Low summer temperatures cause juvenile Atlantic salmon to become nocturnal

Abstract

The diel and seasonal activity patterns of salmonids are predominantly governed by the annual changes in photoperiod and temperature. In winter salmonids become increasingly nocturnal, hiding in refuges by day but emerging to feed at night. This behaviour may be linked to either one of the controlling influences mentioned above or to an inherent annual rhythm. Here we show that the previously described switch by Altantic salmon (Salmo salar L.) from predominantly diurnal to nocturnal activity in winter also occurs at other times of the year in response to low, "winter" temperatures; this is demonstrated both in laboratory experiments and by field observations in glacial rivers. This indicates that there is no underlying inherent annual rhythm to this behaviour nor any photoperiodic influence. Furthermore, in the laboratory experiment this temperature-dependent shift to nocturnalism was explained by a suppression of daytime activity rather than an increase in activity at night.

Daily and annual circarhythms activity in the South American sea lion Otaria flavescens (Carnivora: Otariidae) at the central zone of Chile

“…This fact is associated to the nocturnal feeding habit of the South American sea lion. Behavior which seems to be determined by the feeding habits of their preys which concentrate mainly on the surface during the night (Thorpe et al 1994, Fraser et al 1995, Soto et al 1997. In this sense, the nocturnal feeding of the South American sea lion may be favored by an energetic saving, in terms of a greater accessibility to the preys.…”

Field experiments on stranding in juvenile atlantic salmon (Salmo salar) and brown trout (Salmo trutta) during rapid flow decreases caused by hydropeaking

“…At low temperatures, brown trout prefer more cover, lower flow and seek down into the substrate (Karlströ m, 1977;Cunjak and Power, 1986;Heggenes and Saltveit, 1990). Atlantic salmon fry will also hide in the substrate during winter (Rimmer et al, 1983;Cunjak, 1988;Heggenes and Saltveit, 1990), but for both species this passive hiding behaviour in winter is found only to be a daytime phenomenon, both species being active at night (Fraser et al, 1993(Fraser et al, , 1995Heggenes et al, 1993). This explains the lower stranding tendency found in Atlantic salmon at night during winter.…”

Factors influencing stranding of wild juvenile brown trout (Salmo trutta) during rapid and frequent flow decreases in an artificial stream

“…Sheltering of juvenile trout in the substrate during winter is thought to be a daylight phenomenon, as trout may be active during night. This diurnal change in behaviour occurs when temperatures drop below 5-10 C, and is an antipredator behaviour regulated by light (Fraser et al, 1993(Fraser et al, , 1995Heggenes et al, 1993), but is also a strategy to avoid being frozen in. We link our lack of a diurnal difference during winter conditions in the present study to the fact that despite being able to simulate night conditions, 6 C was not cold enough for brown trout to become nocturnally active.…”