Sorry to hear about the loss of your larger fish.

Sounds like poachers to me. Either otters or the 2-legged kind. (I have the 2-legged kind on my property periodically.)

Do you have a game camera up? Many of the newer cameras have a "time lapse" setting. I now have some more obscured cameras on my property set on time lapse so they don't need to be triggered to catch a picture of an otter or a distant human.

One time, I even had trespassers remove the chip from a camera that was covering my best fishing spot, but not steal the camera. Which is a strange morality.