It might be an otter. Your pond isn't very large and trout size is suitable for this animal to start hunting. Probably you won't ever know if your fish were breakfast for that otter unless you find some evidence of it - some scales or something.
What about this phrase "These were 8-10" trout. I caught out 10 or 11 in April, they were 12"+". How did you catch them? As far as I know, trout is very vulnerable to any kind of injuries. It might cause the death of your fish.
