5. Towards more integrated, upscaled understanding of salmon
population dynamics
Ongoing salmon research, informed by the wider insights into physical
and chemical variability in the catchment environment, has increasingly
revealed more comprehensive and nuanced understanding of long-term
spatio-temporal salmon population dynamics (Figs 9 and 10) which
coincides with increased evidence of the effects of climate change. With
>50 years of age differentiated, multi-life stage data
(Fig. 3), we now know that return rates for adult females as a
percentage of total emigrants are estimated to have declined from an
average of ca. 1.7% in the 1960s to ca. 0.6% by 2020 (Fig. 3b), a
decline of ca. 63% (Glover and Malcolm 2015a). Over the same time
period the numbers of adult females declined from around 100 a year on
average in the 1960s to ca. 12 fish by the early 2020s (Glover and
Malcolm 2015b, Fig. 3c), a reduction of nearly 88%, the same reduction
seen in the river Dee spring (February-May) rod catch (Fig 3a).
Reductions in both female numbers and sizes reduced ova deposition
(Glover et al., 2018) from around 530,000 eggs in the 1966 to only
47,000 eggs by 2022, a reduction of ca. 91% (Fig. 3d). Because survival
between ova and fry is density independent in the Girnock (Fig 10a),
reductions in ova deposition (Fig. 3d) have been approximately matched
by changes in fry production (Fig. 3e, Glover et al., 2018, 2020).
Fortunately, downward trends in parr production (Fig. 3f) have been
buffered by reduced density dependent mortality (Fig. 10b) with declines
of ca. 56% between 1966 and 2015 (Glover et al., 2018).
Changes in emigrant numbers have been further complicated by reductions
in the age of fish at emigration and associated increases in mortality.
The modal age of smolts has changed from three-year old fish to two-year
olds, driven by increased rates of growth due to reduced competition and
warming temperatures (Gurney et al., 2008). These complex changes also
appear to have altered the balance between autumn and spring migrants
(Fig. 3f), so while autumn migrant numbers have declined by ca. 75%,
spring migrant numbers have only declined by ca. 30% with total migrant
numbers now around 55% of their former levels from the 1960s. In the
following sections the current understanding of the ecology, habitat
requirements and controls on the production of each life stage is
summarised and discussed in terms of any implications for management.