There is a fascinating account of River Otter behavior in Dalquest's
Mammals of Washington (I'm pretty sure that's where I read it).

Dalquest and his mammal researching buddies were shooting bats with shotguns
along the shore of one of the San Juan islands. The local River Otters
quickly caught on to what they were doing. They would shoot a bat and, almost
as soon as it splashed down, a River Otter would be on top of it, chomping
it up.

My initial reaction was "I thought River Otters liked seafood". Bats...
yuck.

Then there's the River Otters at Rimrock Lake that attacked the woman last
summer. I know, it was a bizarre, inexplicable anomaly. However, I have
come to the conclusion that River Otters have a temperament that lends
itself to taking a bite out of most anything that might be meaty and
nutritious, including ducks.

Kelly McAllister
Olympia Washington