An estimated 15-meter-long sperm whale spotted near the mouth of the Yodo River in Osaka earlier this week was confirmed dead Friday, local officials said, after it had remained in the same spot for days and was not observed spouting water.

Workers of the Osaka Aquarium Kaiyukan in the western Japan city had approached the large mammal by boat to check if it was still alive, while Tsutomu Tamura of the Tokyo-based Institute of Cetacean Research said that the blowhole that the whale would use to breathe had been submerged underwater since Wednesday.

Photo taken from a Kyodo News helicopter on Jan. 13, 2023, shows a whale near the mouth of the Yodo River being approached by people on a boat. (Kyodo)

The sighting of the whale -- nicknamed "Yodo-chan" on social media -- in Osaka Bay was rare, the coast guard said when the animal was first seen on Monday. The bay is largely enclosed by Awaji Island, which leaves two narrow passages north and south. The southern opening leads to the Pacific.

The cause of death has not been determined. The ICR's Tamura suggested that the whale may have been "weakened for some reason and wandered into the area." It did not appear to have injuries characteristic of trauma inflicted by a ship's propeller, he added.

The whale's length had been earlier estimated at around 8 meters by the coast guard.

Osaka mayor Ichiro Matsui told reporters the city plans to have the whale's carcass removed. The city will discuss how to dispose of it with the Osaka Ports and Harbors Bureau and other organizations.

In July 2021, a whale carcass was discovered off the coast of Osaka. It was towed away and buried on land by the ports bureau, but its skeleton was later excavated and has been stored at the Osaka Museum of Natural History since December.

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Whale spotted near mouth of Yodo River in Osaka