Scientists found caesium in tuna just four months after the Japanese disaster, suggesting the fish carried the material across the Pacific faster than wind or water can.
Standford University scientist Daniel Madigan is one of three researchers who wrote a paper, published this week, on the issue.
He spoke to Pacific Beat and outlined the level of risk this poses to humans eating Bluefin Tuna from the Pacific.
Presenter: Geraldine Coutts
Speaker: Daniel Madigan, Stanford University scientist
MADIGAN: Well by all accounts according to experts in radioactivity of food and food safety, it's not a very big concern. We made that clear in the paper and we compared the caesium we found to what is in these Bluefin naturally in the form of potassium 40, which is in lots of food, and we also compared it to safety limits. And the amount of cesium we found was way below what there's naturally and also well below safety limits for the US and Japan.
COUTTS: Well caesium of course has a lifespan of about 100 years, it doesn't break down, what does this do to the fish themselves?
MADIGAN: So actually we looked at two isotopes of caesium and the half-life of one of them is 30 years, at caesium … and the other one is two years. So the breakdown is slightly faster than that, especially for caesium 134. The levels we measured it's very unlikely that it had any effect on the fish, at least the very large fish. Where the caesium was more concentrated near Japan it's possible that higher levels may have an effect on especially maybe smaller fish, but the levels we found in the size of the Bluefin we found really shouldn't have much of an effect on the animals at all.
COUTTS: And so if it's ingested, Bluefin caught and ingested, no problems?
MADIGAN: It's a tricky issue, people are very concerned about any amount of radioactivity. Like I said according to what's basically been established as safety limits it shouldn't cause any problems. That being said it's clear to me that many people either don't believe the limits or they don't believe that there's any such thing as a safe amount of radiation. So consuming it is really up to anyone, it ends up being really a personal choice of what they want to consume. But again according to research that's been done and the limits that are set, this doesn't pose a safety risk to humans.
COUTTS: How did you go about your study? I mean we're talking about Fukushima in Japan and then being caught off the east coast of California at some considerable amount of the ocean that they've travelled through, so what could it actually tell you about the migration and the extent of the radiation out there?
MADIGAN: Sure, I happen to study the migration patterns of several tuna species in the Pacific and their life history patterns are fairly well known and as far as Pacific Bluefin go they are only born in the western Pacific. And they grow up a little bit in the western Pacific before they migrate over to the eastern Pacific Ocean where they can be caught off California or Mexico. So when you find the small Bluefin, like the ones we tested for this radiation, you can be fairly sure that they migrated from Japan fairly recently. So we selected those on purpose to see if the most likely candidates for transporting radiation from Japan, in this case small Pacific bluefin tuna had any of it in their muscle tissue. We weren't sure at all we'd see the caesium, for it to happen the fish would have had to obviously have left Japan after the accident. They would have had to swim around in contaminated waters for enough time to build up the caesium in their muscle tissue, and they actually don't hold on to it for that long. It's excreted out about two per cent per day. So for example if we looked at those fish a year later around now, you wouldn't detect that caesium anymore, it would be gone. So a lot of things had to happen to see the signals. We were definitely surprised when we saw in all 15 that we sampled. This study didn't tell us anything all that new about migration patterns of bluefin because we selected fish that we basically knew had migrated from Japan. What it did tell us is that the caesium can be used as a tool going forward to look at bluefin and other species and really tell us that a fish has migrated from Japan. So in the eastern Pacific for example we can look at bigger Bluefin and find out if they've recently migrated from the western Pacific Ocean, or if they've been in the waters of the eastern Pacific for quite some time. And we're thinking that this might show up in other highly migratory species that are known to use the waters around Japan in the western Pacific, and we plan to study some of those animals as well, and that includes animals like sea turtles, sea birds, other fish, shark species, basically any species that are highly migratory and might carry the signal, we're really going to try to look into.
COUTTS: How many generations could this span, even though they're excreting it at two per cent a day, when the fish spawn would it be in the eggs as well for the next generation?
MADIGAN: It's unlikely based on how caesium, where it goes in the body that it would be transferred to young. For example there was a study in New Zealand in the last few months where they looked at the chicks of sooty shearwaters, sooty shearwaters migrate to New Zealand to breed from Japan and also from other areas. They tested a good number of chicks and in New Zealand they found nothing, and that's really what you'd expect. Even if the shearwaters were carrying cesium from Japan, which is unknown because the adults were not tested, you wouldn't expect it to transfer to the chicks, and then that's what they found it wasn't in the chicks. So this isn't something that would be passed down to third generations as you put it.