Could be anything, really. Wildlife (and stray cats) that eat fish naturally will try to catch goldfish too. Even bullfrogs have eaten my fish. Just part of having a pond in my opinion and not anything to be too concerned about. I think its good as a pond owner to accept that fish will occasionally be taken. In most cases, if you have a deep enough pond with plants and places to hide, fish will have a good chance of avoiding danger. I think addy has had an issue with a watersnake recently taking a lot of her fish if I remember right, but that seems pretty uncommon with garden ponds otherwise.
