Downstream migration of Atlantic salmon smolt at three German hydropower stations

Abstract

Økland, F., Teichert, M.A.K., Thorstad, E.B., Havn, T.B., Heermann, L., Sæther, S.A., Diserud, O.H., Tambets, M., Hedger, R.D. & Borcherding, J. 2016. Downstream migration of Atlantic salmon smolt at three German hydropower stations. NINA Report 1203: 1-47. The aim of this study was to examine migration routes and losses of Atlantic salmon smolt past three hydropower stations, which were the Unkelmühle (River Sieg), Gengenbach (River Kinzig), and Kuhlemühle (River Diemel) power stations. These three power stations represent the use of different technologies to reduce negative impact on downstream migrating fish. The study was performed by tagging 525 Atlantic salmon smolt with radio transmitters and recording their migration when passing the power stations. In 2015, the loss of downstream migrating smolt due to the power station was 12.8% at Unkelmühle and 3.1-6.3% at the power station in Gengenbach. This represents the percentage of smolt entering the power station area that were lost due to this being a power station instead of a free-flowing river. Immediate mortality for smolt that passed through the Archimedes screw turbine at Kuhlemühle was estimated at 0-8%. Results showed that also reservoirs upstream of power stations can be areas of high mortality. Of smolt entering the reservoir upstream of Unkelmühle, 7.2% and 17.1% (two study years) were lost due to this being a reservoir instead of a free-flowing river. The main reason was likely presence of more fish predators (i.e., larger fishes eating smolt) in the slowflowing reservoir compared to the free-flowing river stretches.