Successful stocking of wild European perch in a reestablished lake with lasting effect over 10 years

Biological manipulation of fish compositions to enhance top‐down control of trophic webs is a common method for improving water and ecological quality in shallow eutrophic lakes, but it often fails. As an alternative to manually thinning unfavored fish stocks, piscivorous fish species can be released to suppress the recruitment of cyprinid fish. Piscivorous pike ( Esox lucius ) have been widely released to Danish lakes, but with small effects, while manual fish thinning mostly has a transient impact. As an alternative to strengthening piscivorous relative to planktivorous fish populations, we released 250 kg (0.3 kg/ha) of wild, local, adult European perch ( Perca fluviatilis ) to the shallow, newly reestablished 900 ha Lake Fil, Denmark. We followed Lake Fil's fish population dynamics and environmental drivers over the following 10 years. Recruitment of perch fry was high immediately after stocking, and perch biomass increased from 0.4 to 2.5 kg per unit effort of standard net catching. Over the following years, the perch biomass has remained relatively high (1.0–1.3 kg) with successful recruitment and fluctuating size structure depending on environmental changes and other prey–predator interactions. Although large densities of piscivorous great cormorants ( Phalacrocorax carbo ) and severe oxygen depletion and fish kill occurred, the perch population was not severely decimated. Our results imply that stocking new lakes lacking piscivorous fish with large wild perch in advance of their spawning period can promote the formation of sustainable piscivorous fish stocks, which are important for reaching and maintaining favorable food webs and ecological conditions in newly reestablished and old lakes.