Effect of brown trout body size on post‐stocking survival and pike predation

A total of 40 (20 age‐3 + 20 age‐4) radio‐tagged hatchery‐reared brown trout (Salmo trutta L.) and 40 wild radio‐tagged northern pike (Esox lucius L.) were released into a regulated river. Age‐3 brown trout were predicted and observed to be highly vulnerable to predation by pike (50% mortality), whereas age‐4 brown trout were predicted and observed to enjoy an almost absolute size refuge from predation (5% mortality). Almost half of the fish from both age groups similarly emigrated and survived from the river within 3 days of the release. However, there was a considerable difference in survival between age groups for fish that remained in the river for a longer period. Of these, all except one age‐3 brown trout were eaten by pike, whereas all but one age‐4 fish survived predation.