The Secret Life of Tuna

Friends, last week, health officials in Europe and the United States announced that certain people--particularly pregnant women, nursing mothers, and young children--should go easy on the tuna. Seems that tuna and other large, top-of-the-food-chain fish contain relatively high amounts of mercury, which is not a good thing to eat.

No one can buy their way out of the problem, either. More expensive albacore, or "white," tuna actually contains three times more mercury than less expensive yellowfin, or "light," tuna fish (shown swimming in non-canned form above). There's something fishy going on here. Who put all that mercury in one of the world's favorite fish?

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Who Put Mercury in Our Tuna Fish?

Fish are mercurial creatures. Practically all of them have a bit of quicksilver in their flesh and fat. But how'd it get there, and why are some, like tuna, more merc'd up than others?

We can blame geology--and ourselves. Mercury may be rarer than uranium, but hot springs and volcanoes (including undersea ones) naturally emit mercury vapor. Burning coal has the same effect. In the United States, coal-fired power plants release about 50 tons of toxic mercury into the air each year. Old American chemical plants, which use mercury to extract chlorine from salt, may release 100 tons more.

Much of that mercury winds up in the water--some directly, some washed out of the air by rain--and the food chain takes over. Bacteria convert mercury into an organic form called methylmercury that is more easily absorbed by living things and, worse, even more toxic. Plants and microbes absorb the methylmercury and concentrate it. Little fish eat these, big fish eat the little fish, and even bigger fish eat them.

In general, the bigger and longer-lived the fish, the more the methylmercury builds up and concentrates in the fish flesh. And tuna are big fish! The largest yellowfin tuna, prior to becoming 6-ounce cans, are 400-pound (180-kilogram) behemoths--almost 7 feet (2 meters) long. That's about the size of a dolphin, which sometimes school with yellowfin. Bluefin tuna, yellowfins' larger cousins, can weigh in at 1,500 pounds (680 kilograms) and stretch 15 feet (4.5 meters).

Of course, such big fish must eat plenty, and tuna do--schools of little fish, plus crab, shrimp, squid, lobster, octopus, and more (including, sometimes, other tuna). A typical tuna can pack away a quarter of its body weight in food each day. And that's a lot of methylmercury, at least relative to what "small fry" like salmon can eat. It builds up.

Lots of tuna fat, however, does not. Tuna are among the fastest and most streamlined fish in the sea, calorie burners built for speed and constant motion. In fact, yellowfin can fold their fins into grooves in their bodies to reduce drag. Doing so allows them to hit speeds up to 40 miles per hour (64 kilometers/hour). They can cross the ocean in a month.

Yet they can't avoid the fishing boats. More than a million tons of yellowfin alone are hauled in each year. Populations of bluefin tuna, considered a delicacy in Japan, have dwindled drastically, and fishing for them has been restricted.

From: KnowledgeNews