A fin whale with a severely deformed spine was recently filmed struggling to swim off the Mediterranean coast of Spain. This extreme case of scoliosis was likely caused by a vessel strike and will probably cause the gentle giant to slowly starve, experts say.

The injured, 56-foot-long (17 meters) fin whale (Balaenoptera physalus) was spotted March 4 by a boat's crew off a beach at Cullera near Valencia. The boat's captain thought the whale was trapped in a fishing net and alerted the Spanish Civil Guard, who sent out a team of biologists and veterinarians from the Oceanographic Valencia aquarium. After arriving on the scene, it became obvious that the whale was not trapped; Instead, it had "scoliosis of unknown origin," according to a Facebook post from Oceanographic Valencia.

The researchers attempted to put a tracking device on the injured animal's back, but it was too deformed for the satellite tag to successfully attach. After "a few hours of attention," the fin whale slowly headed away from the coast and out into deeper waters where it disappeared from view, Oceanographic Valencia representatives wrote.

Experts told Live Science that the scoliosis was probably caused by a vessel strike that broke the whale's back.

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"The term scoliosis simply refers to an abnormal lateral curvature of the spine," Jens Currie, chief scientist of the Pacific Whale Foundation in Hawaii, told Live Science in an email. "The cause of scoliosis can take many forms, but the most common is blunt force trauma."

It's is likely that the whale "was recently struck by a vessel," Currie said, an opinion shared by Erich Hoyt, a research fellow at Whale and Dolphin Conservation (WDC) in the U.K., and Simone Panigada, president of the Tethys Research Institute in Italy. But the experts also noted that it's hard to know exactly what happened.

It's possible for large whales to be born with scoliosis or develop it in their early years. But young whales that develop scoliosis almost never live to adulthood, Currie said.

Baleen whales — a group that includes fin whales, blue whales (Balaenoptera musculus), humpback whales (Megaptera novaeangliae) and others — feed by lunging through large shoals of tiny crustaceans known as krill. To do this, they rely on their enormous tails, or flukes, to rapidly propel themselves through the water. But according to the footage, the injured whale is unable to do this, which means it is probably starving.

"We can see from the video that the whale is already very skinny and beginning to look unhealthy," Currie said. "It is very unlikely it will survive." Baleen whales can survive for many months without properly eating, which means that injuries like this can lead to a "slow and painful death," he added.

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This is not the first case of whale scoliosis linked to vessel strikes. In December 2022, a humpback whale called Moon was spotted in Hawaii with a broken back, after swimming more than 3,100 miles (5,000 kilometers) from British Columbia, Canada using only her flippers, according to The Guardian. Last year, Panigada spotted another fin whale with scoliosis near Barcelona, although its spinal deformity was less severe.

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But most whales don’t survive an encounter with a vessel. In total, around 20,000 whales are estimated to be killed by vessel strikes every year, due to a more than 300% increase in global shipping traffic since 1992, according to Friend of the Sea, a non-governmental organization based in Italy. But it's hard to track this because strikes are often unreported and most killed whales will never be found, Hoyt said.

In addition to vessel strikes, whales are also exposed to a lot of noise from shipping that can disrupt their navigation, feeding and communication. "I would say it [ship traffic] is one of the main problems cetaceans face globally," Currie said.