A possible adaptation of herring feeding migrations to a change in timing of the Calanus finmarchicus season in the eastern North Sea

During the first years after the re-opening of the North Sea herring fisher y in 1983, a relatively high proportion of the catches in early summer was taken in the eastern North Sea. After 1986, the proportion of the catch in the eastern North Sea gradually declined. It is assumed that the high catch es in the eastern North Sea in the early 1980s were due to a delayed migrat ion of the herring from the eastern to the western part of the North Sea. T his delayed departure of the herring from the eastern North Sea could have been caused by favourable food conditions in this area. Data on Calanus fin marchicus from the Continuous Plankton Recorder show that the seasonal cycl e of this copepod in the eastern North Sea was delayed during the period 19 76-1984, which resulted in an prolonged food supply for the herring in this area. It is likely that this extension of the feeding season induced the h erring to delay their departures from the eastern North Sea. When the Calan us season shortened after 1985, the herring advanced their departure from t he eastern North Sea. There was a delay of one or two years between the sho rtening of the feeding season and the earlier departure of the herring from the eastern North Sea. This suggests the existence of a certain conservati sm in the migrations of the herring. It seems that the time of departure fr om the eastern North Sea is based not only on the food situation in the cur rent year, but also on the average timing of food production in earlier yea rs. (C) 2000 International Council for the Exploration of the Sea.