2. The Girnock catchment and its salmon population
The geology of the Girnock’s landscape (Fig. 4) was set
~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
(>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 Sphagnummosses dominate, along with grasses such as Molinia where
minerogenic groundwater drainage occurs. On the podzols and rankers,
Scots Pine (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 (Cervus elaphus ).
Forest now only remains on steeper, more inaccessible slopes or fenced
plantations (Neill et al., 2021). Where the forest has been cleared,Calluna and 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 (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
~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 (<10mm), with larger daily totals
(>25 mm) only occurring 3-4 times per year. Snow usually
accounts for <10% of inputs and snowpack accumulation below
700m generally lasts only a few weeks. Winter temperatures are cold (Jan
mean ~0oC), and summer is mild (July
mean = ~14oC).
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 (>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.