Low summer temperatures cause juvenile Atlantic salmon to become nocturnal

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.