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

Abstract

Ibbotson AT, Beaumont WRC, Pinder A, Welton S, Ladle M. Diel migration patterns of Atlantic salmon smolts with particular reference to the absence of crepuscular migration.Abstract -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°C, hourly rates of migration were significantly lower during the day than at night. When daily mean temperatures exceeded 12°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.

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

“…Whilst, the proportion of smolts migrating during the day increases in the later stages of the smolt migration period (Ibbotson et al 2006), smaller smolts always exhibit a higher probability of migrating at night than the larger smolts.…”

“…Thus, 1 h at sunrise and 1 h at sunset (crepuscular periods) were excluded from the analysis. However, smolt migration rarely takes place during these crepuscular periods (Ibbotson et al 2006) and so few smolts were omitted from the analysis.…”

“…The smolt run of each year, was divided into three separate periods (P1-P3 representing 30 March-14 April; 15 April-30 April; 1 May onwards), to negate the temperature and timing influence on the diel behaviour of smolt migration (Ibbotson et al 2006). A general linear model was used to analyse the effects of time of day, period and year together with their interactions on the mean size of migrating smolts, with smolt length (log) as the dependent variable and time of day (night or day), period (P1-P3) and year (1996)(1997)(1998)(1999) as factorial independent variables.…”

“…The increased body size achieved by the more efficient daylight feeding may decrease size-dependent predation risks during subsequent migration. The smolts tend to nocturnal migration during the first few weeks but then switch to favouring daylight migration at higher temperatures towards the end of the migration period (Ibbotson et al 2006).…”

Identifying when tagged fishes have been consumed by piscivorous predators: application of multivariate mixture models to movement parameters of telemetered fishes

“…Bradford and Higgins [23] also reported lower activity levels for both juvenile Chinook salmon and steelhead trout during the day. Furthermore, Atlantic salmon have also been shown to have a preference for migrating during the nighttime hours rather than during the day [24]. Notwithstanding multiple modes of behavior that would pose difficulty for any classification scheme, our mixture model approach provides a quantitative method for classifying behaviors that are most commonly associated with the movement of predators and smolts.…”