THE MIGRATORY BEHAVIOR OF WILD ATLANTIC SALMON (SALMO-SALAR) SMOLTS IN THE ESTUARY OF THE RIVER CONWY, NORTH WALES

Thirty-two wild Atlantic salmon (Salmo salar) smelts, tagged with miniature acoustic transmitters, were tracked in the River Conwy, North Wales, to describe the freshwater and estuarine patterns of migration. Migration in fresh water was predominantly nocturnal, although there was a seasonal change in this pattern with later run fish moving during both the day and night. Smelts tagged earlier in the study spent significantly longer in the river (mean 456 +/- 43 h) before migrating into coastal waters than fish tagged later in the study (mean 201 +/- 30 h). The movement of smelts through the estuary was indicative of a nocturnal selective ebb tide transport pattern of migration. All of the smelts migrated seawards on an ebb tide close to the surface and within the fastest moving section of the water column. The nocturnal pattern of migration would appear to be the result of an endogenous rhythm of swimming activity that results in the smelts moving up into the water column after dusk and migrating seawards. Smelt migration in the lower portion of the estuary was indicative of active directed swimming and there was no apparent period of acclimation required when moving from fresh to salt water.