The tanker crew was reportedly unaware the whale had been there as they sailed into Mizushima harbor.
Ship strikes are known to be one of the leading causes of death for endangered whale populations.
The huge ship struck in the center of the whale's body, an expert told Insider.
A shocking image shows a dead 39-foot whale hanging limply over the bow of a Japanese tanker in the port of Mizushima, Japan.
The Mizushima Coast Guard's Office confirmed to Insider that the whale found dead was a male Bryde whale, weighing five tons.
Locals caught sight of the whale as the tanker pulled into the harbor in the western city of Kurashiki last month. The images were first published in Yomiuri Shimbun, which is one of Japan's five national newspapers.
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"I've lived for more than 80 years, but it's my first time [seeing a whale]," one bystander who saw the tanker said, according to the Daily Mail.
The ship's crew were reportedly unaware they had been dragging the whale with them as they sailed through the Pacific, according to Yomiuri Shimbun.
A spokesperson from the Mizushima Coast Guard Department said this was the first time they had witnessed anything like this. They would be investigating to see how such an incident can be prevented in the future.
The name of the ship was obscured in the photos distributed by the Coast Guard.
Ship strikes are known to be one of the leading causes of death for endangered and vulnerable whale populations, according to World Wide Fund for Nature (WWF).
Michael Fishbach, executive director and co-founder of the Great Whale Conservancy, an environmental NGO based in North Carolina, told Insider that a dozen whales are killed by a ship for each one that is recorded.
"Because of the negative buoyancy of the whales, they just sink straight to the bottom after they die, except on rare occasions like this one, where the whales are struck in the center of their body, and you have a situation as you see in the above image."
Fishbach told Insider: "There's no question that the number of whales killed by ships each year is in the 1000s each year."
When discussing what needs to change to save these whales, Fishbach said a body designated by the industry that can approach the shipping companies with a "calm, combined effort" to put forward measures and changes "to put a stop to this."
The whale specialist added that approximately 60% of the ships involved in whale strikes are container vessels.
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