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.