The diet of predatory fish in drinking water reservoirs - how can they contribute to biomanipulation efforts?

Efforts to positively influence ecological processes and water quality by manipulating the fish community (biomanipulation) are of particular importance in drinking water reservoirs. One of the principle measures employed is to increase the abundance of predatory fish species as a means of reducing planktonophagous and benthophagous cyprinids. However, there is little information available on the effectiveness of different predatory fish in biomanipulation exercises. We examined the diet of the five dominant predatory species (pike Esox lucius, zander Sander lucioperca, asp Leuciscus aspius, European catfish Silurus glottis, and perch Perca fluviatilis) in five representative reservoirs in the Morava River drainage basin (Czech Republic). Fish prey made up 75 % of total food intake, with undesirable small cyprinids dominant by biomass (40 %). European catfish and asp were not taken as prey and showed no sign of cannibalism. On the other hand, predation on conspecific predatory species (including cannibalism) was relatively high in perch, pike and zander, thereby reducing their net benefit overall. This little-considered aspect of predatory feeding needs to be taken into consideration in future biomanipulation stocking strategies.