A proposed mechanism for the Bohuslan herring periods

During the last 500 years large numbers of herring have occasionally appear ed in winter along the Swedish Skagerrak coast. These "Bohuslan" herring pe riods, sometimes lasting for several decades, were probably caused by a tem porary shift of the over-wintering grounds of North Sea herring. In normal years North Sea herring over-winter in the adjacent waters of the northeast ern North Sea. As yet there is no commonly accepted explanation for the occ asional shifts of the over-wintering grounds towards the Skagerrak. The environmental cause of the Bohuslan herring periods was investigated by looking at the most recent episode of herring invasions in the Skagerrak. In the winters of 1962-1965 large numbers of North Sea herring again entere d the Skagerrak and these invasions showed many features of the old Bohusla n herring periods. The only anomalous environmental parameter at this time was a high Frequency of easterly winds in the autumns of 1962 and 1963. It is assumed that these easterly winds forced surface water out of the Skager rak and thereby strengthened the subsurface Norwegian french Current that f lows into the Skagerrak. This current could easily transport herring from t he normal over-wintering area near Egersund Bank towards the Skagerrak. It is shown that former Bohuslan or Skagerrak herring periods started in years when the North Atlantic Oscillation was in a negative phase and when easte rly winds must have dominated in autumn too. Bohuslan herring periods were more persistent than the periods of easterly winds. The persistence of the herring migrations is explained by "site-fide lity" of the older herring. Once a new migration has been adopted by the po pulation it is repeated in subsequent years even when the original environm ental cause has disappeared. An existing migration pattern can be changed o nly by a new, strong year class, that has not yet developed an attachment t o the traditional over-wintering area. It is shown that the two most recent episodes of Skagerrak invasions were indeed initiated by strong recruiting year classes. (C) 1999 International Council for the Exploration of the Se a.