In the deep and slightly eutrophic dam part of the Římov Reservoir (Czech Republic), the fish use of open water habitat was studied by means of gillnetting, hydroacoustics and diet analyses during the period from April to October 2005. The day and night acoustic surveys revealed majority of pelagic fish present in the upper 5 m of the water column. The highest gillnet catches of planktivorous fish in epipelagic waters were obtained in May and August. The lowest fish catches were recorded in April and October which resulted in significant positive correlation between the gillnet catch per unit effort and water temperature. The majority of captured fish were adults of three cyprinid species: roach Rutilus rutilus, bleak Alburnus alburnus and bream Abramis brama. In the early spring the food of the three species was diversified: bream consumed primarily cyclopoid copepods, bleak fed on terrestrial insects, and gut content of pelagic roach consisted mainly of littoral food components, algae and detritus. In the late spring and summer however, the diets of all three cyprinids were predominated by large cladocerans, thus proving the use of open water habitat abundant in this prey.
The winter diet of the great cormorant (Phalacrocorax carbo) was studied by means of examining regurgitated pellets, individual fish bones and fish remains collected from below the roosting trees in two sites on the River Vltava in Vyšší Brod and at Slapy Reservoir, Czech Republic, and by analysis of stomach contents of birds shot on the River Vltava in Prague. Using diagnostic bones (os pharyngeum, dentale, maxillare, praeoperculare) and own linear regression equations between measured dimension of the diagnostic bone and fish total length (LT), a total of 1152 fish of 22 species and 6 families were identified in the diet of great cormorants and their sizes were reconstructed. At all three localities on the main stream of the River Vltava, roach (Rutilus rutilus), bream (Abramis brama), bleak (Alburnus alburnus), European chub (Squalius cephalus), European perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus) made up at least 74.2 % of the cormorants’ diet. A great potential for fish stock losses was identified for the River Vltava at Vyšší Brod and in Prague where the loss of fish due to overwintering great cormorants was estimated to be 22 kg ha–1 and up to 79 kg ha–1 respectively, i.e. belonging among the highest ever published figures for fish withdrawal caused by great cormorants from any inland waters (carp fishponds excluded). Most probably, both great cormorants and anglers are responsible for the decrease in catches of brown trout (Salmo trutta m. fario) and grayling (Thymallus thymallus) from the River Vltava in Vyšší Brod.