TEMPERATURE-DEPENDENT SWITCH BETWEEN DIURNAL AND NOCTURNAL FORAGING IN SALMON

The activity patterns of animals, whether diurnal, crepuscular or noct urnal, are usually fixed endogenous rhythms, entrained by environmenta l Zeitgebers. Here we demonstrate that juvenile Atlantic salmon, Salmo salar, switch between diurnal and nocturnal foraging solely in respon se to environmental temperature, and independently of photoperiod and season. Above 10-degrees-C juvenile Atlantic salmon fed predominantly during daylight, spending the night exposed in the water column but re latively quiescent. As temperature dropped below 10-degrees-C they bec ame increasingly nocturnal, hiding in refuges by day but emerging to f eed at night. It has previously been shown that parallel physiological changes take place in the retinae of several species of salmonids: th e quantity and composition of the visual pigments change so as to make the fish more dark adapted at low temperatures. As the fish were foun d to be far less aggressive by night than by day at all temperatures, the switch to nocturnal activity was also accompanied by a change in s ocial structure. We suggest that this temperature-dependent strategy m aximizes feeding efficiency in summer but reduces predation risk in wi nter.