4.1 Salmon spawning habitat
The first involvement of a wider network of environmental scientists at
the Girnock sought to understand why salmon spawn in particular places
(Soulsby et al., 2019). Both redd counts and behavioural studies showed
spawning distributions clustered around a small number of relatively
discrete locations, with about 50% of spawning occurring in just five
reaches of the river (Fig. 7). Moir et al. (2004) showed this related to
the distribution of gravel-cobble sized sediments, and a specific
combination of river depths and velocities to provide suitable hydraulic
conditions to assist females with redd excavation and construction. Such
favoured locations were restricted to areas upstream of valley
constrictions or lower gradients. Other smaller spawning sites were more
randomly distributed and related to patches of suitably sized gravels
where hydraulic conditions at the time of spawning were appropriate. In
one particular reach, integration of field observations with hydraulic
models showed that spawning locations under different flow conditions
could have a reasonable degree of predictability (Webb et al., 2001;
Moir et al., 2005).
Nevertheless, inter-annual spatial patterns were time-variant likely due
to both physical and biological controls. In general, high flows allow
fish to access the river earlier (Tetzlaff et al., 2005) and in greater
numbers (Lazzaro et al., 2017) and penetrate further into the river
system, with redds observed up to 8km upstream of the fish trap (Moir et
al., 1998; Webb et al., 2001). However, it is likely not just an issue
of flow, as high numbers of spawning females probably create density
dependent dispersive pressures that result in the use of increasingly
suboptimal hydraulic and sedimentary habitats further upstream.