Wild swimmers face “a risk” to their health from a toxic pesticide discharged into lochs and the sea from over 220 salmon farms around Scotland, according to an expert report for the fish farming industry.
The report — commissioned from consultants by the industry body, Salmon Scotland — concluded that levels of hydrogen peroxide in salmon cages could be 28 times higher than those considered safe for swimmers.
In high concentrations, hydrogen peroxide can be “harmful if swallowed”, and “toxic if inhaled”, the report said. It can cause “severe skin burns and eye damage” as well as “respiratory irritation”.
Campaigners warned that the “poisons” dumped into the water by salmon farms were endangering the health of wild swimmers. They demanded action from regulatory authorities to cut the pollution.
But Salmon Scotland insisted that the “worst-case” estimate in its report “would never occur in real life”. The pesticide would be dispersed and diluted in the water, it argued, and swimmers kept a “safe distance” from industrial sites and vessels.
The Scottish Environment Protection Agency (Sepa) also said it was “highly unlikely” that swimmers would be exposed to harmful levels of hydrogen peroxide. Following concerns raised by swimmers, the risks are being investigated by NHS Highland.
Data previously released by Sepa under freedom of information law suggested that more than 40 million litres of hydrogen peroxide were discharged between 2016 and 2021 from over 220 salmon farms run by eight companies. The chemical is used to kill the sea lice that can plague caged fish.
Salmon Scotland commissioned environmental consultants, WCA, to investigate the “potential risk to human health” of three lice pesticides used by the salmon farming industry. The resulting 73-page report was submitted to Argyll and Bute Council in December 2021.
The council is considering a planning application from the Norwegian salmon farming giant, Mowi, for a new 12-cage farm in North Kilbrannan Sound between Arran and Kintyre. Objectors have raised concerns that the pesticides could put swimmers’ health at risk.
The WCA report reviewed the evidence from dozens of animal tests and estimated the concentrations of the pesticides in water that would have “no observed effect” on people. It set this “no effect” level at one, and calculated the risk level for hydrogen peroxide to be much higher at 27.7.
“This indicates a risk associated with the concentrations of hydrogen peroxide used in the fish treatment baths,” the report said.
“Therefore characterisation of dilution and dispersion factors are likely to be required to be taken into account to demonstrate that discharges of hydrogen peroxide are safe for open-water swimmers.”
The report calculated the risk level of a second pesticide, azamethiphos, at 0.8, just below the “no effect” level. It suggested that the risk level from a third pesticide, deltamethrin, was much lower.
The estimates were based on a series of “worst case assumptions” for an average adult swimmer weighing 71.8 kilograms. These included swallowing 25 millilitres of water an hour and swimming for two hours without a wet suit through a “static plume” of pesticide pollution.
In an addendum to the WCA report, Salmon Scotland modelled the dispersion of hydrogen peroxide in water in order to put the report’s findings “into context”. This concluded that it was “exceedingly unlikely” that swimmers would be exposed to hazardous concentrations.
Hydrogen peroxide would fall below the “no effect” level 30 minutes after use and “generally” within 200-300 metres, Salmon Scotland said. There would be “very few, if any” people who would swim for two hours in Scottish coastal waters, it added.
But the Coastal Communities Network, which brings together 23 groups concerned about the marine environment, warned that the pesticides threatened the health of swimmers.
“The industry’s own report shows that swimmers will be harmed if they swallow very small amounts of water containing two of the poisons being dumped by fish farms,” said the group’s spokesperson, John Aitchison.
“The risk is especially high for women and children who were not mentioned in the report. No other industry is allowed to dump all its pesticides in the sea.”
Aitchison accused Sepa of failing in its duty “to check whether people will be harmed by this disgusting practice”. He highlighted evidence from Canada that hydrogen peroxide did not break down quickly in water.
“We have been asking Sepa to reduce hydrogen peroxide dumping, but it has done nothing about it for years,” he said. He called on the agency to force the salmon farming industry to “stop using the sea as a sewer.”
He added: “More and more people are taking up wild swimming, year round and visiting swimmers support jobs in tourism, but will they want to do so if they risk being exposed to pesticides?”
Since it was formed in 2006 the UK Outdoor Swimming Society has grown from 300 to over 100,000 members. The branch on the island of Bute in Scotland has 600 members.
One the founders of the Bute Outdoor Swimming Society, Theresa Nelson, highlighted that the WCA report had been paid for by the industry that used the pesticides. “It makes assumptions that in no way account for the diversity of swimmers who enter the water for open swimming or all the people who just go in coastal waters to paddle or wade,” she said.
“The bodies meant to protect both citizens and the environment are allowing highly toxic chemicals to be used in dangerous quantities in our coastal waters. The question to be answered is which body will be held responsible when someone is affected or injured by the dispersion of these chemicals?”
The Scottish Environment Protection Agency pointed out that fish farms were now discharging 11 per cent of the volume of hydrogen peroxide they discharged in 2015.
“Improving Scotland’s water environment is a key priority,” said the agency’s head of ecology, Peter Pollard.
“The long-term success of finfish aquaculture sector depends on the protection of the environment being foremost in all the sector’s plans and operations, working together with other users of the marine environment.”
According to Sepa, concentrations of hydrogen peroxide in areas of the sea used for swimming would be “considerably lower” than in salmon cages because of dispersion and dilution. Peak concentrations would fall below harmful levels between 30 and 100 minutes after discharge, it said.
“The predicted maximum distance from a farm that the discharged chemical could travel before concentrations would be below potentially harmful levels was 378 metres. It would be highly unlikely that a swimmer would be exposed to a potentially harmful concentration for long enough for their health to be harmed.”
Salmon Scotland stressed that the industry took its responsibility for fish welfare and the wider environment very seriously. “That is why we commissioned this highly technical report,” said a spokesperson.
“The report estimates a worst-case basis, which would never occur in real life given the safe distance wild swimmers offer worksites and large vessels, and the immediate effects of dispersion and dilution in a massive space.”
The spokesperson added: “Hydrogen peroxide is a naturally occurring substance found in the atmosphere, soil and water, and is used to protect fish from a number of potentially harmful organisms.
“It rapidly breaks down into water and oxygen leaving no harmful by-products in the environment, which is why it is also used by environmental agencies to re-oxygenate polluted rivers.
“There are many varied interests who share the ocean space for business and pleasure, and we all continue to look out for each other’s safety and wellbeing.”
Argyll and Bute Council confirmed that it was considering a planning application by Mowi for a salmon farm in North Kilbrannan Sound. Some objectors were concerned about “the risk that this might pose to the health of wild swimmers,” said a council spokesperson.
In response Mowi had submitted the WCA report to the council. “The council is currently awaiting input from consultees, including NHS Highland, on the content of the report,” the spokesperson added.
NHS Highland and the consultancy, WCA, have been asked to comment. Mowi deferred to its trade body, Salmon Scotland, for a response.