Behavioural interactions between prey (trout smolts) and predators (pike and pikeperch) in an impounded river

Movements of radio-tagged pike Esox lucius (L.), pikeperch Stizostedion lucioperca (L.) and outward migrating sea trout smolts Salmo trutta (L.) were studied in a shallow Danish reservoir to obtain information of predator–prey interactions between these species. Twenty pikeperch (55–74 cm) and 19 pike (52–72 cm) were tagged. Female pikeperch spent more time near the outlet sluice during the smolt run (May) than at other times of the year, apparently actively hunting the smolts delayed in this area. In contrast, male pikeperch did not seem to participate in the smolt predation but remained stationary during the smolt run, presumably guarding their nests. Most tagged pike were present at the spawning grounds during the peak of the smolt run, where they had little chance of smolt encounter. Twenty migrating trout smolts were radio-tagged in the river upstream of the reservoir. Ten of these were located in the vicinity of the outlet sluice at least once, but were unwilling or unable to find and enter the sub-surface outlet sluice. Only one tagged smolt left the reservoir. After 1–12 days in the reservoir, the remaining smolts were eaten by pikeperch or pike and the results indicate that female pikeperch and few female pike have adjusted their behaviour to predation on smolts during the smolt run. The smolt predation in this man-made reservoir is higher than in natural lakes, probably due to the changed physical environment and introduced predators, such as pikeperch. The outlet sluice practice and the temporal overlap between smolt run and predator-spawning may be key factors in smolt survival. Copyright © 2000 John Wiley & Sons, Ltd.