Lots of good advice you are getting on trying to correctly identify the culprit before deciding on the solution.

A good game camera (or video security system if appropriate) might be another good tool for you.

A game camera might not catch a snapping turtle crawling up the bank, since he would not have a heat signature. Therefore, you might have to try some time-lapse shots or video.

However, a visual medium might show you a turtle, small gator, otter, etc. at your pond.

Further, the up-front investment would not be a total waste, since it is always nice to have a little surveillance on your property.