norwegian spring-spawning herring ( Clupea harengus ) in the western part of the Norwegian Sea was studied. Herring stomachs and concurrent zooplankton samples were collected on three cruises in 1995 (April, May, June). Further, the seasonal abundance of zooplankton was examined at two stations located near the shelf break east of Iceland between March 1995 and February 1996. A total of 164 stomachs of herring, ranging in length from 24.1 to 40.0 cm, were examined. The feeding activity was higher in May and June than in April, as indicated by both the higher proportion of stomachs with food in May and June (>95%) than in April (∼82%), and much higher average stomach contents in May and June than in April. Copepods (mainly Calanus finmarchicus, Metridia longa and C. hyperboreus ) were the most important prey, both in terms of numbers (∼25->98%) and biomass (∼50-90%). In June, amphipods (mainly Themisto libellula ) were also important in the diet, both in numbers (∼10-50%) and biomass (∼10-30%). Euphausiids (mainly Meganyctiphanes norvegica ) were the most important as prey in June (∼0-25% and ∼0-10% by numbers and dry weight, respectively). The relationships between herring migrations and the seasonal development of zooplankton are discussed.