"Probably a heron..I always go out first thing inn spring to make sure they all made the winter.I had a few 12" long ,some fancy tailed about 8" long they were fine.Th next week I looked they were all gone.A few times after that I saw a heron fly over house sqawking.He was saying "the fish were good down there........

You can try using a Scarecow to protect your other fish from the same fate. It's motion activated and should work. Just remember to keep it on when your not watching the fish.

Scarecrow didn't work for me after a couple weeks - and I had TWO of them. A tasty (and easy) fish dinner is a pretty strong incentive for a heron. Covering the pond with a net overnight and when you're not around is the only thing that has done the job.