Northern distribution of North Sea herring as a response to high water temperatures and/or low food abundance

Catch distributions and results of acoustic surveys indicate that North Sea herring had an unusually northern distribution during the summers of 1988- 1990. Some of the herring may even have temporarily left the North Sea, and migrated, to the Faroe plateau. The anomalous distribution of the herring in 1988-1990 appears to be related to a short-term climatic variation. The years were characterized by high water temperatures during the preceding wi nter, and by a low abundance of Clanus finmarchicus, the principal food of the herring. The low abundance of C. finmarchicus was probably related to t he high water temperatures. Both the high temperature and the scarcity of f ood could explain the northern distribution of the herring. Apart from the 1988-1990 anomaly, a long-term northward shift of catches oc curred from 1960 to 1990. This northward trend in catches coincided with a gradual increase in winter temperature, and a sustained decrease of C.finma rchicus. The long-term shirt in herring catches, therefore, could signify a gradual change in distribution of the stock, brought about by the same fac tors that caused the 1988-1990 anomaly. If the recent climatic trend toward s higher winter temperatures continues, the anomalous distribution of herri ng in 1988-1990 could become the normal pattern in future years. (C) 2001 E lsevier Science B.V. All rights reserved.