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\begin{document}

\title{Six decades of ecohydrological research connecting landscapes and
riverscapes in the Girnock Burn, Scotland: Atlantic salmon population
and habitat dynamics in a changing world.}



\author[1]{Chris Soulsby}%
\author[2]{Doerthe Tetzlaff}%
\author[1]{I.A. Malcolm}%
\affil[1]{University of Aberdeen School of Geosciences}%
\affil[2]{IGB Berlin}%


\vspace{-1em}



  
  \date{June 14, 2024}


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\selectlanguage{english}
\begin{abstract}
Long-term data are crucial for understanding ecological responses to
climate and land use change; they are also vital evidence for informing
management. As a migratory fish, Atlantic salmon are sentinels of both
global and local environmental change. This paper reviews the main
insights from six decades of research in an upland Scottish stream
(Girnock Burn) inhabited by a spring Atlantic salmon population
dominated by multi-sea-winter fish. Research began in the 1960s
providing a census of returning adults, juvenile emigrants and in-stream
production of Atlantic salmon. Early research pioneered new monitoring
techniques providing new insights into salmon ecology and population
dynamics. These studies underlined the need for interdisciplinary
approaches for understanding salmon interactions with physical, chemical
and biological components of in-stream habitats at different
life-stages. This highlighted variations in catchment-scale
hydroclimate, hydrology, geomorphology and hydrochemistry as essential
to understanding freshwater habitats in the wider landscape context.
Evolution of research has resulted in a remarkable catalogue of novel
findings underlining the value of long-term data that increases with
time as modelling tools advance to leverage more insights from ``big
data''. Data are available on fish numbers, sizes and ages across
multiple life stages, extending over many decades and covering a wide
range of stock levels. Combined with an unusually detailed
characterisation of the environment, these data have enabled a unique
process-based understanding of the controls and bottlenecks on salmon
population dynamics across the entire lifecycle and the consequences of
declining marine survival and ova deposition. Such powerful datasets,
methodological enhancements and the resulting process understanding have
informed and supported the development of fish population assessment
tools which have been applied to aid management of threatened salmon
stocks at large-catchment, regional and national scales. Many pioneering
monitoring and modelling approaches developed have been applied
internationally. This history shows the importance of integrating
curiosity-driven science with monitoring for informing policy
development and assessing efficacy of management options. It also
demonstrates the need of continue to resource long-term sites which act
as a focus for inter-disciplinary research and innovation, and where the
overall value of the research greatly exceeds the costs of individual
component parts.%
\end{abstract}%



\sloppy


\textbf{1. Introduction}

In 1966, two fish traps were installed on the Girnock Burn; a tributary
of the River Dee in northern Scotland draining a
31km\textsuperscript{2}catchment 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 \emph{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).

\textbf{2. The Girnock catchment and its salmon population}

The geology of the Girnock's landscape (Fig. 4) was set
\textasciitilde{}400 million years ago as the granite pluton that formed
Scotland's Cairngorm mountains was emplaced and metamorphosed the
pre-existing sedimentary rocks (Goodman, 2007). Subsequent erosion
exposed granite in the higher elevations of the catchment
(\textgreater{}400m), whilst the lower slopes comprise metamorphosed
schists, some of which are calcareous (Soulsby et al., 2007). The
now-subdued landscape has been subject to many erosion cycles, including
successive glaciations, resulting in a river network occupying
over-widened valleys, extensively covered by glacial and post glacial
deposits (Hall, 2007).

These deposits control the distribution of soils and vegetation (Fig. 4)
(Tetzlaff et al., 2007). Waterlogged peaty soils lie above poorly
drained drifts in the valley bottom (Soulsby et al., 2016). On steeper
slopes, more freely draining podzols form on coarser drift deposits,
whilst poorly developed rankers form along exposed interfluves and
steeper scree slopes. On wetter valley-bottom peats with
\emph{Sphagnum}mosses dominate, along with grasses such as
\emph{Molinia} where minerogenic groundwater drainage occurs. On the
podzols and rankers, Scots Pine (\emph{Pinus sylvestris} ) is the
dominant natural forest vegetation, though this has been extensively
cleared in the past, with re-generation inhibited by grazing Red Deer
(\emph{Cervus elaphus} ). Forest now only remains on steeper, more
inaccessible slopes or fenced plantations (Neill et al., 2021). Where
the forest has been cleared,\emph{Calluna} and \emph{Ericeae} heather
shrubs dominate, and have been sustained by a burning cycle -- typical
on Scottish Highland estates -- which further supresses forest
regeneration to promote feeding habitat for grouse (\emph{Lagopus
lagopus} ) and red deer as game species.

The glacial legacy has a profound effect on river channel morphology and
salmon habitats in the Girnock. The gradient is relatively gentle and
the stream bed is well-armoured by coarse glacial lag deposits, creating
high channel roughness (Moir et al., 1998). This provides hydraulically
complex conditions typical of mountain streams (Fabris et al., 2017)
that contain extensive areas ideal for juvenile salmon (Glover et al.,
2018, 2020). In some places, moraine deposits constrict the river
channel; with short reaches of alluvial channel forming upstream where
the valley gradient is lower. These areas with less coarse sediments are
favoured for salmon spawning, where female fish lay their eggs in open
gravel structures called ``redds'', excavated in the river bed (Fig. 2)
(Malcolm et al., 2005).

Climate in the Girnock is at the temperate/boreal transition, with
\textasciitilde{}1,000mm annual precipitation; evapotranspiration
accounts for 30-40\%, the remainder becomes streamflow. Precipitation is
evenly distributed, though winter months (Nov-Jan) tend to be wettest
and spring (May) driest (Fig. 5). Most rain occurs in small low
intensity events (\textless{}10mm), with larger daily totals
(\textgreater{}25 mm) only occurring 3-4 times per year. Snow usually
accounts for \textless{}10\% of inputs and snowpack accumulation below
700m generally lasts only a few weeks. Winter temperatures are cold (Jan
mean \textasciitilde{}0\textsuperscript{o}C), and summer is mild (July
mean = \textasciitilde{}14\textsuperscript{o}C).

The climate, topography and soil cover result in the Girnock having a
``flashy'' hydrological regime (Fig. 5); with low baseflows during
periods of dry weather sustained by groundwater stored in the drift,
interspersed by rapid rainfall-runoff responses generated as a result of
saturation overland flow from the wet peats in the valley bottom (Fig.
6) (Tetzlaff et al, 2007). Dominant low intensity frontal rainfall
dictates that large runoff events are relatively rare and restricted to
high rainfall events (\textgreater{}25mm) with wet antecedent conditions
which drives non-linear connectivity between the hillslopes and
saturated areas influencing large surface and near surface water fluxes
(e.g. Soulsby et al., 2017a).

This ecohydrological context has sustained an Atlantic salmon population
that probably colonised the Dee catchment soon after de-glaciation
c.10,000 years ago (Cauwelier et al., 2018). Salmon have a complex life
cycle that is adapted to their environment which begins in the
freshwater where fish spawn and lay their eggs in redds in
well-oxygenated river gravels, usually between late October and early
December in the Girnock (Fig. 2). These eggs hatch between late March
and early April the following year where the small fish (alevins) remain
within the gravels until their yolk sac is absorbed. Upon emergence into
the stream in May and June the young, free-swimming salmon become known
as fry, or 0 year old fish as they are spending their first year in
freshwater. In the second year, the fish become known as parr and spend
1-3 years feeding on invertebrates and growing before migrating to sea.
There are two distinct emigrations from the Girnock, in the autumn and
spring (Youngson et al., 1983). Tagging studies show that those
emigrating in spring go straight to sea, whilst those leaving in the
autumn remain in freshwater over winter and migrate to sea the following
spring (Youngson et al., 1994). As the emigrants migrate from natal
rivers towards the sea they undergo a physiological change known as
smolting which allows them to osmoregulate as they move between
freshwater and marine environments. They then spend 1-3 years at sea on
a long migratory path to the north Atlantic where they feed and grow
into adult fish (Malcolm et al., 2010; Gilbey et al., 2021). They then
typically return to their native river system, many to their natal
stream to spawn (Youngson et al., 1994) for the life cycle to start over
again. A notable feature of the Girnock salmon is that they are prized
``spring'' MSW fish that spend 2 or more winters at sea, before
returninf to freshwater early in the year; forming an important economic
component of the fishery in terms of angling in the early fishing
season, an ecological characteristic that is often not seen in other
countries (Youngson et al., 2002).

With this complex lifecycle, salmon are truly ``citizens of the world''
with a lifecycle spanning a large part of the northern hemisphere. As
such they are sentinels of both global and local environmental change.
Our ever-increasing understanding of this, and how salmon populations
might be sustained in the future is informed by trans-Atlantic
monitoring sites collecting similar data to the Girnock (Prevost et al.,
2003; Gurney et al., 2010); although with a few exceptions these rarely
include both detailed long-term multi-life stage census data and
supporting ecohydrological characterisation and understanding.

\textbf{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.

\textbf{4. Ecohydrological research: from characterising salmon habitats
to catchment science.}

Since the second half of the 1990s, the much broader research focus on
salmon and their habitats not only led to better understanding of the
ecology of the species, but also a greater appreciation of the physical
and chemical processes that linked the landscape of the Girnock's valley
(Fig 1b) to the riverscape of its channel network (cf. Hynes, 1975).

\textbf{4.1 Salmon spawning habitat}

The first involvement of a wider network of environmental scientists at
the Girnock sought to understand why salmon spawn in particular places
(Soulsby et al., 2019). Both redd counts and behavioural studies showed
spawning distributions clustered around a small number of relatively
discrete locations, with about 50\% of spawning occurring in just five
reaches of the river (Fig. 7). Moir et al. (2004) showed this related to
the distribution of gravel-cobble sized sediments, and a specific
combination of river depths and velocities to provide suitable hydraulic
conditions to assist females with redd excavation and construction. Such
favoured locations were restricted to areas upstream of valley
constrictions or lower gradients. Other smaller spawning sites were more
randomly distributed and related to patches of suitably sized gravels
where hydraulic conditions at the time of spawning were appropriate. In
one particular reach, integration of field observations with hydraulic
models showed that spawning locations under different flow conditions
could have a reasonable degree of predictability (Webb et al., 2001;
Moir et al., 2005).

Nevertheless, inter-annual spatial patterns were time-variant likely due
to both physical and biological controls. In general, high flows allow
fish to access the river earlier (Tetzlaff et al., 2005) and in greater
numbers (Lazzaro et al., 2017) and penetrate further into the river
system, with redds observed up to 8km upstream of the fish trap (Moir et
al., 1998; Webb et al., 2001). However, it is likely not just an issue
of flow, as high numbers of spawning females probably create density
dependent dispersive pressures that result in the use of increasingly
suboptimal hydraulic and sedimentary habitats further upstream.

\textbf{4.2. Groundwater -- surface water interactions}

Excavation of spawning areas to recover and transplant ova formed part
of experimental work in the 1990s to test whether transplanting ova from
areas of high to low spawner density might increase juvenile production
through reductions in density dependent mortality (Youngson and McLaren,
1998). Indeed, some have advocated for similar approaches more recently,
with a focus on moving wild salmon fry rather than ova (Young, 2017).

Surprisingly high ova morality was found at some of the most heavily
used, and apparently suitable, spawning sites on the Girnock (Malcolm et
al., 2005). It was initially hypothesised by fisheries researchers that
fine sediment infiltration was causing anoxic conditions through low
interstitial velocity in redds. However, the ``fine'' sediment fraction
in the Girnock was found to be comprised mainly of sand (Moir et al.,
2002). Furthermore, interstitial velocities were often high and not
clearly correlated with oxygen concentrations (Malcolm et al., 2011).
Subsequent process-based investigations, including the first use of
continuous optical dissolved oxygen sensors capable of being deployed in
the hyporheic zone (the streambed interface between surface water and
groundwater), showed that some spawning sites were vulnerable to anoxic
conditions where chemically reduced groundwater discharges occurred
through the stream bed (Malcolm et al., 2004a; 2006). These areas of
groundwater discharge coincided with valley constrictions that forced
upwelling groundwater into reaches of the stream where sedimentary
conditions were good for spawning but provided sub-optimum habitat in
terms of sufficient oxygen levels to sustain ova (Malcolm et al., 2005).
Such conditions were shown to be worse in wetter winters, when
groundwater fluxes were higher, and less prevalent in drier winters when
well-oxygenated stream water dominated the hyporheic zone (Soulsby et
al., 2008). So, it seems spawning site selection in the Girnock
represents a trade-off between optimal sedimentary conditions and the
risk of de-oxygenation in some years.

These local investigations into the hydrology of specific spawning
reaches were subsequently placed in a catchment-scale context through
characterisation of deeper groundwater flow paths (Fig. 6) as part of
hillslope hydrology research in the catchment. This has used a
combination of synoptic surveys of environmental tracers in springs and
streams (Soulsby et al., 2007; Blumstock et al., 2015; Scheliga et al.,
2017); sampling deeper and shallower wells (Blumstock et al. 2016;
Scheliga et al., 2018, 2019), geophysical mapping (Soulsby et al.,
2016b) and modelling (Ala aho et al., 2017). Much of this work focused
in the smaller, more accessible 3.2km\textsuperscript{2} Bruntland Burn
sub-catchment of the Girnock, with broadly similar overall landscape
properties (topography, soils, land use etc.) and hydrology where
logistical challenges to monitoring are less severe (Birkel et al.,
2014).

These studies have shown extensive groundwater storage in glacial
deposits in the Girnock which in valley bottom areas can be
\textasciitilde{} 40m deep. Generally, groundwater circulation here is
slow due to the low permeability of these drifts and deeper groundwater
is probably not well-connected to streams. Coarser, shallower drifts on
the hillslopes have higher permeability and more dynamic groundwater
responses. These appear to be fed by fracture networks in the granite
and other rocks exposed on the catchment interfluves which are activated
in wetter periods (Scheliga et al., 2017, 2019). Groundwater chemistry
is highly variable reflecting geology and residence times; but is mostly
strongly alkaline from calcareous rocks in the drifts (Soulsby et al.,
2007). Groundwater in granite is less alkaline (Soulsby et al., 1998).
The spatial and temporal variability of groundwater inputs has a strong
influence on the chemistry of the stream, and the chemistry of the
hyporheic zone (and redd environment) depends on local
hillslope-aquifer-stream connectivity (Malcolm et al., 2005; Soulsby et
al., 2009).

\textbf{4.3. Surface water quality}

The Girnock is an ideal salmon stream in that it is a relatively cool
mountain habitat that has the clean, well-oxygenated water that is
fundamental to the species at all freshwater life stages. The stream is
generally base-rich with a circum-neutral pH. As many salmon life-traits
are closely linked to phenology and seasonal temperature changes (e.g.
ovulation, migration timing etc.) stream temperature was the first
variable measured to complement salmon data. The maintenance of the
stream temperature monitoring has, coincidentally, resulted in the
longest, continuous sub-daily river temperature record in Scotland (Fig.
8). Early studies suggested that autumn and winter river temperatures
decreased by ca. 0.76 \selectlanguage{ngerman}°C and 0.57 °C, respectively, between 1970 and
2000, but that spring and summer temperatures had increased by 1.46°C
and 1.04°C, respectively, over the same period (Langan et al., 2001;
Gurney et al., 2008). Subsequent data collection and updated analysis
has revealed a more complex picture with seasonally varying trends and
the need to correct for operational and instrument related temporal
biases (Jackson et al., In prep.).

Spatial patterns of stream temperatures have also been monitored by a
distributed network of temperature sensors which evolved to be
consistent with the locations of long-term electrofishing sites (Malcolm
et al., 2004b). Mean daily stream temperatures show limited variability
across the catchment, though extreme highs and lows can show substantial
differences between the upper moorland catchment and the lower forested
areas (Malcolm et al., 2008). Where riparian forest cover is present,
day time temperatures are lower and night time temperatures remain
higher than in more open areas upstream (Garner et al., 2014).
Establishment of two automatic weather stations positioned over the
river enabled ground-breaking process-based energy budgets to be
modelled for forest and moorland stream reaches (Hannah et al., 2004,
2008). These showed that riparian tree cover reduces the rate of heating
in the lower catchment by shading which, when combined with cooler water
flowing from upstream overnight (advected heat), produces negative
downstream gradients in observed water temperatures (Garner et al.,
2014). Trees also reduce wind speeds and evaporative heat loss and
reduce net losses of long wave at night, thereby reducing daily
variability in temperature (Hannah et al., 2008).

Stream temperatures are also known to influence hyporheic temperatures
and thus, the environment in which salmon eggs develop. However, this
also varies with catchment wetness and local temporal dynamics in
groundwater hydraulic gradients (Malcolm et al., 2001; 2008). In
general, stream water ingress into the hyporheic zone is dominant in the
upper 10cm or so of the stream bed where permeability is highest, though
temperature variability is moderated (i.e. decrease in summer and warm
in winter) with depth (Birkel et al., 2016). Micro-habitat studies of
spatial variations in temperatures in different morphological features
(e.g. pools and riffles) provide little evidence of thermal refugia in
the Girnock as the stream is generally well mixed and unstratified
(Imholt et al., 2013).

Temperature monitoring and modelling at the Girnock proved prescient,
given emerging concerns over climate change, highlighting the importance
of land use management and riparian woodland. Novel process-based
modelling of the effects of riparian woodland (Fabris et al., 2018)
combined with new remote sensing techniques (Dugdale et al., 2019)
showed that riparian woodland could be an effective climate adaptation
strategy, potentially mitigating lethal and sub-lethal effects of high
temperatures. Such important insights provided the impetus for a
national temperature monitoring network that developed large scale
models to prioritise riparian woodland regeneration to locations where
river temperatures and climate sensitivity are high (Jackson et al.,
2018) and where woodland can have a substantial effect in reducing
temperature extremes (Jackson et al., 2021).

Other important water quality parameters also relate to catchment
characteristics and land use. The peaty soils of the Girnock generate
high Dissolved Organic Carbon (DOC) concentrations during overland flow,
especially during higher flows in the summer and autumn, where organic
acids also reduce the alkalinity and pH of the stream (Dawson et al.
2011). Spatially distributed hydrological models (see below) have been
used to simulate DOC production and transport in different landscape
units and under varying degrees of catchment wetness and connectivity
(Dick et al., 2015).

With a high population of Red Deer, as well as other riparian animals
such as otters, water voles, sheep (in the lower catchment) etc., the
potential for significant microbiological pollution of surface waters
(indexed by Faecal Indicator Organisms (FIOs)) has been shown (Tetzlaff
et al., 2010). Modelling has also used a hydrologically-based spatial
framework and has gone some way to explaining the main drivers of FIO
delivery in terms of temperatures, antecedent wetness and movements of
red deer (Neill et al., 2019). The deer population has been implicated
in other water quality issues; for example, detectable nitrate
concentrations in parts of the Bruntland Burn corresponded to areas
where larger numbers of deer congregate (Blumstock et al., 2015). This
was unexpected in streams draining upland areas which are usually
oligotrophic and any labile nutrients are usually taken up by aquatic
plants.

\textbf{4.4. Primary production, invertebrates and drift}

Continuous stream monitoring for dissolved oxygen (DO) has been carried
out in conjunction with modelled estimates of net primary production
from in-stream plants. Whilst the dominance of near surface runoff
sources, rough channel topography and cool temperatures maintain
relatively high dissolved oxygen concentrations in the Girnock; seasonal
and diurnal variations reflect the balance of primary production and
respiration from heterotrophic consumption in the stream (Dick et al.,
2016). Algae and mosses on the stream bed drive the primary production
through growth early in the year and later in summer, with primary
production being highest in the upper, non-forested parts of the
catchment where radiation inputs are highest (Birkel et al., 2012).
Similarly, respiration is higher in the upper catchment, presumably
reflecting the higher substrate for heterotrophs and warmer temperatures
which make the stream heterotrophic overall.

The Girnock also has a diverse and abundant benthic invertebrate
community, dominated by stoneflies, mayflies and caddis flies (Gibbins
et al., 2016). This largely reflects the good water quality with high
alkalinity, high DO, low nutrient levels and gravel-rich substrate.
These animals graze on the primary producers, detritus and particulate
organic carbon. Despite, strong groundwater influence in the hyporheic
zone in some parts of the catchment, there are few anaerobic-tolerant
invertebrates (Gibbins et al., 2016) implying that overall, strong
well-oxygenated hyporheic flows dominant throughout most of the
catchment for most of the time.

Invertebrates provide the main food for juvenile salmonids.
Investigations into invertebrate drift in the Girnock have shown that it
peaks early in the spring and drives the highest rapid seasonal growth
rates in juvenile salmon. This complex interaction between food
availability and temperature explains the strong disparity between
juvenile growth rates predicted from laboratory-based temperature models
with ad-lib rations (Elliot and Hurley, 1997) and those growth rates
observed in the stream (Bacon et al., 2005). Later in the summer,
reduced food availability and high metabolic costs associated with
higher temperatures constrain growth potential (Jones et al., 2002).
Furthermore, within the river channel, the highest availability of drift
was correlated with areas of higher flow velocity linking hydraulic
habitats with potential for growth, with most bio-energetic models
assuming a trade-off between prey delivery and probability of capture
which reduces at higher velocities. Nevertheless, optimal habitat
utilisation of such physical controls on food availability is also
affected by fish behaviour and the density-dependent competition between
juveniles.

\textbf{4.5. Effects of flow on instream hydraulics and juvenile
performance}

Several studies have tried to better understand how instream hydraulics
affect juvenile salmon habitat. At the catchment scale, higher flows at
spawning and emergence time have been found to have a small positive
effect on fry production; however, there is no evidence that flow
variability has a substantial effect on parr numbers (Glover et al.,
2020) or performance, even during extreme years like 2015 (when Storm
Frank likely generated the highest flood on the River Dee for over 200
years). Early reach scale studies focused on using velocities as an
index of spatio-temporal variations in useable habitat (Tetzlaff et al.,
2005). These simplistically used the concept of critical displacement
velocity (CDV) -- i.e. velocity thresholds at which juvenile fish can no
longer hold station, which varies with fish size and stream temperature
(Fabris et al., 2017). However, CDVs are derived from flume experiments
which may not translate to more complex stream environments. Further,
the 3-dimensional hydraulics of the in-stream environment are complex,
and the high channel roughness results in many boundary layer effects,
that give fish more hydraulic shelter than simple velocity measurements
infer (Höjesjö et al., 2015). Although extreme low flows and short
periods of high flows during larger storm events may temporarily affect
the available habitat for salmon to forage for food, for most of the
time juvenile habitat is not expected to be limited by hydraulic
conditions in the Girnock (Fabris et al., 2017). In addition, while the
spatial distribution of depths and velocities varies in relation to
different types of river reach, as slope and bed roughness change,
essentially there is little difference in terms of useable hydraulic
habitat between reaches that have been studied.

\textbf{4.6 Isotope hydrology}

To understand the influence of instream hydraulics on salmon under
current and expected future climatic conditions, the role of catchment
hydrology as the main driver of flow variability must be recognised.
Short-term, seasonal and inter-annual flow variability depends on
hydroclimate and its effect on the inputs of water as precipitation, how
this water is stored in, and moves through the catchment, and how it
exits as streamflow or evapotranspiration (Soulsby et al., 2016a).

Monitoring the hydrology of a large, complex and remote catchment like
the Girnock is logistically challenging. Over a large heterogeneous
area, it is unclear how the complexities of climate and landscape
characteristics interact to govern the movement and storage of water as
much depends on the subterranean landscape. In 2003, weekly samples of
rainfall and streamflow were taken from several sites around the
catchment over a year and analysed for stable water isotopes to gain a
preliminary understanding of the integrated effects of these processes
(Tetzlaff et al., 2007). This followed previous successful Scottish
studies that used isotopes to understand streamflow generation processes
in larger catchments (Soulsby et al., 2000; 2006). Isotopes of hydrogen
and oxygen in rainfall vary seasonally and on a day-to-day basis,
reflecting air mass sources and energy available to evaporate heavier
isotopes (Fig. 5). Isotopic variation can be traced in the streamflow
response and used to infer water travel times through the catchment and
the ages of streamflow (Tetzlaff et al., 2007). This sampling evolved
into a \textgreater{}10 year programme of daily sampling of isotopes in
precipitation and streamflow which is internationally unique.

Resulting data show that isotopes in the stream follow the seasonality
and short-term variations in precipitation inputs in a highly damped and
lagged way (Fig. 5). Spatial variation in the damping and lagging of the
rainfall signal depends primarily on distribution of soil and drift
which control the dominant streamflow generation processes (see travel
time distributions in Fig. 6) (Soulsby et al., 2007). A constant
groundwater isotope signature dominates base flows, with groundwater
discharging directly into the stream through the hyporheic zone or by
exfiltration from hillslopes, via springs and seeps, around the edge of
the peatlands in the valley bottoms (Scheliga et al., 2016). High flows
are dominated by overland flow/shallow subsurface flow from peaty soils,
though rainfall events largely displace water already stored in the
catchment (Scheliga et al., 2019). The riparian peatlands act as
``isostats'' at high flows, damping the rainfall isotope signal and the
component of ``new'' rain water in storm runoff is usually
\textless{}10\% (Tetzlaff et al., 2014). The isotope studies have shown
that the mean stream water age is around 1.5 years; ranging from a few
weeks at high flows to over 3 years low flows (Soulsby et al., 2015;
Benettin et al., 2017). However inter-annual variability in mean stream
water age between drier and wetter years can be large, ranging between 2
years and \textless{}1 year, respectively (Birkel et al., 2015).

\textbf{4.7 Catchment hydrology}

Initial isotope studies resulted in hypotheses about catchment
hydrological function that were tested via direct monitoring of
hillslope hydrology (Fig. 6), also using tracers, to assess soil
moisture dynamics in the catchment, groundwater responses and generation
of overland flow in storm events (Tetzlaff et al., 2014). Most years,
the Girnock catchment follows a clear annual water balance; in winter,
higher precipitation inputs, coupled with low evapotranspiration, result
in high runoff coefficients (\textgreater{}60\%), whilst in drier
summers the runoff coefficient is \textless{}40\% and evapotranspiration
is the dominant water flux out of the catchment (Kuppel et al., 2020).

Throughout the year, groundwater mostly stored in various glacial drift
deposits sustains baseflows, and the Girnock and its major tributaries
are perennial streams. This isotopic evidence shows the groundwater
mainly recharges from winter precipitation, through the more freely
draining podzolic soils. Monitoring isotopes in soils and groundwater
along a hillslope transect showed that the hillslopes act as
advective-dispersive flow systems (Tetzlaff et al., 2014). However,
connectivity between hillslopes and riparian wetlands during rainfall
events controls the non -linearity of the catchment's storm runoff
response which is dominated by saturation overland flow (Birkel et al.,
2009; Ala aho et al., 2017).

\textbf{4.8. Terrestrial ecohydrology and greenwater fluxes}

Recent research has seen increased focus on the ecohydrology of the
dominant vegetation communities in partitioning rainfall reaching the
land surface (Soulsby et al., 2017b). This has involved direct
monitoring of canopy-water interactions and transpiration, as well as
coupling energy and water budgets in ecohydrological models (Wang et
al., 2017). Evapotranspiration is the main loss of water from the
catchment in most summers (between May and August) and contrasting
vegetation communities have different effects on ecohydrological
partitioning. Evapotranspiration losses are \textasciitilde{}20-30\%
higher from Scots Pine than heather and Sphagnum covered areas, mainly
as a result of higher interception losses from the forest canopy, higher
transpiration losses and higher soil evaporation (Wang et al., 2018;
Kuppel et al., 2020). Isotopic ecohydrology studies also show the xylem
isotopes in pine trees and heather stems, can be largely explained by
the isotopic composition of water in the near surface soils, though
internal storage and mixing in the trees appear to contribute to a more
complex picture (Tetzlaff et al., 2021). In contrast to streamflow,
which tends to draw on older (\textgreater{}3 years) groundwater,
evapotranspiration fluxes recycle much younger soil water. Soil
evaporation is predominantly \textless{} a few weeks old and transpired
waters tend to be a few months old (Kuppel et al., 2018). Transpired
water is older from trees because water ages increase from
\textasciitilde{}weeks/months in shallow horizons to \textasciitilde{}9
months at depth, with tree roots being able to access deeper water
(Smith et al., 2020a).

New insights into terrestrial ecohydrology have informed the evolution
of catchment models used in the Girnock; which have uniquely also used
isotopes and other tracers to improve model realism. These started with
lumped conceptual models (Tetzlaff et al., 2008; Birkel et al., 2010,
2011a,b), which were then spatially distributed (van Huijgevoort et al.,
2016a,b) and have included complex physically-based models (Ala aho et
al., 2017). Most recent has been the development of
EcH\textsubscript{2}O-iso, a new physically-based, spatially distributed
ecohydrological model that includes an isotope mass balance module to
help test and constrain process representation (Kuppel et al., 2018).
Such robust tracer-aided models can then be used for predicting effects
of environmental change in the Girnock, such as climate-driven changes
where drier summers and warmer winters are likely to decrease and
increase respective seasonal flows (Capell et al., 2013, 2014). Very
recent work has examined the potential impacts of re-forestation on the
catchment water balance, showing limited impacts on high flows, but
potentially reduced groundwater recharge and diminished low flows (Neill
et al., 2021). This is significant, given increased momentum behind
re-forestation for re-wilding, nature-based solutions for flood
management, carbon capture storage and biofuel production (Soulsby et
al., 2017a).

Importantly, the tracer-aided modelling developed in the Girnock has
been successfully applied in other studies in catchments as widespread
as Germany (Smith et al., 2021), China (Zhang et al., 2019), Costa Rica
(Correa et al., 2020), Sweden (Smith et al., 2020b), USA (Ala aho, et
al, 2017) and Canada (Piovana et al., 2019). Again, this shows how high
quality data and innovation at a long-term sites can leverage tools that
can be applied elsewhere.

\textbf{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
\textgreater{}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.

\textbf{5.1 Adult numbers and sizes}

Rod catch can be a useful proxy for adult abundance (Thorley et al.,
2005) and these data indicate that the numbers of spring salmon (defined
as fish caught between February and May) returning to the river Dee have
declined since records began in 1952 and are now at around 8\% of their
former levels (Fig 3a). Over the period 1966-2022 (over which the
Girnock has operated) the Dee spring rod catch and number of females
returning to the Girnock have both declined by ca. 88\%, indicating that
temporal trends and processes observed at Girnock are likely indicative
of those across the upper Dee catchment where spring salmon are assumed
to dominate. This makes the Girnock data especially valuable in
understanding population bottlenecks, controlling processes and
evidence-based management options at larger scales.

The relationship between emigrating juveniles and returning adult
numbers is density independent (Fig. 10d) so that greater emigrant
numbers generally result in more returning adults (Gurney et al., 2010).
However, there is also a strong downward trend in return rate,
indicative of declining survival between emigration and adult return
(Fig. 3b). This is particularly concerning given the major reduction in
distant water and domestic commercial exploitation over the same time
period. Indeed, there are now no commercial distant water fisheries for
salmon, local net fisheries have also been removed and catch and release
is compulsory for anglers in spring (before the 1\textsuperscript{st} of
May) and applied almost universally (\textgreater{} 99.5\% in 2022) on
the Dee regardless of the time of year.

The size of female salmon (and thus the number of eggs produced) has
also declined nationally (Bacon et al., 2009) and locally (Glover et
al., 2018). This combination of fewer and smaller females has had a
substantial effect on egg deposition in the Girnock, although the effect
is dominated by reductions in adult female numbers (Glover et al.,
2018). In the last 10 years, it is estimated that ova deposition has
only exceeded the number required to maximise emigrant production on a
single occasion (2017), with major consequences for emigrant production
and subsequent adult returns, particularly in recent years. The factors
driving these changes are multiple and complex. Poor marine survival and
growth are thought to be key factors (Gilbey et al., 2021) resulting
from declining food resources due to climate change (Nicola et al.,
2018). However, commercial bycatch, and predators in both marine (e.g.
seals and dolphins) and freshwater (piscivorous birds, otters and other
fish including brown trout and Pike) have been suggested as additional
potential problems (Marine Scotland and Fisheries Management Scotland,
2023).

\textbf{5.2 Ova -- alevin survival and performance}

As the main spawning locations in the Girnock occur in
geomorphologically stable lower gradient reaches where suitable gravel
accumulates, there is no evidence to suggest winter wash-out of ova is a
significant bottleneck on the salmon population even following extreme
recent events such as Storm Frank (Glover et al., 2020).

As low DO concentrations can result in high ova mortality where spawning
gravels coincide with areas of chemically-reduced groundwater discharge;
consequently conservation stocking has been shown effective at reducing
ova mortality and increasing fry production. However, at least
historically, these early benefits of stocking were offset at later life
stages, not translating to improvements in parr (Glover et al., 2018) or
emigrant production (Bacon et al., 2015). While the reasons for this
failure remain unclear, it emphasises the challenges of trying to
artificially manipulate and improve natural systems even where
underlying processes are extremely well understood. It has also been
shown that low DO concentrations can result in delayed or prolonged
hatch and smaller salmon fry. However, the potential for compensatory
growth when emerging into high DO is also high and again appears to
indicate a strong resilience to naturally encountered environmental
challenges (Youngson et al., 2004).

\textbf{5.3 Fry (age-0 salmon)}

Densities of fry vary spatially, mainly with altitude in the Girnock
(Glover et al. 2019 and Fig. 9). Contrary to previous expectations
(Nislow et al., 2011) they have been shown to occupy a wide range of
habitats (Hojesjo et al., 2016) that were generally only constrained by
proximity to suitable spawning substrate and the ability of fry to
disperse after emergence (Webb et al., 2001). At the catchment scale,
ova to fry survival in the Girnock was found to be density independent
with ova and fry numbers linearly related) (Fig. 10). This was
surprising as the ova to fry stage is usually assumed to be a major
bottle neck to production in salmonid populations, including Atlantic
salmon (Nislow et al., 2004) and anadromous brown trout (Elliot, 1987).
The contrasting findings at the Girnock likely reflect the varying
scales of observation and analysis, with a whole riverscape assessment
of production at Girnock in contrast to more traditional studies
focussed on the scale of individual electrofishing sites where
emigration and mortality are strongly confounded. Of course local
density-dependent mortality could still occur near individual redds, or
groups of redds within the Girnock, and not be detected at the catchment
scale (Einum and Nislow, 2005).

Recent work has demonstrated significant downward trends in fry
abundance both at the catchment scale and at individual long-term
electrofishing sites. However, it was only possible to reliably detect
these downward trends where data were appropriately corrected for
confounding trends in capture probability (Glover et al., 2019). This is
because in such a long-term data sets, the effects of varying staff and
changes in equipment quality, as well as changes in fish density and
size can introduce major biases in the time series (Glover et al.,
2018). As with many novel aspects of research at the Girnock, the
importance of these effects has been recognised more widely with similar
patterns subsequently observed nationally (Malcolm et al., 2023) and in
other countries (Dauphin et al., 2018)

\selectlanguage{english}\textbf{5.4 Parr (age-[?]1 salmon)}

Parr densities exhibit complex spatial patterns of abundance over and
above the effect of altitude that are thought to relate to substrate
characteristics and over wintering habitat, though water quality and
food availability are also likely secondary effects (Fig. 9).
Electrofishing data over five decades show significant downward trends
once the data are corrected for observation bias. The electrofishing
data, combined with advanced population models, show that the survival
of fry to one year old parr is strongly density dependent and this is
where much of the natural population regulation has occurred
historically at the Girnock (Fig. 10b). In recent years low numbers of
returning females mean that ova deposition is insufficient to fully
stock the river resulting in salmon production that is well-below
carrying capacity for fry, parr and emigrants. In these circumstances
any management action aimed at increasing juvenile production would need
to reduce density independent rates of mortality rather than increase
the carrying capacity of habitat. In river systems such as the Girnock
where there is ample cover from large boulders and plentiful available
habitat, making improvements to juvenile habitat quality without
addressing poor adult returns and ova survival are unlikely to be
successful. However, reducing early life stage mortality without
adequate understanding of the constraints on parr survival will also
likely be unsuccessful as shown through previous conservation stocking
experiments (Glover et al., 2018).

\textbf{5.5 Emigrants (autumn parr and smolts)}

There is increasing evidence that migrating juvenile salmon are leaving
river systems earlier in the year (Malcolm et al., 2015). As with the
younger age of smolts, this is likely related to an increase in stream
temperature although the controls on emigration timing within years and
season are hard to determine with confidence given available data
(Buddendorf, 2018). The timing of migration is likely to be important to
subsequent survival, matching arrival at sea with available food
resources and potentially allowing migrants to avoid other pressures
(Thorstad et al., 2012). The consequences of changing migration time
from the Girnock are currently unknown, although the observed patterns
are not unusual in the wider context (Otero et al., 2013).

The relationship between the production of 1-year old parr and emigrants
is not obviously density dependent (Fig. 10c). However, emigrant
production is also affected by density dependent growth of juveniles
(further modified by upward trends in river temperatures), changes in
age at emigration and thus mortality. Although there are clear downward
temporal trends in emigrant numbers, there are also complex temporal
trends in the age composition of emigrants, and timing of migration
(autumn vs. spring). Ignoring these important changes (e.g monitoring
only spring migrants) could provide an extremely biased and misleading
impression of the status of populations. Understanding these complex
processes and their interactions calls for sophisticated numerical
lifecycle models such as those deployed at Girnock previously (Gurney et
al., 2008). Indeed, such lifecycle models are increasingly proposed for
assessing and informing management action for salmon as populations
decline and there is a need for careful consideration of resource
expenditure (Bull et al., 2022). The Girnock remains one of the very few
locations globally where such models can be realistically parameterised
through detailed observational data collected across all relevant life
stages and stock levels.

\textbf{6. Lessons learned over 60 years}

\textbf{6.1 Curiosity drives science}

Whilst the initiation of monitoring in the Girnock catchment resulted
from the need for scientific evidence to inform policy development for
salmon fisheries management, much subsequent research evolved
organically. The motivation for most of the research has been
curiosity-driven science by individual investigators and has usually
been galvanised by hypotheses arising from earlier observations at the
Girnock itself, often informed by prevailing ``hot'' questions in the
wider scientific community. The crucial importance of this motivation
for discovery science is often overlooked, but as with other long-term
sites (e.g. Rose, 2007; Peterken and Mountford, 2017), it is such
motivation for interdisciplinary science that drives the commitment
needed to procure funding, launch projects, manage research teams,
analyse data and then communicate the research in the scientific
literature or at national or international symposia (Burt and Thomson,
2020).

\textbf{6.2 Well-curated data are gold}

A key lesson is that long-term meticulously collected field data has an
inestimable value (Rosi et al., 2022). While ever more sophisticated
models can integrate insights, enhance process-based understanding, test
hypotheses and provide a basis for prediction; to be of most value, they
need to be grounded in high quality data (Blair et al., 2019).

It follows that careful management, quality control, documentation and
curation of data is crucial. Over long periods, staff, field methods,
instruments, data storage capabilities and analytical tools all change.
The accuracy and precision of data can therefore also change and there
can almost never be enough documentation of meta-data to help later
researchers utilise data effectively. Even where data are carefully
quality controlled and archived, it is often only when time series are
analysed in a robust manner that potential errors and biases become
apparent (Glover et al., 2019). In some cases, in the Girnock study
researchers have had to revert to original field notebooks to try and
understand and correct data inconsistencies in very time-consuming data
archaeology (Glover et al., 2020).

\textbf{6.3 Technology transforms}

Since monitoring began in 1966, the technology available for data
collection has inevitably been transformed, with environmental sensors
becoming more accurate, reliable and inexpensive. Likewise, fisheries
assessment tools, such as electrofishing and tagging have improved,
leading to the need to compare methods and re-construct comparable time
series. Fortunately, statistical and modelling tools for data evaluation
are also continuously improving in the era of ``big data'' and often
these can be used in model-data fusion to enhance the maximum
information content extractable from data, as well as quantify
uncertainties associated with historic time series.

\textbf{6.4 Connecting local and global scales}

The research questions prompting the initiation of the Girnock study
were place-based, pertaining to the monitoring of a single site, even if
the site was considered more broadly indicative of spring salmon
habitats. Nevertheless, the process-based understanding is transferable
and this has informed international scientific studies, as well as
management. Likewise, the organic evolution of the research agenda as
largely been driven by emerging local questions arising from
observational science that have led to a more interdisciplinary approach
linking salmon research to the wider environmental sciences. In other
salmon streams around the world, local insights in understanding the
salmon life cycle and its environmental controls have resulted in
comparable time series, though very few have such a wider understanding
of the catchment context. Likewise, environmental observatories without
a specific salmon focus have been seeking similar functional
understanding of catchment hydrology and biogeochemistry that the
Girnock/Bruntland studies have, though in-stream the ecological
implications are generally less well-covered. Thus, the Girnock occupies
a unique position in a number of monitoring sites such as Burrishoole in
Ireland (McGinnity et al., 2004) and Catamaran Brook in Canada (Cunjak
et al., 2013) that allow intercomparison studies to leverage broader
relevance to generic environmental and ecosystem conservation concerns
that are truly global.

\textbf{6.5 Serendipity counts}

Whilst carefully posed research questions are the main drivers of
scientific progress, serendipity is often overlooked as part of
research. In long-term environmental observation, this may include
capturing extreme events (e.g. floods (Soulsby et al., 2017a) or
droughts (Soulsby et al., 2021)) which offer new insights that would
have been difficult to anticipate. Also, visitors to the site have
brought fresh ideas, knowledge and experience that have resulted in
collaborations and step changes in understanding. Even painful mistakes
and errors provide opportunities where new ideas and conceptualisations
emerge. There is a danger if research programmes become too tightly
controlled and highly constrained as budgets tighten, and many benefits
will be lost without the potential for serendipity and the ability to
engage with other researchers.

\textbf{6.6 People are important}

Environmental field research does not occur in isolation, it is
people-orientated and invariably enhanced by teamwork, social
interaction and endless discussion and conjecture involving many
collaborators with different roles. Over almost 60 years, changing
personnel of key team members over successive generations also occurs.
These include scientific ``champions'' of the research site who
formulate key research questions, secure funding and are often
imaginative thinkers with a long-term perspective. Equally, the
intellectual gene pool needs to remain open, new cohorts of PhD students
and post-doctoral researchers provide the tenacity and drive to make
research happen, extend the boundaries of understanding of the site and
disseminate research findings. But the bedrock of successful scientific
function are dedicated, skilled and determined technicians who
year-after-year, day after day, and in all weathers operate the fish
traps, do the electrofishing, download data loggers and empty samplers,
in addition to many other essential services.

\textbf{6.7 Knowledge transfer and sustainable management for the
future}

Concerns about climate change and the risks to salmon populations, as
well as other important issues, such as flood hazards, droughts and
warming streams, are resulting in rapidly changing approaches to
managing salmon streams that are increasingly catchment focussed and
aimed at restoring natural processes and sustainability (e.g. FNLRT,
2021). Extensive riparian planting (to ameliorate stream temperatures),
reintroduction of beavers, installation of large woody structures (to
increase morphological diversity) and restoration of forests and
peatlands are all being actively promoted as ameliorative measures in
Scotland at present. In the right context, there may be much to be
gained from such initiatives. But it is important that the goals of
management are cognisant of the complex biotic and abiotic processes
that sustain wild salmon populations, and in this regard are realistic
about where changes can positively affect salmon production.
Furthermore, management actions must certainly seek to avoid doing
further harm and also avoid the allocation of constrained resources to
actions that have a low likelihood of successful outcomes. In this
context, the continued development of process-based understanding of
salmon populations at low stock levels becomes increasingly important,
as does understanding of the efficacy (or otherwise) of management
actions.

\textbf{7. Conclusions}

The remarkable insights gained in 60 years of ecohydrological monitoring
and research in the Girnock catchment underline the value of such
interdisciplinary, long-term observatories. Of particular importance
are: (1) The unrivalled value of the detailed process data for both
scientific understanding and underpinning evidence for assessment and
management tools. (2) The ability of such sites to test major management
actions through detailed process understanding, these have included
previous stocking assessments and the potential to assess future
management actions such as enhancing riparian woodlands. However, this
work needs to be undertaken in a carefully controlled manner to ensure
the integrity and interpretability of the long-term data for other
purposes. (3) The need for detailed data and process understanding to
parameterise models; both to integrate knowledge, and underpin
assessment and management. These including adult and juvenile assessment
methods and salmon life cycle models as advocated under the Missing
Salmon Alliance's ``Likely Suspects'' framework. (4.) The need for very
detailed monitoring sites where processes are understood so we can more
reliably understand and predict the cumulative effects of future climate
and land use change. Though these requirements and consequent benefits
are widely recognised within the scientific community (e.g. Tetzlaff et
al., 2017), maintenance of funding for such research sites is a constant
and significant challenge. Academic funding agencies often prefer short
term hypothesis-driven research and it is not uncommon for long-term
place-based studies to be erroneously disparaged as ``mindless
monitoring'' or ``case studies'' as noted by Burt (1995). Of course, the
published track record of research at the Girnock shows obvious added
value results from being able to contextualise short-term
hypothesis-driven research at data-rich long-term sites. It is therefore
to be hoped that central public funding continues to support the basic
research infrastructure and core monitoring at the Girnock and
elsewhere, as alternative funding sources are rare and Government is
uniquely placed to sustain and curate such long-term data collection.
Indeed, the exceptional knowledge generated from the Girnock emphasises
the value of collaboration between Government and research organisations
which can bring contrasting but complimentary resources and perspectives
with associated benefits for research and innovation. The commitment to
maintaining such research will be crucial in creating process-based
evidence for understanding how freshwater ecosystems evolve in response
to looming climatic and other (e.g. land use) environmental change. This
will help communicate issues to stakeholders and underpin the evolution
of management strategies that might ameliorate adverse impacts and
conserve important ecosystems that sustain iconic species such as
Atlantic salmon.

ACKNOWLEDGEMENTS

CS is grateful to The Leverhulme Trust the Fellowship entitled
``Atlantic Salmon as environmental sentinels.'' DT thanks IGB for
support. We all thank the many technicians, PhDs, post-docs and senior
scientists who have built the body of knowledge on the Girnock and its
salmon. In particular, we thank Alan Youngson who first invited our
involvement in the work and his generous encouragement and stimulation.



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Soulsby C, Birkel C, Geris J, Dick J, Tunaley C, Tetzlaff D. (2015)
Stream water age distributions controlled by storage dynamics and
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23(1), 51-71

Fig 1 (a) The fish traps on the lower Girnock; these are located c.1km
upstream of the confluence with the River Dee. The lower trap intercepts
immigrating adults; the upper trap emigrating smolts and parr. (b) Upper
catchment some 5km upstream of the traps where the channel provides
juvenile and spawning habitat. Note the lack of riparian tree cover and
montane nature of the upper catchment.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
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\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

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Fig 2 Salmon life cycle with timings for Girnock fish\selectlanguage{english}
\begin{figure}[H]
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\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

Fig. 3 Long-terms trends in the River Dee spring (February \_May) rod
catch (Scottish Government, 2023b) (A), Girnock Burn percentage female
return rates (Glover and Malcolm, 2015a) (B), Female numbers (Glover and
Malcolm 2015b) (C), Ova deposition (D), Fry (0+) and Parr
(\textgreater{}0+) production (E), and production of emigrants (Glover
and Malcolm, 2015c) (F). Fry and Parr production estimates are for day
of the year 240, following Glover et al., 2018.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

Fig 4 Characteristics of the Girnock catchment: (a) topography, (b)
geology, (c) soil cover and (d) land use.\selectlanguage{english}
\begin{figure}[H]
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\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}\selectlanguage{english}
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\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
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\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

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\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

Fig 5 Rainfall -- runoff in the Bruntland Burn tributary of the Girnock
and daily isotope variations in precipitation and stream water in
hydrological years 2011/12-2021/22.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

Fig. 6 Dominant hydrological pathways in typical Girnock hillslope: 1:
vertical percolation through podzolic soils, 2: lateral flow at
permeability boundaries, 3: exfiltration of hillslope groundwater in
saturated valley bottom areas with gley soils, and 4: saturation
overland flow on peat and 5: deeper groundwater flow. Insets show
profiles of dominant soils and travel time distributions as probability
density functions for major soil horizons, groundwater and the stream
channel.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

Fig. 7 Spawning distributions in the Girnock (after Moir et al., 2004)\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Picture1/Picture1}

\end{center}
\end{figure}

Fig 8 Temporal variability in maximum daily stream temperature corrected
for logger bias 1988-2021 (after Jackson et al., in prep.)

\par\null\selectlanguage{english}
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\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/Picture2/Picture2}
\caption{{This is a caption
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Fig. 9 Spatial variability in the predicted densities of juvenile salmon
populations in the Girnock (after Glover et al., 2018). Densities are
illustrated for a median ova deposition level. Panels A and B show
densities of fry obtained from naturally spawned ova (A) and densities
of parr which were independent of stocking effects. Panel C shows the
more uniform spatial variability in fry densities resulting from uniform
stocking. Panel D shows the spatial differences in fry production
between uniformly stocked and naturally stocked years.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image13/image13}

\end{center}
\end{figure}

Figure 10 Girnock stock recruitment relationships between Ova and Fry
(0+) (A), Fry (0+) and Parr (1+) (B), Parr(1+) and Total emigrants
(autumn and spring) (C), and Emigrants and returning females. All data
have been appropriately lagged using information on fish ages obtained
from scale reading. Fry and Parr production estimates are for day of the
year 291 and 241, for fry and parr respectively after Glover et al.,
2020. Years affected by stocking (which enhanced fry production) and low
trap effort (which reduces estimates of emigrant production) are
indicated by varying point colour.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image14/image14}

\end{center}
\end{figure}

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