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Vol. 07 Issue 3, Early Fall 2002

Research Commentary: Two Studies Compare Levels of Contaminants in Farmed Versus Wild Salmon
The Ribbon 

Sandra Steingraber, Visiting Assisant Professor, BCERF

Easton, M.D.L., Luszniak, D., Von der Geest, E. Preliminary examination of contaminant loadings in farmed salmon, wild salmon and commercial salmon feed. Chemosphere 46 (2002):1053-1074.

Iacobs, M., Ferrario, J., Bryne, C. Investigation of polychlorinated dibenzo-p-dioxins, dibenzo-p-furans and selected coplnar biphenyls in Scottish farmed Atlantic salmon (Salmo salar). J Chemosphere 47 (2002):183-191.


Reports about chemical contaminants in popular seafoods have made many women wonder if farmed fish might well be a safer dietary choice than wild fish. Two new studies from Canada and Scotland indicate that, at least in salmon, they are not. Indeed, the results of these two investigations show that salmon raised in fish farms have significantly higher levels of dioxins, chlorinated pesticides, and PCBs than their free-swimming counterparts. Both studies, conducted independently and published in recent issues of the environmental science journal Chemosphere, trace the source of the contamination back to commercial salmon feed.

Fish: Health food or conduit of contaminants?

Fish is the last form of wildlife many Americans still eat. Those who have never called a deer or a wild boar dinner may well enjoy the occasional trout. Those who have never tasted jugged hare, squirrel stew, or broiled grouse may be on very familiar terms with baked haddock, poached halibut, or tuna salad.

And fish is good food. It is low in saturated fat and high in protein, vitamin E, and selenium. Fish oils prevent blood platelets from sticking together, which lowers risk of stroke. Fish is also a leading source of omega-3 fatty acids, which reduce blood pressure and cholesterol, thereby promoting cardiovascular health. These same nutrients also help build healthy brains in our children. During the last trimester of pregnancy, when the fetal brain undergoes a big growth spurt, omega-3 fatty acids are required for the proliferation of neurons and blood vessels.

But fish has also become some of the most chemically contaminated of all human foods. Methylmercury, PCBs, pesticides, and dioxins are just a few of the toxic trespassers found in fish and seafood. Fish advisories now warn women and children against eating sport-caught fish in all of New England's waterways, for example, so contaminated are every single one of them with mercury.

The International Joint Commission of the United States and Canada, which manages our border's water systems, has issued similar warnings for the entire Great Lakes basin. It also recommends that the Canadian and US governments issue advisories directly to women that would plainly state that eating Great Lakes sport fish may lead to birth defects and other serious health problems for children and women of reproductive age. (So far, this directive has gone unheeded.) Human studies in the Great Lakes area clearly show elevated levels of PCBs in the umbilical cord blood of babies whose mothers consumed even moderate amounts of Great Lakes fish during their pregnancies. And PCB exposures during prenatal life are known to place children at risk for cognitive deficits, like lowered I.Q. and shortened attention spans.

As the thousands of advisories for US lakes, rivers, and streams attest, fish that live in fresh water are those most profoundly affected by toxic contamination. But neither are saltwater fish exempt from problems. Since 1991, the Institute of Medicine, a non-profit group affiliated with the National Academy of Sciences, has warned women even considering pregnancy to avoid swordfish because of high levels of mercury contamination. Last year, this recommendation was underscored by the Food and Drug Administration, which extended the warning to shark, swordfish, king mackerel, and tilefish. The FDA is now considering whether restrictions on tuna fish consumption during pregnancy is also prudent. (Several states have already enacted tuna advisories for children, pregnant women, and nursing mothers.) Even Alaskan wild salmon, considered among the cleanest fish in the world, are known to contain persistent organic pollutants at levels sufficient to raise contaminant levels in the lakes into which they migrate and spawn.

How fish concentrate environmental poisons

Fish-farmed or wild-are vulnerable to toxic contamination because watery environments enhance the ability of persistent organic pollutants to biomagnify.

The ecological principle of biomagnification refers to the fact that persistent organic pollutants, such as PCBs or dioxins, concentrate as they move up the food chain. Whether on land or at sea, organisms at the top invariably end up with the highest levels of contamination. Biomagnification follows from two simple laws of physics: the idea that matter can neither be created nor destroyed, and the contrasting proposition that some amount of useable energy is always lost whenever it is transformed from one type to another.

Taken together, these principles mean that fewer and fewer individuals can occupy each ascending link of the food chain because fewer and fewer calories of energy are available to feed them. The total amount of a persistent pollutant doesn't change, however. Thus, as the rarer members of the higher links dine upon the commoners below them, poisons dispersed among the many are drawn up into the bodies of the few. As a general rule, persistent toxic chemicals concentrate by a factor of 10 to 100 with every link ascended.

In water, toxic substances can concentrate to higher levels because food chains are longer than they are on land. The buoyancy of water allows aquatic organisms to survive on comparatively fewer calories than their gravity-bound counterparts. Because they spend less energy holding themselves up, the transfer of energy between one link of the chain to the next is more efficient. With less energy lost between links, more links can be added. Terrestrial food chains rarely have more than three links. Aquatic ecosystems can easily support food chains with six links, and some are known to have as many as twelve.

As a general rule, whenever persistent pollutants are released into the environment at large, people who eat a lot of fish or other aquatic animals will receive the highest exposures. For example, all other things being equal, nursing mothers who are frequent eaters of fish and seafood have higher levels of organic pollutants in their breast milk than women who are meat eaters. (Mothers who have adhered to vegan diets throughout their adult lives, eating no fish or animal products of any kind, have the lowest levels.)

The Canadian study: weekly eaters of farmed salmon at risk

The first study, "Preliminary Examination of Contaminant Loadings in Farmed Salmon, Wild Salmon and Commercial Salmon Feed," examined toxic contaminants in salmon collected from the Pacific Coast of Canada. Its lead author is Dr. Michael Easton, an eco-toxicologist funded by the David Suzuki Foundation of Vancouver. Easton's objective was to survey individual farmed fish, commercial salmon feed, and some wild salmon to learn whether differences in contaminant levels occur among these groups. He and his colleagues were inspired to undertake this investigation by an unexpected discovery in 1987: bottom-dwelling organisms living near salmon pens in British Columbia were found to have significantly elevated levels of PCBs. Subsequent investigation revealed that the source of the contamination was the salmon feed itself.

Because this study was undertaken as a pilot project, the sample sizes are admittedly small. The researchers analyzed five types of commercial salmon feed, four farmed salmon obtained from retail fish outlets in Vancouver, and four wild salmon purchased from a Vancouver fish company. The researchers tested each for many different contaminants: 112 PCB congeners, 41 polybrominated diphenylethers (PBDEs) (flame retardants), 25 different organochlorine pesticides, 20 types of polycyclic aromatic hydrocarbons (PAHs), as well as methyl and inorganic mercury.

The results were striking. While methyl mercury was not notably different between the wild and farmed fish, the farmed salmon showed consistently higher levels of PBDEs and pesticides (except toxaphene). Most dramatically, levels of PCBs were ten times higher in farmed fish. Elevated levels of contaminants were also found in all types of five salmon feed.

The authors' discussion focuses on the human health implications of eating farmed salmon. None of the salmon collected for analysis in this study had levels of contaminants that exceeded government-approved safety levels. However, individuals who consume farmed salmon on a regular weekly basis would, according to this analysis, easily exceed the World Health Organization's tolerable daily intakes for persistent organic pollutants.

The Scottish study: the culprit is fish oils in feed

Like the Canadian study, the Scottish study compared contaminant loads in wild and farmed salmon obtained from commercial fish markets. The sample size was nine. In this investigation, however, the fish were analyzed only for dioxins, furans, and seven different PCB congeners. Miriam Jacobs, the leading investigator in this study, "Investigation of Polychlorinated Dibenzo-p-dioxins, Dibenzo-p-furans and Selected Coplanar Biphenyls in Scottish Farmed Salmon (Salmon salar)", a toxicologist at the University of Surrey, worked in conjunction with the US Environmental Protection Agency.

Using methods of analysis similar to those employed by the Canadian group, the researchers obtained similar results. Farmed salmon had significantly higher levels of dioxins, furans, and PCBs. As in the Canadian study, PCB levels were roughly ten-fold higher in the farmed fish. And once again, the authors deduce, regular consumption of farmed salmon could lead to intakes above the tolerable weekly intake for these chemicals, especially for PCBs and especially for children under five.

What makes the Scottish study particularly interesting reading is its discussion of the fish meal manufacturing industry and the salmon aquaculture industry. Farmed salmon are fed a diet far richer in fish oils than their wild cousins enjoy. This lipid-rich diet allows them to grow more quickly and reach market size sooner. The typical oil fed to salmon is herring oil, which can come from many different parts of the world, depending on price and availability. Because herring is a naturally oily fish, it is known to be comparatively high-both on a whole weight and lipid-adjusted basis-in dioxins and PCBs, which are oil-soluble substances. The authors favor an aquaculture diet based more heavily on vegetable oils, which, they contend, have fatty acid compositions more closely resembling the invertebrates that comprise the natural diet of salmon in the first place.

Final thoughts

It is easy to convince ourselves that we can somehow opt out of food chain contamination by selecting dietary items, like farmed fish, that are produced under controlled conditions. But all fish, including those confined to watery pens, have to eat. And whenever animals are raised for human consumption, the economic incentive to speed growth by offering a high-fat, high-energy diet means that they are vulnerable to contamination by fat-soluble pollutants. At the same time, economic globalization means that animal feeds can derive from ingredients gathered from all over the world. Thus, a farmed salmon bought in a Canadian fish market may well contain PCBs from herring caught in the Baltic Sea. As noted by the authors of the Scottish study, the farmed fish on your dinner plate may actually be part of many more marine food chains than a wild fish.

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Last Update 09.12.02