PARTIAL SEGREGATION IN THE TIMING OF MIGRATION OF ATLANTIC SALMON OF DIFFERENT AGES

The timing of the seaward migration of Atlantic salmon, Salmo salar, of different ages, sizes and sexes, and the return migration of the same groups to coastal western Norwegian waters and up the River Imsa during 1976-1988 were investigated. The majority of postspawners descended the River Imsa during two periods, December-January and March-May. Males descended on average earlier than females, whereas age and size groups did not segregate in time of descent. The survival of the spawners in the river was 64·5% (males) and 85·3% (females). The smolts (the stage when the juvenile salmon is physiologically adapted for sea life) migrating to sea during the main smolt run in April-June were on average smaller and younger than those that descended during January-March. When returning from the feeding areas in the Norwegian Sea during summer, fish that had spent one winter at sea (grilse) entered Norwegian home waters significantly later in the season than older salmon, although the grilse sscended the River Imsa earlier, and at lower water discharges, than older and larger fish. Males and females of corresponding sea-age and size ascended the river concurrently. The results lend support to the hypothesis that older individuals show circannual changes in the life history earlier in the year than younger ones. © 1990 The Association for the Study of Animal Behaviour.