Changes in the species composition and biomass of aquatic macrophytes in lakes and streams have dramatic impact on the physio-chemical and biological conditions. They strongly influence the plant-associated invertebrate fauna. The aquatic macrophytes have a central position in the network of ecological relations between nutrients, plankton and macro-invertebrates. Therefore, they also determine the carrying capacity for the population of fish species like pike, most cyprinids and eel. The negative effects of eutrophication on the fishery conditions are mediated through these aquatic plants. Research directed towards quantifying the effect of habitat change on fisheries should be aware of the direct and indirect relations between fish and aquatic plants.
Research on fisheries often is carried out independently and it is rarely fitted within the framework of limnological studies covering also the lower trophic levels (phytoplankton, epiphytes, plants and macro-invertebrates). Research aimed at improving habitats for fish should link up with ecological research engaged in "ecosystem restoration".
Nowadays there is a certain consensus about the models of interaction between the invertebrate grazers, macrophytes, phytoplankton and nutrients derived from isolated process studies and small scale experiments. In cases where long-term base line studies are available on larger water bodies, the impact of these mainly hypothetic interactions between fish, invertebrates and plants have to be tested on the ecosystem level by whole lake manipulations. These ecosystem experiments should be designed carefully and ought to include meaningful control or reference objects (Likens, 1985).
Removing large parts of the fish population may improve water quality. Although for the short term this type of biomanipulation appears of no importance for fisheries, these experiments may result in the long term in the restoration of fish habitats. Stocking with piscivorous fish and other means to manipulate the young-of-the-year fish populations should be implemented in these large-scale ecosystem experiments. Besides these large-scale ecosystem experiments, the painstaking study of relations between fish, grazers and macrophytes is important.
This research should be carried, out by teams of scientists investigating different trophic levels (fish, macro-invertebrates, aquatic plants, plankton and their physico-chemical environment). Because restoration will serve the multiple purpose of water bodies, these teams should be financially supported by the different organizations (recreational and commercial fishery, Water Boards and other governmental organizations) involved in water management. International cooperation is needed, because problems in the different industrialized countries have much in common. The exchange of know-how by means of small workshops is probably the most rapid and effective way to steer this research.
Acknowledgements.
During .the preparation of this review I had discussions with Cees den Hartog (Dept. Aquatic Ecology University of Nijmegen) and Wolter van der Zweerde (Centre for Aerobiological Research. Wageningen). Earlier drafts of the manuscript have been commented by Elly Best (Centre for Agrobiological Research). Wim van Densen (Dept. of Fisheries. Agricultural University Wageningen). Brian Moss (School of Environmental Sciences. University of East Anglia). Jan Osse (Dept. of Zoology. Agricultural University Wageningen). Wim van Vierssen (Dept. of Nature Conservation. Agricultural University Wageningen) and Koos Vijverberg (Limno1ogica1 Institute. Tjeukemeer Laboratory). Further on I got much information from the manuscript of Steven Carpenter and David Lodge, who reviewed the ecological research on submersed aquatic macrophytes.
Moreover I thank the Directorate of Fisheries of the Netherlands for funding this study and for the support by Bert Steinmetz, as well as the Agricultural University of Wageningen for the provided facilities.
