Diel migration patterns of Atlantic salmon smolts with particular reference to the absence of crepuscular migration

The real-time diel pattern of Atlantic salmon smolt migration was observed for 8 years using automatic resistivity counters verified by video surveillance. A clear dominant nocturnal migration was demonstrated early in the migration period, later becoming increasingly diurnal, until rates became approximately equal at day and night. Migration patterns were related to water temperature, such that when mean daily temperatures were below 12 degrees C, hourly rates of migration were significantly lower during the day than at night. When daily mean temperatures exceeded 12 degrees C, there was no significant difference between diurnal and nocturnal migration rates. Migration patterns showed a distinct suppression of migration at dawn and dusk throughout the migration period. It is hypothesised that this behaviour is an active decision and/or an adaptive strategy either to take advantage of increased food in the form of invertebrate drift or to reduce predation risk from actively feeding piscivores or both.