Probably, the discharge from the nuclear plant might have aggravated some issues but I’d bet the main problem is related to temperature increases -> more bacteria -> less oxygen
Something similar happened on Brazil a while ago on some sections of the Amazon river
Earlier this month, an estimated 1,200 tonnes of sardines and mackerel were found floating on the surface of the sea off the fishing port of Hakodate in Hokkaido, forming a silver blanket stretching for more than a kilometre.
On Wednesday, officials in Nakiri, a town on the Pacific coast hundreds of miles south of Hokkaido, were confronted with 30 to 40 tonnes of Japanese scaled sardines, or sappa, which had been observed in the area a couple of days earlier.
Japanese government officials have blasted a report in the British newspaper the Daily Mail that appeared to link the phenomenon to the release of treated water from the Fukushima Daiichi nuclear power plant.
The report noted that dead fish had begun washing ashore almost four months after the plant began discharging the water – which contains small quantities of the radioactive isotope tritium – into the Pacific.
The International Atomic Energy Agency approved the plan, stating in a safety review that discharging the water would have “a negligible radiological impact on people and the environment”.
China, which opposed the release and imposed a ban on Japanese seafood, has been accused of hypocrisy since its own nuclear plants routinely pump wastewater with higher levels of tritium than that found in Fukushima’s discharge.
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Probably a shitty combo of the daiichi discharge with climate change warming waters and fucking up marine ecosystems. In a healthier ecosystem the fishery mightve been more resilient but having both happen probably means the animals and plants that live there can't handle perturbations and any small change in pollution anymore.