Abstract

To describe the activity patterns of juvenile Atlantic salmon, I monitored the behaviour of 35 age-0+ (emerged from gravel in the spring) and eight age-1+ (emerged from gravel in the spring of the previous year) fish over an eight-week-summer field season. Active fish were visible on their home range whereas inactive fish were assumed to be sheltering. My data were consistent with the prediction of the Asset Protection Principle that larger individuals will take fewer chances in order to reduce risk of predation. Indeed, 1+ fish (larger and older) were more active at night than during the day, whereas 0+ fish were almost exclusively active during the day. In contrast to my expectations, however, daytime activity did not peak at the optimal water temperature of 17C̕ found in laboratory studies of other populations. Rather, the activity of 1+ fish peaked at 20.7C̕, whereas the activity of 0+ fish continued to increase until 23C̕ and then leveled off between 23-27C̕. Once individuals were active, season and light intensity were the variables that most influenced their foraging rate. There was considerable individual variability within an age-class in how fish responded to environmental variables. The causes of this individual variability and the consequences for growth deserve further study.