A comparison of Acoustic Deterrents Devices used by The Scottish Salmon Company at Ardyne fish farm on the Clyde

The Scottish Salmon Producers Organisation (SSPO) released a statement that “There are currently no ADDs being used on Scottish fish farms”. This broad statement seems to have been withdrawn and/or accompanied with the caveat that they “would consider redeploying newer devices that were compliant with Marine Scotland and the US Marine Mammal Protection Act”. The Scottish Salmon Company, a member of SSPO and largest operator of fish farms on the Clyde, are also reported to have stated that they have been operating new generation devices since March 2021. 

We are deeply concerned about the unlicensed use of ADDs on fish farms and the direct impact this having on our seals and non-target species like porpoise. We have been collecting acoustic data to help understand how porpoises are impacted by the ADDs and intrigued by The Scottish Salmon Companies claims that their new generation devices are somehow more compliant and do not require a European Protected Species licence to operate them…..

So we compared acoustic survey data from supposedly ‘new generation’ 2021 ADDs with those in operation in 2019. 


We compared Passive Acoustic Monitoring data from 4 different surveys of Ardyne fish farm when  OTAQ SealFence ADD system was  operating in 2019 and 2021 to compare ‘new generation’ with previous ADD noise emissions. 

Acoustic Deterrent Device survey transects at The Scottish Salmon Company Ardyne fish farm siteAll survey transects were completed at a similar distance to fish farms (50-100m). No seals were sighted and no porpoises acoustically detected anywhere near the fish farm when ADDs were active. Multiple baseline surveys were also completed, between 2016 and 2021, on days when ADDs were not activated and cages were not in production. All acoustic data and photographic evidence have been offered to regulators for analysis and corroboration. Independent witnesses participated in all surveys. The most recent Ardyne basin survey (20 June 2021) was conducted with a former Marine Scotland Compliance Officer on board the survey vessel.

Comparison of ADD Frequency Spectrum

Comparison of ADD output at Ardyne fish farm

  • The first 4 high amplitude peaks represent peaks at frequencies around 9.7kHz, 19.5kHz, 29.3kHz and 39.0kHz (x-axis). The high peaks indicate that most of the energy is contained at these frequencies, which can be heard by marine mammals. Sounds of lower frequencies (peaks on the left of the graph) propagate further through sea water compared to higher frequencies which are attenuated over a much shorter distance.
  • The frequency content has not changed significantly between 2019 and 2021.
  • The difference in the amplitude of the peaks is likely due to the small differences in  distance at which the recordings were made and physical differences in water quality parameters.

Number of ADDs Activated

Use the slider on image to compare 2019 and 2021 data. Each black dot represents a sources of sound. The dark ones represent the noise from the ADDs as the vessels pass by them.  At any given point along the time axis (x-axis) the black dots at towards the top of the screen represent signals forward of the hydrophone (0 to 90 degrees) and black dots at bottom represent signals behind the hydrophone (90 to 180 degrees). Black dots along the midline represent signals that are 90 degrees to hydrophone. The lower displays represents the wave form and frequency content from  individually selected noises.

  • The PAMGuard bearing/time display screen is used to graphically represent porpoise detections provides a good representation of the number of ADDs activated during transect of the fish farm site (i.e., the virtual parallel lines created by consecutive black dots).
  • The data suggest that there were 2 more ADDs activated during the 2021 survey day compared to 2019. More ADDs operating at similar amplitude levels (i.e., loudness) produce more noise pollution and can impact porpoises to a greater extent.

Comparative Amplitude (dB)

  • Todd et al. (2021) provides source level (loudness of the sound measured at 1m of the source) measurements of OTAQ SealFence system as: Standard Mode 189 dB and Patrol Mode 186dB (dB re 1uPA RMS@1m).
  • Marine Scotland have provided advice that source levels above 120 dB will disturb porpoises and require EPS licensing to operate ADD equipment at fish farms.
  • The PAMGuard amplitude/time display suggest that even at distances over 50 m the loudness (in this case, received levels) exceeds the 120 dB for which European Protected Species licence should be required.

Us the slider on image to compare 2019 and 2021 data. Each black dot represents sources of sound and displayed based on their amplitude (received level). The very dark and louder ones are the ADDs as the vessels pass by them. The lower display represents the signal in time, the frequency content, and the spectrogram.

  • The observations indicate comparable sound levels across the survey days and in line with Todd et al (2021) results. These levels are well above the dB (loudness) levels advised by Marine Scotland, irrespective of any calibration issues, and present a significant auditory impact risk to non-target marine mammals like porpoise, minke, orca and dolphins etc. 

Comparison of Operational Mode

Use the sider to compare 2019 and 2021 data. The screen shows audio snippets from 2019 and 2021 surveys. The spectrogram display can be used and useful to compare ADD emissions and how they are functioning over time. . 

  • Operating characteristics were observed and recorded between the survey dates.
  • The number of times per second a single ADD was activated was found to be similar in 2019 and 2021.
  • However, our additional survey data indicate there has been a tendency for farms to utilise more ADD units and often have multiple devices firing simultaneously, creating a continuous source of noise for extended periods of time. 
  • On the vast majority of surveys between 2016-2021 our observations, video and photographic records, indicate that no attendants were observed on the fish farms.  

Marine Mammal Audiograms

  • Threshold audiograms are used to graphically illustrate sensitive frequencies and hearing threshold values for different marine mammals. The lower areas of each audiogram curve represent the most sensitive frequencies and high amplitude (loud) noise at those frequencies will likely impact marine mammal hearing, leading to a temporary or permanent deafness depending on exposure times. Deafness means the animal is not able to detect and catch prey and can suffer from starvation.
  • The audiogram below (Pamguard.Org) represents the hearing range and threshold values for various marine mammals. The 9.7kHz, 19.5kHz, 29.3kHz and 39.0kHz spectrum signals produced by Ardyne ADDs have been overlaid at the bottom of the audiogram to estimate whether these frequencies will be audible to non-target species.

Audiograms of marine species with OTAQ frequencies (adapted from Pamgurad.Org)

  • Harbour porpoises, bottlenose dolphins, and killer whales all have more sensitive hearing and so require lower noise levels to suffer temporary or permanent hearing damage compared to seals.
  • The noise produced by Ardyne ADDs will, without any doubt, be perceived by these non-target species that are resident and visit the Clyde.
  • It stands to reason that ADD amplitude levels that are used intentionally to disturb seals will also disturb the non-target marine mammals that have greater hearing sensitivity and lower auditory threshold levels than seals.
  • The ADD manufacturer have only provided information on the 9.7kHz (c.10kHz) frequency emission which is cited in the literature but the other unreported frequencies’ also have have potential to contribute to hearing damage in cetaceans. 


  • The acoustic data indicate there is no significant difference between 2019 and 2021 ADD signal outputs (i.e., frequency content). based on this evidence we dispute any assertion from operators that this ‘new generation system’ used in 2021 is somehow more compliant to ADD outputs used in 2019.
  • The ADDs are designed to intentionally ‘disturb’ seals. Non-target species with greater hearing sensitivity and lower auditory threshold values will also (and more markedly) be disturbed.
  • The ADD source levels are well above the 120dB value advised by Marine Scotland to cause disturbance and warrant immediate application for EPS license.
  • The ‘adaptive management strategy’ employed by farm operators are failing to protect porpoises and other non-target species from disturbance and potentially damaging levels of noise pollution.
  • We have contacted the ADD manufacturer OTAQ for information about their SealFence system but have failed to respond with any information. 

Clyde Porpoise PAM ADD data sets are currently being analysed by impartial acoustic specialists and will become available for peer review as resources allow. Raw data is available to Marine Scotland, Nature Scot and Sea Mammal Research Unit and academic partner for their independent analysis/corroboration.

It is Clyde Porpoise CIC opinion that the unlicensed use of this ADD system, constitutes a Wildlife Crime and incumbent on regulators and decision makers to 1) immediately ban their use in aquaculture setting 2) proceed with wildlife prosecution. Our surveys support the overwhelming scientific evidence base that porpoises are being impacted at population and individual level by these unlicensed devices.