3. Early salmon research
The first decade of research at Girnock focused on building a basic
understanding of salmon biology and taking steps towards population
assessment with an annual census of returning adults and emigrating
juveniles. Size, and sex (for adults) were also recorded, and scale
samples taken to determine ages via scale reading (Buck, 1975; Youngson
and Hay, 1996) (Fig. 3). This early work necessitated pioneering
development of electrofishing and tagging techniques as fisheries
science evolved. Emigrating smolts were tagged to assess return rates
(Youngson et al., 1994), as well as to track marine migratory routes and
determine rates of exploitation when capture by commercial and
recreational fisheries. This identified the areas around West Greenland
and Faroe islands as important feeding grounds during their marine
migration (Malcolm et al., 2010). However, it also highlighted the
prevalence of Girnock salmon in the rod fisheries on the river Dee with
peak numbers caught in February, particularly in the lower and mid-Dee,
and declining catches through the spring and summer.
As these core datasets increased in duration and variability, an
understanding of population dynamics began to emerge. Of particular
interest were the relationships between stock levels (e.g. number of
females or ova deposition) and subsequent recruitment of offspring.
These relationships can be modelled in a number of ways using different
lifestage combinations, providing information on the carrying capacity
of habitats, population regulation and bottlenecks to production. The
relationships between returning adult numbers (and ova deposition) and
subsequent production of emigrants was of particular interest in the
early years (Buck, 1976; Buck and Hay, 1984). Stock-recruitment (S-R)
curves are an important step in fisheries management (Prevost et al.,
2003) and the Girnock is one of the very few places with accurate S-R
data in Scotland, and one of only two places with detailed ova-emigrant
estimates over a large range of stock levels. These data are considered
to be of particular value because they exclude the noisy (and trending)
density independent marine phase of the lifecycle that is incorporated
when working with adult-adult or ova-ova stock recruitment relationships
(Gurney et al., 2010).
Girnock S-R data (particularly ova-ova, for consistency with a wider
range of rivers) have been used to try and scale production estimates
and develop management tools for salmon across the UK and Europe (e.g.
Prevost et al., 2003), and continue to be used in support of adult based
assessment methods to manage exploitation of salmon in Scottish rivers.
Girnock stock-recruitment data have also been used to provide an
important independent check to “benchmark” juvenile densities used to
assess the status of juvenile salmon stocks in Scotland (Malcolm et al.,
2019) through the National Electrofishing Programme for Scotland: NEPS
(Malcolm et al., 2023).
Aside from the core focus on the development of assessment data, other
aspects of basic salmon ecology became apparent as studies progressed.
For example, the number and spatial distribution of salmon redds was
related to interannual differences in female numbers (Hay, 1984). It was
initially assumed that one female salmon produced one redd, where she
laid her eggs. The story was shown to be much more complex, with females
often producing multiple redds, that may extend pre-existing structures,
and where the eggs of individual females were commonly being fertilised
by more than one male. This included small precocious parr that sexually
mature early in freshwater occasion (Buck and Youngson, 1982; Taggart et
al., 2001) and may contribute to the population on more than one
occasion.
Wider salmon-environment interactions also became apparent: Application
of novel radio-tracking to adult salmon showed the inter-relationships
between hydrology and spawning, as fish movements into the Girnock from
the Dee related to periods of increased flows following rainfall events,
as indeed were many subsequent movements of fish upstream to spawning
sites in the Girnock’s headwaters (Webb and Hawkins, 1989).
Environmental cues for the outmigration of juvenile fish were also
investigated by Youngson et al. (1983) who found complex interactions
between stream temperatures, flows, phases of the moon, cloudiness and
the timing of smolt in springtime runs that were difficult to
disentangle. The outmigration of parr occurring in autumn was also
discovered and subsequently found to form an important component of the
Girnock’s returning adult population (Youngson et al., 1994).
Observational studies also contributed to an understanding of the
behaviour of both adult and juvenile salmon. Radio-tracking of spawning
salmon was accompanied by three years when the river network was walked
daily during the spawning season to map spawning locations and the
timing of spawning events (Webb et al., 2001). Further work on spawning
sites used emerging genetic techniques to better understand the role of
sexually mature male parr in the fertilization of eggs (Jordan and
Youngson, 1992). There was also a suggestion that Girnock salmon may
represents a distinct breeding population within the Dee system (Jordan
and Youngson, 1991). This assertion was supported by a high proportion
of returning adults known to have migrated from the catchment as
juveniles, (Youngson et al., 1994) increasing the likelihood of distinct
local breeding populations.
For juvenile fish, behavioural studies were carried out in an artificial
channel near the Girnock traps. This channel was historically
constructed to take water from the stream to power a now-disused
sawmill, but was later used for experiments to assess fish responses to
habitat manipulation (e.g. changing flows and substrate etc.).
Behavioural studies using both direct observations and pit-tagged fish
showed strong organisational patterns of juvenile salmon for foraging
space, with larger fish able to compete better for areas where food
resources (invertebrates carried by the in-stream current) were
delivered (Armstrong et al., 1999). These studies also showed that fish
may only have limited knowledge of total habitat availability and may
therefore be unable to respond to changing conditions (e.g. decreasing
streamflows or food sources) (Armstrong et al., 1997; 1998).
The early studies in the Girnock provided the basis for intercomparison
of fish population performance with other sites. This has included an
examination of the broader context of salmon in other tributaries of the
river Dee (Shackley and Donaghy, 1992). Importantly, the annual Girnock
adult numbers (Glover et al., 2018, Fig. 3c) are strongly and
significantly correlated with spring rod catches in the Dee (Youngson et
al., 2002 and Fig. 3a), showing that population trends in Girnock salmon
are highly likely to be more generally representative of those
experienced in the upper tributaries of the Dee and other rivers where
spring stocks dominate. Further, the Girnock acts as a key index
monitoring site for comparison with salmon populations elsewhere in
Europe and North America (Freidland et al., 2009).
The development of the Girnock as a more inter-disciplinary research
site was initiated in the early 1990s, when FFL senior scientist Alan
Youngson looked to involve scientists from other disciplines to develop
more integrated research agendas aimed at an improved understanding of
complex salmon-environment interactions. This coincided with growing
concern from the government, fishery owners and anglers, over declining
salmon catches generally and those of spring fish in particular (see
Section 5 below). Such concern focused minds towards developing stronger
evidence-based, inter-disciplinary approaches for sustainable management
of salmon. These in turn fed into government policy at a national level
through the Scottish Salmon Task Force (e.g. Scottish Office, 1997) and
more locally with a Salmon Action Plan for the river Dee, launched at a
scientific meeting about enhancing the Dee’s stocks (Youngson, 1995). As
part of this knowledge transfer, salmon research from the Girnock was
summarised in The Lives of Salmon (Youngson and Hay, 1996) book which
targeted a non-specialist audience.