FOOD-WEB MANIPULATION IN THE MALTANSKI RESERVOIR

The Maltanski Reservoir (64 ha, mean depth 3.1 m) was emptied in Septe mber 1992, and all fish were harvested. Following refilling in the spr ing of 1993, fry of eel (Anguilla anguilla), pike (Esox lucius) and we ls (Silurus glanis) were introduced, in total 236 fry ha(-1). In 1994 the reservoir was additionally stocked with adult pike in spring (10 k g ha(-1)), with pike fry in summer (156 specimens ha(-1)), and pike an d pikeperch (Stizostedion lucioperca) fry in the autumn (413 specimens ha(-1)). High water transparency and low concentration of chlorophyll -a were observed throughout 1993 except for a short period of water bl oom caused by Aphanizomenon flos-aquae in summer. In the spring of 199 4 only a short water bloom caused by diatoms was observed. Not only zo oplankton grazing but also low phosphorus content limited the growth o f phytoplankton biomass in this period. At the beginning of summer cla doceran biomass decreased, leading to a strong water bloom dominated b y Planktothrix agardhii. During this event water transparency decrease d to 0.3 m. This was a result of insufficient introduction of predator y fish, which could not ensure effective control of the rapidly increa sing plankivorous fry numbers.