Longitudinal training dams mitigate effects of shipping on environmental conditions and fish density in the littoral zones of the river Rhine

Highlights: • Impacts of passing ships on abiotic conditions in the littoral zone of rivers were reduced in shore channels behind LTD. • Flow stability was enhanced in the shore channel along the LTD compared to traditional groyne fields. • Fish densities in the littoral zone of the LTD shore channel were significantly higher compared to traditional groyne fields. • Fish densities in stony habitats along the LTD increased linearly with distance to dynamic sections (such as in- and outlet). • LTDs allow for ecological rehabilitation of littoral zones of navigated rivers while enabling multiple uses and flood safety. The stability of habitat conditions in littoral zones of navigated rivers is strongly affected by shipping induced waves and water displacements. In particular, the increase of variability in flow conditions diminishes the suitability of these habitats for juvenile fishes. Recently, a novel ecosystem based river management strategy has resulted in the replacement of traditional river training structures (i.e., groynes) by longitudinal training dams (LTDs), and the creation of shore channels in the river Waal, the main, free-flowing and intensively navigated distributary of the river Rhine in the Netherlands. It was hypothesized that these innovative LTDs mitigated the effects of shipping on fishes by maintaining the natural variability of habitat conditions in the littoral zones during ship passages whereby shore channels served as refugia for juvenile fishes. Measurements of abiotic conditions showed a significantly lower water level fluctuation and significantly higher flow stability in shore channels compared to groyne fields. Flow velocity did not differ, nor did the variation in flow velocity fluctuation during ship passage between these habitats. Densities of fish were found to be significantly higher in the littoral zones of shore channels compared to nearby groyne fields. Moreover, electrofishing along the inner side of the newly constructed LTD showed a significant linear relationship between fish density and distance from highly dynamic in- and outflow sections and to lowered inflow sections in the LTD. Results of our field sampling clearly indicate successful ecological rehabilitation of littoral zones that coincides with a facilitation of navigation in the main river channel and increased flood safety.