1. Introduction
In 1966, two fish traps were installed on the Girnock Burn; a tributary of the River Dee in northern Scotland draining a 31km2catchment dominated by mountains and moorland (Fig. 1). The traps were operated by the Freshwater Fisheries Laboratory (FFL) of the Department of Agriculture and Fisheries for Scotland, for monitoring as part of international efforts to understand and manage stocks of Atlantic salmon. The River Dee, like many large Scottish rivers, provides a renowned freshwater habitat for Atlantic salmon (Salmo salar ): an iconic, keystone species both important for conservation and popular for angling, leading to financial significance to the rural economy (PACEC, 2017). The Girnock was identified as having an important population of “spring” salmon; fish that predominantly spend more than one year at sea (multi-sea winter fish (MSW)) and return to freshwater early in the year, spawning in the following autumn and early winter (Fig. 2). One fish trap was designed to capture out-migrating juveniles leaving the freshwater habitat for the marine phase of their life cycle (Bacon et al, 2015). The other trap was designed to capture their later return as adult fish to spawn in their natal headwater stream. This initiated an ongoing unique time series of the annual outmigration of juvenile salmon and returning adults (Glover and Malcolm, 2015a,b). Together with regular and pioneering electro-fishing, behavioural studies, stocking experiments and genetic analyses, this has resulted in a unique, globally important long-term record of an Atlantic salmon population. This data set has captured quantitative changes in the return rates, distribution, size, growth and age of salmon; giving a comprehensive picture of the demographic structure and population dynamics in the stream which is rivalled by very few other studies (Glover et al., 2020). Unfortunately, it also shows a worrying record of declining fish numbers (Fig. 3) indicating the loss of salmon populations from some small sub-catchments such as the Girnock is not inconceivable in the coming decades if marine survival remains poor (Fig. 3b).
These salmon studies stimulated a wider range of ecohydrological research aimed at better understanding the physico-chemical habitat used by salmon at different life stages and its temporal variability in relation to climate and other factors such as land use. Initially in-stream studies, encompassing hydrology, geomorphology and water quality, focused on the dynamics of in-channel habitat at the river-reach scale (e.g. Malcolm et al., 2004a). But to fully understand these complex interactions, investigations linking the river network to the wider landscape have facilitated comprehensive catchment-scale, process-based understanding of the energy and water budgets that drive the fluctuations in streamflow and water quality that maintain salmon habitats (e.g. Fabris et al., 2018). These studies linking the landscape to the riverscape have increasingly relied on fusing high-resolution environmental observation data into spatially distributed models (e.g. Ala aho et al., 2017). These more extensive studies – which are unusual for a salmon research site - have begun to inform fish population analysis and brought about a much richer understanding of the ecological functioning of the stream. Such broader advances have enabled a multitude of more local, site-specific studies over the monitoring period to be better contextualised and understood in an integrated way. To facilitate this, advances in statistics and ecological modelling have been used to overcome the limitations in, and increase the value of, historic data (Glover et al., 2019).
Here, we provide an review of this integration of salmon and associated ecohydrological catchment research at the Girnock. We highlight notable inflection points in this trajectory, major shifts of focus and significant research breakthroughs. Imposing a narrative on what has been an organic process of scientific exploration (see Soulsby et al., 2019) is inevitably somewhat artificial, but for convenience, after a brief description of the Girnock itself, we sub-divide the research review into three parts; (i) early fisheries research in the first 30 years, followed by (ii) the increased environmental context in second half of study period, and then (iii) consideration of how this has informed a more integrated understanding of salmon-environment interactions. We finally consider the lessons learned from this near-60 year journey which may contribute a wider reflection on the future goals and value of the increasing number of long-term studies in environmental science (e.g. Holmes and Likens, 2016; Tetzlaff et al., 2017; Rosi et al., 2022) that are perpetually threatened by reduced funding and constrained resources (Laudon et al., 2017, Rosi et al., 2023). We do this from the perspective that we are in unchartered waters with regards to the declining status of Atlantic salmon, associated climate change and the biodiversity crises. This recognises the urgent need to develop and assess appropriate management responses (Scottish Government, 2023), but also strongly advocates that robust data-driven science is essential to provide an evidence base to inform rational policy and management decisions that do more good than harm (Oreskes, 2019).