Cormorant predation overlaps with fish communities and commercial-fishery interest in a Swedish lake

The increase of the fish-eating cormorant (Phalacrocorax carbo sinensis) in Europe has resulted in conflicts with fisheries. In Lake Roxen, Sweden, cormorants are blamed for causing a decrease in fishery catches. To study and describe the potential effects that cormorants may have had on fish in the lake, their diet was analysed in relation to fish catches in gill-net surveys and fishery catches. Estimates of predation were achieved by ‘tag and recovery’ on eel, pike-perch and perch. Cormorants predated on the most common species and sizes, which were mainly smaller perch, ruffe and roach (mean sizes of 9, 8 and 13 cm respectively). Tag recoveries from perch, eel and pike-perch detected predation estimates of 14, 7 and 15% respectively. From a highly eutrophic state, the lake has shown improvements in water quality and a development towards larger predatory fish was expected, but the results from gill-net surveys did not show this. Results indicated that cormorants and fisheries may both be responsible, but because cormorants remove more fish, they may be the main factor for the lack of recovery of large predatory fish. Their predation keeps recruitment high, but the number of fish that reach large sizes remains low.