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 winter, and by a low abundance of Calanus finmarchicus, the principal food of the herring. The low abundance of C. finmarchicus was probably related to the high water temperatures. Both the high temperature and the scarcity of food could explain the northern distribution of the herring. Apart from the 1988-1990 anomaly, a long-term northward shift of catches occurred from 1960 to 1990. This northward trend in catches coincided with a gradual increase in winter temperature, and a sustained decrease of C. finmarchicus. The long-term shift in herring catches, therefore, could signify a gradual change in distribution of the stock, brought about by the same factors that caused the 1988-1990 anomaly. If the recent climatic trend towards higher winter temperatures continues, the anomalous distribution of herring in 1988-1990 could become the normal pattern in future years.
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