Long‐term climate forcing of European herring and sardine populations

Records of the herring, Clupea harengus, fishery off the Swedish coast of Bohuslan, in the Skagerrak, date back to the 10th century. Nine periods, each lasting several decades, are known during which large quantities of herring were caught close to the shore. In the 1895–96 season, more than 200 000 tonnes were landed. During the `interim' periods, which stretched over 50 or more years, the herring fishery played little role in the economy of this region. Several other herring fisheries in European waters overlap with recent Bohuslan periods whereas the Norwegian spring-spawning herring and some sardine, Sardina pilchardus, fisheries exhibit alternating periods. A study of the climatological/hydrographic scenario of all Bohuslan periods and those of herring in the English Channel and the Bay of Biscay showed that, on a decadal scale, they coincided with times when there was a strong ice cover off Iceland, severe winters in western Europe with extremely cold air and water temperatures, a reduction of westerly winds as indicated by negative anomalies in the North Atlantic Oscillation (NAO) index and a minimum of south-westerly winds over England in response to meridional migrations of the belt of westerly winds. Periods of the Norwegian spring-spawning herring and sardines in the English Channel coincided with inverse climatological/hydrographic situations. It is concluded that climate variation governed the alternating herring and sardine periods.