Researchers are reportedly 'startled' that bluefin tuna have carried radioactive contamination from Japan right across the huge expanse of the Pacific to the US west coast.
It had been previously thought that these large fish would have metabolised and shed the contamination, but levels of radioactive caesium have been found at ten times the previous amounts measured in the fish found off the California coast. The results, say experts, show 'unequivocally' that the radioactive contamination originated from the Japanese Fukushima  nuclear reactor disaster.
Experts do say that even at these levels the fish are perfectly safe for us to eat as the levels found are well within the safety limits set by both the Japanese and US authorities.
The bluefin tuna can grow to be ten feet in size and weighing some 1,000 lb. Tuna also have a unique metabolism (seagrant.gso.uri.edu/factsheets/tuna.html) that keeps their body temperature slightly higher than the ambient temperature, which boosts their muscles and as a result they can get up to speeds of 40 mph for short distances. The upshot is that they get around fast.
But despite the pronouncements that the levels of Caesium found so far are safe, some have doubts about this in the long run.
Two caesium elements have been found in the tuna: Caesium -134 and caesium 137. The latter has a radioactive half life of 30 years (news.lucaswhitefieldhixson.com/2011/04/bioaccumulation-why-fukushima-matters.html), can be mistaken by the body for potassium and absorbed into the blood and 'bioaccumulation' can occur (note the linked article was written in April 2011).
There is also the issue of 'biomagnification' where caesium can concentrate more the further up the food chain it goes.
Then thinking seems to be that, as time goes on, the concentration of contamination in the sea will reduce and that the metabolism of the tuna will outstrip the levels they take on when eating their prey.
But we must always remember that we are also predators as we eat the tuna and we are at the top of this particular food chain.
By aes256 [CC-BY-SA-2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons