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.