The proposed Mowi salmon farm raises serious questions about the protection of Scotland’s seabirds, the integrity of marine conservation designations and the scientific evidence required before industrial development takes place in sensitive marine habitats.
A protected seabird colony facing an uncertain future
At the southern entrance to the Firth of Clyde, Sanda Island and its neighbouring islands support one of Kintyre’s most important seabird colonies.
The Sanda Islands Site of Special Scientific Interest (SSSI) is designated for its maritime cliffs and eleven breeding seabird species, including black guillemots, razorbills, puffins, common guillemots, European shags, kittiwakes, Manx shearwaters and storm petrels.
For these seabirds, the surrounding sea is not simply water between nesting sites. It is an essential part of their habitat, providing access to fish, feeding grounds and the marine ecosystems on which successful breeding depends.
Much of this marine environment is protected as the Clyde Sea Sill Marine Protected Area (MPA), designated partly because of its importance as a feeding ground for black guillemots.
Yet Mowi Scotland is proposing a substantial new salmon farming development off the eastern side of Sanda Island, within the MPA and close to the protected breeding colony.
The company’s published proposals include six cages, each measuring 200 metres in circumference, with an anticipated maximum permitted biomass of approximately 4,000 tonnes.
This raises a fundamental conservation question:
Why should a large industrial aquaculture development be considered within a marine area specifically designated to protect the feeding habitat of a seabird population already requiring recovery?
Black guillemots are already in trouble
NatureScot’s December 2025 Conservation and Management Advice for the Clyde Sea Sill MPA provides particularly concerning evidence.
It reports that black guillemot numbers at the associated breeding colony declined from 443 birds in a 2003 count to 220 birds in the 2015–2021 survey period — a reduction of approximately 50%.
The conservation status of the MPA’s black guillemot feature is consequently assessed as unfavourable, with a formal objective to recover the population to favourable condition.
The causes of the decline are not fully understood. NatureScot identifies possible factors including mink predation and changes in prey availability.
There is no evidence establishing that salmon farming caused this historical decline, and it would be scientifically inappropriate to suggest otherwise.
But that uncertainty makes the proposed development more concerning, not less.
Where a protected population has already experienced a substantial decline, introducing new pressures before understanding its existing vulnerabilities carries obvious conservation risks.
Read NatureScot’s December 2025 MPA conservation advice
The missing evidence: where exactly do Sanda’s seabirds feed?
Black guillemots are particularly interesting because they frequently forage relatively close to their breeding colonies, diving to capture small fish and other marine prey.
A 2015 RSPB study commissioned by Scottish Natural Heritage tracked black guillemots around Orkney and demonstrated that individual birds repeatedly used particular feeding areas.
Some travelled several kilometres, but their movements were not random. Birds returned to preferred feeding locations, often associated with relatively shallow seabed habitats.
This has important implications for Sanda.
The proposed salmon-farm footprint lies within an MPA recognised for its black guillemot foraging habitat. However, the precise overlap between the proposed cages, moorings, associated infrastructure and the feeding areas used by individual Sanda birds has yet to be established from publicly available tracking evidence.
This distinction matters.
An environmental assessment should not assume that birds will simply move elsewhere if a preferred feeding area becomes disturbed or inaccessible.
Alternative feeding grounds may be farther away, less productive or already used by other individuals. Additional travel and reduced feeding opportunities could impose energetic costs during breeding.
At present, we do not know whether these effects would occur at Sanda, or how substantial they might be.
The scientifically responsible approach is to establish where the birds feed before deciding that development in those waters is acceptable.
Read the RSPB black guillemot tracking report
Fish farms can change seabird behaviour
Scientific research demonstrates that seabirds do interact with marine fish farms, although the effects vary between species, locations and operating practices.
An especially relevant Scottish study by Carss (1993), examining European shags at fish farms in Argyll, found that birds consumed wild fish aggregating around the cages. Their feeding rates were not significantly different from those observed in natural habitats.
Other international studies have documented increased bird abundance around aquaculture installations, changes in bird-community composition and interactions with farm infrastructure.
These observations should not automatically be interpreted as environmental benefits.
Attraction to artificial structures can change natural foraging patterns and may increase exposure to nets, ropes, vessel movements and disturbance.
Equally, the published evidence does not demonstrate that every fish farm causes seabird displacement or population decline.
What emerges is a complex relationship that requires careful, species-specific investigation.
At Sanda, where seabird conservation is already an explicit management objective, the question is not simply whether birds can be observed around salmon cages.
It is whether the development would alter their feeding success, breeding performance, survival or access to important habitat.
Underwater noise: an overlooked conservation pressure?
One of the less understood aspects of aquaculture development is underwater noise.
Modern salmon farms are not acoustically silent. Depending on their equipment and operations, underwater sound can be generated by workboats, feeding and maintenance activities, net-cleaning systems, pumps and other machinery. Acoustic deterrent devices, where proposed or used, require particular scrutiny.
Noise emissions can vary substantially in frequency, intensity, duration and operating pattern.
This is an area of particular interest to Clyde Porpoise.
We are undertaking underwater acoustic monitoring around Sanda Island, including the area of the proposed salmon-farm development.
Our objective is to improve understanding of the existing underwater soundscape and provide evidence against which potential additional anthropogenic noise can be assessed.
The implications extend beyond marine mammals.
In 2020, researchers demonstrated that common guillemots (Uria aalge) responded behaviourally to experimentally presented underwater sound. Although the study did not involve black guillemots or operational salmon-farm noise, it challenges the assumption that diving seabirds are necessarily unaffected by underwater acoustic disturbance.
There are currently substantial evidence gaps concerning the underwater hearing and behavioural sensitivity of black guillemots.
Further questions arise concerning noise effects on small fish and other prey organisms. Disturbance to prey behaviour or distribution could potentially produce indirect consequences for feeding seabirds even where the birds themselves show no obvious response.
These remain plausible pathways requiring investigation, rather than proven impacts at Sanda.
For this reason, acoustic assessments should consider the complete operational cycle of the proposed farm, including intermittent high-noise activities, rather than relying solely on measurements of routine background operations.
Read Hansen et al. (2020), The common murre, an auk seabird, reacts to underwater sound
Nets, entanglement and the loss of feeding habitat
Aquaculture infrastructure also introduces physical structures into the water column, including containment nets, protective netting, mooring systems and associated equipment.
Historical research has documented seabird deaths associated with entanglement at Scottish fish farms, although historical mortality figures cannot be assumed to represent the risks associated with modern cage systems.
NatureScot’s 2024 review of measures to support seabird recovery also identifies aquaculture-net entanglement as an issue where improved monitoring and evidence are needed.
For diving seabirds, the potential risks are particularly relevant because they actively pursue prey underwater.
Modern engineering and properly maintained protective systems may reduce these risks. However, the absence of reported entanglement incidents is not necessarily evidence that no incidents occur, particularly where systematic monitoring is limited.
NatureScot’s December 2025 advice for the Clyde Sea Sill MPA expressly identifies finfish aquaculture as a potential source of entanglement, disturbance and reduction of prey-supporting habitat affecting black guillemots.
It recommends measures including careful site selection, appropriate net design, entanglement monitoring, vessel management and protection of feeding habitats.
These concerns have therefore already been recognised by Scotland’s statutory nature conservation adviser.
They should be central to the assessment of any proposed development within the MPA.
A significant scientific evidence gap
Perhaps the most troubling issue is how much remains unknown.
NatureScot’s own December 2025 advice identifies the need for updated black guillemot counts, better information about their distribution within the MPA, improved understanding of their local prey and supporting habitats, and further research into the reasons for their decline.
These are not peripheral scientific questions.
They concern the very species for which part of the marine environment around Sanda has been designated for protection.
Mowi has reported undertaking environmental baseline surveys, including seabird surveys, to inform its Environmental Impact Assessment. Those data deserve careful consideration when they become publicly available.
However, the adequacy of the assessment should be judged by whether it answers the relevant ecological questions, not simply by whether surveys have been carried out.
In particular, a survey recording birds flying or resting near the proposed development would not necessarily establish where individual breeding birds obtain their prey, how frequently they use the development footprint or how changes in prey availability might affect them.
The distinction between recording bird presence and understanding ecological dependence is fundamental.
Cumulative impacts cannot be ignored
Sanda’s seabirds do not experience environmental pressures in isolation.
Their populations are influenced by changing marine food webs, climate-related changes, disease, predation, fisheries, vessel activity and other pressures.
A new salmon farm would introduce additional infrastructure, aquaculture operations and vessel movements into this already changing environment.
A robust assessment must consider whether these additional pressures could interfere with the recovery of protected bird populations.
It must also examine impacts on seabed communities, fish habitat, water quality, wild salmonids and marine mammals.
An assessment that finds each individual pressure to be small does not necessarily establish that their combined effects will be insignificant.
For an MPA with a protected feature already in unfavourable condition, the relevant question is whether development is compatible with achieving recovery.
What should happen before any development is approved?
Clyde Porpoise believes that important scientific questions must be answered before a development of this scale is permitted within the Clyde Sea Sill MPA.
At a minimum, the environmental assessment should provide:
- Detailed seabird foraging studies, establishing the distribution, seasonal use and ecological importance of waters around the proposed cages and moorings, with particular emphasis on black guillemots.
- A transparent assessment of habitat overlap, showing the proposed infrastructure, vessel routes and potential pressure zones in relation to SSSI and MPA boundaries and identified seabird feeding areas.
- A comprehensive underwater acoustic assessment, including representative measurements or defensible modelling of routine operations, vessel activity, net cleaning and any proposed acoustic equipment.
- A species-specific assessment of entanglement and disturbance, including monitoring arrangements, protective-net design and contingency measures.
- An assessment of prey and supporting habitats, including potential changes in fish distribution, benthic communities and access to foraging areas.
- A meaningful cumulative-impact assessment, considering existing environmental pressures and the MPA’s conservation and recovery objectives.
- Independent scrutiny and public access to the underlying evidence, enabling researchers and conservation organisations to evaluate the assumptions used to justify the proposed development.
Where material ecological uncertainties cannot be resolved, alternative locations outside sensitive protected foraging habitat should be properly considered.
Protection must mean more than a line on a map
Mowi has stated that it believes the proposed Sanda development can operate without adversely affecting the MPA’s protected black guillemots and seabed features.
Those claims must be tested against transparent scientific evidence.
A Marine Protected Area is not necessarily a blanket prohibition on development. But its designation should have practical consequences for the location, design and assessment of activities that could affect protected features.
In this case, the conservation objective is not merely to prevent further deterioration. It is to recover a black guillemot population already assessed as being in unfavourable condition.
That requires particular care.
Sanda’s ecological importance has been recognised for decades, and the surrounding marine habitat is central to the purpose of the Clyde Sea Sill MPA.
It would be difficult to justify introducing a substantial new industrial development into those protected waters without first demonstrating, through sufficiently robust and publicly accessible evidence, that it would not undermine the conservation objectives.
We should not have to demonstrate that damage has already occurred before taking the protection of an important seabird feeding ground seriously.
For Clyde Porpoise, the position is clear: where the scientific evidence remains inadequate to assess significant environmental risks, uncertainty should favour protecting the ecosystem rather than assuming that the development will be harmless.
Sanda’s seabirds deserve more than a promise that they will be unaffected.
They deserve an environmental assessment capable of demonstrating it.
Scientific references and supporting documents
- NatureScot (2011). Sanda Islands SSSI — Official Citation.
- NatureScot (2025). Clyde Sea Sill MPA — Conservation and Management Advice.
- Owen, E. (2015). Black guillemot tracking in Orkney, 2013 and 2014. SNH Commissioned Report 903.
- Carss, D.N. (1993). Shags at cage fish farms in Argyll, Western Scotland. Bird Study, 40, 203–211.
- Ronconi, R.A. & St. Clair, C.C. (2002). Management options to reduce boat disturbance on foraging black guillemots. Biological Conservation, 108, 265–271.
- Hansen, K.A. et al. (2020). The common murre, an auk seabird, reacts to underwater sound. Journal of the Acoustical Society of America.
- NatureScot (2024). Potential Actions to Support Recovery of Seabird Populations in Scotland. Research Report 1370.
- Mowi Scotland (October 2025). Sanda Island Development Update.
Clyde Porpoise is an independent marine research and conservation organisation investigating the underwater acoustic environment and marine wildlife of the Firth of Clyde and western Scotland.