5.3 Fry (age-0 salmon)
Densities of fry vary spatially, mainly with altitude in the Girnock (Glover et al. 2019 and Fig. 9). Contrary to previous expectations (Nislow et al., 2011) they have been shown to occupy a wide range of habitats (Hojesjo et al., 2016) that were generally only constrained by proximity to suitable spawning substrate and the ability of fry to disperse after emergence (Webb et al., 2001). At the catchment scale, ova to fry survival in the Girnock was found to be density independent with ova and fry numbers linearly related) (Fig. 10). This was surprising as the ova to fry stage is usually assumed to be a major bottle neck to production in salmonid populations, including Atlantic salmon (Nislow et al., 2004) and anadromous brown trout (Elliot, 1987). The contrasting findings at the Girnock likely reflect the varying scales of observation and analysis, with a whole riverscape assessment of production at Girnock in contrast to more traditional studies focussed on the scale of individual electrofishing sites where emigration and mortality are strongly confounded. Of course local density-dependent mortality could still occur near individual redds, or groups of redds within the Girnock, and not be detected at the catchment scale (Einum and Nislow, 2005).
Recent work has demonstrated significant downward trends in fry abundance both at the catchment scale and at individual long-term electrofishing sites. However, it was only possible to reliably detect these downward trends where data were appropriately corrected for confounding trends in capture probability (Glover et al., 2019). This is because in such a long-term data sets, the effects of varying staff and changes in equipment quality, as well as changes in fish density and size can introduce major biases in the time series (Glover et al., 2018). As with many novel aspects of research at the Girnock, the importance of these effects has been recognised more widely with similar patterns subsequently observed nationally (Malcolm et al., 2023) and in other countries (Dauphin et al., 2018)