Effects of fish length and condition on spawning migration in Norwegian spring spawning herring (Clupea harengus L.)

In general the following facts apply for the Norwegian spring spawning herring in the 1990s. It does not feed from the onset of wintering (September-October) in Vestfjorden, northern Norway (68 degrees N), until spawning is completed (March-April). In mid-January it migrates towards spawning grounds within a range of approximately 1500 km along the coast (58 degrees-70 degrees N). The relative weekly energy loss is 3-4 times higher during the spawning migration than during the wintering, and it increases with decreasing fish length. Survival of progeny may increase southwards due to beneficial environmental conditions. It is widely accepted that herring return to spawn at the same spawning grounds year after year (homing). However, this study demonstrated that Norwegian spring spawning herring may deviate from this general homing tendency, due to a trade-off between survival of progeny and physiological migration constraints related to fish length and condition (energy storage). During the spawning seasons in 1995 and 1996 the mature herring were mainly distributed between Sogn and Lofoten (61 degrees-70 degrees N), with the shelf area off More (62 degrees-64 degrees N) being most important. Both the fish length, condition and stage of maturity increased southwards, but the condition appeared to be the most important variable influencing the distance migrated and spawning time.