An otter feeds on fish along the Kallang Basin in Singapore, Oct. 14, 2016. Getty Images

A man swimming near Yellowstone National Park got an unexpected and unwelcome surprise in the form of two angry otters earlier this week. Stew Larsen, a triathlete from Washington, was swimming in Cliff Lake in Montana Tuesday when he encountered the animals.

Larsen said he wanted to take advantage of swimming at such a high altitude as part of his triathlete training. When he reached about 500 yards out from the shore, Larsen said he noticed an otter about 10 feet away from him looking in his direction.

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“It wasn’t barking, snapping, anything like that, just looking at me,” Larsen told the Idaho State Journal Wednesday. “So I kind of splashed and yelled.”

An otter feeds on fish along the Kallang Basin in Singapore, Oct. 14, 2016. Getty Images

The splashing had its intended effect: The otter went on its way and Larsen continued his swim. But on his way back, at around the same point, Larsen spotted two otters lurking even closer to him. And despite his splashing, the creatures didn’t leave him alone.

“It didn’t deter them at all that time,” said Larsen, who noted he was making a “real commotion.”

Both otters then attacked. Larsen “connected with one of them” and was bitten on his right thigh hard enough to draw blood as the otter’s fangs went through his wetsuit.

“I had been thinking that if they’re protecting a den, I’d be better off in the middle of the water and away from shore,” he said. “But I finally figured that if I’m going to be bitten, I need to on land and I swam with everything I had.”

Larsen was able to make it to some nearby rocks where he was picked up in a boat.

Otter attacks, while uncommon, are not unheard of in Montana. A woman swimming in Madison River was attacked, albeit far worse than Larsen, in 2013. Sydney Sainsbury was floating in the river with some friends with the creatures bombarded her.

“I spotted the otter on the other side of the river and it just came at me,” she told the Bozeman Daily Chronicle at the time. “It was still attacking me under the water and out of the water. It just kept biting me, that was the scariest part.”

Sainsbury emerged from the attack with a broken right hand, torn ligaments, bites, scratches and torn tendons. She was taken to the hospital where she had pins put in the joint of her hand thanks to the otter biting clean through the ligament.

“I was terrified,” she said. “I’ve never been afraid of the water and I also didn’t do anything out of the ordinary to the otter. If I were to float again, there’s nothing I could do differently to have scared it away.”

There were only about 39 violent encounters between otters and humans from 1875 to 2011, according to a 2011 Oceanographic Environmental Research Society study of otter attacks. Most happened during the last 20 years, with almost half having occurred in Florida.

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The carnivorous creatures usually only reach up to about 30 pounds and five feet long. And while they typically don’t want to have a confrontation with humans, according to experts, they are extremely territorial.

“They are apex predators which can make a meal out of alligators,” Dr. Daniel Allen, an animal geographer and the author of the book “Otter,” told Outside Online in 2014. “Do not expect a waterside encounter with a placid, cuddly creature.”

A family of otters sits along the Kallang Basin in Singapore, Oct. 14, 2016. Getty Images