Trying to paint the picture here.
I am trapping a branch of a larger river and at its peek, this river branch is a glorified creek. Right now, it is drying up BADLY and only a 100yd stretch is holding water yet. I have been trapping muskrats.
First check- 3 muskrats
Second check- No muskrats but found a LARGE shiner with it head eaten off
Third check-3 muskrats (One still alive, one COMPLETELY shredded into a gut and fur pile, one with its stomach ripped open)

I would also like to say we have a big population of leopard frogs, and there are many scattered about with bite marks, missing legs, etc in the water along the edge.

Typically, I would say mink because where I grew up, that was the only thing. However, there are really old remnants of broken turtle shells in that area which would make me think otter (however, that was from awhile ago)

You have any blue heron around?

Heron.... like Bruce said.

Crap, yes. It is there every time I check traps

Never saw a heron just take the head off a shiner. Here they eat them whole no matter how large.
I'm thinking mink. But I know very little of otter

The torn up muskrat and the muskrat with its stomach ripped open is screaming blue heron to me.

mink in our area love the small frogs, I find parts of them lying every where. In the water bed and on the bank edges. Herons eat the whole thing in a fast swallow.