BOOTHBAY HARBOR — Naturalists from two whale watching companies based in Boothbay Harbor confirmed the sighting this afternoon of an endangered blue whale, the largest mammal on Earth.

Naturalists on the Pink Lady II, which is operated by Cap’n Fish’s Whale Watch, and the Harbor Princess, operated by the Boothbay Whale Watch, confirmed the sighting of the blue whale.

The whale was believed to be “logging” or sleeping in waters about 15 miles offshore from Boothbay Harbor.

Dominique Leclerc, a marine biologist based on the Pink Lady II, took video of the whale as it slept and posted it on the company’s Facebook page. In the video, the whale surfaces one time. Its grayish-blue skin can been seen just beneath the ocean’s surface.

Leclerc said the whale watch vessel, which had about 150 people on board, floated near the whale for 45 minutes. She estimated the whale to be about 80 feet long.

It’s rare to see a blue whale in the Gulf of Maine, Leclerc said. They normally swim in the deeper waters of the North Atlantic.

Blue whales can grow to enormous sizes, weighing on average 400,000 pounds and possessing tongues that weigh as much as an elephant.

“The passengers were cheering and clapping and having a ball,” Leclerc said. “It was the chance of a lifetime. I have never seen a blue whale.”

Another whale watch vessel, the Harbor Princess, also came upon the whale around the same time.

The naturalist on board the Harbor Princess posted a comment on the company’s Sept. 11 Web log that said, “A 70-80 foot Blue Whale! What an amazing sight!”

Whale watch boats based in Maine typically spot fin, humpback and sei whales in the Gulf of Maine.