The migratory behaviour of wild sea trout ( Salmo trutta L.) smolts in the estuary of the River Conwy, North Wales

Abstract

Nineteen wild sea trout ( Salmo trutta L.) smolts, tagged with miniature acoustic transmitters, were tracked in the River Conwy, North Wales, to describe the freshwater and estuarine patterns of migration. Migration in freshwater was predominantly nocturnal (mean times 23 h 59 min and 00 h 2 min in 1992 and 1993, respectively), although there were changes in this pattern in the lower reaches of the estuary with fish moving during both the day and night. The nocturnal pattern of migration in freshwater would appear to be the result of a rhythm of swimming activity, triggered by light levels, which resulted in the smolts moving up into the water column after dusk and migrating seawards. Smolts tagged earlier in the study spent significantly longer in the river before migrating into coastal waters than fish tagged later in the study. The movement of smolts through the estuary was indicative of an ebb tide transport pattern of migration. All the smolts migrated seawards on an ebb tide (mean time after High Water 2 h 48 min–4 h 11 min), close to the surface and within the fastest moving section of the water column. Smolt 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 saltwater.