The finding reported last week that bluefin tuna caught off southern California carried low levels of nuclear radiation from Japan was another signal the world's food chain could be compromised. It also showed how efficient natural systems are at distributing radioactivity from the Fukushima-Daiichi plant that blew up last year.

Bluefin tuna spawn only off the coasts of Japan and the Phillippines, and some migrate to the waters off southern California and Baja. The tuna caught off southern California, according to the study published in the

, contained elevated levels of cesium 134 -- a radioactive isotope otherwise absent in the Pacific Ocean and linked with nuclear production. The fish also contained elevated levels of cesium 137, already present in the eastern Pacific.

The findings were a surprise to the researchers. But they prompted no warnings that eating bluefin tuna would pose an increased cancer risk, because even the elevated radioactivity levels detected were well below thresholds considered safe. Meanwhile, the fish tested were never intended for sale to food markets, so it seems as safe to order tuna sushi in Portland now as it was two or four years ago.

But the unmappable spread of radioactive particles reveals some large knowledge gaps. The migratory bluefin tuna was never before identified as a transport vector for radioactive particles.

Japan fell into a food emergency within days of the Fukushima-Daiichi failure and remains so. Radioactive particles quickly showed up in vegetables, beef, fish and even rice. Thousands of Japanese citizens, misled by their government's early underestimates of radiation exposure, insist on the use of Geiger counters at markets and restaurants to ensure safety.

Five months ago, cesium was reported in baby formula manufactured north of Tokyo but far from the nuclear site, prompting a recall of 400,000 cans. An ocean and a continent away, the premier New York City seafood restaurant Le Bernardin now refuses all fish from Japan, while using a Geiger counter on fish from other sources.

Yet Oregon and the West Coast are between New York and Japan. And radiation, which shrinks the world as it spreads, won't be gone soon. Post-Chernobyl research established that radioactive particles travel far and last for years. Fifteen years after that disaster, cesium 137 was found in high concentrations in reindeer in Norway, in some locations posing a genuine health threat to those eating large quantities of the animal's meat.

Now, however low-level the counts, it's in bluefin tuna. As Oregon and Washington watch for debris released by the tsunami that triggered the nuclear failure, Japan's radiation seems to be closer to home and even inescapable.

"Just what is safe?" asks

, which sources fish and ingredients only locally but encounters the occasional Portland diner worried about even trace amounts of radiation. "And who monitors? We're trying to find out."

The job going forward will be to smartly detect radiation and properly scale its risk to humans, who pick up small amounts every day just by breathing. Increased vigilence, without panic, should rule.