Previous studies on plastic pollution of aquatic ecosystems focused on the world's oceans. Large rivers as major pathways for land-based plastic litter, has received less attention so far. Here we report on plastic quantities in the Austrian Danube. A two year survey (2010, 2012) using stationary driftnets detected mean plastic abundance (n = 17,349; mean ± S.D: 316.8 ± 4664.6 items per 1000 m−3) and mass (4.8 ± 24.2 g per 1000 m−3) in the river to be higher than those of drifting larval fish (n = 24,049; 275.3 ± 745.0 individuals. 1000 m−3 and 3.2 ± 8.6 g 1000 m−3). Industrial raw material (pellets, flakes and spherules) accounted for substantial parts (79.4%) of the plastic debris. The plastic input via the Danube into the Black Sea was estimated to 4.2 t per day.

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. Copyright © 2013 John Wiley & Sons, Ltd.

The dispersal patterns of marked larvae of the nase carp (Chondrostoma nasus L.) were observed alongside dissimilar shoreline configurations in the main channel of the free-flowing Austrian Danube and compared with those of floating particles to investigate the mode of dispersal (active–passive). Individuals of different larval stages and floats at similar densities were released at an artificial rip-rap with groynes and a rehabilitated gravel bar. In both habitats, marked individuals were recaptured during the sampling period of 4 d after release. Relevant shoreline attributes for larval dispersal, such as the accessibility of nursery habitats, connectivity between adjacent habitats, and retention potential, were more pronounced at the gravel bar than at the rip-rap. At the gravel bar, larvae moved upstream and downstream within the connected bankside nurseries and displayed longer residence times. Larvae settled in groyne fields along the rip-rap as well; however, longitudinal dispersal was disrupted by groynes, forcing larvae to enter the main channel. Rather than settling in subsequent groyne fields, we assume that these larvae are displaced downstream and potentially lost from the local population.

Navigation alters the ecology of large inland waterways via engineering and more directly by emission of hazardous substances and wave wash. We investigated the potential effects of the latter on the early life history stages of riverine fish, which have their nursery zones generally located in shallow areas with low current and higher water temperatures. Ship induced wave wash causes the following impacts on fish during their early life history stages: i) short-term dislocation of suitable larval and juvenile fish habitats due to wake and splash; ii) water velocities during ship passages frequently exceed maximum swimming performances of 0+fish; and iii) suspended solids concentrations in the inshore habitats increase dramatically and limit the foraging efficiency of young of the year fish (YOY). In situ conditions during ship passages were investigated in three different inshore zones of the Austrian Danube with respect to wake and splash patterns, changes in water velocity and turbidity. These navigation induced conditions in the main nursery zones of riverine fish were set in comparison to results on swimming performances and capacities of larval and juvenile fish obtained in experimental studies. Engineering in large river systems in Europe and USA has resulted in a strong reduction of natural shoreline structures. Within this altered situation, navigation induced wave wash has a strong and cumulative effect on riverine fish recruitment and, hence, contributes to the general ongoing decline of native fish stocks in inland waterways. These aspects require particular consideration in river restoration programs to mitigate the effects of navigation, especially with respect to the intended expansion of European water ways within EC projects like TEN-T and NAIADES.

No abstract