Effects of water level fluctuations on the early life stages of fish in the littoral zone of lakes

In a mesocosm study, somatic and otolith growth of common dace (Leuciscus leuciscus) and bream (Abramis brama) differing in body size and body shape were studied in a wave and a no-wave control regime. A low-intensity wave treatment, causing hydrodynamic stress comparable to that resulting from distant ship traffic or moderate wind, caused significant changes in somatic growth and otolith daily increment width (ODIW) of the experimental fish. Three different reaction types could be distinguished, correlating with the body morphology of the six fish groups investigated in this study. Small and fusiform dace and bream benefited from low-intensity waves, showing higher somatic growth rates and greater ODIW in the wave treatment. In small, deep-bodied bream, growth and ODIW were reduced by waves. Finally, in larger fusiform dace and deep-bodied bream, ODIW was decoupled from somatic growth, with larger ODIW in waves, but reduced somatic growth, suggesting that these fish were obliged to allocate considerably more energy to activity when exposed to the wave treatment. The results show that even moderate hydrodynamic stress from low-intensity waves has a serious impact on lake habitat quality and the competitive fitness of littoral fish species in lakes.