Did you look around on the ground just in case? If there is a hole big enough for a cat to reach in and pull out the fish, then he could have jumped out of it. It is also possible that he did jump out and the cat found him before you did. Regardless of how it happened, I'm very sorry for your loss.
I use canvas craft mesh, cut it to size, and strong tap to attach it to the lid. Works good for covering any openings.
Edit - Sorry, just saw that you found him