Summer diel activity and movement of adult brown trout in high-elevation streams in Wyoming, USA

The variation in diel activity patterns was determined by implanting transm itters in adult brown trout of various sizes in two montane streams in two summers in the central Rocky Mountains, U.S.A. More brown trout tended to b e active, and to move greater distances, each hour during twilight and nigh t than during the day. Mean hourly light intensity was significantly negati vely correlated with the proportion of fish active and with the distance mo ved. Maximum water temperature was not correlated with any measure of diel activity. Brown trout displayed fidelity to diurnal positions within a give n diel cycle. Overall, fish were active an average of 11 h, had an average home range of 41 m, and moved an average of 121 m over the diel cycle. Fish length and distance moved were significantly positively correlated. Water temperature, predator avoidance, and foraging tactics may influence these p atterns of diel behaviour in adult brown trout in U.S. streams. (C) 1999 Th e Fisheries Society of the British Isles.