(CNN) -- Fish and plankton collected from the Pacific Ocean near the Fukushima Daiichi nuclear power plant contain elevated levels of radioactive materials, but below levels that pose a threat to public health, researchers reported Tuesday.
Levels of the long-lived nuclear waste cesium-137 were 1,000 times higher in seawater samples taken three months after the accident than they were before the meltdowns at Fukushima Daiichi, said Nicholas Fisher, a marine science professor at New York's Stony Brook University. Zooplankton, which get carried by currents, collected in those waters had levels of cesium-137 and the shorter-lived cesium-134 that were on average 40 times higher than the surrounding water, he said. They also had much higher levels of a radioactive form of silver produced by nuclear reactions.
But the readings amounted to a fraction of the amount of radioactivity sea life is exposed to from naturally occurring potassium in seawater, Fisher said.
"The total radiation in the marine organisms that we collected from Fukushima is still less than the natural radiation background that the animals already had, and quite a bit less," he said. "It's about 20%."
The findings were among several reports on the Fukushima Daiichi accident that were presented at an ocean science conference in Salt Lake City held this week.
That's not to say that all fish taken around the crippled plant are safe, however. Japan's agriculture and fisheries ministry reports 13 types of fish have been caught off their shores with cesium levels above regulatory limits in recent weeks, and those limits are scheduled to go down.
Fisher said the cesium results were about what researchers expected, but the presence of the radioactive silver was a surprise.
The three operating reactors at Fukushima Daiichi suffered meltdowns after the plant was hit by the tsunami that struck after Japan's historic earthquake in March 2011. Scientists believe most of the radioactive materials released in the disaster ended up in the Pacific -- either by blowing out to sea or leaking directly from the plant, located about 240 kilometers (150 miles) north of Tokyo.
Fisher said that when his group took samples in June 2011, they were not allowed within 30 kilometers (19 miles) of the plant, an exclusion zone set up in the week surrounding the accident. The samples were taken at points in and around the Kuroshio Current, the Pacific version of the Gulf Stream, and involve surface water, not the coastal sediments where a portion of contaminants are likely to have settled.
"There's a lot of key missing information about the sediments," he said.
Cesium-137 has a radioactive half-life of 30 years, while cesium-134 loses half its radioactivity after two. Fisher said concentrations of the isotope might undergo a "moderate buildup" in marine life higher up the food chain, but it's not likely to concentrate as much as some dangerous industrial toxins like mercury.
Fisher said test results are still pending for other reactor byproducts believed to have been released by the plant, such as strontium-90 -- which builds up in bone -- and heavy elements like plutonium.