The impact of water level fluctuations on the year class strength of roach: Implications for fish stock management

Water level fluctuations in a drinking water reservoir (Saidenbach Reservoir, Saxony, Germany) and length–frequency data of roach (Rutilus rutilus) derived from gillnet catches were studied over a period of eight succeeding years and subsequently analysed with respect to level change and cohort establishment. The results indicate a strong impact of water level on the year class strength of roach. Constantly high water level with large areas of inundated plants led to a very strong roach year class. Conversely, decreasing water level shortly after the spawning period was found to result in a total loss of the new roach year class. Any other possible factors influencing year class strength of roach (such as food, temperature, winter duration) may only play a role if the water level after spawning remained relatively stable. Consequently, water quantity management in reservoirs can be used as a biomanipulation tool to reduce the planktivorous fish stock and to improve water quality, which should be an interesting option especially in drinking water reservoirs.