Agriculture and Environment: Salmon

Environmental Impacts of Production: Contamination with Toxic Compounds

The farming of fish high up the food chain can tend to concentrate contaminants (Staniford 2002).

The artificial food chain built by feeding oil-rich and animal-derived diets to salmon has resulted in elevated levels of such contaminants as dioxins and polychlorinated biphenyls (PCBs) in farmed salmon compared to their wild counterparts.

Banned PCBs still effect us
The term dioxins refers to over 200 different polychlorinated dibenzo- para- dioxins and dibenzofurans, seventeen of which are considered toxic. Dioxins are produced as unwanted by-products, while PCBs are manufactured for use in transformers and insulators (CFIA 2002).

Chlorinated hydrocarbon compounds can accumulate in the fatty tissues of fish, so fish oil has relatively high levels of these compounds (especially if derived from fish from contaminated areas). Any of these toxins can pose serious risks to human health.

PCBs and many organochlorine pesticides (which have been found in aquaculture salmon) have been banned in most of the world, but they still affect humans through their diet. European farmed salmon can be a significant source of these toxins in the diet (Jacobs et al. 2002).

Contaminated fish posing health risks
The European Union's Scientific Committee on Food found that fish represent up to 63% of the average daily exposure to dioxins. The Food Standards Agency of the United Kingdom recommends that people consume only one portion of oily fish per week (Staniford 2002).

A recent study of PCB concentration in salmon showed that some farmed salmon had relatively high concentrations of he compound. However, wild salmon captured from polluted water had even higher levels of PCBS. Variation in farmed-salmon PCB levels is attributed to the variation in the level of contamination in fish meal.

Fish meal from Peru had PCB concentrations 10-20 times lower than those from Denmark and the Faroe Islands (Jacobs et al. 2002). Farmed salmon in Scotland were shown to have relatively high concentrations of dioxins and PCBs, presumably due to the sources of the fish meal and oil used for feed. Concentrations of the compounds in salmon were higher than those of other species such as cod, because salmon have a higher fat content than other species.

More fatty fish = more toxins retained
Thus, salmon retain more toxins per pound of fish than do fish with lower fat levels since the compounds accumulate in the fatty tissues of the fish (Jacobs et al. 2002). In addition, farmed salmon have 4-5 times more fat content than wild salmon (Staniford 2002).

In addition to these contaminants, toxic heavy metals can also accumulate in the fatty tissues of fish. These metals an be concentrated further through the rendering of fish meal and fish oil and further still in the animals that eat feed made from them. Mercury is a good example; once consumed by humans, it s readily absorbed into the gastrointestinal tract. Symptoms associated with the consumption of low levels of heavy metals may not appear until later in life (Quig 2002).

Many studies have examined the concentrations of toxins in fish, fish meal, and fish oil. Results vary considerably. One study in Canada showed that fish meal and fish oil do not contain high levels of dioxins, PCBs, DDT, or mercury (CFIA 2002), while the authors of a study in Scotland recommend that measures be taken to lower these levels because they are too high.

Toxins in almost all food types
An analysis of dioxin toxicity of thirteen categories of food (such as beef, chicken, ocean fish, freshwater fish, butter, eggs, etc.) found that the freshwater fish (in which the study included many farmed species and salmon) had the highest dioxin toxicity. In fact, freshwater fish toxicity was 50% higher than butter, which had the second high toxicity. All of the other products had less than half the toxicity of butter (Schecter et al. 2001).

Credits

Extracts from "World Agriculture & Environment" by Jason Clay - buy the book online from Island Press

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