Thousands of sardines and mackerel were reported to have washed up dead on a beach in Japan.

The waves swept the fish ashore on Japan’s northernmost main island of Hokkaido on Dec. 7, reported Associated Press.

Residents take advantage of fishy situation

The dead marine life littered along the beach turned the shoreline into a mass grave spanning about one kilometre, said Japan Today.

Locals in the area stated that they'd never witness such a phenomenon before.

Some were seen laboriously collecting the fish to sell or to eat.

But the authorities have posted an online notice advising residents against consuming the fish, reported Japan Today.

Cause unknown

Takashi Fujioka, a Hakodate Fisheries Research Institute researcher, also did not recommend eating the fish as the reasons behind the phenomenon remains a mystery.

While the expert has heard of similar phenomena before, he had never witnessed it, he told Associated Press.

Pondering over the possible reasons for the dead fish, he speculated that the fish may have been chased by larger species, gotten tired due to a lack of oxygen while travelling in a densely packed school, and were thus swept up onto the beach by the currents.

Another theory could be that the fish had swam into cold waters in the midst of their migration, he said.

This incident could also impact the surrounding marine environment, the researcher noted, as the decomposing fish could lower oxygen levels in the water.

Somewhat related story

Top images via @tamate_doshin/X and Associated Press