Raccoon would be the number one culprit. Something could have scared it off before it finished it's meal. Sometimes they catch more than they can eat too. It is common to find bones, fins or scales when they've dined. Birds eat fish whole. Raccoons don't. Fishing cats aren't common. Fantails don't swim very well, they wiggle at the top of the water and are more likely to be eaten by predators than comets , commons or shubunkins who can all swim faster and deeper.

You should probably contact somebody who does this professionally. Often filling in a hole won't work, as the animal will simply move back in. Depending on the animal there may also be regulations regarding what can and can't be done.

You know what works for me and don't think I'm crazy but we also have loads of critters in our backyard and most I don't mind but we have a skunk living under the shed of my next door neighbour which is right next to my property line. So I pile dog poop around the perimeter of my property. It's in the woods so piles of dog poop aren't unsightly and they will dissolve into the ground with time but the dog poop seems to tell the critters that the dog lives here and you best not try to claim the dog's territory. We no longer have bunnies, coons or skunks (so far, knock on wood) and the only critters that still come are the squirrels but they are up in the trees so they are unconcerned by the poop. But maybe putting another animal's poop at the entrance of the hole might tell the animal not to use that exit any more and then you can close it off after a while. I also used this technique for mice that were living under my shed. The mice left but moles moved in instead and aren't bothered by the poop (go figure) lol! But the moles are cool...they eat grubs, slugs and other nasty ground dwelling bugs that most gardeners don't want around anyway and they aren't vegetation eaters, so double plus for me. Anyway, just a thought if you wanna take a "natural approach" to closing off your critter hole. Oh btw... we had groundhogs too (the babies were darling!) but we didn't want their burrow on our property and the dog poo worked there as well. You will know a groundhog hole as it has a musky odor and gnats will gravitate to the musk and fly around the hole at dawn and dusk. Let us know how it all turns out. Barb southern Ontario, CANADA

Could be a "sick" animal...most often with some pathogen like GIARDIA which can be transferred to humans or pets through contact. Obviously you aren't going to touch it on purpose but just be wary that sickness is a distinct possibility esp. if you have any pets that also access this area. Our dog had Giardia and it's something you don't wish on your WORST enemy. If pets are sniffing around or even eating animal excrement they are in a position to get diseases carried in the intestinal tract of wild animals. I would be inclined to take a shovel out there and cover over it with fresh soil IF you have a pet who roams in the area. Barb