July 2, 2012
-- There is no doubt about the immense value of protein and essential fat in fish, yet fish is contaminated with man-made non-biodegradable toxins that tend to accumulate up the food chain. A survey of salmon, published in 2004 in Science magazine, showed that all salmon have some contamination with PCBs (polychlorinated biphenyls), dioxins (industrial pollutants that don’t biodegrade), dieldrins and toxaphene which are pesticides. Fish caught in the Pacific have consistently lower levels than fish caught in the Atlantic. The survey also showed that farmed salmon consistently had higher levels than wild salmon.
While nutritionists generally recommend eating more fish, it’s easy to get confused by recommendations to avoid fish because of the contamination mentioned above. In the case of dioxins and PCBs, these are in the food chain, and other pesticide residues are likely to accumulate more from feed fed to farmed fish. Organic fish, which are fed less-contaminated feed, have a diet that consists of at least half aquatic origin, meaning other fish. Because of the non-biodegradable toxins transmitted up the food chain, organic fish are not exempt. It depends on the sea in which they swim that determines the level of toxins.
Then there is the issue of mercury. All fish contain mercury, and generally the larger the fish, the more mercury exists. The highest content of mercury is found in shark (0.68 mcg per pound), next highest is swordfish (0.64), then marlin (0.49), and tuna (0.18). Salmon and trout tend to be very low (around 0.02 mcg per pound). Contrary to popular opinion most of the mercury does not come from man-made pollution but from the ocean bottom where mercury deposits from volcanic action along the edges of the tectonic plates. We all have been exposed and on average we consume 1 mcg of mercury a day from food, water, and the air.
The problem of mercury is substantial only if either you are eating a lot of large carnivorous fish or you are already mercury toxic, possibly from a mouthful of amalgam fillings. It is advisable to have the mercury amalgam fillings removed and consume less large fish.
So, what’s the bottom line on fish? Given that it’s a great source of protein and essential fats, it is advisable to limit consumption of large fish such as tuna and swordfish meaning eating them only twice a month and, where possible, eat wild Pacific salmon, followed by Atlantic salmon, then organic farmed salmon or other smaller carnivorous fish three times a week. Sardines are an excellent choice because they are small and therefore are less likely to have accumulated toxins than their larger cousins.