Effects of turbidity on feeding and distribution of fish

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 the 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 the 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 a consequence of either optimal foraging behaviour or a trade-off between foraging and predator avoidance. In turbid lakes, diel horizontal migration patterns could differ since turbidity can act as a refuge and affect the predator-prey interactions. Laboratory experiments were conducted with perch (Perca fluviatilis (L.)) and white bream (Abramis bjorkna (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. Muller), and Chaoborus flavicans (Meigen) were used as prey in different experiments. To clarify the role of turbidity in the 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 the distribution of perch and roach in these two lakes over a 15 year study period. The 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 are probably better at using turbidity as a refuge compared to Daphnia. Increase in turbidity probably has a stronger effect on perch predating on plant-attached prey. The main findings of this thesis show that turbidity can play a significant role in the 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. Floatingleaved macrophytes probably provide good refuge 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. The turbidity of water should be taken into account when studying the diel horizontal migrations and distribution of fish in shallow lakes. LIST OF ORIGINAL PUBLICATIONS 6 CONTRIBUTION OF AUTHORS 7