Downstream migration of Atlantic salmon smolts at Unkelmühle power station and Buisdorf dam in 2016
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- NINA Rapport/NINA Report 
Havn, T.B., Økland, F., Heermann, L., Thorstad, E.B., Teichert, M.A.K., Sæther, S.A., Tam-bets, M. & Borcherding, J. 2018. Downstream migration of Atlantic salmon smolts at Unkelmühle power station and Buisdorf dam in 2016. NINA Report 1412. Norwegian institute for nature research. Background and study aim The aim of this study was to examine migration routes and losses of Atlantic salmon smolts past the Unkelmühle hydropower station in the Sieg. Results from the study in 2016 are the main focus of this report. The results from 2016 are also compared with results from similar studies at Unkelmühle in 2014 and 2015. Technical facilities at Unkelmühle are designed to facilitate safe passage of downstream migrating fish, including ten different bypass routes where fish can pass outside the turbines, and narrowly spaced racks installed in front of the turbine intakes to prevent fish from entering the turbines. The efficiency of these measures are evaluated. Downstream migration past Buisdorf dam was also examined in 2016. This enabled comparison of loss and migration speeds of smolts between the Unkelmühle hydropower station and a weir without a hydropower station at Buisdorf. Methods The study was performed by tagging 227 Atlantic salmon smolts with radio transmitters and recording their migration in the river and past Unkelmühle power station and Buisdorf dam. Their movements were recorded 1) on free-flowing reference stretches upstream of the power station and dam, 2) on impounded stretches upstream of the power station and dam, 3) when they passed the power station and dam, and 4) on downstream river stretches. Migration routes used by tagged fish when they passed the power station and dam were mapped in detail by using networks of automatic, stationary receivers. The loss of downstream migrating smolts due to impoundments and past the power station and dam was calculated by comparing losses in these areas with losses on the reference stretches. This is based on the assumption that the loss per km recorded on the reference stretches (termed “reference loss” in this report) is representative for the developed stretch (stretch affected by hydropower development) if it had been a free-flowing river instead of being impounded by a reservoir and having a power station or dam. To examine if the location of the reference stretches affected the estimated loss caused by hydropower development, we compared two estimates of loss due to Unkelmühle power station based on reference loss on two different reference stretches. Results and conclusions The loss of downstream migrating smolts due to Unkelmühle power station was minimum 2.9% during the study in 2016. This represents the percentage of smolts arriving at the power station area that were lost due to this being a power station area instead of a free-flowing river. The loss estimate represents direct loss at the power station and delayed mortality due to the power station on the stretches downstream (7.5 km). There was no difference in loss between fish using the headrace to pass the power station and those passing over the weir. Loss due to the Buisdorf dam was minimum 3.4 and 5.7% (two different estimates) and not significantly different from the loss at Unkelmühle. The loss estimates are minimum estimates, because fish injured when passing the power station or dam can experience delayed mortality at later stages than recorded in this study, and the total mortality might therefore have been higher. There was no turbine mortality at the power station, because none of the smolts passed through the bar racks in front of the turbines, as expected due to the narrow bar spacing (10 mm) of the racks. Hence, the extra loss of smolts passing the power station was likely related to physical injuries in bypass routes aimed at guiding smolts outside the turbines, and increased predation. Loss due to the hydropower station was lower in 2016 compared to the two previous study years (minimum 9.9% in 2014 and 12.8% in 2015). Although total loss due to the power station was highest in 2015, loss in the bypass route that leads smolts outside the turbines was higher in 2014 than in 2015 and 2016. This was likely caused by smolts becoming trapped in an area of the bypass route where debris and branches piled up in 2014, but not in 2015 and 2016. Water discharge was higher in 2016 compared to the previous study years, and the high water discharge was probably an important factor for reducing loss of smolts passing the power station in 2016. Results showed that the reservoir upstream of the power station can be an area of high mortality for downstream migrating smolts. Of all smolts entering the reservoir upstream of Unkelmühle, 7.2% in 2014, 17.1% in 2015 and 4.4% in 2016 were lost due to this being a reservoir instead of a free-flowing river. The reservoir upstream of Unkelmühle is 2.3 km long, with slow-flowing water, and more resembling a lake than a river. The main reason for the extra loss in the reservoir is likely presence of more fish predators in the slow-flowing reservoir compared to the free-flowing river stretches. These results show that reservoir mortality may vary among years, probably due to variation in the predator community. In contrast to at Unkelmühle, where fish were delayed at the power station, fish moved at the same speed past Buisdorf dam as on unimpounded stretches. This difference in migration speed was also found when comparing only fish that migrated through the spillway gate at Unkelmühle with fish that used the weir at Buisdorf. No fish spent time in the turbine intakes before swimming back upstream and using the spillway gate to pass the power station, so behaviour upstream of the power station did not seem to explain why fish moved slower past the power station compared to the dam. However, smolts seem to follow the main water flow when navigating past power stations, and the observed differences may be caused by a higher proportion of the total water discharge running over the weir at Buisdorf compared to the proportion running through the spillway gate at Unkelmühle, making navigation over the weir and exit of the tailrace faster at Buisdorf compared to Unkelmühle.