HOMING OF ATLANTIC SALMON (SALMO-SALAR L) TO A TRIBUTARY SPAWNING STREAM IN A MAJOR RIVER CATCHMENT

At the Girnock Bum, a tributary of the River Dee in Scotland, a substantial part of the annual production of migrant juvenile salmon (Salmo salar L.) leaves the stream in autumn rather than in spring. Previously, the status of the autumn migrants was uncertain because Carlin tagging indicated that they returned to the stream with a lower frequency than smolts tagged in spring. The difference was not attributable to the smolting of autumn migrants outside of the Gimock Bum and their homing to these new locations since adults tagged as autumn paff were also under-represented in fisheries distant from the home stream. Instead, the differences were probably artifactual and attributable to a seasonal effect of the Carlin tagging procedure itself. When juveniles were tagged instead with wire microtags, recapture rates were similar for autumn and spring migrants. This is consistent with the hypothesis that the autumn migration is an aspect of smolting. Microtagging of all juvenile migrants (paired with adipose fin clipping) indicated that the homing of adult spawners to the Gimock Bum was substantially accurate. Over the period of study 45% of fish of the sea-age classes that were expected to bear adipose fin clips did so.