The migratory behaviour of wild Atlantic salmon (Salmo salar) smolts in the estuary of the River Conwy, North Wales

Abstract

Thirty-two wild Atlantic salmon (Salmo salar) 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 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. Smolts 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 smolts through the estuary was indicative of a nocturnal selective ebb tide transport pattern of migration. All of the smolts 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 smolts moving up into the water column after dusk and migrating seawards. 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 salt water.

Linking Behavior, Physiology, and Survival of Atlantic Salmon Smolts During Estuary Migration

“…Penobscot River estuary (Fried et al 1978;Moore et al 1995). The reduction in movement rates through estuaries might also result from the reversal of migratory direction during the passage of tidal estuaries and bays (Kocik et al 2009;Dempson et al 2011;Halfyard et al 2013).…”

“…This finding is corroborated by results of studies in the northeastern Atlantic Ocean, where smolts acclimated in net-pens within an estuary showed no improvement in survival over fish released directly into the mouth of the river (Thorstad et al 2012a). Other laboratory experiments have found no period of acclimation occurred for smolts upon reaching salt water (Moore et al 1995). It is, however, possible that fish use tidal movements to minimize energetic costs, especially because this is the period of migration during which smolts are thought to transition from passive to active migration (Hedger et al 2008;Martin et al 2009).…”

Experimentation at the interface of fluvial geomorphology, stream ecology and hydraulic engineering and the development of an effective, interdisciplinary river science

“…This is of interest because juvenile salmonids form schools during migration (McCormick et al, 1998) and yet for many species this occurs primarily during hours of darkness (e.g. Tytler et al, 1978, but see Soloman, 1978Moore et al, 1995). The results of the smolt experiment contradict previous suggestions that nocturnal migration reflects a passive downstream displacement when dark because fish are either unable to maintain a visual fix on surrounding features (Thorpe et al, 1988 for juvenile Atlantic salmon) or become disoriented (Pavlov, 1994 for subyearling Cyprinidae).…”

A size-dependent migration strategy in Atlantic salmon smolts: Small smolts favour nocturnal migration

“…One study has observed the same individual smolts migrating both night and day later in the smolt run (Moore et al 1995), possibly responding to a need for smolts to reach their destination at the optimum time (Hansen and Jonsson 1989). This may explain the observation of this study that from the 1 May onwards (P3) the probability of migrating at any particular time of day was not dependent on fish size.…”