That depends on which fish you eat. While fish are a healthy alternative to red meat for lowering heart-disease risk, some of the waters in which they swim are not so pure. Many of the chemicals and pesticides used on land leach into the lakes, rivers, streams, and coastal waters where they are ingested by simple forms of marine life. These lifeforms, in turn, are consumed by medium-sized fish, which then are consumed by larger fish. Each step of this food chain – from plankton to trout – further concentrates chemical contaminants in fish tissues.
The chemicals of biggest concern in fish include pesticides such as DDT and dioxin, mercury, and PCBs or polychlorinated biphenyls. DDT and PCBs were discontinued in the 1970s, yet decades later residues are still found in samples of domestic fish. Granted, levels are gradually receding in the United States, but DDT is still used in other countries and its wide distribution in the environment and slow disintegration means that DDT and its breakdown products will be around for decades to come.
Despite these concerns, fish and shellfish remain one of the healthiest, low-calorie sources of many nutrients, such as protein, special fats called omega-3 fatty acids, the B vitamins, fluoride, iodine, zinc, and iron. Canned salmon and sardines eaten with the bones are excellent non-dairy sources of calcium. Oysters are the best dietary source for zinc (one large oyster supplies an entire day’s requirement for this trace mineral) and a good source of copper.
So how to get the benefits and reduce the health risks?
1) Select lean fish (most chemical contaminants concentrate in fatty tissues). Cod, flounder, haddock, Pacific halibut, ocean perch, pollock, and sole, are relatively safe from chemical contamination.
2) Select Pacific- and offshore-caught fish (limit or avoid near-shore saltwater or inland-caught freshwater fish). Salmon (caught in the Pacific, or farmed in Chili or Norway) also is safe.
3) Choose small, young fish. It’s the older, fattier fish that have had time to bioaccumulate pesticides and PCBs.
3) Cook shellfish and select clams and oysters harvested on the Pacific coast, if possible. (Food poisoning cases are caused by natural toxins or microorganisms that migrate into seafood because of poor handling practices. The greatest health risk is with raw oysters and other mollusks, accounting for up to 85% of all food poisonings caused from seafood.)
4) Limit tuna, swordfish, and shark to one serving a week; women who might become or are pregnant should limit these fish to once a month, since these types of fish may contain mercury, a toxic metal known to cause birth defects. Canned tuna contains less mercury than fresh tuna steaks, but limit intake to no more than two medium-sized cans a week (about 6 sandwiches).