A concentration of ducks in an enclosure is like a chicken pen, it will
attract predators. Rivers otters prey on ducks occasionally. So do eagles
and falcons. If somebody raises tasty animals for food or show they have to
deal with predators, it's part of animal husbandry. River otters can dig,
have pretty agile bodies, and of course can swim pretty much anywhere. Any
animal that gets easy food will return, this is why bird feeders work. A
duck enclosure thus becomes an otter feeder unless you can exclude the
otters. In my experience raising chickens, killing predators doesn't really
solve the problem of a non-secure enclosure.

Rob Sandelin
Naturalist, Writer, Teacher
Who is overrun with mice and wishes he had some weasels