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HABITAT MODIFICATION MEASURES IN NORWEGIAN REGULATION SCHEMES 1,2 John E. Brittain, Jon Arne Eie, ZiBige Brabrand, ZSvein Jakob Saltveit and 2Jan Heggenes 1 Environmental Section, Norwegian Water Resources and Energy Administration (N\/E), P.O. Box 5091 Majorstua, 0301 Oslo 3, Norway. ZFreshwater Ecology & Inland Fisheries Laboratory, University of Oslo, Sars gate 1, 0562 Oslo 5, Norway. During recent years there have been increasing efforts in Norway to develop new ways of reducing the environmental effects of future schemes as well as rehabilitating the landscape in and around already regulated rivers. Much of the earlier emphasis was on the development and construction of weirs. Studies of the long-term ecological consequences of weirs are continuing, but additional means of reducing the adverse effects of regulation are also being developed and tested within the framework of the Norwegian Biotooe Adiustment Programme. Among the measures being developed and tested are the improvement of wetland habitats for birds by constructing _ artificial islands, trials for establishing vegetational cover in the drawdown zone of reservoirs and the improvement of fish habitat by changing substrate and 'flow conditions. The programme is illustrated by reference to studies in the Lesjaleirene, drained and channelized to provide additional agricultural land. The river has a very homogeneous sand substrate and areas of rocks and stones have been placed in the river to improve fish habitat. The effect of these measures on fish distribution, density and recruitment is being followed and preliminary results will be presented. The economics of such measures will also be evaluated. ANALYSIS OF THE CHANGES IN FISH COMMUNITY COMPOSITION CAUSED BY RIVER REGULATIONS - THE USE OF THE ABUNDANCE BIOMASS COMPARISON METHOD. 3. Coeck et al., Institute of Nature Conservation, Kiewitdreef 3, B-3500 Hasselt, Belgium. WARWICK (1986) suggested that the distribution of numbers of individuals among species should behave differently from the distribution of biomass among species when an ecosystem is influenced by disturbance. Using combined k-dominance plots for species biomass and numbers, three situations can be distinguished; undisturbed, moderatly disturbed and grossly disturbed. This technique is called the ABC method (Abundance Biomass Comparison). in this study, the applicability of the ABC method is evalua- ted for the assessment of disturbance in water courses, using fish populations. The biomass and density of the fish popula- tions was determined by electrofishing in a number of low- landrivers. Also the level of pollution and the physical habitat was measured at the sample sites. The sample sites represent a range of physicaly disturbed watercourses (e.g. chanalization), going from natural undisturbed sites to heavi- ly channelized reaches. Field data from undisturbed sample sites (good water quality and absence of physical disturbance) support the ABC model. For the disturbed water courses the model was supported as well, but for the rivers that suffer from pollution it was difficult to make a distinction between physical disturbance and disturbance by pollution. There are indications that the ABC method is applicable for the assessment of disturbance in natural fish populations in rivers. However this method gives indications of the physical disturbance together with the disturbance caused by pollution. It seems that the ABC method could be useful to evaluate the disturbance of fish population in unpolluted rivers caused by physical modifications of the river habitat. The ABC method may also give an indication for the succes of the rehabilita- tion of the fish populations in both natural and human induced river restaurations. 16