Food-web manipulation in the Maltanski reservoir

The Maltanski Reservoir (64 ha, mean depth 3.1 m) was emptied in September 1992, and all fish were harvested. Following refilling in the spring of 1993, fry of eel (Anguilla anguilla), pike (Esox lucius) and wels (Silurus glanis) were introduced, in total 236 fry ha(-1). In 1994 the reservoir was additionally stocked with adult pike in spring (10 kg ha(-1)), with pike fry in summer (156 specimens ha(-1)), and pike and 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 bloom caused by Aphanizomenon flos-aquae in summer. In the spring of 1994 only a short water bloom caused by diatoms was observed. Not only zooplankton grazing but also low phosphorus content limited the growth of phytoplankton biomass in this period. At the beginning of summer cladoceran biomass decreased, leading to a strong water bloom dominated by Planktothrix agardhii. During this event water transparency decreased to 0.3 m. This was a result of insufficient introduction of predatory fish, which could not ensure effective control of the rapidly increasing plankivorous fry numbers.