It is related to the decay product of the nearly five billion tons of uranium present in the ocean since the evolution of oxygen in the planetary atmosphere, specifically 210Po, an isotope of polonium that is present in the decay series of 238U.
I discussed the very stupid media drive brouhaha about fish in the north pacific here:
Deluded Scientists Think They Can Reassure the Public on the Fukushima Tuna.
The Fukushima tuna fish was one of the best indicators of my often stated (partially) joking assertion that one cannot get a degree in journalism if one has passed a college level science course with a grade of C or better.
The concern over radioactivity in fish, particularly the very, very, very stupid carrying on about tritium releases, is absurd and silly to the extreme. However the upside is that fear of radioactivity in fish may be environmentally advantageous inasmuch as it may help restore depleted fish stocks. The concentration of tritium in seawater has been falling dramatically since the abandonment of atmospheric nuclear testing by the US and the former Soviet Union in 1963. People have been eating fish in the entire period between 1963 and the present day, with the environmental result, again, of the depletion of certain fish stocks.
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