. >> reporter: yamazaki says the level of contamination doesn't pose a threat to human health, but he says he wants to share his data with the u.s. researchers to figure out how the tuna pick up the radioactive material. yamazaki says it takes time for tuna to accumulate radioactive substances since they're at the top of the marine food chain. tiny creatures such as plankton absorb radioactive substances first. small sh then eat the plankton and big fish like tuna eat the smaller ones. recent studies show blue fin tuna spend their juvenile period in japan's coastal waters. the fish then take one to four month to migrate across the pacific to the u.s. west coast. yamazaki says he thinks he can figure out where and how the blue fin tuna accumulate radioactivity by studying fish on both sides of the ocean. he asked the u.s. researchers to collaborate with his team. >> translator: japan needs to work with people from different sides to gather and assess the same oat of data. we need to provide the public with reliable information. >> reporter: researchers at stamford university in april s