| Summary: | Many watersheds have lost stocks of migrating fish species because of the development of hydroelectric plants and other fragmenting barriers. The remaining stocks of migrating Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in River Klarälven, Sweden are five times smaller compared to one hundred years ago. A fish-trap at the hydroelectric plant in Forshaga has been vital for the continued existence of the stocks. This is the first hydroelectric plant of eight, blocking the way to the spawning grounds. Spawners are collected in the trap for both hatchery and transport by truck upstream and past seven more hydroelectric plants to the spawning grounds, where they are released for possible natural reproduction. Past studies show that different hydrological parameters are important for successful migration and the function of a fishway, but the fishtrap in Forshaga has never been evaluated scientifically under different environmental conditions. In my study I investigated the importance of discharge on catches of Atlantic salmon and brown trout with the fish-trap using already existing data over a period of ten years. The results show that discharge is important, partly because of the number of weeks the fish-trap was open decreased with increased discharge on a yearly basis, partly because of a negative correlation between catch size and discharge on both annual and weekly basis, partly because the trap was closed during high discharge levels. It also seems like spill water could have a negative effect on the catch-size, because the catch-size was smaller when the hydroelectric plant exceeded its maximum capacity, and that is when spill water takes place. Många vattendrag har förlorat sina bestånd av migrerande fiskarter till följd av vattenkraftsutbyggnad och andra fragmenterande barriärer. De kvarvarande bestånden av vilda migrerande laxar (Salmo salar) och öringar (Salmo trutta) i Klarälven motsvarar bara en femtedel av vad som fanns för hundra år sedan. Helt avgörande för att bestånden idag ...
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