Varlation in migration and abundance of norwegian spring spawning herring (Clupea harengusL.)

Abstract

The Norwegian spring spawning herring has undergone great fluctuations in abundance and changes in migration. In the middle of this century the stock was in a productive phase, after which a serious depletion took place and the stock collapsed by the end of the 1960s. The primary factor for the collapse was high fishing pressure. A gradual, but small increase in the adult stock took place between 1974 and 1986. In this period the adult herring did not leave the coast after spawning to feed in the Norwegian Sea, as in previous years, but remained near shore to feed and later migrated into the fjords for wintering. The only numerous year class between 1970 and 1987 was that of 1983. When this year class became mature a significant increase in the spawning stock took place. At the same time a change in the migration pattern was recorded. From 1989 spawning extended southwards along the Norwegian coast, and during the 1990s the Norwegian Sea was increasingly reutilized for feeding. However, wintering still took place in the fjords. Two strong year classes, those of 1991 and 1992, started to recruit to the spawning stock in 1994 and 1995. The subsequent three year classes are weak and a decline in the spawning stock size is expected after 1997–98. Without agreement on total allowable catches (TACs), which limit fishing mortality to a low level, the spawning stock may again decrease below the minimum biological acceptable limit, and the potential largest fish resource in the North Atlantic may again be depleted.