Thousands of tons of dead fish have washed up on a beach in northern Japan and nobody knows why. The haul – mostly sardines and some mackerel – washed ashore in Hakodate on the northernmost main island of Hokkaido on Thursday morning.

It created a sliver blanket stretching half a mile along the coastline. Locals said they have never seen anything like it. Officials posted a warning online urging people not to consume the fish after some bagged them up to eat or sell.

Takashi Fujioka, a Hakodate Fisheries Research Institute researcher, said he has heard of similar phenomena before, but it was his first time to see it.

‘One possible cause is that the school of fish was chased by a larger fish, became exhausted, and was washed up by the waves,’ he suggested. ‘Another possible cause is that the school of fish entered cold waters during their migration and were then washed ashore. But details are uncertain.’

The decomposing fish could lower oxygen levels in the water and affect the marine environment, he said. ‘We don’t know for sure under what circumstances these fish were washed up, so I do not recommend eating them,’ Fujioka added.