Can feeding of European catfish prevent cyprinids from reaching a size refuge?

The size structure of a predator population can cause differences in the relative survival of different prey length classes whereby the biggest prey may reach a safe size because of the size-dependent character of predation. In the present study, the diet of European catfish was investigated to examine if catfish feeding can prevent cyprinids from reaching such a size refuge. In the lake studied, catfish was stocked for biomanipulation purposes to reduce unwanted roach and bream populations. Crayfish and roach were the most important prey items of catfish. If only species composition in the diet was considered, no clear changes were recorded in relation to catfish size. However, the length of roach as prey significantly increased with catfish length. Catfish were significantly larger than the other piscivorous fish in the lake, but took relatively smaller roach in comparison with similar-sized pike or pikeperch. Nevertheless, because of the high mean length of catfish, roach cannot reach a size refuge. For unknown reasons, the expected and intended predation on bream was not observed. Catfish took smaller prey than could be expected from mouth gape data. By extending the relationship between catfish as predator and roach as prey beyond the predator length range currently found in the lake, it could be shown that even catfish of 150 cm length will probably feed upon only 65% of all available bream length classes. Therefore, stocking with catfish cannot prevent a size refuge for the bream.