“This is unprecedented,” Delphine Eloi, the head of the GECC mammal research institute, told Reuters after her team confirmed reports shared on social media that the animal could be circulating in the freshwater area of the Normandy Bridge close to Le Havre.

Eloi added that although rorqual whales were not unusual in the English Channel, it was the first time a mammal of that species had been spotted swimming in the Seine river.

The whale was first spotted by sailors on Thursday, the office of the Seine-Maritime prefecture said in a statement shared on Twitter, adding that an emergency meeting with researchers and state officials had been held immediately afterwards to discuss footage of the mammal.

The team confirmed the animal was likely an adult common Minke whale, a rorqual subspecies, about 10 metres long, the prefecture said, adding that the animal seemed to be in a good physical condition.

Last month, an ailing 4-metre orca, identified as a male, was spotted kilometres upstream in the Seine, something that had never been observed before.

The animal died after an attempt to guide it back to the Channel failed.

“We need to absolutely make sure this won’t happen again,” GECC researcher Eloi said on Thursday, adding that her institute and authorities were now monitoring the mammal, which was still relatively close to the sea.

(Reporting by Tassilo Hummel. Editing by Sandra Maler.)