Hydraulic forces impact larval fish drift in the free flowing section of a large European river

Dispersal of fluvial freshwater fish larvae occurs commonly in heavily regulated rivers. Studies increasingly indicate that drifting young fish have an active component to their movement and so are capable of coping with the dynamic hydraulic forces typical of rivers. We investigated hydraulic-drift relationships of the young stages of fish over one breeding season along a gravel shore of the Austrian Danube using stationary drift nets from the first occurrence until the end of the seasonal peak (April-June 2008). Observed densities of families and developmental stages were related to the hydraulic parameters flow velocity (in three spatial directions), turbulent kinetic energy and water depth, derived from a three-dimensional hydrodynamic model of the sampling site. We detected distinct responses of drifting young fish to several hydro-physical factors under conditions (low light level, over-critical currents), which were considered to cause washouts and passive drift. In general, drift densities decreased with water depth and flow velocity. Weak swimmers (e.g. bull head Cottus gobio and early-stage cyprinid larvae) avoided turbulent flows, as their abilities to orientate and hold position may be limited. Early larval as well as early juvenile stages of cyprinids used lateral currents directing to the bank, potentially to drop out of the flow and reach inshore areas. Our study indicates that there are family-specific and stage-specific responses to hydraulic variables and that fish actively disperse, probably to minimize mortality and maximize successful dispersal.