In aquatic systems, the ability of both the predator and prey to detect each other may be impaired by turbidity. This could lead to significant changes in the trophic interactions in the food web of lakes. Most fish use their vision for predation and the location of prey can be highly influenced by light level and clarity of the water environment. Turbidity is an optical property of water that causes light to be scattered and absorbed by particles and molecules. Turbidity is highly variable in lakes, due to seasonal changes in suspended sediments, algal blooms and wind-driven suspension of sediments especially in shallow waters. There is evidence that human activity has increased erosion leading to increased turbidity in aquatic systems. Turbidity could also play a significant role in distribution of fish. Turbidity could act as a cover for small fish and reduce predation risk. Diel horizontal migration by fish is common in shallow lakes and is considered as consequences of either optimal foraging behaviour for food or as a trade-off between foraging and predator avoidance. In turbid lakes, diel horizontal migration patterns could differ since turbidity can act as a refuge itself and affect the predator-prey interactions. Laboratory experiments were conducted with perch (Perca fluviatilis L.) and white bream (Abramis björkna (L.)) to clarify the effects of turbidity on their feeding. Additionally to clarify the effects of turbidity on predator preying on different types of prey, pikeperch larvae (Sander lucioperca (L.)), Daphnia pulex (Leydig), Sida crystallina (O.F. Müller), and Chaoborus flavicans (Meigen) were used as prey in different experiments. To clarify the role of turbidity in distribution and diel horizontal migration of perch, roach (Rutilus rutilus (L.)) and white bream, field studies were conducted in shallow turbid lakes. A clear and a turbid shallow lake were compared to investigate distribution of perch and roach in these two lakes in a 15-year study period. Feeding efficiency of perch and white bream was not significantly affected with increasing clay turbidity up to 50 NTU. The perch experiments with pikeperch larvae suggested that clay turbidity could act as a refuge especially at turbidity levels higher than 50 NTU. Perch experiments with different prey types suggested that pikeperch larvae probably use turbidity as a refuge better compared to Daphnia. Increase in turbidity probably has stronger affect on perch predating on plant-attached prey. The main findings of the thesis show that turbidity can play a significant role in distribution of fish. Perch and roach could use turbidity as refuge when macrophytes disappear while small perch may also use high turbidity as refuge when macrophytes are present. Floating-leaved macrophytes are probably good refuges for small fish in clay-turbid lakes and provide a certain level of turbidity and not too complex structure for refuge. The results give light to the predator-prey interactions in turbid environments. Turbidity of water should be taken in to account when studying the diel horizontal migrations and distribution of fish in shallow lakes.Increase in turbidity in aquatic systems has become a major problem during the last century. Turbidity is caused by particles that are suspended in the water such as clay or phytoplankton. Turbidity is highly variable in aquatic systems, due to seasonal changes in suspended sediments, algal blooms and wind-driven suspension of sediments especially in shallow waters. There is evidence that human activity has increased erosion leading to increased turbidity in aquatic systems. Besides the negative affects of turbidity on drinking water or recreational use of lakes, turbidity can significantly affect the predator- prey interactions in aquatic systems. Most fish use their vision for predation and the location of prey can be highly influenced by clarity of the water environment. I have investigated the effects of mainly clay turbidity on the feeding of fish species as perch (Perca fluviatilis L.) and white bream (Abramis björkna (L.)). Perch and white bream are common species found in lakes in Europe. Laboratory experiments showed that high turbidity levels affected the feeding of perch while white bream were not affected by increasing turbidity levels. The effect of turbidity changed with the type of prey the fish were feeding on. Fish larvae used turbidity as refuge while zooplankton were not protected by predation at even high turbidity levels. I also investigated the distribution of fish with changing turbidity levels within a lake and between two lakes. Turbidity could act as a cover and protect small fish from predators such as large fish and birds. Fish show diel migration in lakes in order to avoid predators or to find food. The small perch seemed to be using turbidity as a refuge more compared to macrophytes which are usually good refuge habitats for small fish in lakes. Also small perch and roach distributed around the lake when lake switched from a clear to turbid stage. The results give light to the predator-prey interactions in turbid environments. Turbidity of water should be taken in to account when studying the diel horizontal migrations and distribution of fish in shallow lakes
