Just stopped in for a visit and saw your post. What do the bites on the heads of the fish look like? How big are the fish? Are you seeing herons or cormorants piling out of there when you pull up? How about latrines/otter scat? If it's otter, you'll find scat somewhere around that pond, esp if they've been there for 10 days. If otters are living in/or around the pond, is there any structure around that they're denning up in or under (pump house, enclosed well head. etc.)? If you're miles from the nearest waterway, those otters aren't commuting each day... that is, if otters are the culprit.
If the pond is muddy, you may have a problem with using either a submerged (white) bucket set (MN trappers web site has instructions) or a sumberged (or floating) cage trap with 1 way door, baited with live fish (check your state regs.). A guy by the name of Bill Weeks has a company in Okeechobee FL, who makes this type of cage trap for dealing with conflict otters. We've been using this to help solve problems with pond owners here, which is what he developed it for. Sorry I don't have the contact info with me. Again, check your state regs. on the use of colony-type traps, and the use of live bait. We put 4" dia PVC pipes (white) with glued end caps running the length of the trap (3 feet) as floats. If you want to get really fancy, try using one of those kid carnival strobe light rings or earrings that flash multi-colors. They are powered by watch batteries, and you see kids walking around with them at concerts, carnivals, etc. Turn it on in the evening and attach it to the top of the cage as an attractor to run all night. Otters are pretty curious, it might work, who knows.
Good luck, and Happy New Year
ODNR, Division of Wildlife