Pacific bluefin tuna which have migrated from Japan to California have been found to be contaminated with radioactive cesium from the Fukushima nuclear accident, researchers from Stanford’s Hopkins Marine Station in Pacific have reported in PNAS (Proceedings of the National Academy of Sciences). Despite radiation contamination, levels so far detected are well below those considered hazardous for human health, the authors emphasized.

The researchers have no doubt that the fish caught of the San Diego coast in 2011 were contaminated with radiation that originated from the nuclear disaster.

Seafood distributors have been assuring the American public that their bluefin tuna should not be affected by radiation because they are fished thousands of miles from Japanese waters.

Bluefin tuna migrate, and on this occasion were caught by sports fishermen off the Californian coast. Daniel J. Madigan, a marine ecologist, senior author in this study, added that the affected tuna had not been headed for the consumer market in the USA.

Thunnus orientalis (Osaka Kaiyukan Aquarium)
Pacific bluefin tuna

Madigan explained that he and his colleagues had been studying bluefin tina migratory patterns; fish radiation levels had not been the aim of their overall Pacific fish migration study.

San Diego had given them muscle samples from 15 fishes, average age 2 years, by sports fishermen in August last year. Routine tests detected radioactivity in one of the samples. All samples were subsequently sent to Nicolas Fisher at Stony Brook University on Long Island for further testing. Fisher is a radiation hazards experts of international repute.

The tuna were found to be contaminated with radioactive cesium; two forms:

  • Cesium-134 isotope
  • Cesium-137 isotope

These two isotopes are only produced in nuclear explosions, and do not exist naturally.

Cesium-137 takes thousands of years to break down, while Cesium-134 is shorter-lived.

Madigan said that radiation contamination, if it is also present in other migratory sea animals, such as sharks, turtles, and even birds and seals, acts as a tracer and helps researchers better track their migratory patterns.

Madigan and team say they are about to collect samples from another group of bluefin tuna that has migrated to Californian waters recently. Their aim is specifically to find out whether these fishes are also contaminated with radioactive cesium.

The tuna in this new group have spent a year longer off the Japanese coast after the nuclear accident. The scientists want to find out whether their radiation levels have dropped or increased.

Fisher said:

“We don’t think there will be any public health concern from the results of the new tests, but if we do see any higher concentrations of cesium, we will certainly alert public health agencies again.”

Written by Christian Nordqvist