My sympathies, we are also duck breeders and it must be a painful experience.
We lost our runners and campbells a short while after moving here - local gamekeepers said it would have been otter which have seen off all the mink in our area - dog otters are big, but it may have been a fox, badgers are as bad, or even a large stoat or even ferret?
We now only stock Muscovy ducks as they fly shortish distances and roost on shed roofs, fences and window boxes and now nothing gets taken.