Predatory rainbow trout, while not confined to any part of the lake, appear to move into areas of greatest shoal development at a time when their prey, redside shiners, are most heavily concentrated there. A complex movement pattern apparently governed by shoal water temperature leads to a seasonal distribution of shiners which brings the prey into contact with the predator in midsummer.Distribution of the prey on the shoals around the lake shows prey to be most heavily concentrated over areas of extensive shoal and only small numbers present along steep shore areas. In areas of heavy concentration shiners show a definite vertical and horizontal distribution of sizes; the larger prey always found farther from shore or deeper than smaller shiners. In addition, there are vertical seasonal movements of shiners at shoal edge and a diurnal pattern of movement from shoal to deep water after dark.The effect of the movement patterns of predator and prey are manifested in the diet of the trout. In August, shiners constitute over 90% of the diet of predatory trout (those over 10 inches in length), but shiners are a negligible item in the winter diet.