Every day, hundreds of tons of fish and seaweed are bought and sold at Tokyo’s seafood markets. The markets are still bustling, but prices have fallen sharply amid concerns that some products might be contaminated with radioactive material leaking from Japan’s troubled Fukushima Dai-ichi nuclear plant. How likely is that?

NPR posed the question to Masashi Kusakabe, director of the Nakaminato Laboratory for Marine Radioecology not far from Tokyo. The research center is devoted to figuring out precisely what happens to radioactive material that gets into the ocean.

Kusakabe says what’s been getting into the Pacific Ocean near Fukushima is mostly radioactive iodine. It dissolves in water, and experiments have shown that the iodine tends to concentrate in algae. Then it gets even more concentrated as it works its way up the food chain.

Kusakabe says that might sound bad, “but the iodine we’re talking about now is iodine -131, which has a very short half-life at eight days.”

Every eight days, half of the iodine goes away. So after a few weeks, there’s not much iodine-131 left in a fish.

Kusakabe says radioactive cesium is a lot worse: Its half-life is measured in decades, not days. But so far, much less cesium has gotten into the ocean at Fukushima.

Also, the ocean is so vast that radioactive materials are heavily diluted by the time they travel even a few miles.

So the Japanese fish most likely to become contaminated are the ones that spend their entire lives right near the Fukushima power plant. And the government isn’t letting fishing vessels anywhere near the place.

But what about the ocean-going fish that show up on sashimi platters — fish like salmon and tuna? Might they be contaminated by radioactive material from the power plant?

“I don’t think so,” he says, “because tuna move everywhere. They travel, you know, maybe hundreds of kilometers, so they never stay there.”

A tuna might swim by the Fukushima plant. But it wouldn’t hang around long enough to become seriously contaminated.

Kusakabe says the biggest threat to the Japanese fishing industry right now isn’t radiation. It’s fear.

“Most people now think, ‘Oh, it’s very dangerous to eat fish in Japan or fish around this coast.’ But I think it’s very safe. So now is your chance to eat fish, because it’s cheap,” he says.

Asked if he is still eating fish, Kusakabe replies, “Oh, of course. Why not?”

Kusakabe says once people realize that Japanese fish are safe, he expects the price of Pacific Bluefin to go back up. Copyright 2011 National Public Radio. To see more, visit http://www.npr.org/.