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)