FOOD-CONSUMPTION AND PREY ORIENTATION OF PISCIVOROUS BROWN TROUT (SALMO-TRUTTA) AND PIKEPERCH (STIZOSTEDION-LUCIOPERCA) IN A LARGE REGULATED LAKE

Food consumption and prey orientation of stocked brown trout, Salmo tr utta, and pikeperch, Stizostedion lucioperca, were studied in Lake Oul ujavi, a regulated lake situated in northern Finland. A total of 454 b rown trout and 451 pikeperch were sampled throughout 1994-96. Vendace was the dominant prey for brown trout, and smelt for pikeperch. There appear to be significant differences in prey handling: brown trout ate significantly more prey fish head first whereas pikeperch consumed pr ey equally head or tail first. Estimated by bioenergetics models, 9.1- 56.7 % of the annual age 0 + vendace biomass could be consumed by the two predators. However, when the vendace year class is low, a shift to other prey species or larger size vendace is likely to take place. If fishing mortality on brown trout in particular is reduced, the consum ption of vendace can increase to the extent where an entire year class may be depleted. We stress that the adjustment of the stocking rate o f predators to correspond to the abundance of prey fish is, therefore, one of the key factors in fisheries management.