Migration, growth patterns, and diet of pike (Esox lucius) in a river reservoir and its inflowing river

The pike population in the Løpsjøen reservoir and the accessible 21 km section of the inflowing River Rena was investigated from 2003 to 2013. Telemetry and mark-recapture with Floy tags demonstrated that most fish had an annual home range of less than 2 km, while some fish performed quite extensive migrations (up to 14.4 km). Of fish radio-tagged in the reservoir, 57% were positioned in the river at least once. The population of pike ≥25 cm was estimated at 1002 fish, or 3.9 fish (3.17 kg) ha−1. Mean length at age was significantly larger for pike caught in the river than in the reservoir for almost all age groups. CPUE during boat electro-fishing indicated significantly lower population densities and a lower proportion of juvenile fish in the river than in the reservoir. Diet analysis revealed that almost all available prey fish species were taken by pike, and that there was a clear diet differentiation between pike size classes, and between fish from the river and reservoir habitats. Small pike (<25 cm) in the reservoir had eaten mainly invertebrates, whereas smaller pike and Alpine bullhead (Cottus poecilopus) dominated in small pike from the river. Nine prey fish species were identified in the stomachs of medium sized pike (25–50 cm) in the reservoir, with brook lamprey (Lampetra planeri) and brown trout (Salmo trutta) as dominant. Medium sized pike in the river had taken mainly lamprey and Alpine bullhead. Larger pike (>50 cm) in the reservoir had taken brown trout as well as a number of other prey fish, while river pike of this size had taken burbot (Lota lota), lamprey and brown trout. Species like perch (Perca fluviatilis), roach (Rutilus rutilus) and in particular grayling (Thymallus thymallus) were rarely found in pike stomachs. The construction of the reservoir in a fast-flowing river caused the establishment of a healthy pike population and additional predation pressure on the rheophilic salmonids in the system, both in the reservoir and in the inflowing river.