The “wall of sound” created by acoustic deterrent devices targeted at seals could harm porpoises up to 28 kilometres away, a study has found.
The devices, known as ADDs or “seal scarers”, are used to deter seals from attacking fish farm pens by emitting a sound in the water that seals find unpleasant, as an alternative to the use of lethal force. They are also used to keep marine mammals away from offshore construction sites.
Studies of the effects of ADDs have typically focused on single devices. The research, carried out by the Scottish Association for Marine Science (SAMS), modelled the cumulative effect of all the ADDs known to be deployed until recently by 120 fish farms along a stretch of the Scottish coast from Cape Wrath to the Clyde. The Scottish Salmon Producers Association (SSPO) announced earlier this year that devices of this type are no longer being used by its members.
The study area included the Inner Hebrides and the Minches Special Area of Conservation (SAC) for harbour porpoises, the largest of its kind in Europe. The study found large areas within the SAC were regularly exposed to high noise levels from ADDs.
Using data from 2017 the scientists found that the accumulative level of noise could exceed thresholds in harbour porpoise hearing, which may result in temporary impairment at the lower end of their hearing range. The findings also show that, within the study area, sound from ADDs could remain exceed those thresholds for for the porpoises up to 28 kilometres from a farm.
It is not known what long-term impact this might have on porpoises, but like other marine mammals they use sound to navigate and to locate predators and prey.
The research made use of data collected by Scottish Natural Heritage in an earlier report released in 2014, as well as mathematical modelling and measurements gathered at sea to test the model’s accuracy.
Charlotte Findlay, the report’s lead author and a final year University of the Highlands and Islands PhD student based at SAMS, said: “Previous studies on noise from ADDs have tended to look at individual farms, or areas in isolation. We modelled all farms that were known to be using ADDs in 2017 across the entire west coast to find out the potential accumulative output and assess impacts on a habitat scale.
“We estimated the potential for ADD noise to damage the hearing of harbour porpoises and at the frequencies we modelled (2-40 KHz) there is the potential for temporary hearing loss, based on thresholds defined by the science community. Although the ecological consequences of these findings for harbour porpoises are not fully understood, the large extent of impact zones within a protected area for the species demand careful consideration by policymakers and industry.”
The need to comply with requirements set out by Marine Scotland and the US Marine Mammals Protection Act – which bars seafood imports from regions where seals are killed or targeted by ADDs deemed to be harmful – was a key factor in persuading the fish farmers to stop using these devices. Another factor is that seals can become habituated to a constant or frequent sound even if it is unpleasant for them.
Reducing the noise level, the regularity at which ADDs produce noise (the so-called “duty cycle”) and limiting the number of ADDs on a farm could all help to reduce the overall noise in the ocean, said Findlay.
Some farmers are experimenting with a new generation of deterrents based on acoustic startle response (ASR), which are designed to meet the standards set by the US. These typically operate for very short bursts, are triggered by sensors that spot when seals come close and produce a frequency that specifically targets the seals’ hearing range but not that of other marine mammals.
Dr Denise Risch, a co-author of the study added: “Alternative solutions, such as the use of more robust netting material and better tensioning of the net pens, do exist and have been proven successful in reducing seal predation in some areas. Their expanded use should be further explored and encouraged as a way to reduce the overall noise footprint from ADDs on the Scottish west coast.”
A spokesperson for the SSPO commented: “The Scottish salmon sector is no longer using any acoustic deterrent devices (ADDs) that may be considered to cause disturbance to European Protected Species. Our sector remains committed to prioritising innovation and investment in the health and welfare of our fish and the environment in which they are reared.
“Scotland’s salmon producers farm in the most responsible way and any potential future deployment of acoustic devices will be done through a science-led approach, compliant with Marine Scotland and the US Marine Mammals Protection Act (MMPA), and with total confidence that no harm will be caused to protected species.”