Aquaculture and Pesticides

You may not realize it, but there's a good chance that the salmon sizzling on your barbecue wasn't raised in the wild, but in a sprawling net pen off the Atlantic or Pacific coast—gill-to-gill with up to 15, 000 other cage mates destined for the dinner table. Over the past decade, fish farming has become big business in Canada—so big, in fact, that salmon have leap-frogged potatoes as New Brunswick's main crop.

Like any farm, these high-density pens are vulnerable to pests, and with salmon those pests include sea lice. These voracious, shrimp-like crustaceans feed on the surface of the fish, decreasing their commercial value and, in some cases, killing them. In 1995 alone, sea lice infestations cost salmon farmers in Atlantic Canada more than $20 million in lost revenue.

To control these infestations, many farmers turn to pesticides, which they apply by enclosing the pens with giant tarps, adding the pesticide solution, and then releasing it to the surrounding environment after the fish have bathed in it for an hour or so. There are concerns that such releases pose a potential risk to non-target aquatic organisms—particularly crustaceans, such as lobsters and shrimp, which are close relatives of sea lice.

In 1996, reports of a major die-off in a New Brunswick lobster pound showed the presence of cypermethrin—a synthetic pesticide authorized for use in the United States and a number of other countries, but not in Canada. Spurred by the incident, and by a growing industry push to register cypermethrin in Canada, toxicology and pollution prevention experts from Environment Canada's Atlantic Region worked with the Department of Fisheries and Oceans to measure the fate and toxicity of pesticides in simulated treatments in the Lower Bay of Fundy, New Brunswick.

Three treatments were done with cypermethrin, and three with azamethiphos—an organophosphorous insecticide that was temporarily registered for use by the federal government's Pest Management Regulatory Agency (PMRA). Dye was added to the pesticide solutions to track their dispersing plumes, and water samples were analyzed for pesticide content and toxicity to a small, bottom-dwelling crustacean related to the lobster.

Most samples taken after the releases of azamethiphos were not toxic to the test organisms in short-term bioassays, and no samples taken at time periods greater than 50 minutes after release were toxic. In contrast, almost all of the samples taken after the release of a single cage treatment of cypermethrin were toxic—even up to five hours after release. The cypermethrin plumes moved distances of up to one kilometre while still above lethal concentrations—indicating that the operational norm of treating multiple cages with the pesticide could have serious area-wide effects on sensitive, non-target species. The studies also showed that cypermethrin contacted the ocean floor in the intertidal zone, where it could have an effect on bottom-dwelling species, particularly crustaceans. Other studies showed that cypermethrin binds to organic matter, which could subsequently be deposited in sediments.

Since the results of the study have been provided to the PMRA, salmon growers, the pesticide manufacturers and local environmental associations, azamethiphos has been fully registered for use in Canada. Cypermethrin use remains illegal, and the effort to register the pesticide has declined. Instead, the salmon farming industry has begun looking at a variety of non-pesticide methods of controlling sea lice—many of which have already proven successful in Europe. These include reducing the number of fish in the pens, separating age classes at each site, leaving the pens fallow for a year, and disturbing the sediment beneath the cages.

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