Review on impacts of artificial rapid and frequent flow changes on salmonids in fast flowing waters

Salmonid fishes are keystone species in riverine environments in temperate and polar environments. Salmonid fishes have naturally evolved to cope with severe fluctuations in important niche parameters like variations in water level, but hydropower regulation may represent extreme and unnatural situations especially with respect to flow changes (e.g. hydro peaking). Rapid flow fluctuations affect salmonids differently depending on life stage where time of day and season are strong interacting factors. Therefore there is increasing need for making seasonal operational guidelines for hydropower plant operators. This can be illustrated by: daytime feeding is dominating in summer contrary to winter nighttime feeding and available habitat for small young-of-the-year is dominated by shallow margins. The winter life of riverine salmonids appears to be dominated by energy saving where the fish uses shelters contrary to the spring situation when salmonid fishes store large amounts of energy during relative short time periods. The macroinvertebrate riverine fauna may be severely affected by the same hydrological changes as fish are affected by. This can again lead to changes in food availability and feeding patterns with changes in interspecific or intraspecific competition. Knowledge is missing to predict effects on whole populations and their long term survival and performance. Modeling such population aspects should become a major task for future investigations.