AS a keen wild swimmer, I’m clear in my mind that there are places I would never swim on the grounds of safety – and among them is the vicinity of a salmon farm. That’s been the case ever since I wrote an article, last year, covering an objection to a fish farm application on the grounds that the pesticides it would release into the water might represent a risk to human health.

So I wasn’t greatly surprised to see a report in the Ferret this week, which stated: “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.”

When, in 2021, I wrote about objections to Mowi’s North Kilbrannan Sound fish farm application, what was apparent was that there was almost no research into the dangers for humans swimming in water contaminated by those pesticides. Those in the recent report include hydrogen peroxide (bleach), azamethiphos (an organophosphate which is toxic to crustacea) and deltamethrin. Though open water swimming has boomed in recent years, there was no guidance or studies held by Sepa (the Scottish Environment Protection Agency) on this issue.

Now, however, there is this one report, created by the chemical risk assessment consultancy WCA, commissioned by the industry body Salmon Scotland, and lodged with Argyll & Bute council in relation to a Mowi planning application for a fish farm in the North Kilbrannan Sound.

What the report found was on some levels slightly better than I expected, on some slightly worse. What I concluded from it was that this is a risk that should not be dismissed and needs to be still further researched and addressed before further CAR (controlled activities) licences are issued to salmon farms by Sepa.

One of the problems acknowledged by the consultancy compiling the report is that it’s hard to model exactly what kind of exposure a swimmer might get to these chemicals, since different factors like currents and distance from source will have an impact. Hence, WCA has made the worst-case scenario assumption of swimmers coming right up to the release area and being exposed to the chemicals at the concentration that the fish are, around the time that pesticdes are being used.

Risk is quantified on a scale in which a level of one is regarded as showing no reason for significant concern or action, with deltamethrin calculated as present at levels of negligible concern, and azamethiphos risk calculated at 0.8, just below one.

However hydrogen peroxide has a risk level of 27.7, suggesting that even if a swimmer were some distance from the release site, a danger might exist. The report authors write: “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.”

A spokesperson for Salmon Scotland (quoted in the Ferret) said: “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.”

But we still don’t know what is safe. The report takes as its example a swimmer weighing 71.8kg, and makes no account for those who might weigh less, for whom dangerous concentrations will be different. John Aitchison, a BBC wildlife cameraman, and critic of salmon farming methods, said to me:“If it’s a risk you should be protecting the most vulnerable, not the typical person.”

The risk remains too little understood. And in the meantime it seems nothing is being done by Sepa – with judgment on the matter currently passed to NHS Highland.

This is particularly concerning given the scale of use of hydrogen peroxide. Anti salmon-farm campaigner Don Staniford gained access to data via FOI that showed hydrogen peroxide usage over the past six years in Scottish salmon farms at a shocking 40 million litres. Though usage has been declining significantly in recent years, 2.2 million litres still slooshed out into the sea in 2021.

And, of course even if these pesticides do not represent a significant threat to human health, they impact on the life of the oceans. Toxic to sea lice, which are crustacea, they are also toxic to other crustacea, including shrimp and lobster. One 2020 study for instance showed that azmethiphos and deltamethrin were "acutely toxic to European lobster larvae", and that deltamethrin represented a significant risk to lobster larvae living near fish farms.

As we stand poised for further expansion of the aquaculture sector, what’s needed is more research into the many impacts of this form of farming, not just on those humans that swim by it, but the marine life that is bathed by its effluent.