Thousands of dead fish have washed up on a beach in northern Japan.

The fish, mostly made up of sardines and some mackerel, washed ashore in Hakodate on the northernmost main island of Hokkaido on Thursday, covering a stretch of beach about half a mile long.

Local residents said they have never seen anything like it and have been urged not to consume the fish.

Image:
Pic: AP

The cause of the incident remains unknown, but Takashi Fujioka, a Hakodate Fisheries Research Institute researcher, suggested the animals may have become exhausted while being chased by larger fish.

He said a lack of oxygen while moving in a densely packed group could have led to them washing up on the shore.

Dead fish in Japan
Image:
Pic: AP

Dead fish in Japan
Image:
Pic: AP

The incident coincides with sardine migration southwards, Mr Fujioka said, adding that the fish may have suddenly entered cold waters.

Read more from Sky News:
Large volumes of dead fish ‘dumped’ by bottom trawlers
Deepest fish ever recorded revealed by scientists
Seafood sold in UK supermarkets may have a dark side

He said he has heard of similar phenomena before, but had never seen it in real life.

Sardines and mackerels are seen washed up on a beach in Hakodate, Hokkaido, northern Japan Thursday, Dec. 7, 2023. Thousands of tons of dead sardines have washed up on a beach in northern Japan for unknown reasons, officials said Friday. (Kyodo News via AP)
Image:
Pic: AP

Dead fish in Japan
Image:
Pic: AP

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

The day after the wash-up, city officials were seen inspecting the site and trying to collect the fish, while residents collected them to sell or eat, despite warnings not to.