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.