Many species are so high in contaminants like mercury that their health benefits are outweighed by their health risks. Others are flown in from halfway around the world, but given labels that make you think they were caught fresh earlier that morning. And still others are raised in filthy, overcrowded pools and loaded up with chemicals to keep them alive.
Two years ago, scientists had for the first time - discovered Bluefin tuna that were contaminated by the Fukushima nuclear crisis in Japan last year - swimming off the coast of California. Radioactive cesium ten times above the normal level was found in the fish. Hazardous radionuclides such as iodine-131, caesium 137, and other isotopes currently being released in the sea and air around Fukushima bio-concentrate at each step of various food chains (for example into algae, crustaceans, small fish, bigger fish, then humans; or soil, grass, cow's meat and milk, then humans).
Mercury contamination of all seafood is a widespread public health concern. In fact, pregnant women, children and women who might become pregnant should avoid the consumption of swordfish, tuna and orange roughy due to their high methyl-mercury content. The EU's food safety authority (EFSA) "EFSA recommends that women of childbearing age...select fish from a wide range of species, without giving undue preference to large predatory fish such as swordfish and tuna," it said in a statement.
In addition to being toxic for humans, swordfish and many other species of fish are being caught in ways that are devastating ocean habitats and fisheries. Longline fishing, the fishing method used to catch swordfish, kills thousands of sea turtles per year.
"If the fish is sustainable, then it is likely to be healthy to eat too," said Leah Gerber, an associate professor and senior sustainability scientist at Arizona State University.
"In general, larger longer-lived fish are more likely to have exposure to toxins due to the length of their lives and their place on the food chain," Gerber explained. "So you might be best served to stay away from them -- like Bluefin Tuna or Swordfish. Besides they already are over fished."
Around 95 percent of all salmon in existence are now farmed, and domestication has made them very different to wild populations, each of which is locally adapted to its own river system. Just two years ago a Purdue University scientist urged federal officials to decide favorably on allowing genetically engineered salmon into the food supply arguing that not doing so may set back scientific efforts to increase food production. The argument came in direct contradiction to statements made by the same scientist who found that releasing a transgenic fish to the wild could damage native populations even to the point of extinction.
A June 2013 report from the Earth Policy Institute noted that worldwide production of farmed fish now not only exceeds the production of beef, but that consumption of farmed fish is soon expected to exceed consumption of wild-caught fish.
There are dozens of species unworthy of consumption, however due to their popularity and accessibility, here are 7 you should stay away from.
Why It's Bad: At .976 ppm (parts per million), it has the highest mercury content of any fish out there. The bio accumulation of methyl mercury is worse the higher up the food chain that you go hence swordfish is fairly bad because it's higher up the food chain. The same argument can be justified for shark and marlin. Mercury pollutants have to be made into bio-available methyl mercury by anaerobic sulfur based bacteria. These only exist in certain regions so if you can get swordfish that's from an area without the deep water sulfur based bacteria then you can avoid the high contamination levels. Problem is that these anoxic regions are growing every year due to warmer waters which carry less oxygen thereby favouring anaerobic organisms over aerobic.
Why It's Bad: At .639 ppm, it's a close second to swordfish, especially big eye and blue fin tuna. The New York Times found that Atlantic bluefin tuna has the highest levels of mercury of any type of tuna. To top it off, bluefin tuna are severely overharvested, to the point of reaching near-extinction levels, and are considered "critically endangered" by the International Union for Conservation of Nature. Rather than trying to navigate the ever-changing recommendations for which tuna is best, consider giving it up altogether.
#3: ORANGE ROUGHY
Why It's Bad: At .554 ppm, orange roughy is still very high in mercury toxicity. Like most deep sea dwelling fish, orange roughy take a long time to grow to maturity, and are easy to threaten with overfishing. Extensive deep sea fishing for over two decades resulted in severe depletion of adult stocks. Orange roughy do not generally breed until they reach 30 years of age. Scientists predict it could take decades for orange roughy populations to recover from heavy fishing which have decimated the species. The Environmental Defense Fund has issued a health advisory.
Why It's Bad: One of the reasons for the popularity of tilapia is that they are short-lived and primarily vegetarian and therefore do not accumulate substantial amounts of mercury by consuming other fish, as other common predatory food fish (such as tuna) do. This factor also means, as reported in a June 2013 National Geographic article, that tilapia are more efficient to farm because they eat lower on the food chain. This causes a world of problems to the detriment of human health.
Dibutylin levels, a chemical used in PVC plastics is said to be 2 times higher in farm-raised tilapia compared to wild ones. Dibutylin is toxic and can impair immune system function while also contributing to inflammation. Dibutylin may be the reason as to why there is a rise in asthma, obesity, allergies and other metabolic disorders in recent years.
Dioxins are also typically higher in farmed tilapia. The problem with dioxins is that once it enters our system, it can take a very long time until it is removed from the body. The half life of dioxin is about 7 to 11 years.
#5: SALMON (Both Wild-caught and Farmed)
Why It's Bad: Although salmon is typically lower in mercury concentrations, the recent devasting events in Fukushima, Japan, have many scientists questioning whether any Pacific salmon should ever be consumed again due to high radioactive isotope levels. It's actually illegal to capture wild Atlantic salmon because the fish stocks are so low, and they're low, in part, because of farmed salmon. Salmon farming is very polluting: Thousands of fish are crammed into pens, which leads to the growth of diseases and parasites that require antibiotics and pesticides. Often, the fish escape and compete with native fish for food, leading to declines in native populations.
A study with mice proved that a diet high in farmed salmon contaminated by persistent organic pollutants - POPs - contributes to weight gain and increases the risk of diabetes.
Adding to our salmon woes, the U.S. Food and Drug Administration is moving forward with approving genetically engineered salmon to be sold, unlabeled, to unsuspecting seafood lovers. That salmon would be farmed off the coast of Panama, and it's unclear how it would be labeled. Currently, all fish labeled "Atlantic salmon" come from fish farms. They're also fed pellets that contain pink dye--that's how they get their color.
#6: FLATFISH (Flounder, Sole and Halibut)
Why It's Bad: This group of fish includes flounder, sole, and halibut that are caught off the Atlantic coast. Although flounder and sole have relatively low levels of mercury, halibut still remains on the higher end at .252 ppm. Flatfish have found their way onto the list because of heavy contamination and overfishing that dates back to the 1800s. According to Food and Water Watch, populations of these fish are as low as 1 percent of what's necessary to be considered sustainable for long-term fishing.
Why It's Bad: Considered as a healthier fish due to it's high mineral magnesium, mackerel is just as bad as the top five when it comes to mercury. Most of the world eats Spanish Gulf mackerel which is quite high in mercury toxicity at 0.454 ppm. No more than 4 ounces of mackerel should be consumed at one time and preferably on once per month.
Marco Torres is a research specialist, writer and consumer advocate for healthy lifestyles. He holds degrees in Public Health and Environmental Science and is a professional speaker on topics such as disease prevention, environmental toxins and health policy.