(Chairmen: M.P. Grimm, J. Vostradovsky)
In highly productive lakes a high abundance of cyprinids and a relatively low predator stock leave room for the speculation that enhancement of pike-perch population by stocking advanced pike-perch fingerlings might be an option in reducing planktivorous fish (van Densen). If not, stocking of pikeperch fingerlings may smooth down strong year class variations in this species. Intensive rearing appears biologically possible, but its economics needs to be assessed.
Biological control by predator species primarily means transfer of energy from planktivorous to the predator compartment of the food chain. Considering the preference of most predators for a preyfish length of 5–15 cm, it can be expected that competition among predators counteracts the impact of stocking additional predators (van Densen).
The growth of pike and perch can be related to the structural complexity of the environment: in a lake where vegetation disappeared in mid-summer, growth of pike was retarded (Walker). This confirms the experimental work of Persson.
The predation of the exotic Nile perch on fishes of Lake Victoria, East Africa, has resulted in substantial changes in the whole ecosystem, which became unstable. In spite of the mass removal of the predatory Nile perch through intensive fishery, the organic matter in the lake continues to increase, as well as the volume of deoxygenated water. Fish kills are becoming more frequent. Lack of limnological data for the pre-Nile perch situation does not allow a good assessment of changes in the ecosystem (Gophen).
The discussion focused on the structural complexities of ecosystems and recognized the levels of nutrients as important steering components. More attention needs to be paid to research on critical nutrient levels to determine optimal situations for an improved management of stressed ecosystems.
