Trophic structure of nine Czech reservoirs regularly stocked with piscivorous fish

For nearly 20 years, most Czech reservoirs supplying drinking water have be en under statutory protection which permitted reservoir managers to manipul ate fish stocks in order to maintain a sustainable water quality. The most common biomanipulative measure adopted was stocking with piscivorous fish ( mostly 5 cm fry) using an annual stocking level of approx. 25 000 fish per reservoir. Nine reservoirs were studied for signs of top-down food web effe cts, as predicted by the trophic cascade hypothesis based on levels of tota l phosphorus (TP), chlorophyll a (Chl a), zooplankton biomass (ZB) and zoop lankton community structure. In all nine reservoirs, only small Daphnia spe cies were recorded, such as D. galeata and D. cucullata. The proportion of large-bodied daphnids retained on a 0.71 mm sieve was less than 10% of the total crustacean biomass in all reservoirs. The relationship of Chl a level - TP, and of ZB - Chl a, was positive under enhanced piscivory and did not differ statistically from the relationships in other reservoirs with natur al fish stocks. This implies that bottom-up forces remained stronger than t op-down ones in the studied reservoirs, despite the stocking of piscivorous fish. The failure of this attempt at biomanipulation may be due to an insu fficient stocking rate of predatory fish and/or inadequate data on the resi dent planktivorous fish levels.