Hell, most militaries dumped tons of nerve agents directly into deep waters, within just a few decades ago — "official policy" doesn't even surprise me anymore!
Hoping to be wrong.
While it's understandable that someone's mind would jump to Fukushima as a possible cause, since it's been in the news recently, it is likely just due to natural phenomena such as the fish being chased ashore by a pod of dolphins.
[1] https://news.yahoo.co.jp/articles/a5e8d11585ad014f14b45ad0bf...
The radioactive water that was released was treated and diluted, to have a radioactivity way below what is allowed and safe for ocean release. The release was not exceptional, neither for Fukushima nor for the many other nuclear power plans that do similar water release.
China especially routinely releases water from its nuclear power plant with higher tritium concentration than Fukushima.
Ex of discussion: https://www.reddit.com/r/NuclearPower/comments/169ekpv/gener...