Rising sea temperatures linked to the climate crisis are thought to be behind the dwindling salmon population.

A tour boat operator in Shiretoko, a UNESCO World Natural Heritage site, spotted a starving brown bear scrounging for food among rocks and seaweed.

Brown bears typically wait at estuaries for pink salmon swimming upstream to lay eggs between August and October - but this year they have been swimming in the sea trying to find fish, the Asahi Shimbun reported.

"Some bears have grown really thin, and they are having a tough time," cruise operator Katsuya Noda told the newspaper. "There are no fish in the rivers, just like last year."

The shortage of salmon has been compounded by a poor acorn harvest - another food source bears use to fatten up before the winter.

Masami Yamanaka, a researcher at the Shiretoko Nature Foundation, told the Asahi Shimbun: "An estimated 70 to 80% of the cubs born this year are dead.

"It's really a serious situation."

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Last year, the pink salmon catch was just 5% of the previous good catch in 2020.

Around 500 brown bears live in Hokkaido's Shiretoko area and pink salmon are an essential food source in late summer.

Bears start to fatten up on the fish before going to the mountains and preparing for hibernation.

Brown bear attacks on humans in Japan are at their highest level since records began in 2007, with authorities warning the acorn shortage could drive more attacks.