5.1 Adult numbers and sizes
Rod catch can be a useful proxy for adult abundance (Thorley et al., 2005) and these data indicate that the numbers of spring salmon (defined as fish caught between February and May) returning to the river Dee have declined since records began in 1952 and are now at around 8% of their former levels (Fig 3a). Over the period 1966-2022 (over which the Girnock has operated) the Dee spring rod catch and number of females returning to the Girnock have both declined by ca. 88%, indicating that temporal trends and processes observed at Girnock are likely indicative of those across the upper Dee catchment where spring salmon are assumed to dominate. This makes the Girnock data especially valuable in understanding population bottlenecks, controlling processes and evidence-based management options at larger scales.
The relationship between emigrating juveniles and returning adult numbers is density independent (Fig. 10d) so that greater emigrant numbers generally result in more returning adults (Gurney et al., 2010). However, there is also a strong downward trend in return rate, indicative of declining survival between emigration and adult return (Fig. 3b). This is particularly concerning given the major reduction in distant water and domestic commercial exploitation over the same time period. Indeed, there are now no commercial distant water fisheries for salmon, local net fisheries have also been removed and catch and release is compulsory for anglers in spring (before the 1st of May) and applied almost universally (> 99.5% in 2022) on the Dee regardless of the time of year.
The size of female salmon (and thus the number of eggs produced) has also declined nationally (Bacon et al., 2009) and locally (Glover et al., 2018). This combination of fewer and smaller females has had a substantial effect on egg deposition in the Girnock, although the effect is dominated by reductions in adult female numbers (Glover et al., 2018). In the last 10 years, it is estimated that ova deposition has only exceeded the number required to maximise emigrant production on a single occasion (2017), with major consequences for emigrant production and subsequent adult returns, particularly in recent years. The factors driving these changes are multiple and complex. Poor marine survival and growth are thought to be key factors (Gilbey et al., 2021) resulting from declining food resources due to climate change (Nicola et al., 2018). However, commercial bycatch, and predators in both marine (e.g. seals and dolphins) and freshwater (piscivorous birds, otters and other fish including brown trout and Pike) have been suggested as additional potential problems (Marine Scotland and Fisheries Management Scotland, 2023).