By Clyde Porpoise | Marine Conservation and Underwater Acoustic Research
Scotland’s salmon farming industry increasingly relies on powerful underwater machinery to maintain its fish pens. But what happens when industrial net-cleaning equipment introduces noise into waters occupied by harbour porpoises, dolphins and other marine wildlife? And is the existing regulatory system adequately protecting these animals?
For decades, concern about underwater noise from Scottish salmon farms has concentrated on acoustic deterrent devices (ADDs), which were widely used to discourage seals from approaching fish pens.
However, ADDs are not the only potential acoustic pressure associated with aquaculture. High-pressure net washers, underwater robotic cleaning systems, vessel machinery, thrusters and associated electronic equipment all have the potential to introduce sound into the marine environment.
Recent research has demonstrated that net cleaning can be one of the noisiest routine activities undertaken at a salmon farm.
This raises an important conservation question: are the acoustic impacts of routine industrial maintenance being properly assessed, particularly where multiple fish farms operate within habitats used by legally protected cetaceans?
Net cleaning is not necessarily a quiet operation
Salmon farm nets accumulate marine organisms, including algae, hydroids and other fouling species. Regular cleaning helps maintain water circulation and prevent excessive biological growth.
Modern cleaning equipment can employ high-pressure seawater jets, remotely operated vehicles, electric thrusters and powerful pumps.
These processes generate underwater noise through several mechanisms, including water-jet turbulence, machinery vibration and the movement of equipment through the water.
Some underwater robotic systems also use acoustic sensing or positioning technology. Such systems must be distinguished from the mechanical cleaning equipment itself: a net cleaner equipped with cameras and sensors does not necessarily contain an active sonar transmitter.
The important issue is the total underwater acoustic output of the equipment actually being operated.
New scientific evidence from 2026
A study published in Aquaculture International in March 2026, by Jónsdóttir and colleagues, examined underwater sound levels during routine activities at a Norwegian Atlantic salmon farm.
Researchers monitored an open sea cage over several months, comparing ordinary operating conditions with net cleaning and delousing activities.
They found that net cleaning consistently elevated underwater sound levels, particularly above 200 Hz.
During days involving cleaning of the monitored cage, the 95th-percentile broadband sound pressure level reached approximately 150 dB re 1 µPa, measured across 20 Hz–12.8 kHz.
The researchers suggested that net cleaning may represent a significant acoustic stressor for farmed salmon, while emphasising that more research is required to understand the biological effects of repeated exposure.
These measurements cannot be directly translated into a harbour porpoise disturbance radius or hearing-injury threshold. Nevertheless, they establish that industrial net cleaning can materially alter the underwater soundscape.
Harbour porpoises live in an acoustic world
Harbour porpoises depend heavily on sound to navigate, locate prey and interact with their environment.
Their narrow-band, high-frequency echolocation clicks are particularly important when hunting small fish.
Unlike animals that can rely substantially on vision, porpoises must operate within an underwater acoustic environment that is increasingly influenced by human activities.
Underwater noise may affect cetaceans through several pathways:
- Behavioural disturbance: Animals may change swimming direction, dive behaviour or feeding activity.
- Habitat displacement: Animals may avoid areas repeatedly exposed to disturbing sound.
- Acoustic masking: Noise can interfere with the detection of biologically important sounds when frequencies and other acoustic characteristics overlap.
- Hearing effects: At sufficient exposure levels, some sounds can cause temporary or permanent changes in hearing sensitivity.
- Repeated disruption: Even exposures below auditory-injury thresholds may affect animals if they repeatedly interrupt essential activities.
It is important to distinguish these possible outcomes. Not every audible sound is harmful, and not every behavioural response represents a significant conservation impact.
However, absence of demonstrated hearing damage does not establish absence of disturbance.
The cumulative noise problem
Perhaps the greatest concern is not a single net-cleaning operation but the combined acoustic footprint of an increasingly industrialised marine environment.
Consider a coastal area containing several salmon farms.
Each farm may generate underwater noise from vessels, generators, pumps, feeding operations, cleaning equipment and other activities. Additional sound may come from commercial shipping, fishing vessels, recreational boats and scientific or navigational sonar.
Even when individual operations are relatively short, repeated or overlapping activities may create persistent pressure.
A harbour porpoise exposed to sound at one farm may encounter additional sources as it moves through neighbouring waters.
This is especially relevant to animals repeatedly using the same feeding grounds.
In its December 2025 UK Cetacean Conservation Strategy, the UK and devolved governments acknowledged that the combined effects of multiple underwater noise sources may be greater than those of individual activities.
The strategy also recognised that substantial knowledge gaps remain concerning the effects of cumulative disturbance on reproduction and survival.
This creates a difficult conservation problem: how can regulators establish that repeated industrial activities are environmentally acceptable without adequate information about their combined acoustic effects?
An assessment confined to one piece of equipment or one fish farm may fail to capture the total exposure experienced by animals moving through a wider coastal habitat.
UK Cetacean Conservation Strategy: technical report
Evidence from Scottish aquaculture already warrants caution
The potential for fish-farm noise to disturb cetaceans is not a new concern.
Research published in Biological Conservation in 2024 investigated the effects of chronic ADD noise on harbour porpoises around Scottish aquaculture installations.
The study found changes in porpoise acoustic activity associated with ADD exposure and estimated that approximately 34% of porpoises could potentially be displaced from the areas experiencing the most intense ADD noise.
This study concerned acoustic deterrent devices, not net-cleaning machinery. Its results must not be treated as evidence that net cleaners have equivalent acoustic characteristics or biological effects.
It does, however, demonstrate that noise associated with aquaculture can affect the distribution or acoustic behaviour of harbour porpoises.
Harbour porpoises respond to chronic acoustic deterrent device noise from aquaculture (2024)
The Scottish Government and NatureScot have previously recognised the importance of managing ADD disturbance, including the risks associated with repeated exposure and overlapping noise sources.
The question now is whether a similarly precautionary approach should extend to other emerging or poorly characterised sources of aquaculture noise.
Clyde Porpoise investigations at Ardyne
Clyde Porpoise has been investigating underwater acoustic activity associated with fish-farm operations in the Firth of Clyde.
On 14 September 2026, passive acoustic monitoring near the Ardyne fish farm recorded high-frequency acoustic signals during a period of activity associated with a workboat at the installation.
Harbour porpoise detections were also recorded during the monitoring period.
These observations justify further scientific investigation, but important questions remain unresolved.
In particular, the source of the high-frequency signals must be positively identified before they can be attributed to a specific cleaning or sonar system. Their received levels, propagation characteristics and potential biological significance also require further assessment.
The presence of porpoise detections during an industrial operation does not, by itself, demonstrate disturbance or displacement. Equally, continued detections do not necessarily demonstrate that exposure had no effect.
Clyde Porpoise is seeking to establish what equipment was operating, its acoustic characteristics and whether the activity was assessed for possible impacts on protected marine mammals.
This work forms part of our wider investigation into the underwater acoustic environment of the Clyde and the potential cumulative impacts of industrial marine activities.
Is underwater net-cleaning equipment legally licensable?
Potentially, yes — but different activities and legal protections must be distinguished.
The Scottish Government confirmed in a 2024 Environmental Information Regulations response that there is no single set of regulations specifically governing every form of net washing.
Depending on the activity, relevant controls may include planning permissions, environmental discharge authorisations and marine licensing requirements.
The Marine (Scotland) Act 2010 requires marine licences for defined categories of activity, such as certain deposits, construction works and dredging. The generation of ordinary underwater noise is not, by itself, a general category of licensable marine activity under that Act.
However, a separate and particularly important legal framework applies where protected marine mammals may be disturbed.
European Protected Species legislation
All cetaceans occurring in Scottish waters, including harbour porpoises, are European Protected Species.
Within Scottish territorial waters, they are protected by the Conservation (Natural Habitats, &c.) Regulations 1994, as amended.
Regulation 39 prohibits the deliberate or reckless disturbance of any dolphin, porpoise or whale, subject to the law’s exceptions and licensing provisions.
Consequently, an industrial activity that would otherwise be lawful may require a European Protected Species (EPS) licence if it is likely to cause disturbance or injury falling within these protections.
For relevant commercial marine activities, EPS licensing is administered by the Scottish Government’s Marine Directorate – Licensing Operations Team.
The licensing regime includes demanding tests relating to the purpose of the activity, satisfactory alternatives and the conservation status of the species concerned.
Importantly, a fish farm’s existing planning permission or environmental discharge authorisation does not automatically authorise unlawful disturbance of cetaceans.
Conservation (Natural Habitats, &c.) Regulations 1994, Regulation 39
Scottish Government: Marine EPS licensing
Could operating noisy net-cleaning equipment constitute a wildlife crime?
Yes, in principle. But the operation of noisy equipment is not automatically a criminal offence.
A possible offence would have to be considered against the specific circumstances and the statutory requirements.
Relevant questions include:
- Was a protected cetacean disturbed or injured?
- Is there sufficient evidence linking the alleged disturbance to the operation?
- Was the conduct deliberate or reckless within the meaning of the legislation?
- Did the operator hold any relevant authorisation?
- Do any statutory exceptions or defences apply?
An acoustic recording demonstrating a powerful underwater signal could provide important evidence. However, sound detection alone does not prove that a wildlife offence has occurred.
Evidence of an animal’s exposure, behavioural response, the source and operation of the equipment, and the operator’s knowledge of the risks may all be relevant.
Any determination of criminal liability belongs to the appropriate investigating and prosecuting authorities.
The conservation issue is therefore not whether every noisy cleaning operation is criminal. It is whether operators and regulators are taking sufficient precautions to identify and prevent activities that could unlawfully disturb protected animals.
Are there gaps in underwater-noise regulation?
One concern is that existing monitoring frameworks are not necessarily comprehensive inventories of all anthropogenic underwater sound.
For example, the UK’s Marine Noise Registry records specified categories of impulsive noise activity, including ADDs and certain sonar systems. Other equipment, including single-beam echosounders and some higher-frequency systems, falls outside its normal reporting categories.
This does not make those activities unlawful. Nor does it mean the equipment is necessarily harmful.
It does mean that the absence of an activity from the registry cannot be taken as evidence that it makes no contribution to underwater noise.
Modern aquaculture increasingly employs sophisticated equipment whose acoustic characteristics may not be fully described in publicly accessible environmental assessments.
If regulators are to assess cumulative impacts properly, they need reliable information about what equipment is operating, how frequently it operates and what acoustic energy it introduces into the marine environment.
Joint Nature Conservation Committee: Marine Noise Registry
What should change?
Clyde Porpoise believes that underwater noise from aquaculture maintenance deserves a more systematic and precautionary approach.
We recommend:
1. Independent acoustic characterisation. Manufacturers and operators should make available suitable measurements of underwater sound emitted by net cleaners and associated equipment, including frequency spectra, source levels where reliably established, operating modes and duty cycles.
2. Cumulative impact assessment. Fish-farm environmental assessments should consider overlapping vessel operations, maintenance activities, ADDs and other relevant sound sources rather than examining equipment in isolation.
3. Transparent operational records. Information about equipment type, operating times and relevant acoustic systems should be available to regulators and, subject to legitimate restrictions, to independent researchers.
4. Species-specific protection. Assessments should consider harbour porpoise hearing, feeding ecology, behavioural responses and the potential significance of repeated disturbance.
5. Practical noise reduction. Wherever practicable, operators should use quieter technology, avoid unnecessary acoustic transmissions and minimise activities during periods of particular sensitivity.
6. Clear licensing guidance. The Marine Directorate should clarify when routine aquaculture maintenance may trigger EPS licensing requirements, how operators should demonstrate compliance, and how possible disturbance incidents will be investigated.
Protecting Scotland’s underwater environment
Scotland’s coastal waters are not simply industrial operating areas. They are productive marine ecosystems that support protected wildlife, commercial fisheries and coastal communities.
Harbour porpoises have occupied these waters for thousands of years. They depend on an acoustic environment in which they can detect prey, navigate and communicate.
The growing use of industrial equipment below the surface introduces pressures that are often invisible to the public and may be poorly captured by conventional environmental monitoring.
Modern net-cleaning technologies offer operational advantages to the aquaculture industry. That does not remove the need to understand and manage their environmental effects.
The precautionary principle should mean identifying and reducing credible risks before significant harm occurs, rather than waiting until damage has been conclusively demonstrated.
Where protected cetaceans are present, underwater noise should be treated as an environmental pressure requiring proper evidence, transparent assessment and effective regulatory oversight.
Clyde Porpoise will continue investigating the acoustic impacts of marine activities in the Firth of Clyde and supporting evidence-based protection for Scotland’s marine wildlife.
Independent underwater acoustic monitoring and marine conservation research — Clyde Porpoise.
Scientific and regulatory references
- Jónsdóttir, K. E., Mathiassen, J. R., Svendsen, E. & Volent, Z. (2026). Long-term underwater passive acoustic monitoring of the soundscape in an Atlantic salmon sea cage. Aquaculture International, 34, article 118. https://doi.org/10.1007/s10499-026-02522-1
- Harbour porpoises respond to chronic acoustic deterrent device noise from aquaculture (2024). Biological Conservation, 293, 110569. https://doi.org/10.1016/j.biocon.2024.110569
- Scottish Government (2025). UK Cetacean Conservation Strategy: Technical Report. https://www.gov.scot/publications/uk-cetacean-conservation-strategy-technical-report/
- Scottish Government (2024). Salmon farming net washing: Environmental Information Regulations release. https://www.gov.scot/publications/eir-202400398306/
- Environmental Standards Scotland (2023). Use of Acoustic Deterrent Devices: Summary Report. https://environmentalstandards.scot/our-work/our-investigation-reports/use-of-acoustic-deterrent-devices-summary-report/
- Scottish Government. Marine European Protected Species and basking sharks: licensing. https://www.gov.scot/publications/marine-european-protected-species-and-basking-sharks-licensing/
- Conservation (Natural Habitats, &c.) Regulations 1994, Regulation 39, as amended. https://www.legislation.gov.uk/uksi/1994/2716/regulation/39