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 win ter 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 an nual rhythm. Here we show that the previously described switch by Alta ntic salmon (Salmo salar L.) from predominantly diurnal to nocturnal a ctivity in winter also occurs at other times of the year in response t o low, ''winter'' temperatures; this is demonstrated both in laborator y experiments and by field observations in glacial rivers. This indica tes that there is no underlying inherent annual rhythm to this behavio ur nor any photoperiodic influence. Furthermore, in the laboratory exp eriment this temperature-dependent shift to nocturnalism was explained by a suppression of daytime activity rather than an increase in activ ity at night.