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).