The Cairngorms national park has some of Scotland’s wildest rivers. The Spey, Dee and Tay are famous for salmon fishing. The mountain headwaters of these rivers have reliably provided cold, clean water and suitable rearing habitat for Atlantic salmon for thousands of years. These are among the least disturbed, most highly protected rivers in Europe, designated as special areas of conservation for the preservation of salmon and other species.
Paradoxically, these rivers have recently seen unprecedented levels of heavy engineering as part of expensive, publicly funded schemes intended to help salmon conservation in response to climate change. Unfortunately, this probably won’t enhance salmon populations and may do more harm than good. So what is going on?
Most salmon rivers have seen a dramatic decline in catches over recent decades. This probably relates to the complex, far-reaching effects of climate change.
In Scotland, warmers winters means less snowfall and lower summer river flows, with stream temperatures increasing by about 1°C over the last 50 years and now sometimes approaching harmful levels for salmon. Recently, increased winter rainfall has caused unprecedented flooding and changes to river channels.
So, fisheries managers and conservation organisations have tried to increase the resilience of salmon streams.
The resulting projects focus on installing high numbers of artificial, engineered wooden structures in rivers. These comprise the trunk and root networks of several dead, wind-blown trees. Proponents claim – based on assumptions rather than scientific evidence – that this will enhance salmon populations by providing shade, shelter from floods and predators, and diversifying salmon habitat by creating new patterns of erosion and deposition.
Salmon ecology is complex and relies on rivers providing a diverse mix of habitats to sustain them at different life stages. Fish need spawning gravels in the stream bed for adults laying their eggs, areas of fast currents for juvenile feeding and slow-flowing areas and pools for shelter.
But, there’s little evidence that lack of such suitable habitat currently limit salmon production in designated Cairngorm streams or that re-engineering rivers with wood will enhance populations.
Unfortunately, the strongest climate change impacts probably occur in the Atlantic feeding grounds where juvenile salmon migrate to grow into mature adults. Here, warming temperatures have altered food webs, affecting prey species such as sand eels. As a result, salmon populations have declined and, in extreme cases, numbers of adults returning to spawn have fallen by 90%. This triggers a downward spiral: fewer returning adults mean fewer eggs, fewer young fish then migrate to sea and even fewer adults return the following year.
This has socio-economic and political implications. During their upstream return journey, adult salmon are sought by fishermen, with angling generating £135 million annual revenue and sustaining more than 4,000 jobs in rural areas. So, the decline of salmon creates political pressure to act.
In response, the Scottish government’s 2022 strategy for wild salmon advocates the protection and recovery of salmon rivers. This coincided with the governments Nature Restoration Fund, administered by NatureScot, providing over £50 million for projects addressing the climate and bioversity crises. These initiatives rapidly stimulated a growing number of landscape-scale, often salmon-focused, habitat “restoration” projects affecting over 100km of rivers in the Cairngorms national park.
These usually centre on tree planting along river banks to provide shade and offset increasing temperatures. But proponents also argue that trees eventually die and fall into rivers, further diversifying the habitat in ways that may benefit salmon. As this will take decades, managers seek to accelerate the process with engineered wood.
Engineered wood features are usually fixed in the stream bed or banks. This requires using 14-tonne diggers to excavate many tonnes of stream sediments for installation.
Examples of recent schemes involve up to 100 engineered wood structures spread over a few miles of stream. Often in rivers that already provided high-quality salmon habitat. The ecological consequences of such large scale interventions have not been established.
Disturbance is damaging
However, such extensive engineering risks damaging the pre-existing habitat long-used by salmon that management usually seeks to protect. Movement of heavy machinery can damage fragile in-stream and riverside habitats. Fine sediments disturbed during wood installation can be lethal to salmon and degrade downstream habitats when deposited.
Fixing engineered wood structures in upland salmon streams is highly questionable given most recent science. Fixed structures restrict a river’s freedom to adjust to disturbances such as floods, which probably makes channels less able to accommodate expected climate extremes and maintain salmon habitats.
So why haven’t these rivers been protected from such unnecessary engineering? Inadequate regulation seems to be an issue. Placing wood in streams is exempt from licensing by the Scottish Environment Protection Agency which encourages small-scale use of engineered wood for restoration of degraded rivers.
However, availability of large amounts of restoration funding has rapidly accelerated upscaling of such engineering projects in our least disturbed streams without enough environmental assesment. Although many such rivers retain conservation designations under residual EU legislation, it is unclear what protection this now provides.
While managers might feel pressure to do something in relation to salmon declines, the notion that adding engineered wood to upland streams will improve climate resilience needs scrutiny. Projects need to be evaluated on the basis of clear evidence. This needs thorough assessment of risks, scientifically robust monitoring and provision for adaptive management. At present, expensive and weakly regulated “restoration” schemes in Scotland risk doing salmon more harm than good and opportunities to learn from practical experiences are being missed.
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Chris Gibbins received a grant from UK Environment Agency and United Utilities that funded the restoration of the River Ehen (Cumbria).
Chris Soulsby has received funding from NERC and the Leverhulme Trust.
This article was originally published on The Conversation. Read the original article.
