A growing number of whales are becoming stranded in a western Japan bay known for its maze-like passages, with experts warning the marine mammals are increasingly likely to wander into the area as global warming progresses.

On Monday, the carcass of a sperm whale measuring 13 to 14 meters and weighing 25 to 30 tons was found in Osaka Bay. The discovery followed reports of sightings off Kobe in Hyogo Prefecture a week earlier, with the whale stranded in the bay since late January.

In January last year, a sperm whale died after being spotted struggling near the mouth of the Yodo River in Osaka. The body of the 15-meter-long male, referred to affectionately on social media as Yodo-chan, was later sunk off the Kii Peninsula south of Osaka Bay.

According to the local ports bureau, all whales that have strayed into the bay have died after being unable to return to the Pacific Ocean. While metallic sounds can be used to chase away whales, the risk of agitating them means the solution is not ideal.

File photo taken on Feb. 19, 2024, shows a dead whale floating in Osaka Bay, western Japan. (Kyodo)

"The only thing we can do is consider in advance how to dispose of the carcasses," a bureau official said.

Yasunobu Nabeshima, chairman of an Osaka Museum of Natural History community club, attributed the increase in whales wandering into Osaka Bay to global warming, which has caused the temperature difference between the Pacific Ocean and bay to decline.

The difference has been further minimized by the development of low-temperature eddies in the Pacific caused by the large meander of the Kuroshio Current, which begins off the Philippines and flows northeastward past Japan, since 2017, Nabeshima said, adding it has also led to more dolphins and sea turtles in the area.

The structure of Osaka Bay also makes it easy for whales to become trapped, with the coast of Osaka and Sakai cities comprising many intricate passages.

While Kobe harbor, where the latest whale was initially sighted, has a simple structure, Sakai-Semboku port, where its carcass was found Monday, leads to a dead end.

Whales use sound waves to navigate and Osaka Bay becomes "a place from which they cannot escape once they enter," said Nabeshima.

"Local authorities should collaborate with research institutions to conduct ecological surveys and seek effective measures," he added.

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