(CNN) -- Scientists hope to test new samples of Pacific bluefin tuna after low levels of radioactive cesium from Japan's Fukushima Daiichi nuclear accident turned up in fish caught off California in 2011, researchers reported Monday.
The bluefin spawn off Japan, and many migrate across the Pacific Ocean. Tissue samples taken from 15 bluefin caught in August, five months after the meltdowns at Fukushima Daiichi, all contained reactor byproducts cesium-134 and cesium-137 at levels that produced radiation about 3% higher than natural background sources -- but well below levels considered dangerous for human consumption, the researchers say.
Cesium-137 has a radioactive half-life of about 30 years, and traces of the isotope still persist from above-ground nuclear bomb tests in the 1950s and '60s. But cesium-134, which has a half-life of only two years, "is inarguably from Fukushima Daiichi," Stanford University marine ecologist Dan Madigan told CNN.
Madigan is the lead author of a paper published in this week's edition of Proceedings of the National Academy of Sciences. One of his co-authors, Nicholas Fisher, said levels of both isotopes detected in fish caught in August 2011 are one-thirtieth the amount of naturally occurring radioactive potassium found in all marine life. It's also about 2.5% of the more restrictive limits Japan imposed on fish caught for human consumption after the accident.
But neither thought they were likely to find cesium at all, they said. And since the fish tested were born about a year before the disaster, "This year's fish are going to be really interesting," Madigan said.
"There were fish born around the time of the accident, and those are the ones showing up in California right now," he said. "Those have been, for the most part, swimming around in those contaminated waters their whole lives."
Scientists don't yet know whether this year's catch will have more or less cesium in their bodies, said Fisher, a marine science professor at New York's Stony Brook University. The particles that blew into the ocean could have been diluted by the vast Pacific, or the fish could have taken in more of them as they grew up.
Even if there's no change, the presence of cesium in the fish can be useful for scientists like Madigan who track the migration of species like the bluefin.
"We've established that this marker can be used as a tracer to follow which fish came over from Japan," he said.
Before the accident, there was no trace of cesium-134 in bluefin tuna. And samples of West Coast yellowfin tuna, which tend to stay off the U.S. and Mexican coasts, show no signs of cesium today, Madigan said.
Pacific bluefin tuna are among the largest and fastest fish in the world. They're heavily fished and higly prized for sushi and sashimi; one nearly 600-pound specimen sold for a reported $700,000 at Tokyo's Tsukiji fish market in January.
The samples Madigan, Fisher and colleague Zofia Baumann examined came from fish caught by recreational anglers near San Diego. The concentrations of both isotopes of cesium totaled about 10 becquerels per kilogram of dry weight, according to their findings.
By comparison, naturally occurring potassium-40 levels average about 350 bq/kg. A becquerel is a unit of radioactivity equal to one nuclear disintegration per second.
Madigan said the concentrations were likely higher in smaller fish, but shrank as the bluefin grew during their migration and processed some of the cesium in their bodies. Japanese government figures estimate cesium levels in fish caught off its shores at between 61 and 168 bq/kg.
"This year's study will be much higher sample size across a greater range of fish, ages and sizes," Madigan said. And if any fish are found with dangerous levels of radioactive material in their tissue, "It would be our responsibility to report it right away," he said.
The three operating reactors at Fukushima Daiichi melted down after the earthquake and tsunami that struck Japan in March 2011, creating the worst nuclear disaster since Chernobyl. Most of the radioactivity released by the plant blew out to sea.
Last week, the plant's owner, the Tokyo Electric Power Company, raised its estimate of the amount of radioactive material released from the plant in the first weeks of the crisis to 900,000 terabecquerels -- about a fifth the size of the release from the 1986 Chernobyl disaster. A terabecquerel is equal to one trillion becquerels.
The revised figure more than two and a half times what was estimated in April 2011, when Japan declared Fukushima Daiichi a top-level event on the international scale that ranks nuclear disasters.