Pond Boss Forums AQUATIC VEGETATION Controlling unwanted plants HELP....fish kill caused by dying duckweed!!!??.

Lost about 20 fish, more than half of which were LMB over 14"

Added fluridone 2-3 weeks ago. It has started killing off the duckweed that covered 2/3 of my 1-acre pond, but now I'm losing LMB at an alarming rate, including some over 20" as well as a half dozen big BG. The weather has been well into the 90's for the past week, and the pond is shallow, 4-5' at the deepest parts. Water is almost like bathwater and for the first time smells bad. Would it be worthwhile to grab a couple Bioverse bacteria spheres at Tractor Supply to aid in the breakdown/digestion of nutrients from the decaying duckweed? I had been using another brand of mail order beneficial bacterial but ran out of it after 6 months of use. It's frustration throwing so much money at pond chemicals, an aeration system, etc. And then finding trophy LMB serving as turtle bait. I've been aerating 24/7 since spring, after ramping up as suggested here.

This happened even with an aeration system?

Yes--I have 2 9" diffusers spaced apart in my shallow, sort of triangular pond and have never even seen a dead fish until yesterday and today. My guess is that the sudden heat wave and the decaying duckweed caused a perfect storm. This is an old pond and since buying the property last fall, I've been trying to do what I can to reduce muck, boost oxygen and clear up the unwanted duckweed and FA. Our primary intentions are casual fishing and, hopefully some day, swimming. Right now I'm pretty disgusted and discouraged.

Are the diffusers running 24/7 I think they could be actually doing bad for a pond that small in a sense super heating it.. I'd only run it at night for a few hours to mix the warm surface water with the cool bottoms.. A pond that shallow probably don't have a thermocline to worry about, and constant aeration wouldn't be needed.. Just a thought..

Also being that you just bought the property this problem could of been a reoccurring thing before you arrived..

Yes, diffusers are running 24/7 and have been since working up to it this spring. It's possible that thisnhas happened in the past when we didn't own it, but the number, size, and, before this week, health of the LMB and BG seems to suggest otherwise. At least a dozen more floaters tonight

I don't think aeration is needed 24/7 for a pond that small/shallow.. Your in my opinion heating the pond instead of cooling it only multiplying your problems. Anyone else agree with me on that?

If it kills off every couple years you'd still have big fish dying.. Just think now your smaller fish aint competing with the big boys since their dead..

Well...shutting off the aeration before I left this morning didn't help. My wife and son removed about 200 dead fish in 2 hours today, and there are probably 50-100 more dead ones on the pond right now. I'm tempted to go buy a trash pump and start pumping cooler water from a creek that is 50 yards away to raise the water level and cool things down. Or I could sit back and watch all of the remaining fish die.

Looks as if all LMB are gone. BG are the ones dying off now.

SML, I'm hating this for you. Hope you get it figured out soon.

Maybe a water test also?

We literally removed a couple hundred more dead fish today, mostly BG and a few crappies as the LMB were already gone, at least those with any size. While exploring under our deck that overlooks the pond I found an electric pond waterfall pump from the previous owner who had used it until 2010 for a large waterfall feature in the smaller pond on our property. It was stuck back on the corner by the heating oil tanks. The pump is a Tsurumi 90gpm model and has a 2" outlet, which I necked it down to 1.25" so I could use plastic coil pipe from Lowe's to pump cool water from the creek 50 yards away into my dying 1-acre pond. Just got it up and running an hour ago. Wish I'd have found the pump and thought of this a week ago. Hopefully by morning I will gain at least a few inches in depth and drop the temperature enough to stop the kill. My wife came up with another factor that probably attributed to our perfect storm: killing off so much duckweed also eliminated some of the "shade" that helped to keep the temp down. After having dumped an unbelievable amount of dead fish over the past 3 days, my thinking is that the pond was probably way overpopulated for its size and, especially, shallow depth, and the central PA heatwave we're having was just too much, too suddenly. This 4th of July holiday (and my birthday on the 5th) was not a good one for me. Hoping for better days ahead...

I don't think it's as much of a temperature issue as a DO issue from the dying duckweed.. Also running your aerators only at night will get your pond as cool as its gonna get without adding lots of cold water.. And if I remember duckweed actually holds in heat or maybe that was FA I can't remember but I know I read it here on the site somewhere.. I'm really surprised you haven't got more response its getting overlooked I quess..

Any remote assessments of this situation are somewhat speculative; but I'll add this: I've never encountered a DO-crash due to the die-off of duckweed - whether the die-off occurred quickly or slowly (as with fluridone treatments).
Unless the surface-layer of duckweed was really dense, it may not represent enough biomass to cause a DO crash (on its own) - unlike a dense population of coontail or Najas, or a thick mat of algae, which often dies/decomposes very quickly when treated - and sometimes prompting a spike in BOD due to microbial decomposition.
BGK, I'd probably place a bet on your initial assessment: That is, running the aerator 24/7 increased the pond-water's temperature. Warm water inherently retains less dissolved oxygen than cooler water. So, if the 24/7 aeration dramatically increased the water-temp, the intended benefits of this action were possibly negated.
Just my speculative opinion.

Since we've started pumping the creek water in yesterday, it looks as if there are few, if any, newly killed fish. I guess I'll stop aerating during the day and allow the creek water to form a cool section in the pond, and then aerate at night. What time period do you recommend for nighttime aeration....like 10pm to 6am or what? Thanks!

Looks like your getting some good responses. Let me add my take without having all the data of the aeration system.If this one acre pond has an average depth of 4 ft lets say you have 1.4 million gallon total. Lets also presume your two discs are the most efficient on the market and are lifting 750 gallon per minute each for 1500 gpm or 2.16 million gallons per 24 hours.This divided into your total gallons would be a lift of 1.5 times per day which mathematically is fine "but" if you have them in a shallow end they may have little effect on the total as far as satisfying the total oxygen demand.As far as temperature is concerned lets also assume that without aeration (circulation) your bathwater is at 86F and lets use 85% saturation for the oxygen level which is common.That would give you 6.375 ppm DO which is enough oxygen.Now lets start aeration as assume you were circulating the entire water body which would be the correct thing to do and you could get the water to 95F at the same % saturation your DO would only drop to 5.865 ppm DO.This would still be plenty of oxygen.I think your pond has a high BOD (biological or biochemical oxygen demand)and you may not be circulating enough or do not have the air stations in the correct areas or the correct amount of air going to them or they may be a type that are lifting only a few hundred gallons per minute. Shallow ponds require more equipment to aerate than the same size surface acreage ponds that are deeper. I agree with the notion that the duckweed biomass and fluridone probably is not the cause of the DO depletion. We routinely treat for duckweed at 45 ppb active fluridone which is called for and where water meal or other plants require more fluridone are present. Thus 90 ppb are added and even though I see a more rapid response to the DW dieoff in that case I also have not ever had a fish kill by DO depletion even when 100% covered. Although I will not treat those ponds without proper aeration.Hope this does not make your situation more confusing and frustrating.

Personally....I would not have used a broad spectrum herbicide (fluridone) to treat such a small area of duckweed. (1/2 gallon of Tribune at around $25-$30 would have worked fine) Unless you were very accurate with the dosing, the fluridone most likely was affecting other aquatic plants as well. Shallow water = organic rich (sapropelic) muck with EXTREMELY high BOD.

I believe that an oxygen meter is EXTREMELY valuable in these situations. Everything else is guessing and theoretical calculations that mean nothing to a dying fish from lack of oxygen. Inadequate aeration in a organic bottom rich shallow pond can be more deadly than a pond that is not being mixed with artificial aeration.

Very sad to read that such a healthy fish population has been eliminated with good intentions.

In my experience, I have not seen a fish kill due to increased water temperatures through aeration unless we are dealing with cold water fish. Even in the most severe heat waves and dry conditions in Texas... if anything, aeration is more important than ever when temperatures increase. I am very sorry you lost your fish ~ it does appear to be oxygen related since the big bass went first. Unfortunately once that chain of events is set into motion, there is little you can do to stop the kill. I personally believe dying vegetation could have increased your BOD to the point where the aeration system couldn't keep up...

Sue what about the fact his pond is only 4' deep?

During the drought in TX, the lakes and ponds were drying up and the temps were over 110 degrees ~ increased water temperatures did become a concern, but after doing a little research it was determined that the aeration was the only chance for fish survival. I'm not a fisheries expert, but I believe that LMB and BG can handle pretty warm temperatures.

Surface aeration is what Overton uses in some of his shallow ponds with high biomass.
I use both bottom diffuser aeration (circulators) and surface aeration.
Bottom diffusers in daytime and surface aeration at night and cloudy days.

A two acre perfect pond, shrunk to one acre pond in extreme Texas drought leads to dead trophy fish.

Kudos to you george1 for further explaining the differences in aeration (other posts).

I believe the OP would have benefitted from using surface aeration rather than bottom diffuser aeration in his shallow (<5 feet max depth) pond. Too many people simply believe at having any type of aerator is serving their particular needs.

For deeper stratified larger lakes, I like a process called "hypolimnetic aeration". It is perfect for those times when you want to maintain a cold water fisheries in a stratified lake.

Thank you for all of the comments and insights.

We have gained several inches of pond depth since I started pumping creek water in on Sunday. I do not believe we lost any more fish yesterday or today, and the pond is noticeably cooler now. I am aerating 24/7 again.

One thing I have learned on this forum is that there are lots of different opinions. I'll never know the exact cause or whether there were simply too many pounds of fish to survive a sudden heatwave. One particularly puzzling aspect is that we never observed any fish gulping for air, as I would have expected in an oxygen crisis.

There is another possible factor that I forgot to mention. After the 16 oz. Fluridone treatment and a week or so before the heatwave, we were overrun with a flock of Canada geese for several days until I had a day off from work and was able to encourage them to move on. They crapped all over the pond and it's banks. That dose of "nutrients" plus the decay of DW certainly didn't help.

The geese contributed to the problem. IIRC one goose poops once every 10 minutes, and contributes 1# of poop per goose per day. Pretty rich in nutrients too.

Must have lost a very large percentage of fish. A friend came by yesterday with his 2 young sons and they couldn't even manage to catch a BG, which used to be almost a certainty.