AGGRESSIVE-BEHAVIOR OF UNDERYEARLING RAINBOW-TROUT IN SIMULATED WINTER CONCEALMENT HABITAT

The behaviour of wild underyearling rainbow trout Oncorhynchus mykiss in concealment habitat in a laboratory stream aquarium at 2 and 6 degr ees C was recorded daily with an infrared video camera for 90 min over dawn. Aggressive behaviours (threat nips, nips and chases) were frequ ent during this time as fish entered concealment habitat. Aggressivene ss varied widely among fish groups, with a range of 1 to 45 aggressive acts being initiated during a 90-min filming period. Larger fish init iated most of the encounters and removed a higher proportion of fish f rom concealment than did smaller aggressors. Thirteen per cent of the aggressive acts resulted in the recipient being completely removed fro m concealment habitat. The highest combined frequency of aggressive ac ts was 0.64 per visible fish per 10 min period and occurred al relativ ely low light levels (300 Ix). The data suggest that when fish density is high, such as when habitat is limited, aggressive behaviour in win ter may cause some underyearling salmonids to be excluded from conceal ment. (C) 1996 The Fisheries Society of the British Isles