The Associated Press reports that thousands of tons of dead fish were found on the northern coast of Japan and no one seems to know why.  The sardines and mackerels were found on an over half-mile stretch of beach in Hakodate, Hokkaido on the country’s northern main island.

Some residents started collecting and selling the fish, prompting local officials to post a warning advising against doing so. Officials said Friday they didn’t know the reason behind the die-off.

Takashi Fujioka, a Hakodate Fisheries Research Institute researcher, told the Associated Press that multiple possibilities could have caused the die-off, including predatory avoidance or oceanic temperature changes.

“We don’t know for sure under what circumstances these fish were washed up, so I do not recommend” eating them, Fujioka said.

Thousands of dead fish wash up on beaches in Japan

Thanks to Alaric Bond for contributing to this post.