Thousands of tons of dead fish wash ashore in Japan – three months after the nation released treated Fukushima radioactive water into the sea

Thousands of tonnes of dead fish have washed up on a beach in northern Japan, prompting speculation that the release of treated radioactive water from the Fukushima nuclear power plant has wreaked havoc on local ecosystems.

The sardines and some mackerel washed up Thursday morning in Hakodate on Japan's northernmost main island, Hokkaido, creating a disturbingly thin blanket covering nearly a mile of coastline.

An official announcement did not provide any explanation for the phenomenon.

Takashi Fujioka, a researcher from the Hakodate Fisheries Research Institute, posited a number of theories about why the fish might have died en masse.

He said they may have become exhausted from a lack of oxygen while moving in a tightly packed school in shallow waters, or perhaps suddenly encountered cold waters during their migration and succumbed to shock, he said.

There are several recorded cases of similar phenomena occurring on different parts of the Japanese coastline.

But this particular phenomenon occurred just three months after Japanese authorities began dumping treated radioactive water back into the sea – a move that angered neighboring countries including China and South Korea.

China has since banned Japanese seafood, criticizing the country as 'extremely selfish and irresponsible'The Global Times, the flagship newspaper of the Chinese Communist Party, writing this could open “Pandora's box” and raise fears of a 'real Godzilla'.

South Korean protesters also tried to enter the Japanese embassy in Seoul with banners reading “The sea is not Japan's garbage bin.”

The sardines and some mackerel washed up Thursday morning in Hakodate on Japan's northernmost main island, Hokkaido.

The phenomenon created a disturbingly thin blanket covering nearly a mile of coastline

A closer look at TEPCO's Fukushima Daiichi nuclear power plant in Okuma

Local residents observing the coastline in Hakodate said they had never seen anything like it.

Some collected the fish to sell or eat, prompting the city to urge residents not to consume the fish in a message on its website.

The decomposing fish could lower oxygen levels in the water and harm the marine environment, Fujioka said.

“We don't know for sure under what circumstances these fish washed up, so I don't recommend eating them,” he concluded.

In March 2011, the Fukushima power plant was destroyed after an earthquake and the ensuing tsunami destroyed the plant's cooling systems, causing three reactors to melt down.

Now an undersea tunnel is being used to drain the radioactive water treated by the Advanced Liquid Processing System, which uses a process called isotopic dilution to make the water less dangerous.

This process involves adding tritium – a less harmful radioactive isotope – to the contaminated water, which is then mixed with 'clean seawater', diluting the concentration of more harmful substances.

Japanese officials claim the treated water is safe.

But critics say a lack of long-term data means it's impossible to say with certainty that tritium poses no threat to human health or the marine environment.

Greenpeace said radiological risks have not yet been fully assessed and that the biological effects of tritium are “being ignored.”

Activists take part in a protest against Japan releasing treated radioactive water from the destroyed Fukushima nuclear power plant in Busan, South Korea

More and more South Korean protesters are taking to the streets to complain about the treatment of toxic water

The Fukushima power plant, pictured after it was damaged, was crippled by an earthquake and tsunami in 2011

A look at Japan's Fukushima power plant and its four nuclear reactors. A kilometer away, an underwater pipe will be used to drain the toxic water