That's a biiig fish. Kinda sad really.
Doubt the fallout from Japan is responsible for it though. It was probably alive when it happened (judging by it's size) and radiation isn't like the movies. Things don't turn super size like the hulk. They just die if there is too much.
And if it wasn't born (which I would think to be unlikely), radiation does do mutations, but being of perfect form but just massive probably wouldn't be likely. Would be more likely to have a munted fin like nemo or something.
And if radiation made it massive, slice me off a piece of that and serve it with some toast. Could use another growth spurt!