Evidence of radioactivity released during the 2011 nuclear disaster at Fukushima in Japan has been found in tuna caught off the coasts of Washington and Oregon, experts say.

However, those levels are low, they say, so low a human would need to consume 700,000 or more pounds of the tuna to receive the level of radiation humans are exposed to naturally everyday from the atmosphere, cosmic rays and similar sources.

Researchers at Oregon State University analyzed Pacific albacore taken from Oregon and Washington waters to compare radiation levels before and after the earthquake and tsunami disaster that damaged the Fukushima nuclear power plant and released radioactivity into the Pacific.

While levels of one specific radioactive isotope, radiocesium, did increase, it reached only 0.1 percent of levels the U.S. Food and Drug Administration consider as meriting concern or intervention, they reported.

"You can't say there is absolutely zero risk because any radiation is assumed to carry at least some small risk," says Delvan Neville of the university's Department of Nuclear Engineering and Radiation Health Physics. "But these trace levels are too small to be a realistic concern.

Pacific albacore were chosen as the focus of the study because of their Pacific Ocean migrations between America and Japan and back.

Fish that were four-years-old showed slightly higher amounts of radiation than some younger fish, the researchers say, suggesting the younger albacore had made only one migration across the Pacific whereas the older fish might have traveled through the radiation plume from Fukushima twice.

Many if not most of the fish three-years-old or younger showed no signs of any Fukushima radiation, they say.

When the fish mature at about five years of age they no longer migrate, moving south to the western and central Pacific, not returning the U.S. West Coast.

Since migration patterns in young tuna before they show up in U.S. waters at about three-years-old is little understood, the radiation findings could yield significant scientists information on the fish, the researchers say.

"Fukushima provides the only known source for a specific isotope that shows up in the albacore, so it gives us an unexpected fingerprint that allows us to learn more about the migration," study co-author Jason Phillips says.

Still, people worried about the presence of radiation in a valuable food source should be comforted by the findings, Neville says.

"I think people would rather have an answer on what is there and what isn't there than have a big question mark."