Scottish river restoration schemes which are currently receiving large sums of public funding are unlikely to be successful, are based on weak scientific evidence and risk significant degradation of salmon habitat, a leading scientist has warned.
Professor Chris Soulsby from the University of Aberdeen and two of Scotland’s most experienced salmon biologists, have set out concerns about the impact of engineering projects in upland areas like the Cairngorms in a commentary paper published in the journal Hydrological Processes.
He argues that the conservation of mountain streams in Scotland is being compromised by weakly regulated engineering projects which aim to restore salmon habitat but are instead risking significant in-stream degradation that could do ‘more harm than good’.
The Cairngorms region includes some of Scotland’s most famous salmon fishing rivers including The Spey, Dee and Tay.
In recent years salmon restoration projects in many Scottish rivers, including those withing the protected Cairngorms National Park, have involved extensive engineering to install dozens of Large Wood Structures (LWS) usually incorporating several dead trees. These have been employed as part of bids to halt declining salmon numbers.
Proponents claim 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. However, while such engineering may have benefits in highly degraded urban and agricultural streams, the scientists say there is no clear evidence that salmon habitat is limiting in many of the upland rivers affected or that engineering such rivers will actually help salmon.
In the paper, Professor Soulsby and colleagues set out that ‘these schemes - which can affect rivers with the highest conservation designations in protected landscapes - have been subject to limited environmental assessment.’
They question the risks engineering activities pose to juvenile salmon, and suggest existing high quality salmon habitat may be degraded.
Heavy machinery disturbing rivers is known to cause sediment pollution, siltation of downstream habitats and compaction of the river bed in areas where salmon live.
Professor Soulsby said: “In most conservation work, protecting existing high-quality habitat, rather than disturbing it, is the most effective way of helping endangered species.
“Many of these engineering schemes are not effectively regulated by Scotland’s environmental management agencies despite being in protected areas like National Parks and involving a protected species.
“One of the reasons for weak regulation seems to be that the engineered structures incorporate dead trees. It is naively assumed that this makes the stream more ‘natural’ as much of Scotland was afforested before human influence and streams would have contained dead wood.
“However, engineering fixed large trees into rivers is increasingly recognised as being a poor analogy for natural streams.”
In Scotland and other North Atlantic countries, salmon populations have declined in recent decades; climate change and reduced food sources in marine feeding grounds are the likely main causes, Professor Soulsby explains.
“As declining salmon numbers is essentially a marine problem, engineering in upland streams will not address this fundamental cause of the problem,” he adds.
“Salmon ecology is complex and relies on rivers providing a diverse mix of habitats to sustain them at different life stages. Many salmon-rearing streams in the Cairngorms already do this.”
The authors say fixed wood like LWS should only be used as “a last resort” and reserved for the most severely degraded streams lacking habitat variability. Whilst planting trees along river banks as part of restoration schemes - to help shade salmon streams - will help mitigate the effects of a warming climate, engineering will actually restrict rivers from adapting to expected climate extremes, the scientists warn.
They are calling for a more evidence-based approach, scientifically-robust monitoring of the impacts and adaptive management where necessary.
‘It is striking that these components are still absent for most projects in Scotland,’ they conclude.