CONDITIONS FOR EFFECTIVE BIOMANIPULATION - CONCLUSIONS DERIVED FROM WHOLE-LAKE EXPERIMENTS IN EUROPE

Since indirect effects, feedback mechanisms and time lag play an important role in top-down manipulated food webs, conclusions regarding the reliability of biomanipulation as a management tool should be derived only from whole-lake studies, the results of which were followed for at least three to five years. There are only a few experiments and applications of that type from which the following conclusions can be drawn: (1) Food web manipulation should be combined with resource limitation measures (nutrients or light) if a 'biomanipulation-efficiency threshold of the phosphorus loading' is exceeded. (2) The critical biomass of undesirable fish shows a very wide range of variation (20-1000 kg ha-1) and depends on several factors, like species and age of fish, total food assemblage, refugia etc. (3) The maximum possible stability of food web manipulation is probably achieved at a biomass of undesirable fish a little below the critical biomass rather than by extermination of these fish. (4) High piscivore diversity increases stability and reliability of food web manipulations. (5) Various techniques of food web manipulation should be combined, except poisoning the entire fish community. © 1990 Kluwer Academic Publishers.