She was healthy and my other koi look fine from what I can see.

Does anyone have an idea what this might be? A parasite or a heron attack?

Those injuries are definitely from an attack by a predator. Since the holes are not scattered as they would be from heron strikes, my guess is they were made by a claw and the positioning suggests a raccoon but there are other possibilities. If that is the case there will likely be other attacks.

Check your pond for access. Steep sides and deep shelves make access more difficult. Make sure there are lots of places for your fish to hide away from the shelves. Don't encourage your fish to come to the side where they could be trapped.Shallow ponds are a magnet for raccoons. They like to play with the fish even more than eating them. They are fascinated by flashing scales and movement. Deep water with cover is the best protection. Check into an electric fence.

Turn on security lights around the pond but no lights in the pond at night unless you are right there. Even then the little monsters may approach the pond once they get accustomed to your presence. I had to keep a hose on full force handy to keep them wary when I sat by the pond at night. A water cannon with a motion and heat sensor like the Scarecrow works most of the time if you are not there.

Do a search on this forum for more info on raccoons and other predators. Don't get discouraged. It took three years to convince our local raccoons my pond was not their playground. Good luck.

Shortly after a fish dies redness normally dissipates from wounds and even gills.

Ulcers obviously don't kill overnight, they also don't form overnight. Just because OP didn't notice the marks before the fish died doesn't mean the marks weren't there for a long time. Even an experienced keeper may not have seen these on a live fish as they're so low on the fish and mostly covered by the pectoral.

But like I said, pretty much impossible to tell from a single picture...at least for me.

Since we're questioning diagnosis I'm happy to play too...Predator attack seems very unlikely. There don't appear to be any missing scales except maybe right where the marks are, hard for me to tell from the picture.

Marks from Heron attacks I've seen were a nearly vertical straight mark on each side of the fish, missing scales, scratch. I don't see how a Heron could have made marks low on the body and behind the pectoral unless the fish was swimming at least on its side, almost upside down. And what appear to be at least 4 marks means the Heron made the awkward attach at least twice in almost the same spot. Pretty unlikely imo.

The "puncture" marks appear to be about at least 1/8" in diameter if that's a normal adult hand in the picture. A puncture type wound would always appear smaller than whatever caused the wound as the flesh stretches during the injury and then closes back somewhat. So given the wound size and the size of raccoon claws I'd say we'd have to be looking at a raccoon larger than an SUV. Maybe OP lives near the Susquehanna power plant?

Bear or eagle attack? That would be possible in NE PA. Seems unlikely as these were the only marks. Normally in an attack there are marks on the other side of the fish as well because if you poke a fish in water it's difficult to get a puncture as water provides little resistance. If you've ever watched raccoons or bears fish they normally use both paws to clamp onto the fish. And even eagles and osprey have to get talons on either side of the fish to clamp down. Herons have to get a mandible on either side of the fish to clamp down, they don't normally stab fish.

If we go just by odds the smart money is going to be on ulcers from poor water quality. Very common. Given the weak evidence of attack I wouldn't spend my time installing lights and fences and would instead consider water quality issues first. But that's just me.

I know nothing about ulcers in koi. I just checked our pond water. The pH is at 7.5 which is typical. Even our well water is always in that range. Ammonia tested out a 0 ppm; again very much the norm for our pond. I collected a water sample in a clear glass jar and there is no discoloration in the water. We've had a good amount of rain this year so mother nature has been providing us with fresh water for the pond pretty frequently.

The only thing different this year is that we have almost half the pond covered by water hyacinths and a small water lilly. As we have had 90 degree sunny days all summer I allowed the hyachinths to sprawl over more of the pond than normal just so the koi had some relief from the hot sun. Additionally I was under the impression that hyacinths help improve water quality. Our pond is on the smaller side; about 800-900 gallons.

I was able to examine the fish pretty well while she was still alive and in the water. The "puncture" wounds did not have any redness. They looked much the same as the photo.

The theory of a predator attack is also puzzling. Once the fish had died (12 hours later), I brought her in to the kitchen so I could thoroughly examine her. There are no other wounds on the other side like you would expect if something grabbed her. There were no scales missing.

I am an adult woman however I have small hands. My husband estimates the punctures were about 1/16th of an inch in diameter. The one directly behind the pectoral fin looked rather deep.

We live in eastern PA and have a population of raccoons, bears, sharp shinned hawks, and an occasional eagle.

This particular koi loved to hang around under the water fall very close to the edge of the pond so she certainly could be easy prey.

Waterbug and Sleepless, I truly appreciate your insight. Any other info is welcome. If I need to make some changes to ensure the health and safety of our koi, I am certainly open to any suggestions.

Maybe you can tell I have some experience with raccoons. In Wisconsin, we had seven adult females to rehab one year and 25 babies between them to keep fed. The mommas had been injured in illegal traps. Here in Indiana we have as many as seven we can identify that drop by, among many other critters. Last night we had a huge male chomping on some cat kibble that got left out by accident. He has been very well fed and must weigh close to 40 pounds. BTW, this is a residential area where you don't have a lot of open space.

By the pond, wet footprints are easy to identify if you catch them before they dry out in the morning sun. Do a Google search for raccoon tracks to confirm. Adolescent to young adult raccoons have a front paw length of about 2.25 inches and about as wide with the middle pad of 5 often longer than the rest. A full grown adult male may have a paw almost 4 inches spread out. The digits are very long and strong and feel like bony pencils if they grab you. They are very dexterous even without opposable thumbs.

In a residential area they get used to humans very easily. It is not a good idea.

Thanks, Ilene

This not a bacterial infection. If it was the fish would still be alive as it would be way too small to kill the fish. I would say that most likely Sleepless is correct. It looks like wound closest to the gill may have punctured the fish far enough to have hit a vein.
Mike

I know this has nothing whatsoever to do with Ilene's problem, but I hate making mistakes like that for some reason.

I agree that this has to be an attack and a raccoon seems to be the likely culprit.

Thanks everybody!

Raccoons make a huge mess. They knock over pots and push rocks into the water. They spend hours going over everything with their hand like paws looking for food. They do not just sit on the edge and swipe at the fish. They are mostly looking for crayfish, snails, frogs, and they eat waterlily rhizomes too. I deal with raccoon damage on a monthly basis despite having an electric fence around my fish pond. They hit my barrels and even some plants I had potted up for sale on Craig's List that were in a bigger pot with water in it....They never just take fish. They always trash the entire pond. You'll find it muddy and looking like a hurricane hit. We find their foot prints in mud and water when we get up all the time. Hubby watched a mother and three kits the other morning. They put their paws where they can't see and try to pry up rocks...

800-900 gallons is not a lot of water for koi. Especially multiple koi. Water quality and pond suitability for koi is not always about test strips. They don't get proper exercise for best growth without at least three foot depths and they can be stressed for a long time without showing symptoms. I think Waterbug is correct that it is ulcers....