2.4 | Rotenone treatment and fish reestablishment
The rotenone treatments of River Fusta were performed on August 18th 2011 and August 17th 2012 using a rotenone formulation CFT-Legumine with a 2.5% active gradient. In both treatments the release of rotenone took place over a 7-hour period from two stations in the river, in the upper and in the middle part (Stensli & Wist, 2014). The upper release was situated approximately 1 km upstream of our uppermost sampling station and the release in the middle part was just downstream of our lowermost sampling station. In addition, riverbanks of slow flowing areas, adjacent ponds, oxbow lakes and streams were rotenone treated with local point releases. The total use of CFT-Legumine in River Fusta was 754 and 573 L in 2011 and 2012, respectively. The water temperature was approximately 15° C during both treatments and the water flow was 12 m3s-1 in 2011 and 11 m3s-1 in 2012. In the lower part of River Fusta, treatment concentration of CFT­­-Legumine was higher than 0.5 ppm over a period of several hours, which was the desired minimum concentration, periodically reaching up to 7 ppm (Adolfsen et al., 2014).
The rotenone treatment of Lake Fustvatnet took place between October 16th and October 20th 2012. The total use of CFT-Legumine in Lake Fustvatnet was 139,200 L, of which 96,600 L were used for the surface waters and 42,600 L were used for deeper areas. A CFT-Legumine formula of 3.3% rotenone was used, except for the surface layer where the treatment included 25,800 L with 2.5% rotenone (Moen & Bardal, 2014). The 3.3% formula did not contain the synergist piperonylbutoxid and the solvent N-metylpyrrolidone since the former increases the toxicity to invertebrates, and not to fish (Finlayson et al., 2010). Measurements of rotenone concentration were performed over scattered sites in the surface water and at different depths during and after the treatment. On October 21st2012 no measurements were below 0.5 ppm CFT-Legumine, and the average concentration was 0.68 ppm. Due to low water temperatures, the breakdown of rotenone was slow. In April 2013, the average concentration of CFT-Legumine was 0.1 ppm and 0.3-0.4 ppm in the surface layers and in the deeper parts (>10m), respectively. In June 2013, the concentration was below 0.1 ppm at all depths. No rotenone was detected in mid-October 2013 (Adolfsen et al., 2014).
Fish was reestablished by stocking of Atlantic salmon (Salmo salar ) in River Fusta and Arctic charr (Salvelinus alpinus ) and brown trout (Salmo trutta ) in Lake Fustvatnet. Three thousand Atlantic salmon smolts were released in the river in 2013 (Lo & Holthe, 2014) followed by planting of 187,000 Atlantic salmon eggs in 2014. In both 2015 and 2016, 12,000 Atlantic salmon smolts were released, as well as 384,000 young of the year (YOY) Atlantic salmon in 2015 and 120,00 YOY in 2016 (Holthe et al. 2018). In Lake Fustvatnet 35,000 Arctic charr, mainly 2-year old, were released during the spring of 2014, as well as 93,000 brown trout YOY (Lo & Holthe, 2014, Holthe et al., 2015).