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.