INFLUENCE OF MACROINVERTEBRATE DRIFT AND LIGHT ON THE ACTIVITY AND MOVEMENT OF COLORADO RIVER CUTTHROAT TROUT

Most studies of the diel pattern of trout foraging have focused on exa mining gut fullness at different times of the day. We used radioteleme try coupled with stomach content and macroinvertebrate drift sampling to compare the diel activity of Colorado River cutthroat trout Oncorhy nchus clarki pleuriticus with variation of invertebrate drift and ligh t intensity on six dates during summer 1993 in the North Fork Little S nake River, Wyoming. The percentage of trout active was greater during midday than at night (medians, 100% versus 40%) and active fish had f ewer transmitter signal strength fluctuations per minute during crepus cular periods than during the remainder of the day (medians, 8.8 versu s 12.3). Light intensity and daytime macroinvertebrate drift rate were significantly correlated with trout activity but 24-h drift rates wer e not. Stomach samples collected in the afternoon weighed significantl y more than samples collected in the morning. Despite declines in dayt ime drift rate and stream discharge, the diel movement of trout also d eclined as summer progressed. Drift and diet significantly overlapped on all six dates, but selection occurred for certain taxa, especially terrestrial insects. We believe that Colorado River cutthroat trout in this stream forage primarily during the day in summer and that low li ght intensities prevented trout from exploiting the nocturnal increase in drifting invertebrates.