The influence of predation risk on habitat selection and food intake by Arctic charr, Salvelinus alpinus (L)

Large piscivorous fish are assumed to affect habitat selection and food intake of prey fish. To study the effects of cannibalistic Arctic charr, Salvelinus alpinus (L.), on smaller stunted charr, we sampled the prey fish in littoral and pelagic habitats using gill nets, before and shortly after the release of large charr in a small lake (0.52 km(2)). In the habitats where the risk of predation was highest, the catch per unit effort decreased from 13.3 to 4.8 fish per 100 m(2) of gillnet after release of predators. The large decrease in numbers of charr <18 cm corresponded with the predicted vulnerable prey sizes, according to a model based on the size distribution of predators. The occurrence of planktivorous fish and weight-specific food intake decreased in the high risk habitat and remained unaffected in the low risk habitats. Changes in the food intake of prey fish could not be explained in terms of fish length, indicating that prey fish changed diet when the risk of predation was high.