Deep pools of the Danube River: ecological function or turbulent sink?

Danube main channel deep pools are in-stream habitats of high ecological relevance. We used dual-frequency identification sonar to investigate seasonal and diel fish abundance in two hydrodynamically different deep pools in the main channel of the Danube River in 2008. In general, fish of different species were present in both deep pools throughout the year, irrespective of water level. High fish abundance was recorded during autumn and winter, low abundance during spring and summer. During low discharge and low temperature and particularly during low flow velocity, low kinetic energy and low shear stress, high fish abundance in densely packed aggregations were observed, a clear indication for the refuge capacity of deep pools. In contrast, during turbulent hydraulic conditions with high flow velocity, high turbulent kinetic energy and high shear stress, low fish abundance occurred, mainly consisting of single fish, small fish shoals and benthic fish specimens. Furthermore, we could reveal that calm as well as turbulent pools fulfil the requirements as refuge and feeding habitat of eurytopic, piscivorous catfish throughout the year. The application of a multiple linear model based on the hydrological parameters discharge and water temperature as well as the hydraulic parameters flow velocity, turbulent kinetic energy and shear stress provided highly significant evidence between the observed and the expected seasonal fish abundance in the deep pools (R ² = 0.77, P < 0.001). For the first time these results make clear that the main channel deep pools are important refuge and resource habitats of different species of the Danube fish community. Deep pools are key habitats for the in-stream fish community of the Danube River.