You don't say what State or Climate Zone you are in, which may give a clue to what could be eating your fish.
That being said, since the fish's carcass is still there, albeit damaged, it could be a crow that snatched them up or a cat got them.
A Heron or other water bird, even a Raccoon, would eat all of the fish, as would a Beaver, Muskrat etc.
My bet would be on bird or cat damage.
"Horton"

Raccoons do not always eat the entire fish. I've found plenty missing heads, eyes, fins etc. They are notorious for pulling the heads off chickens too.

Cats are a much less likely predator of fish. The link between cats and fish is strictly man made because other meats were considered too good to waste on catfood in previous times. Fish is not a natural food of cats nor is it even good for them.

BOS,The several times raccoons have raided my ponds, the only evidence left behind from the fish, were a few scales.
No heads or tails, they ate the lot.
They did leave a bullfrog ripped wide open on the grass and the ponds full of mud and broken plants.
Maybe our raccoons have bigger appetites or your raccoons are just picky eaters. LOL
Don't get me wrong, I like cats, but not in my flower beds, near my pond or bird feeders.
I have seen them trying to scoop a fish or two out of the pond. I don't think they succeeded, only because I chased them away, but the did try a few times.
"Horton"

A raccoon got one of my HUGE bluegill and ate only the head. He was kind enough to leave the rest of it inside the garden shed..............in 90+ degree heat.......I FOUND IT, BELIEVE ME!!!

rofl

I just noticed the same question was posted by the same poster on different days. :)
Oh well!
"Horton"

Before we put up the electric fence because of the raccoons, I observed many cats at our pond many, many times. While they did reach out a paw towards them, the minute the paw hit the water they jumped back and shook the paw. It was actually quite amusing.

They would sit for hours and watch them but I never saw any evidence that they actually scooped any from the pond. My own cats and the neighbors and stray cats are fascinated by the fish swimming, in the front ponds with no fence or fish there are treefrogs and tadpoles and they are all fascinated by them too but we have not found any froggie corpses. Nor seen them actually catch one. Again when the paw gets wet, they jump back and shake it.

We watch them for hours out the front window. Having the "nature" ponds out front is almost better than TV. We've seen opossums drinking too but raccoons are the only ones who do any damage.

They do present us with dead rats, mice and an occasional house sparrow though. I'd like to pay them a bounty on the dang starlings. IF they got a fish or a frog, I have no doubt that we would find the corpse in the house or on the back porch where they leave everyone else.....

The only fish I would truly worry about being killed by cats are fancy goldfish like fantails that don't swim well at all and have to stay near the surface. They wiggle so enticingly that they are impossible to resist and they can't dive like comets or koi do to escape. I can then imagine the cat playing with it until it died and then walking away thinking all the fun has gone.