Effects of experimental flooding on brown trout (Salmo trutta fario L.): The River Spol, Swiss National Park

Following the construction of two large reservoirs in the late 1960s on the River Spol, Swiss National Park, flow was greatly reduced and regulated at a relatively constant discharge. The regulated flow regime resulted in the elimination of river changing floods, causing altered and degraded habitat conditions for the brown trout (Salmo trutta fario L.). Although food resources (i.e., zoobenthos abundance) increased after flow regulation, trout spawning areas were greatly reduced by the clogging of coarse sediments. Consequently, the National Park in agreement with power authorities initiated an experimental flood program in 2000 to improve the fisheries potential of the River Spol. Fish abundance was not reduced by the floods and relatively few fish (< 2 %) were killed or stranded by the floods. In fact, the quality of fish habitat, spawning grounds in particular, was noticeable improved, even though food resources were altered to some degree by the floods. The results showed that the condition of trout in the Spol remained relatively constant, but the number of redds has increased three-fold since initiation of the flood program.