Pike predation on hatchery-reared Atlantic salmon smolts in a northern Baltic river

The effect of pike Esox lucius predation on the mortality of newly stocked Atlantic salmon Salmo salar smolts was investigated in the Pyhajoki River, Finland. The number of smolts eaten by pike was assessed by estimating the size of the pike population (mark-recapture experiment) and studying the stomach contents of pike. Before recapturing the pike, approximately 39,700 smolts were stocked upstream of the 2.5-km-long (89-ha) research area. The estimated size of the >40-cm pike population was 1507 (95% CL 1012-4731) individuals (17 pike and 29.8 kg.ha(-1)). Pike were estimated to eat 29% of the released smolts during 1 week. The diet of the pike in the research area consisted almost entirely of smolts, whereas in the reference area with no stocked smolts, the meal sizes were significantly smaller and the importance of smolts as prey was substantially lower. Pike < 40 cm had not eaten any smolts, probably indicating a size refuge for the smolts, or alternatively fear of intraspecific interactions or cannibalism of pike.