Why the proposed salmon farm near Sanda Island raises important questions about underwater noise, seabird conservation and the future of the Clyde Sea Sill Marine Protected Area.
The waters surrounding Sanda Island, at the southern entrance to the Firth of Clyde, are home to one of Scotland’s most distinctive diving seabirds: the black guillemot (Cepphus grylle).
With its striking black summer plumage, white wing patches and bright red feet, the black guillemot is a familiar sight around rocky Scottish coastlines. Yet much of its life takes place beneath the sea surface, where it uses its wings to pursue fish amongst rocky reefs, seaweed beds and seabed habitats.
These underwater feeding grounds are essential to its survival.
As Mowi Scotland pursues proposals for a large salmon farm near Sanda Island, an important environmental question deserves closer examination:
What effects could increased underwater noise have on the black guillemots that depend on these waters?
A protected seabird population already in difficulty
Sanda Island, Sheep Island and Glunimore Island support breeding black guillemots associated with the Clyde Sea Sill Marine Protected Area (MPA).
Black guillemots are one of the MPA’s designated conservation features. Their conservation status is currently classified as unfavourable, and the stated objective is to recover the population.
NatureScot’s conservation advice identifies a historical site reference population of 440 birds, based on counts from 1998–2003. The most recent count cited in that advice was 220 individuals in 2021.
The reasons are not fully understood but this represents a substantial reduction relative to the historical reference population and any new development should be assessed in the context of a protected population already requiring recovery..
Black guillemots live in an underwater world
Unlike many seabirds that capture prey at the surface, black guillemots are pursuit divers.
They descend through the water, propelling themselves with their wings, and feed on fish such as butterfish, blennies, sandeels and other small species associated with seabed habitats.
NatureScot identifies nearshore waters around Sanda, Sheep and Glunimore as particularly important. Black guillemots predominantly forage in relatively shallow waters, often diving to depths of up to approximately 50 metres.
They are also resident birds, remaining around their breeding colonies throughout the year.
During the breeding season, adults must repeatedly catch fish and return to their nests to feed their chicks.
A disturbance that interrupts feeding, even temporarily, could therefore have consequences for the amount of energy a bird expends or the food it delivers to its offspring.
Whether underwater noise actually causes such effects in black guillemots remains an important scientific question.
Can guillemots hear underwater?
Until relatively recently, underwater-noise research concentrated mainly on marine mammals and fish.
Diving seabirds received comparatively little attention.
However, a significant scientific study published in the Journal of the Acoustical Society of America in 2020 demonstrated that a closely related auk, the common guillemot (Uria aalge, also called the common murre), responded to underwater sound.
Researchers exposed two common guillemots to controlled underwater acoustic signals, including broadband sound bursts and mid-frequency naval sonar signals.
The birds consistently responded to the experimental sounds, while comparable responses were absent during control trials.
The received sound-pressure levels ranged from approximately 110 to 137 dB re 1 µPa.
For one bird, the number of responses increased as the sound level increased.
This was an important demonstration that an auk can detect and react to underwater acoustic disturbance.
However, the experiment involved only two common guillemots under controlled conditions. The reported sound levels are not established disturbance thresholds for black guillemots and cannot simply be transferred to a salmon farm.
We still lack sufficient species-specific evidence to determine precisely which underwater frequencies black guillemots detect, the levels that provoke disturbance, or the long-term consequences of repeated exposure.
That uncertainty should be recognised in environmental assessments rather than treated as proof that disturbance cannot occur.
What underwater noise does a salmon farm produce?
Modern salmon aquaculture involves considerably more than stationary floating cages.
Depending on the design and operation of a particular site, potential underwater sound sources include:
- Workboats, wellboats and other servicing vessels.
- Propellers, propulsion systems and manoeuvring equipment.
- Pumps, compressors and mechanical equipment.
- Net-cleaning machinery and associated vessels.
- Underwater positioning, telemetry and acoustic navigation systems, where installed.
- Acoustic deterrent devices, where proposed or used.
Their acoustic characteristics can differ enormously. A vessel propeller may produce substantial low-frequency noise, while some underwater positioning systems operate at much higher frequencies.
This distinction matters because the biological effects of noise depend not simply on how loud a signal is, but on its frequency, duration, repetition, propagation and the hearing abilities of the receiving animal.
A 2025 study of Norwegian salmon farms found that the underwater soundscape of open sea cages was highly variable and strongly influenced by vessel activity.
More recent research at a salmon farm in Chile also documented increased underwater noise associated with daytime operations.
Neither study establishes the acoustic conditions at Sanda Island, but both demonstrate why operational noise should be considered as part of aquaculture environmental assessment.
How might underwater sound affect a diving seabird?
There are several potential pathways through which underwater noise could affect black guillemots.
Disturbance during feeding
A bird hearing an unexpected underwater sound might interrupt its dive, alter its swimming behaviour or leave an otherwise productive feeding area.
Even a short interruption could increase the time and energy needed to obtain food.
Displacement from feeding habitat
If repeated acoustic disturbance caused birds to avoid particular areas, they might need to travel farther to find alternative feeding grounds.
This could be important for breeding birds that depend on regular access to prey close to their nesting sites.
Effects on prey
Underwater sound can also affect some fish and invertebrates. Depending on the species and exposure, it may change movement, behaviour or habitat use.
Any resulting changes in prey availability could indirectly affect a diving predator.
Cumulative disturbance
A single passing vessel and a location experiencing regular servicing, mechanical operations and other underwater sound sources represent different exposure patterns.
The combined effects of frequency, timing and repeated activity deserve consideration, particularly during the breeding season.
These are credible potential impact pathways. They are not evidence that Mowi’s proposed Sanda development has already caused them.
Why Sanda Island deserves particular attention
The Clyde Sea Sill is an ecologically important region where water from the North Channel interacts with water from the Clyde.
Its tidal fronts and associated habitats support marine wildlife, including black guillemots feeding around the islands.
NatureScot’s conservation guidance emphasises the importance of maintaining access to suitable feeding and resting habitat and avoiding significant disturbance.
This is especially relevant because the local black guillemot population has already declined relative to its historical reference.
An environmental assessment of the proposed salmon farm should therefore consider more than the physical space occupied by the cages.
It should examine whether operational activities could influence the surrounding waters used by feeding birds, whether important habitat might be avoided, and whether potential disturbance could interfere with the MPA’s recovery objective.
The precise relationship between the proposed farm footprint, acoustic exposure areas and black guillemot feeding locations must be established from site-specific evidence.
What should Mowi’s environmental assessment demonstrate?
Clyde Porpoise considers that the following questions warrant clear, evidence-based answers:
- What is the existing underwater soundscape? Are baseline measurements available across relevant seasons and tidal conditions?
- Which acoustic sources will operate? Have the types of vessels, machinery, net-cleaning systems and any hydroacoustic equipment been fully identified, including their frequencies, sound levels and operating patterns?
- Where might the sound travel? Has suitable propagation modelling examined potential exposure within important guillemot feeding areas?
- How could the birds respond? Does the assessment recognise the available scientific evidence from diving seabirds while clearly explaining uncertainties specific to black guillemots?
- What about cumulative exposure? Have repeated vessel movements, maintenance activities and other existing underwater noise sources been considered together?
- How will effects be checked? Are appropriate monitoring, mitigation and review measures proposed if disturbance is identified?
These questions are particularly important because underwater noise is generally invisible to people observing a development from the shore.
A site may appear relatively undisturbed at the surface while containing a complicated and changing underwater soundscape.
A precautionary approach to an uncertain risk
The scientific literature does not yet demonstrate that routine salmon-farm noise causes population decline in black guillemots.
Equally, research has shown that related diving seabirds can respond to underwater sound, and important questions remain about the implications of repeated exposure.
The Scottish Government’s 2025 Seabird Vulnerability Report acknowledges underwater-noise sensitivity in common guillemots, while emphasising the limited available evidence.
For Sanda Island, the appropriate response to uncertainty is a transparent, proportionate and scientifically defensible assessment.
The important questions are whether biologically relevant sound is likely to be produced, whether black guillemots will be exposed to it, and whether any resulting disturbance could compromise their access to feeding habitat or the conservation objectives of the MPA.
Protecting what makes Sanda Island special
Sanda Island is not simply a convenient stretch of coastal water for industrial development.
It is part of a wider marine ecosystem supporting protected wildlife, important seabed habitats and seabird populations whose recovery is already a conservation priority.
Clyde Porpoise believes underwater noise deserves careful consideration alongside the more familiar environmental concerns associated with salmon farming, including seabed impacts, fish health, pollution and interactions with wild marine species.
The proposal provides an opportunity to ask an important question that extends beyond Sanda itself:
Are environmental assessments of modern aquaculture adequately considering the underwater acoustic environment experienced by diving seabirds?
The science is still developing. That is a reason to investigate potential effects carefully, not to overlook them.
Scientific references and further reading
NatureScot (2025). Conservation and Management Advice: Clyde Sea Sill Marine Protected Area. Site-specific conservation objectives and black guillemot population information.
Anderson Hansen, K. et al. (2020). The common murre (Uria aalge), an auk seabird, reacts to underwater sound. Journal of the Acoustical Society of America, 147, 4069–4074.
Scottish Government (2025). Scottish Seabird Conservation Action Plan: Vulnerability Report. Assessment of seabird pressures, including underwater noise.
Bittelli, D. et al. (2026). Preliminary characterization of underwater noise from a salmon farm in northern Patagonia. Marine Pollution Bulletin, 230, 119860.
Scottish Government (2026). Clyde Sea Sill MPA — Site and Area Proposal Document.
Featured Image Credit: Black guillemot (Cepphus grylle), Oban Harbour, Scotland. Photograph © Jacob Spinks, 2013. Licensed under CC BY 2.0: https://creativecommons.org/licenses/by/2.0/ . Original image: https://commons.wikimedia.org/wiki/File:Cepphus_grylle,_Oban_Harbour_1.jpg
Published by Clyde Porpoise — Independent marine research, underwater acoustics and conservation in Scotland’s west coast waters.