Five months after a magnitude-9 earthquake triggered a tsunami that badly damaged the Fukushima Dai-ichi reactors causing leak of radioactive substances into the seas of Japan, researchers from Stony Brook University in New York have reported that Pacific tuna caught off the coast of San Diego had elevated levels of radioactive isotopes Cesium-134 and Cesium-137.
The levels were said to be 10 times higher than in previous catches off the California coast.
Although previous catches also registered already higher levels of the radioactive substances for the smaller fish and planktons, this is the first time they have discovered it in big fishes, and migratory ones at that, when tissue samples from all 15 tuna captured were analyzed.
The discovery, however, surprised the researchers since bluefin tuna, known for its size, speed and migratory distance traveled, continued to retain in its body radioactive isotopes instead of being dissipated or metabolized, as it should happen, having traveled almost 10,000 kilometers to the shores of the US from Japan and from across the Pacific ocean.
One of the largest and speediest fish, the Pacific bluefin tuna, spawns off the Japan coast and rapidly swims east to the waters off California.
Long before the Fukushima disaster, bluefin tuna that migrated to California and yellowfin tuna that are native to California waters were tested for radioactive substances and both indicated negative results.
This time, however, only the yellowfin tuna showed clean, but the bluefin tuna had signs of the presence of the radioactive isotopes, which indicated that it could only have come from Fukushima.
According to doctoral student Daniel Madigan, who studies the migration patterns of tuna at Stanford University, the amount of Cesium 134 and 137 detected in the fish “didn’t come close to exceeding safety limits.”
Madigan said, noting that what was in the fish, per gram, is lower than the amount of naturally occurring radioactive potassium found per gram in a banana.
Nevertheless, the presence of radioactive substances in big fishes is a warning that should nudge the researchers to monitor closely and do more sampling on migratory fishes, especially the expensive bluefin tuna, for the benefit of the consuming public.