The Canary in the Coal Mine

Why focus on salmon? Apart from their iconic status, importance to the Scottish economy, incredible life cycle and exceptional inherent beauty, salmon are a very useful species to focus our attention on.

Born in a wee bed of gravel wherever there’s a suitable flow they can be found as far up into the hills as salmon can reach. Only major waterfalls or man made dams are likely to stop their ambitions to get as high into the river system as possible.

Bu even these waters that they can’t reach can affect the redds (nests) and nursery areas downstream, so as the young salmon hatch, live and grow in this flowing freshwater world they are influenced by the quality and temperature of the water coming from both upstream and off the surrounding land.

The young salmon spend as little as one year or as many as seven in their home river – juvenile fish less than 12cm long represent 90% of the entire Atlantic salmon population. When they’re ready they travel downstream through the river system, journeying through lochs and tributaries into the main stem, down into the estuaries, passing through sea lochs, traversing inshore waters and then on out to the open ocean.

All going well, they’ll return anytime between one and six years later as adult salmon ready to create the next generation of young fish. Some returning salmon hang about in the river for nearly two years before spawning themselves. So the vast majority of the world’s stocks of Atlantic salmon can be found in our rivers at any one time. If there’s a deterioration in their environments, it will show in their health and numbers.

As such, salmon are an important indicator species – the Canary in the Coal Mine.

Hot water

The Highlands of Scotland are not generally noted for high temperatures, winter or summer. However, Scotland’s rivers – even those in the Highlands – do reach temperatures that can kill salmon, and predictions say that our future will be hotter yet.

The temperature maps produced by Marine Scotland Science – thanks to the recent addition of temperature loggers to many rivers – tell an alarming story. We may have as little as 30 years before rising temperatures make these rivers – in their current wide, shallow unshaded state – uninhabitable for salmon.

Salmon eggs, which are usually laid in their nests (known as redds) between November and February, can survive temperatures up to a maximum of 16C. The beginning of 2022 saw us very close to that limit –
Scotland recorded the mildest New Year’s Day on record thanks to a flow of warm subtropical air from the Azores.

The Met Office confirmed temperatures reached 15.9C (60.6F) at Achnagart in Glenshiel, Kyle, on the morning of 1 January 2022. But the Met Office say warm weather records are being broken nine times more frequently than in previous years, a clear sign of an over-heating planet

While air temperatures don’t immediately result in the same water temperatures, sustained periods of warmer winter weather will jeopardise salmon egg survival.

During the summer we’ve also been breaking temperature records. It’s now common for us to spend several summer weeks with daytime temperatures above 20C. Both young and adult salmon struggle at anything above 20C. Above 23°C and the adults can’t find the energy to put on that burst of speed they need to leap the falls – salar, the leaper (the salmon’s scientific name is Salmo salar), gets stranded downstream.

Young salmon at the same temperatures will be suffering thermal stress and behavioural change, stopping feeding and seeking out shade. If there’s a lack of shaded areas, they may have to share those spaces with bigger fish that might still be feeding and like the look of the lunch swimming their way.

At 33°C a previously healthy salmon wouldn’t survive much more than a minute.

During a study of causes of death associated with Catch and Release (C&R) angling it was found that thermal shock (in water temps above 18C) was the top killer. You might think it would be squeezing of the fish by the over-excited angler, or suffocation from being out of the water, or fungal infection from losing their protective slime, but it was thermal shock which trumped them all as the most deadly.

        “mortality rates of C&R Atlantic salmon tend to be less than 12% when water temperatures are <18°C (Brobbel et al. 1996, Dempson et al. 2002, Thorstad et al. 2007, Havn et al. 2015), but can be as high as 80% when the water temperature exceeds 18°C” (

A Fish of the Forest

Six thousand years ago, a rich, dynamic woodland ecosystem stretched across 60% of Scotland’s land area. These were diverse forests of Scots Pine, Oak, Rowan, Ash, Birch, Hazel, Alder, Blackthorn, Crab apple, Aspen, Willow and more – each of which has good numbers of associated insect species.

Both the trees and their insects have adapted to live here over millennia. There wasn’t a Sitka spruce in sight. Our native Oak, Birch and Willow have over 200 associated insect species each, and all these species of tree produce leaf litter which becomes aquatic insect food. Sitka spruce, on the other hand – a non-native species now planted widely across Scotland – has fewer than 40 associated insect species and only produces needles which are acidic and actually reduce insect productivity.

These ancient forests had plenty of dead wood – a home for still more insects – which.along with abundant leaf litter, produced rich soils. The rivers had regular additions of fallen trees, not just to the banks but crucially in mid-stream and all over the river corridor. Those trees, with no one to winch them out in an attempt to ‘tidy up’, would decay either on site, or wherever flood waters took them, to accumulate in piles of woody debris which would become biodiversity hotspots. Decaying wood is nature’s own bug hotel.

Wooded river banks provide shade that helps keep rivers cool and productive. Picture a river with trees on the banks. Those banks, even if they become undercut, will be held together by the root network and associated fungi. Those undercut banks are shaded and filled with roots. The roots themselves, of trees like alder and willow which can live in the water, provide excellent cover from the sun and hidey-holes for fish during high flow events.

If those trees die, when their roots decompose the bank is less stable. This often results – particularly where no trees grow to replace the dead ones – in the banks falling in. The effect upon the river is more erosion. Fine sediments which now dump into the water choke the oxygen out of fish redds, and the river becomes wider and shallower, and may lose some riffle-and-pool diversity.

Shallow water heats up more quickly than deep water, and evaporates faster. Abstraction for hydro power can also worsen this situation. Whatever was living in that river is now in warmer, shallower water, more exposed and less likely to survive. If the change from slow flow to fast or from cool water to warm happens gradually the fish have a better chance of acclimatising and surviving. Trees on the banks and in the water can not only buffer these fluctuations to make them slower but also reduce their peak extremes.

Beaver in our Neolithic scene would have been creating wetlands, raising the water table and coppicing their favourite species on the banks, providing dappled shade over the water – the best kind for fish. Coppicing of trees extends the life of that tree, rather than foreshortening it. Territorial creatures, such as salmon, are limited in where they can go. That’s why having the options of light or dark, hot or not, in close proximity is particularly valuable. Such options are only provided by dappled shade.

The creatures that fed on salmon such as bear, osprey and otter were excreting those fishes’ precious marine nutrients into the surrounding environment. Phosphorous, found in salmon carcasses, is a precious and limited commodity in the soils of much of Scotland. Not having enough phosphorous limits plant growth, and limited plant growth results in limited food and habitat for other creatures.

The South Esk in Angus, with nicely wooded riverbanks

The Way Ahead

Today, just 3% of that original native forest cover remains, and despite an increase in will and action to preserve what remains, we’re still in decline. It’s worth noting that the European average native woodland cover is 30-40%. Despite their reputation for beauty and drama, Scotland’s glens where our rivers run are often bare and treeless, reflecting the centuries of ecological decline that we have come to accept as normal.

If a river and the creatures within it have the wood removed they are left exposed. Exposed to floods, exposed to droughts and worst of all exposed to direct sunlight and increasing heat. The general decline in insect numbers has been noticed by most of us. A lack of trees reduces insect numbers further and fewer insects means less food for all the animal life forms that feed upon them, and for plants that require them for pollination.

If present at all on our rivers, trees are often limited to a single line, which is never replaced by natural regeneration because the young saplings are often eaten by sheep or deer before they can get above a few inches – long before they’re big enough to coppice.

The stratified habitat that trees offer is often overlooked. Canopy, understory, shrub layer and herb layer all provide living space for different creatures as do the unique conditions on a forest floor, and still more habitat is created below the soil. Though no vision for a healthy landscape would be 100% closed canopy native tree cover, a coverage of 30% would represent a colossal upturn in life potential, climate breakdown resilience and carbon sequestration, as well as having wide-ranging benefits for Scottish biodiversity.

We’re looking at 40 years of rising temperatures, a decline in insects, salmon and trees. What can we do?

Given that more than 90% of the world’s salmon are, at any given time, in rivers and given that actions on the river are more immediately achievable than the issues they face at sea, increasing native tree cover is a significant step toward recovery. Those trees need to be not just in the river corridor, but also on the surrounding land.

The presence of trees means more rainwater is absorbed and the land can act like a sponge. Compacted land however, where rainwater doesn’t soak in to be cooled by the soil and released gradually, but instead runs off overland and straight into the nearest waterway, does little to help water quality, temperature or flood risk.

Trees represent the salmon’s shield from the fireball above, and our best hope of resilience in the impending heat to come from climate change. Trees also bring the associated benefits of better water retention during droughts, reduced effects from agricultural runoff, greater abundance of food species and overall landscape health.

We need to get trees back on our river banks as quickly as possible. Natural regeneration does it best, but for this to work we need to have a seed source and either fewer sheep and deer or less access for them to the river banks. Failing that, protected planting of carefully chosen native trees will have to do.

We must repopulate our rivers and glens with trees and, in so doing, with life.

Duncan Pepper | River Revivers (