"Fallbacks" - betydelsen av vandringstid för Atlantlax (Salmo salar L.) som faller nedströms vandringshinder

In Lake Vanern, Sweden, there is an endemic population of Atlantic salmon (Salmo salar L.) that lives its entire life in fresh water. The salmon is hindered from migrating to its natural spawning sites in the northern part of the River Klaralven by nine hydro-electric power plants. None of the hydro-electric power plants have fishways so that salmon migrating upstream or downstream may pass. In the current situation, the upward migrating salmon are caught in a fish trap at the most downstream located power plant in Forshaga. From there, the salmon are driven in a truck, past eight power plants and released a few kilometers upstream of the eighth power plant at Edsforsen. This study investigated the difference in the proportion of fallbacks between early (June-July) and late (August-September) migrating salmons in 2011-2013. The results showed that there was a significant difference in the proportion of fallbacks between the early and the late-migrating salmon. Fifty-one percent of the early salmon and 13% of the late salmon fell back during these 3 years. Further, there was no significant difference between males and females that became fallbacks. Nor was there any difference in the length of early and late-migrating salmon or fallbacks and non-fallbacks.