Video: thousands of dead fish wash up on Japanese shore

Thousands of dead fish washed up in northern Japan across a stretch of more than a kilometre-long beach, according to local media.

Tons of mostly sardines and some mackerel washed up Thursday morning on a shore of Hakodate in Japan's Hokkaido prefecture.

The cause behind the wash-up is still uncertain.

The timing of the incident, however, coincides with the season of sardines migration southward from Hokkaido, according to Takashi Fujioka, a researcher at Hakodate Fisheries Research Institute.

“One possibility is that the school of fish was chased by a larger fish, became exhausted, and was washed up by the waves,” says Fujioka.

“Or they entered a mass of cold water.”

The day after the wash-up, city officials were still seen inspecting the site and are trying to collect the fish.

Fujioka said he doesn't recommend eating them.

"We don’t know for sure under what circumstances these fish were washed-up," he said.