Pond Boss Forums GENERAL QUESTIONS Help Large Carps are dying in pond. Anything I can do?

I have small pond 40 ft by about 100 ft, deep 8 to 6 feet across. This Spring friend of mine gave me about 10 common carps. The size is from 12'' to 16'' or so. They been doing great, eating and growing all summer. About month ago I found one big carp dead. I assumed it's rotten leaves issue and started to aerate 2-3 hours at night. It was some ice, but weather was mild. I found one more dead carp week later and started to aerate whole night. I have pretty powerful 1.5 HP pump for the size of my pond. Couple days later I found 3 more dead carps. So, DO was not apparently the issue. I stopped aerating but week after found one more dead carp. They appeared healthy, no spots or parasites that I can see. I have some bluegill and several Large Mouth . Non of those seem to get affected.
I'm devastated. Those carps survived one hour trip in trash containers with little water, hot weather, fall floods, herons visiting the pond and now dying like crazy. Water is crystal clear. Pond is fed by spring that goes in on one side and excess water is coming on other side.
My first thought was that I dropped temp too much melting all the ice with aeration, but fish is still dying with no aeration at all.
Is there anything I can do at this point? Why do you think fish is dying?
Thank you for advice.

You may not be aware, but most folks would love a way to selectively remove invasive European carp from their ponds. If this is a disease, you might contact your fisheries extension folks and see if they might want to isolate the disease organism to use as a biological control agent.

are these typical brown carp found in every puddle, pond or lake? or are these colorful carp, Koi?

What was the advantage or desirable features of your carp that you miss?

LOL , it's a brown carp. This is my third year with pond , I recently made it bigger and deeper and now ina process of getting some life in it. So, I'll take anything live and swimming.

So, You think it is some kind of disease?

No idea if it is a disease, but a disease that is specific to common carp would be a great tool for getting this invasive exotic out of ponds (you wrote that your BG and LMB are OK).

Yes, not sure we are aware of carp specific diseases. In fact, carp are so hardy and tolerant of poor water quality and low oxygen that they pretty much can almost come to the top and breathe air through their gills when under stress. Usually they survive long after all the other 'sportfish' are dead.

However, there are studies that show that a fairly low concentration of insecticide using the common ingredient bifenthrin or permethrin or related chemical is a selective poison for carp and not for other kinds of fish. An accidental spill or run off of insecticide into the water would take out the carp and not the other fish if the concentration was high enough for long enough.

Was not aware of the specificity of some pyrethroids. Good info!

Update: This paper shows BG more sensitive than Carp to bifenthrin (Table 12.4)

https://books.google.com/books?id=vsjECQAAQBAJ&pg=PA314&lpg=PA314&dq=pyrethroids+most+sensitive+fish+%22carp%22&source=bl&ots=IFkuF-jQ_z&sig=ACfU3U14xBFuKc9rZWj37or1T1mD4FD09A&hl=en&sa=X&ved=2ahUKEwis4tG-rannAhVLJt8KHUVWApg4ChDoATAGegQIChAB#v=onepage&q=pyrethroids%20most%20sensitive%20fish%20%22carp%22&f=false

different pyrethroids have different potency (synthetic vs natural).

Different studies have reported widely differing toxicities even in subspecies of carp

Also, you would need a targeting mechanism. It wipes out the whole invetebrate/insect chain as those smaller critters are not immune to the nerve poison effect. So there are studies using corn or bread balls saturated in the solution to try to target the carp only.

Now I understand. The bait is specific rather than the pyrethroid.

Since the carp are the largest fish in the pond, they could possibly all have died at the same time from a low O2 event and you are just seeing them now. Since the bass and bluegill are smaller, even though they need a bit more O2 to stay alive, being much smaller they might have survived where the larger fish didn't.

I was surprised that I lost all of my CC, but LMB and BG survived a winterkill. Larger sized fish do seem more vulnerable. I did think that carp were particularly resistant to low O2?

https://journals.sagepub.com/doi/10.1354/vp.43-3-302

Also see Table A3 here:

https://journals.sagepub.com/doi/10.1354/vp.43-3-302

I'm not an expert, so I just report on a personal experience. In my pond there are grass carp (20-25 lb), common carp (0-13 lb), LMB (0-4 lb), crucians (0-3 lb) and other smaller fish.
Some summer ago, due to a sudden change in atmospheric pressure during a very hot period, there was a sudden drop in DO which led to a serious fish kill.
About 200 fish died and I had to remove them to avoid hygiene and odor problems. Out of about 200 dead fish there were:
- 1 22 lb grass carp,
- almost all the larger LMB (about forty),
- about eighty LMB between 0.5 and 1 lb,
- about eighty large crucians.
No common carp died, neither large nor small.

Another thing related to common carp: when friends ask me for their ponds, being quite big fish and having to transport many of them every time, I transport them "dry", that is I put them all together in a large tub without water, then I put wet jute clothes and ice on it. I made hour-long trips like this and the carp always came safely.

All above to say that is not always true that the biggest fish is the less resistant to low DO.