Maybe it was otter or a hungry neighbor but they can be birds too.
A relative of mine was owner of a rainbow trout pond and he has similar problems with some kind of eagle. It visited his pond often and took away large trouts...
He calculated that eagle takes away even more than 1000 kg fish per year (if I'm not wrong that even 1500 kg!!!). Sounds much, huh, 3-4 kg per day? But probably this bird doesn't always eat whole fish but only a part of it and goes for another one. And when it has it's young ones, then eagle has to feed them too.
What did my relative do? Actually nothing, because his pond is for commercial fishing and bird attracts visitors :)He calculated that his loss isn't that great when additional fishermen come.
Conclusion - birds can also ruin your fish population because trout are easy to catch from air (I have heard about cases when even hawks catch rainbow trouts - normally fish isn't in his menu because it's created for hunting birds and small animals).
