Abiotic Characterization of Brown Trout (Salmo trutta f. fario) and Rainbow Trout (Oncorhynchus mykiss) Spawning Redds Affected by Small Hydropower Plants Case Studies from Austria

Salmonid rivers in Austria are considerably regulated by small hydropower facilities, resulting in potential declines of the spawning habitats of salmonids. To assess the restrictions and possible quality of hydropower-influenced river sections for salmonid, spawning redd densities of brown trout and rainbow trout were monitored in two rivers in 2014 and 2015. The results showed spawning close to small hydropower facilities for both investigated species with similarities in redd characteristics like pit and tail length. Differences occurred concerning the distance of redd construction to the next shore. Brown trout spawn close to the banks in comparison to rainbow trout which use the entire active channel width. In addition to the preference of brown trout for certain cover types, it turned out that the presence of high quality spawning gravel in the river is the dictating abiotic variable (probably bottleneck) in the control of salmonid populations even for river reaches impacted by small hydropower plants. Moreover, the assessments of spawning redd densities enabled a discussion of different opportunities for spawning habitat enhancement of salmonids in river sections regulated by small hydropower facilities. Here, in conclusion, it was found that the fill-up of the backwater sites by transported sediments or the structural modification (e.g. boulder placement) in the tail of the backwater could improve the spawning situation in a sustainable way. Copyright (c) 2016 John Wiley & Sons, Ltd.