Session E3: Overall Downstream Migration Success

Abstract: The Rhine countries identified 552 major bottlenecks (height \u3e 2 m) for downstream migration along the river Rhine and its bigger tributaries (\u3e 2500 km², map to be shown). Only few obstacles have downstream migration facilities installed (29 out of 552; about 5%). Little is known exactly on mortality during downstream migration, even if downstream migration facilities have been installed. Though there is an expected positive effect of those facilities. In France, some estimations are made on the downstream migration success. The effect of hydropower on fish migration does not only depend on individual installations, but also on the amount and type of installations in a certain river stretch. A study on the Moselle (2009) showed that Eels that have to pass all hydropower installations only have 3% chance of reaching the mouth of the Moselle unharmed. A recent study on the Ill (2014) on Eel and Salmon (Smolts) took into account the spread of the populations in the river basin, their periods of downstream migration and the hydrological data during those periods, as well as the data on the hydropower installations in the river basin. It concluded a reduction of the production of the Salmon with 26% and of the Eel of 29% because of hydropower. A combined German-Dutch study to the success rate of downstream migration of Smolts from the Sieg river using telemetry, showed that only 18% successfully reached the North Sea although on this route, there are very few hydropower installations