Then there is that dependence of otters on fish. That confuses me. To me otters seem to have enormous pride. They live in a world filled with things to eat and yet insist on eating only what they can get by diving into the water. Their cousins the mink will commonly eat their terrestial neighbors, like muskrats.
Raccoons like to eat turtles.
Muskrats eat shellfish. There is nothing physical preventing otters from doing the same thing. Indeed, otters do at times eat ducks, seagulls, rabbits, and other things. But they simply don't make a practice of it. To further complicate the matter, I think otters don't have any great compunction about not killing mink or muskrats.