4.5. Effects of flow on instream hydraulics and juvenile performance
Several studies have tried to better understand how instream hydraulics affect juvenile salmon habitat. At the catchment scale, higher flows at spawning and emergence time have been found to have a small positive effect on fry production; however, there is no evidence that flow variability has a substantial effect on parr numbers (Glover et al., 2020) or performance, even during extreme years like 2015 (when Storm Frank likely generated the highest flood on the River Dee for over 200 years). Early reach scale studies focused on using velocities as an index of spatio-temporal variations in useable habitat (Tetzlaff et al., 2005). These simplistically used the concept of critical displacement velocity (CDV) – i.e. velocity thresholds at which juvenile fish can no longer hold station, which varies with fish size and stream temperature (Fabris et al., 2017). However, CDVs are derived from flume experiments which may not translate to more complex stream environments. Further, the 3-dimensional hydraulics of the in-stream environment are complex, and the high channel roughness results in many boundary layer effects, that give fish more hydraulic shelter than simple velocity measurements infer (Höjesjö et al., 2015). Although extreme low flows and short periods of high flows during larger storm events may temporarily affect the available habitat for salmon to forage for food, for most of the time juvenile habitat is not expected to be limited by hydraulic conditions in the Girnock (Fabris et al., 2017). In addition, while the spatial distribution of depths and velocities varies in relation to different types of river reach, as slope and bed roughness change, essentially there is little difference in terms of useable hydraulic habitat between reaches that have been studied.