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).