You're not doomed at all. You know where `it' is feeding, and approximately what time it's feeding. Is your pond: Surrounded by woods, fed/overflow through drainages extending into the woods, or are the banks and surrounding area completly cleared?
Since `IT' is not going into the traps, I'd grab the rifle and look for sign along the most likely approaches to the pond. If it is a raccoon, and it has been feeding pretty regularly at one location, you should find scat and, if you find where it's hanging out, feathers, etc. near the tree where it's sleeping (in general they like a specific tree and don't move around unless the food runs out).
Probably the easiest would be something toxic mixed with a can of catfood, set out at the far end of the pond at 10pm and picked up, at 1am if nothing eats it. I prefer to retire them on the instant and would park myself in a comfortabe location near the feeding zone at 11pm with the .22,, a spot, and an undoctored can of stinky cat food as an aiming point.
We had a Royal Palm hen working on a nest no more than 60yd. from the house. She hadn't gone broody (still laying an egg a day) and was flying back every evening to roost in the shed. We found her in the run (she'd fly in and out over six ft. fence like it wasn't there) mortally wounded one afternnoon. Went out to check her nest and found eggs broken and scattered around. Set HAH trap, at nest location, and caught a coon the following afternoon. When rooting around in the woods a wk. later, I found the big hickory snag where the coon had been staying (eggshells/scat/hair) about 75yd. from where turkey had been nesting.
We have three HAH's set at all times, the coon could have gone for the bait, but it was on a daytime schedule and the turkey hen, like your poor ducks, was even EASIER MEAT. And coons like easy meat.
Given your location, and the fact you don't mention having actually SEEN what is scarfing your ducks up, you might want to take both a .22 and a .307 if you decide to ambush whatever it is.
Good luck!
ed: bad `gwamma'