A proposed mechanism for the Bohuslän herring periods

During the last 500 years large numbers of herring have occasionally appeared in winter along the Swedish Skagerrak coast. These “Bohuslän” herring periods, sometimes lasting for several decades, were probably caused by a temporary shift of the over-wintering grounds of North Sea herring. In normal years North Sea herring over-winter in the adjacent waters of the northeastern North Sea. As yet there is no commonly accepted explanation for the occasional shifts of the over-wintering grounds towards the Skagerrak. The environmental cause of the Bohuslän 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 entered the Skagerrak and these invasions showed many features of the old Bohuslän 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 Skagerrak and thereby strengthened the subsurface Norwegian Trench Current that flows into the Skagerrak. This current could easily transport herring from the normal over-wintering area near Egersund Bank towards the Skagerrak. It is shown that former Bohuslän or Skagerrak herring periods started in years when the North Atlantic Oscillation was in a negative phase and when easterly winds must have dominated in autumn too. Bohuslän herring periods were more persistent than the periods of easterly winds. The persistence of the herring migrations is explained by “site-fidelity” of the older herring. Once a new migration has been adopted by the population it is repeated in subsequent years even when the original environmental cause has disappeared. An existing migration pattern can be changed only by a new, strong year class, that has not yet developed an attachment to 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.