River Regulation Causes Inter-Annual Variation in Dispersal Patterns of Riverine Fish Larvae (Chondrostoma nasus)

The fate and survival of small fishes depends greatly on the successful dispersal from spawning sites to nursery habitats. Dispersal patterns in rivers are shaped by the interplay of behavioural reactions and hydraulic forces. The latter can be influenced by river regulation. Alterations in river morphology (e.g. channelization and shoreline embankments) modify the conditions under which dispersal takes place and may impinge on the survival of young. Artificial structures, such as groyne fields and rip-rap, might offer favourable conditions for young fish under certain (low) discharge regimes; however, this may change during rising and higher - below bankfull - levels when hydraulic and erosive forces increase wash-out events. Near natural, complex shoreline configurations presumably dampen those effects and provide shelter for young fishes even at fluctuating hydraulic/hydrological conditions. We tested this hypothesis in a field experiment in the Austrian Danube. In two consecutive years, with different flow characteristics (low-flow, mean-high flow), marked fish larvae were released alongside a near-natural and an artificial shoreline. Inter-annual discrepancies in dispersal and settlement patterns revealed the effects of man-made river obstructions on the most vulnerable early developmental stages of riverine fishes.