The attack left her with dozens of stitches and injuries to her face, arms and legs.
Lynnette Bellin, a certified Infant Swimming Resource instructor who teaches self rescue lessons, said she was on her weekly swim in the lake when the encounter occurred.
She said she had been trying to get back to teach a self-rescue lesson when the animal attacked.
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"It ripped the goggles off my face. It got me on the temple, on my cheek, my nose. You can kind of see claw marks there, full on bite marks on this arm. But my foot is the worst because it has 22 stitches," Bellin said while showing the injuries.
Bellin said she posted videos to social media immediately afterward showing blood on her body.
She described the animal pulling her underwater before she realized it was an otter.
"And right before I got there and I got grabbed by the lake, yanked under water. It was so powerful it felt like a human. That's when I saw the otter and it just did not hesitate. It started coming after me. It was teeth and claws and I was pushing it away," Bellin describing what she says went on for about 5 minutes.
She said she eventually escaped by relying on her swim training and climbing onto a rock. Bellin then recorded video while walking about three-quarters of a mile to Hidden Beach. People who saw her along the way contacted authorities. Bellin said officials responding to the incident were surprised by her account.
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"Every single authority that was coming to do a report on this was like, we've never had an otter attack. are you sure it was an otter. Sure looks like what I was fighting off," she said.
In a statement, the Nevada Department of Wildlife said, "River otters are native predators and are found in Lake Tahoe and its surrounding waterways. While attacks on people are uncommon, otters are wild animals and can become aggressive if they feel threatened, cornered or are defending young."
Bellin said she believes the encounter occurred because she was swimming close to shore to avoid boat traffic and that the otter may have been defending its young.
"I think the most disconcerting thing is that lake Tahoe is always my sanctuary, my peace. I go there to get mental calm and just to regulate myself. I have 38 stitches right now. I can't swim until those are all healed up. But September is a great time to be swimming in the lake so I'm sure I'll find myself in there eventually," Bellin said.
The Nevada Department of Wildlife said it is working with Nevada State Parks to post signs in the area alerting visitors to the reported incident and reminding the public to be aware of their surroundings and give wildlife plenty of space.