The isotope is produced during nuclear fission – the process that drives both atomic power stations and atomic bombs. Because it has a half-life of about two years, any caesium-134 that was released into the atmosphere by previous bomb tests or reactor disasters (such as Chernobyl) has long since decayed away.

Therefore, any caesium-134 found in anything at the moment can only have come from Fukushima.

So, yes, a radioactive nasty from Japan did end up in a fish in Canada. However, there is much more to the story than that.

First off, scientists have always predicted that radioactive stuff from the damaged reactor would spread around the world, through the oceans and the air.