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

Abstract

Migration theory states that migration behavioural strategies should be optimised to maximise fitness. Many studies have shown that in downstream migrating Atlantic salmon Salmo salar L. smolts, mortality from predation is high and negatively size dependent. The most common predators are birds and piscivorous fish that are mainly daylight feeders. Given the high mortality during this stage we should expect to observe smolts to follow predator avoidance strategies that may be affected by body size. We tested the hypothesis that small smolts have a higher tendency to exhibit predator avoidance strategies (i.e. nocturnal versus diurnal migration) than larger smolts. The number and size of out-migrating/downstreammigrating wild Atlantic salmon smolts was recorded as they passed through a glass-sided channel during April-May, 1996-1999. In all years, the mean size of nocturnal migrating smolts was significantly lower than the mean size of diurnal migrating smolts. Analysis of the size of smolts, during early and late stages of the migration period showed size-dependent nocturnal migration behaviour up to the end of April. After this, no such size dependent migration pattern was observed. However, small smolts (<100 mm) were absent during this period. We suggest that nocturnal migration is an adaptive behaviour that small Atlantic salmon smolts have to avoid predation by large daylight feeding visual piscivorous predators (e.g. pike Esox Lucius L. and fish eating birds).

Diel migration pattern of Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) smolts: an assessment of environmental cues

“…Ibbotson et al (2011) found that nocturnally migrating smolts were smaller than diurnally migrating smolts. Ibbotson et al (2011) found that nocturnally migrating smolts were smaller than diurnally migrating smolts.…”

“…For instance, in the River Frome, England, Ibbotson et al (2006) found that most Atlantic salmon smolts migrated downstream during night at the beginning of the annual run. Furthermore, Ibbotson, Beaumont, and Pinder (2011) reported that small more than large smolts migrate at night when they move from the river to the ocean for feeding. This change in diurnal behaviour pattern may be related to the ability of ectotherms, such as smolts, to more easily escape endothermic predators, such as piscivorous sea birds and mammals, in warm than in colder water.…”

Predator swamping reduces predation risk during nocturnal migration of juvenile salmon in a high‐mortality landscape

“…Atlantic salmon ( Salmo salar ) smolts migrate nocturnally as well, but such behaviour is only prominent for small individuals and with reducing prevalence throughout the season (Ibbotson et al . ; Ibbotson, Beaumont & Pinder ). Thus, the predation risk of smolts in the current study might be more extreme and/or consistent to result in adaptive behavioural mechanisms across smolt sizes and seasonal timing.…”

Under what circumstances does the capture and tagging of wild Atlantic salmon Salmo salar smolts affect probability of return as adults?

“…Ibbotson et al . () recorded the size of smolts as they passed East Stoke on the River Frome using video equipment. In all years, the mean size of nocturnal migrating smolts was significantly lower than the mean size of diurnal migrating smolts.…”