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 © 2016 John Wiley & Sons, Ltd.     

Modelling changes in trout habitat following stream restoration

Stream restoration was implemented on the Upper Arkansas River near Leadville, Colorado, to improve brown trout (Salmo trutta) populations. Metals pollution and channel disturbance associated with historic mining, land use, and water development degraded aquatic and riparian habitat. Changes in instream habitat quality following restoration were investigated with a before–after–control–impact study design. Baseline, as‐built, and effectiveness surveys were conducted in 2013, 2014, and 2016, respectively. Two‐dimensional hydrodynamic modelling with River2D was used to estimate weighted usable area (WUA) for adult, juvenile, fry, and spawning brown trout across a range of flows. WUA was calculated from habitat suitability curves for velocity, depth, and channel substrate. Foraging positions (FP) and habitat heterogeneity were also evaluated as indices of habitat quality. All results were analysed with analysis of variance. At impact sites, WUA increased by 12.2% from 2013 to 2014 but decreased by 10.2% from 2014 to 2016, whereas FP increased by 24.8% from 2013 to 2014 but decreased by 26.1% from 2014 to 2016. Spawning habitat increased 53.3% from 2014 to 2016 at impact sites. The 15.4% increase in depth variability from 2013 to 2016 indicates that habitat heterogeneity was enhanced at impact sites. No changes in WUA, FP, or habitat heterogeneity were observed at control sites. Although changes in WUA and FP suggest that initial habitat improvements were not sustained, increased spawning habitat and depth heterogeneity suggest otherwise. Our results highlight the value of monitoring strategies that utilize multiple lines of evidence to evaluate restoration effectiveness, inform adaptive management, and improve restoration practices.