Effectiveness of a fish-guiding device for downstream migrating smolts of Atlantic salmon (Salmo salar L.) in the River Piteälven,northern Sweden

Abstract: Hydropower poses a major threat to both upstream and downstream migrating Atlantic salmon (Salmo salar L.). To ensure a relatively safe route for migrating fish, fish ways and different guidance structures are constructed to help fish to bypass the turbine intakes to power stations. The success of these constructions varies and is generally dependent on the local river conditions. In 2010, a fish guiding structure was installed in Sikfors power station, River Piteälven, to improve the downstream migration for smolts of Atlantic salmon. In the year of installation, a study aimed to evaluate the success of the guidance structure failed due to technical problems. The aim of this thesis was to evaluate the fish guidance structures ability in helping downstream seaward migrating salmon smolts in Piteälven with a safe passage through the power station and the dams. During the period May 25th to June 15th, a total of 117 hatchery reared smolt were marked with radio-tags and then released 2. 6 km upstream the hydropower station. Tagged and released smolts had an average length of c. 201 mm. To understand the effect of the guidance structure before and after installation, the obtained data was analyzed together with data from previous studies conducted in Sikfors (Lundstrom et al., 2015, Rivinoja, 2005a). 74 smolts (85 %) made the passage through the spill gates while 13 smolt (15 %) passed through the two Kaplan turbines. The survival was higher for fish passing through the spill gate (c. 80 %) compared to fish passing through the turbines (c. 69 %). Flow greater than 100 m3/s resulted in a higher mortality rate for passing smolt and thus the flow regime seemed to be an important abiotic factor to consider when discussing safe fish passages and guidance structures. Of the original tag group with 117 individuals, 37 completed the downstream seaward migration. These data give in short a relatively good picture on smolt passage effects that can affect the whole lifecycle. The result on survival was significantly better for downstream migrants when the guidance structure was installed compared to a system without this structure. With the guidance structure in place a majority of the released smolts was directed into the spillway instead of having to pass through the turbines on their seaward migration.