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Radiation reveals secrets of tuna migration

06 Mar 2013
Traces of radiation in bluefin tuna could give insight into the fish’s migratory habits. Credit Julie Bedford/NOAA

Traces of radiation in bluefin tuna could give insight into the fish’s migratory habits. Credit Julie Bedford/NOAA

Scientists at Stanford University are using traces of radiation in juvenile Pacific bluefin tuna to gain insight into the fish’s migratory habits.

In May last year scientists found traces of radiation in 15 bluefin tuna caught in California following Japan’s Fukushima disaster – now follow-up research has found that young Pacific bluefin tuna are still arriving in California carrying two of Fukushima's signature radioisotopes, cesium-134 and cesium-137.

The idea is that the Fukushima radioisotopes can be used to determine the previously unknown trans-oceanic movements of juvenile Pacific bluefin tuna - information that could be used to prevent tuna from being overfished.

Researchers have sampled 50 Pacific bluefin tuna that were between one and four years old.

Because the fish are born near Japan, the smallest fish were certain to have swum in Japanese waters only in the year following the Fukushima accident. The presence of cesium in these fish would support the claim that they are still carrying radiation from Fukushima eastward, and that this measure can be used to trace bluefin migrations.

Analyses revealed that every one of the smallest fish contained low but detectable cesium-134 levels, making it clear which fish had recently spent time swimming near Japan.

The measurements might also shed light on how long ago the fish left the cesium-contaminated waters near Japan. Researcher Daniel Madigan showed that measuring the ratio of two of the isotopes discharged by Fukushima, cesium-134 and cesium-137, could indicate how much time had passed since a fish left Japan. This approach could make it possible to tell which fish have swum from Japan to California and then back Japan to breed before returning to California.

"We plan to look for this tracer in hundreds of samples of Pacific bluefin tuna to get a better idea of their retrospective movements," Mr Madigan said. "It's not that easy to tell what a fish did in the past, but this gives us a forensic tool."

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Traces of radiation in bluefin tuna could give insight into the fish’s migratory habits. Credit Julie Bedford/NOAA

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