During the first years after the re-opening of the North Sea herring fishery 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 catches in the eastern North Sea in the early 1980s were due to a delayed migration of the herring from the eastern to the western part of the North Sea. This delayed departure of the herring from the eastern North Sea could have been caused by favourable food conditions in this area. Data on Calanus finmarchicus from the Continuous Plankton Recorder show that the seasonal cycle of this copepod in the eastern North Sea was delayed during the period 1976–1984, 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 herring to delay their departures from the eastern North Sea. When the Calanus season shortened after 1985, the herring advanced their departure from the eastern North Sea. There was a delay of one or two years between the shortening of the feeding season and the earlier departure of the herring from the eastern North Sea. This suggests the existence of a certain conservatism in the migrations of the herring. It seems that the time of departure from the eastern North Sea is based not only on the food situation in the current year, but also on the average timing of food production in earlier years.
- North Sea