5.2 Ova – alevin survival and performance
As the main spawning locations in the Girnock occur in
geomorphologically stable lower gradient reaches where suitable gravel
accumulates, there is no evidence to suggest winter wash-out of ova is a
significant bottleneck on the salmon population even following extreme
recent events such as Storm Frank (Glover et al., 2020).
As low DO concentrations can result in high ova mortality where spawning
gravels coincide with areas of chemically-reduced groundwater discharge;
consequently conservation stocking has been shown effective at reducing
ova mortality and increasing fry production. However, at least
historically, these early benefits of stocking were offset at later life
stages, not translating to improvements in parr (Glover et al., 2018) or
emigrant production (Bacon et al., 2015). While the reasons for this
failure remain unclear, it emphasises the challenges of trying to
artificially manipulate and improve natural systems even where
underlying processes are extremely well understood. It has also been
shown that low DO concentrations can result in delayed or prolonged
hatch and smaller salmon fry. However, the potential for compensatory
growth when emerging into high DO is also high and again appears to
indicate a strong resilience to naturally encountered environmental
challenges (Youngson et al., 2004).