A beluga whale wearing a harness that read "Equipment St. Petersburg" appeared in Norway in 2019.
Officials said they believed the trained whale was a Russian navy asset that may have escaped.
On Sunday, the whale appeared in Sweden, moving further away from its natural habitat.
A friendly beluga whale that was first observed wearing a harness four years ago and is believed to be a Russian spy reappeared in Sweden this week, puzzling scientists.
The beluga first popped up off the coast of Norway in 2019, spotted by fishermen who noticed the whale was wearing a harness equipped with camera mounts. A fisherman alerted Norway's Directorate of Fisheries, who said a clip on the whale's harness read "Equipment St. Petersburg" — indicating it may have come from Russia.
"The whale seemed playful but our instincts said that it was also asking for help to get out of the harness," Jorgen Ree Wiig, a marine biologist told CNN at the time. He said officials believed the whale had come from Russia and was trained by the Russian navy, which has "been known to train belugas to conduct military operations before."
Biologists said belugas in the past had been used to guard naval bases, help divers, and find lost equipment. They also said in the Cold War Russia used beluga whales to sniff out mines and torpedoes.
Researchers said it was clear the whale had been trained, noting it was approaching boats, raising its head above water, and opening its mouth, suggesting it was waiting to be fed as a reward. It was unclear how the whale ended up in Norway, but one theory was that it somehow escaped its marine pen.
The whale was nicknamed Hvaldimir, a play on the Norwegian word for "whale" and Russian President Vladimir Putin's first name, and the harness was removed by Norwegian officials.
Hvaldimir also made headlines for retrieving and returning the phone of a woman who had dropped it in the water.
After spending several years traveling south down Norway's coast, the beluga whale sped up to quickly cover the southern half of the coastline and appeared off the southwest coast of Sweden on Sunday, The Guardian reported.
"We don't know why he has sped up so fast right now," Sebastian Strand, a marine biologist with OneWhale, told the outlet. He also noted it was puzzling because the whale was traveling "very quickly away from his natural environment."
The closest beluga whales live farther north, in the Arctic Ocean and the frigid waters north of Norway and around Greenland.
"It could be hormones driving him to find a mate. Or it could be loneliness, as belugas are a very social species – it could be that he's searching for other beluga whales," Strand said.
Russia has never addressed reports that the beluga could be a Kremlin spy.
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