Northern river otters are members of the weasel family and weigh up to 9kg — as heavy as a small dog — and up to 1.2m in length. They primarily eat fish.
They can use their teeth and claws to bite and scratch, Jacobsen said.
“If folks are attacked by an otter, our recommended response is to fight back, get away and get out of the water,” he said.
Warning signs were posted at access points along the Jefferson River in the area of the attack.
No otters have been seen there since Wednesday and there will be no efforts to catch or remove any of the animals because it’s believed to have been a defensive attack, he said.
Two years ago a 12-year-old boy on an inner tube was attacked but not seriously hurt by an otter on Montana’s Big Hole River.
Last month in California, a sea otter gained widespread attention for aggressively wrestling surfboards from surfers off the coast of Santa Cruz.