Mustalids routinely win against larger prey. Stout kill full size rabbits and a wolverine has been recorded to have killed a polar bear. Don't give the otter an excuse.
Otters are apex predators simply because they have the ability of tactical genius working in strike group squads. If you've ever seen the videos of them taking down and drowning crocodiles, then you know. They are not omg adorbs so cute!! They can and will destroy you. Walking navy seal copypastas.
There's a video on YouTube where otters were housed with monkeys, the monkeys trolled the shit out of the otters for the Lulz... Eventually the otters all sort of snapped, together, grabbed one and absolutely mauled the shit out of it.
Of all the things to get attacked by in Montana. Not a bear, not a mountain lion, or a wolf, or a moose. Nope, a fucking otter is the thing that gets you. Jesus.
By the grace of god you made it out of the river? By the grace of god? You dumb shitheel an otter bit your fuckin' ear off. If anything your stupid ass got smote.