Composition and biomass of the fish stocks in various European reservoirs and ecological consequences

Fish stocks in seven Czech, four British and three Dutch reservoirs were su rveyed quantitatively once or several times, in a total of 21 surveys. The open water of the reservoirs was studied by mobile survey with scientific e chosounders using both vertical and horizontal beaming. The littoral region s were sampled by means of quantitative shore seining carried out on the sa me night as the acoustic survey. The estimated biomass of fish older than 0 + for different reservoirs ranged from 5 - 600 kg/ha. This wide range is a likely result of site-specific conditions. Although the trophy of the studi ed reservoirs spans over a wide range of total phosphorus concentrations (3 0 - 1000 mg.m(-3)), there is no link between the fish biomass and reservoir trophy. Fish abundance and biomass were very low in the basin-like banksid e reservoirs whose sides are covered with concrete slabs or asphalt. Two ot her factors can lead to low fish stocks, namely hydropeaking power generati on and long-term biomanipulation, although to a lesser extent. Reduction of the fish stock to < 100 kg/ha is accompanied by an increase in the body si ze of zooplankton. Further reduction to < 30 kg/ha results in (a) dense gro wth of filamentous algae in the littoral margins either due to increased wa ter transparency or little fish disturbance and (b) to the dominance of per ciform fish over cyprinids. In densely populated reservoirs without the abo ve limitations, perciform fish are always less common than cyprinids. Conse quences of the ratio between inshore and offshore fish are discussed.