Do you have a game camera?
Otters are well-insulated in their fur, so they may NOT trigger the camera if they are even moderately far away.
If that appears to be the case, you might gain some valuable information by putting your camera on "time lapse" with a short interval. Step One is to find out exactly what species of thieving rat b*astards are devastating your pond! Hopefully, you can catch otters, cormorants, etc. in at least one picture over the period where they are actually on your pond.
(I agree with the experts above, that it is probably otters. However, if you are not observing any otter sign around your pond it is more difficult to place your traps. If you catch otters on the camera, and they have a favored entry point, then you can really get going on eliminating them!)
P.S. Personally, I would not hesitate to take a shot with the .243 at an otter on sloped ground (a backstop), if there was zero expectation of humans in the line of fire.
Good luck on your fish re-population project.
