The latest (but not the only) cautionary tale about the wisdom of this practise, has just emerged from Norway.
A team of researchers there exposed (in the lab), an important food source for the fish, to varying levels of hydrogen peroxide (H202). It's the active ingredient in several such products.
The food source was a zooplankten called Calanus spp.
It's abundant in coastal waters where many salmon farms are located and is a key component in the North Atlantic food web. It's important, not only to young farmed fish, but to wild herring and cod, as well.
The lab results were convincing.
In just one hour, at only 10% of levels the farmers would apply, 92% of the juvenile Calanus spp. and all of the adult females died. And, at much lower doses (1% or less), the ability of the organisms to take in oxygen was greatly reduced. Their “escape response” was destroyed, making the likelihood of them being eaten by predators, "extremely high."
The researchers concluded, "Present recommended levels of application underestimate the impact of the pesticide on non-target crustaceans."
I interviewed the lead author of the study, Rosa Escobar Lux of the Marine Austevoll Research Station, Norway.
PinP: Do you have any evidence that the abundance of Calanus spp. may be affected to the degree that the fish themselves are becoming "food-deprived?"
Dr. Escobar Lux: "No. Our experiments were done in a laboratory which can answer some of our questions but it does not give definite answers to what is happening in the wild. Also, there's a need for dispersion models to help us understand the real magnitude of the effects."
The findings of her team were published recently in the Canadian science journal, FACETS.
Is evidence of harm confined to the lab?
Another study from Norway published just last month, takes us beyond the lab, into open waters (or "the wild" as Dr. Escobar Lux puts it). It reveals, elevated levels of the pesticide diflubenzuron (DFB) are being found in commercially-valuable northern shrimp (Pandalus borealis) in Norwegian fjords. Salmon farms there use a medicated feed containing that product. Lab tests have shown it can be lethal to the shrimp, and actually becomes more toxic in warmer waters. This raises added concerns in a world that is heating up fast.
Many Norwegian fishers report, they're catching fewer shrimp in fjords where salmon farms are operating. Experts want further studies to find out if shrimp populations are already crashing.
And yet another recent study reaches a similar conclusion, that
hydrogen peroxide's toxicity may already be making itself felt in the open ocean, on northern shrimp. Conducted by mostly Norwegian researchers, they find hydrogen peroxide may be causing "gill damage and delayed mortality" to the shrimp, more than a kilometre from fish farms there.
Even older studies, some done in Canada, point to several other marine creatures being vulnerable to aquacultural pesticides, too. These not only include zooplankten like the kind already referred to, but commercially valuable catches such as lobster and shrimp!
Last year, experts "rounded up" those studies and combined them in a single, "systematic and exhaustive" review.
They concluded that hydrogen peroxide wasn't the only suspect product. Three others, cypermethrin, deltamethrin and azamethiphos - each used extensively in the industry - had similar effects.
The review concludes, "Aquaculture has consequences for the environment. Salmon and trout cage culture has required the use of large quantities of pharmaceuticals. Our results show clear negative effects at concentrations lower than those used in treatments against sea lice in all of the species studied."
Despite all of this, in 2016, quite some time after much of this research was known, Health Canada granted "full registration for the sale and use" of pesticides using hydrogen peroxide as their active ingredient" for the treatment of sea lice on Atlantic salmon reared in marine aquaculture sites." That was at least five years after the first warnings about the pesticides I was personally able to find…warnings that our government officials must have been aware of.
A year later, the Department also registered azamethiphos for an identical use, giving identical reasons for doing so.
In its documents approving registration of both products, Health Canada concludes, "Under the approved conditions of use, the products have value and do not present an unacceptable risk to human health or the environment. <They are> relatively benign products that pose little or no risk to salmon, the marine environment, non-target species, or human health."
It went on to recommend that the industry, facing one of its worst years for sea lice that year, be allowed one treatment more of H202 than was usually allowed.
In addition to H202, Canada has, for at least a decade, also permitted the use of deltamethrin, at least in Atlantic Canada. Cypermethrin, however, is prohibited.
In the course of my investigation, I was only able to find out how much hydrogen peroxide is being used in aquaculture.
Health Canada figures (see table) show more than one million kilograms were sold in 2016. That placed the active ingredient among the top ten best-sellers that year (9th). It did not register in the top ten the following year. (The government counts products sold for aquaculture as "agricultural." 
