Increased metabolic cost of swimming and consequent alterations to circadian activity in rainbow trout (Oncorhynchus mykiss) exposed to dietary copper

This study tests the hypothesis that rainbow trout (Oncorhynchus mykiss) compensate for the metabolic cost of dietary Cu exposure by reducing swimming activity at particular times during the die] cycle. Fish were exposed to excess dietary Cu for three months (726 mg Cu.kg(-1) dry weight) and simultaneously oxygen consumption (MO2) and spontaneous swimming activity were measured. Rhythmicity in swimming activity was examined by videorecording fish behaviours for 48 h. Standard metabolic rate estimates (R-s) of 7.2 and 8.7 mmol O-2.kg(-1).h(-1) (15degreesC) were measured for control and Cu-exposed fish, respectively. MO2 was higher in Cu-exposed fish at any chosen speed compared with control Cu-exposed trout, which decreased activity (mean speed) by at least 75%, spent more time at lower speeds, and lost circadian periodicity in these parameters compared with controls. Mean growth rates were normal, although Cu-exposed fish showed a narrower range of body weights and fewer mortalities than control groups, suggesting a suppression in social behaviour in Cu-exposed fish. Overall, the increased metabolic cost of swimming in Cu-exposed fish was fully compensated by a reduction in activity, particularly at night and dawn. However, this behavioural strategy suggests that spatial and temporal aspects of ecologically important social behaviours may be compromised in Cu-exposed fish.