July 1, 2013 | Vol. 69, No.07

Fukushima Tuna Study May Lead to Solving Missing Albacore Mystery

Michael A Moore

Pacific coast Albacore fishermen may soon have answers to the puzzle of why the catch is better some years in the northern Pacific albacore grounds than it is in the southern.

A recently published scientific report made the news for its statement that California caught Pacific bluefin tuna are safe for human consumption in spite of migrating from Japanese waters contaminated with radiation from the damaged Fukushima reactors. The main focus of that report was the fact that the same radioactive substances that are dangerous for humans make great tracers to help scientists track and understand the migration habits of the Pacific bluefin tuna and other highly migratory sea creatures such as whales, turtles, sharks and seabirds and albacore.

That report, Radiocesium in Pacific Bluefin Tuna Thunnus Orientalis in 2012 Validates New Tracer Technique, was published in the February 11, 2013, edition of the scientific journal Environmental Science and Technology. The study was prepared by an international team of scientists led by Dan Madigan, a doctoral student at Stanford University’s Hopkins Marine Station in Pacific Grove, California.

The idea that Fukushima radiation could be used to track the Pacific bluefin tuna came to Madigan when he bought some fresh caught tuna steaks from fishermen on the dock in San Diego in 2011. Madigan wondered if the tuna might be contaminated by radiation from Fukushima. He went back to the dock and bought more tuna, which he sent to a scientist friend who had a laboratory where radiation measurements could be made.

The results from this preliminary investigation, using 15 fish, came back positive for two radioactive isotopes, cesium 134 and cesium 137, both known components of radioactive waste from Fukushima.

Madigan realized the presence of the radioactive cesium represented an opportunity to use isotopes to track the movements of cross Pacific migratory animals, especially the Pacific bluefin tuna. He decided to do a more in-depth report to confirm that opportunity.

This time the sample size was 50 Pacific bluefin tuna that were donated by recreational sport fishermen and caught within 300 km of San Diego, California. They also sampled 5 yellowfin tuna as controls. Yellowfin tuna are known from electronic tagging studies to be residents of the California Current Large Marine Ecosystem migrations and do not migrate from the western Pacific.

There are two main reasons the presence of the radioactive isotopes allows the migrating tuna to be tracked without tagging. The first is that the Pacific Ocean contains a low level of Cs-137 background radiation, fallout leftover from the US atomic testing that ended in the early 1960’s. The Cs-134 from those atomic tests has long since disappeared.

The presence of the Cs-137 background radiation is due to its 30-year half-life, the time it takes to lose one half of its radioactive energy. The half-life of Cs-134 is a little over two years, meaning it quickly loses its radioactivity. The only Cs-134 present in the Pacific basin comes from Fukushima.

The Pacific Bluefin tuna spawns in the western Pacific, and the juveniles forage along the Japanese coast before migrating to across the ocean to the California Current Large Marine Ecosystem.

It also means that those same fish would have levels of Cs-137 higher than the background. Only those fish that had been in Japanese waters since the Fukushima incident would test positive for Cs-134. If a fish tested positive for levels of Cs-137 above the background levels but showed no Cs-134, that meant it had been to Fukushima but more than a year had passed since it was in Japanese waters.

The amounts of cesium detected were minimal, far below the quantities that would make the tuna unsafe for human consumption.

The scientists tested small, younger bluefin tuna, known to be migrants from Japan, to determine if all migrants would demonstrate a measurable radiocesium signal from Fukushima. Older fish (age was determined by a formula based on length) were tested to see if migrants or residents could be discerned by the absence of Cs-134 (due to excretion during a year or more in California coastal waters) or the presence of Cs-134 (due to recent migration from Japan). The ratio of Cs-134 to Cs-137 was used to estimate time of departure from Japan in recently migrated Pacific bluefin tuna.

Those results confirmed Madigan’s original hunch that the radiation seeping into the ocean from Fukushima’s damaged reactors could be put to good use to track migratory fish and marine mammals across the Pacific. They also showed that the amounts of radiation present in Pacific bluefin tuna were far below the safety levels for human consumption.

“This research was extremely important to do now,” says Dan Madigan. “By taking advantage of the presence of the cesium radionuclides in the water, we can not only track different species of migratory marine animals while the radiation lasts, we can use those results as a baseline to validate other tests we have been doing using naturally occurring stable isotopes.”

“The use of the Cs-134 is better than tagging in many ways because it gives us the ability to know where and when a migratory marine animal has been, plus it costs a lot less,” said Heidi Dewar, a tagging expert with the National Marine Fisheries Service who worked on the study with Madigan. “There was a 1,323 pound mako shark recently caught off of Huntington Beach that we are going to [test with] the cesium analysis to get an idea of where it has been. We are looking to expand cesium tracing to study the migratory habits and patterns of marine mammals, albacore, blue sharks, salmon sharks, swordfish, albatross, turtles and other marine migratory animals we know very little about.”

“This knowledge will give us a better understanding of the habits and lifecycles of migrating marine animals. We will be able to better manage the fisheries as a result of this,” said Dan Madigan. We plan to focus on the mystery of why albacore catches differ between the northern and southern regions. We don’t know if the differences are due to southern region migrations or what other factors may influence the marked differences between the two fishing areas. The ability to track without tagging will be a great tool to have at our disposal.”