HORIZONTAL AND VERTICAL MOVEMENTS OF TELEMETERED RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) IN LAKE WASHINGTON

In summer and fall 1989, six rainbow trout (Oncorhynchus mykiss) were tracked in Lake Washington with ultrasonic transmitters for a total of 349 h to determine their movements in relation to the distribution of possible prey species. The trout moved primarily in the nearshore are a at an average velocity of 12.4 cm/s (similar to 0.25-0.3 body length s/s). Five of the six fish made one more rapid (>1 body lengths/s) exc ursion across the lake, then continued moving in the nearshore zone. T he trout were generally inactive, staying close (<50 m) to shore at ni ght, and became more active near dawn; however, the highest average ve locities were at dusk. They spent over 90% of their time in the top 3 m of the water column and 10% in brief (2 min), shallow (mean 6.6 m) d ives. Dives occurred most frequently at dawn and during the day (0.8/h ), less often near dusk (0.5/h), and seldom at night (0.1/h). The dept h distribution and movement patterns suggest that the trout were feedi ng on Daphnia pulicaria during the day, in both nearshore and offshore areas, supplementing this diet with nearshore fishes such as prickly sculpins (Cottus asper). Predation on pelagic planktivores (longfin sm elt, Spirinchus thaleichthyes, and juvenile sockeye salmon, Oncorhynch us nerka) was unlikely because the trout were primarily found nearshor e and near the surface, whereas the planktivores are primarily offshor e and closer to the bottom.