During the summer of 1992 and spring and summer of 1995 and 1996, the fish stocks of four Czech reservoirs, Lipno, Orlik, Slapy and Rimov, were surveyed acoustically. Two acoustic systems (BioSonics model 102 dual-beam 420 kHz echosounder and SIMRAD EY500 split-beam 120 kHz echosounder) were employed to survey the whole bodies by vertical and near-surface horizontal beaming with the transducer at a depth of 40-60 cm. Day and night acoustic surveys were carried out during the spring and summer when fish were unlikely to be undetectable ’hiding’ in the ’acoustically-blind’ zone near the bottom. Fish more than one pulse length from the bottom could be detected in the sonar beam. The main finding of this study was that vertical beaming must be supplemented by horizontal beaming in all the water bodies as the fish were usually confined to depths of 0-4 m due to attraction to warm surface waters in spring and, later, due to avoidance of de-oxygenated hypolimnions. This applied to all the coarse fish species (mainly Perca fluviatilis, Rutilus rutilus and Abramis brama) apart from their larval stages. Fish densities estimated by vertical beaming were two to fifty times lower than those estimated by horizontal beaming due to avoidance of the boat. Split-beam tracking showed that fish remained very close to the surface even within the strata surveyed by the horizontal beam. (C) 1998 Elsevier Science B.V.