Pond Boss Forums INTRO Questions & Observations fish kill after treatment for weed control

I had my pond treated for the first time this year to control surface weeds and some bottom rooted weeds. I hired a well known local company who have been in business for a number of years. As I understand they treated the pond with copper sulphate and Sonar. This treatment was accomplished with a 6 wheel amphibious vehicle with a small outboard motor because there is no way to easily launch a boat in the pond. The point of entry into the pond at the shallow end is heavily silted. They fired up the outboard and stirred up a huge cloud of silt as they made their way to deeper water. Other than the muddied up pond there were no apparent problems with the treatment. The surface weeds died within 2 or 3 days. Hard to tell about the bottom weeds because the pond was muddied up. They returned 2 weeks later and retreated with the same chemicals. They also muddied up the pond again which had still not fully cleared up from the first time. I arrived the day after the 2nd treatment to find a muddy pond with 4 dead grass carp in the 17lb. range, 2 dead channel cats in the 4 lb. range, and a couple of dead bluegill, probably last years babies. I tried to call the company but they are closed on the weekends. I ended up losing all 6 grass carp and a couple more bluegill over the next 3 days.
When I finally talked to the company and explained what was going on they agreed to come out to test the water to find what caused the kill. The tests showed no overdose of chemicals but a very low oxygen level. I have never had an oxygen problem in the pond even during a serious drought, with very low water levels a couple years ago. They stated that the low oxygen would kill the larger fish first when I asked why no bass and very few bluegill had died. My question is whether the heavily muddied up pond could have caused the low oxygen level resulting in the fish kill? It has been about 3 weeks since the last treatment and the pond is just starting to clear back up. I think the first round of muddy water stressed the fish without killing them and the second dose of muddy water pushed them over the edge. Does this make sense?

Dieing vegetation pulls oxygen out of the water as it decays. DO sags tend to be at their worst just before sunrise. Treating aquatic vegetation always poses a risk of a DO sag once the weeds start to die off, an aeration system can help mediate that risk.

With just a small number of fish that died I think you should be OK.

The death of the plants could have caused an 02 crash. One of the experts is sure to give you a better answer. When I applied chemicals to my pond to treat Elodea I was advised not to treat more than 20-25% of the pond at one time to avoid an 02 crash.

If it makes any difference there was not a huge amount of weeds at the time of the treatment. Very few rooted weeds and maybe 5% or less of the total surface area covered in floating weeds.

Sooner or later, we all have fishkills. It sounds like yours was not too horrific.

Do you think the muddied up water was a factor or not?

When you use chemicals you not only kill the weeds you can also kill plankton. Even though they are microscopic they will still use oxygen as they decay. Plus you no longer have them in the water making oxygen during the day.

The low oxygen was due to the oxygen demand placed on the pond by the dead and dying plant material and the lack of oxygen production through photosynthesis both through the chemical treatment as well as the muddied water not allowing sunlight to the plants.

As 'jeffhasapond' said, I too usually do about 20-25% at a time. I did mine last week with some cutrine and had excellent results...just have to be careful. I've done it myself for 3 years now with great results. Sounds like the hired guy overdid it, I don't know how big your pond is, but I would do it myself next time. I stray away from the copper sulfate at this time.

I think muddy water by itself won't cause that many problems. When I renovated the pond last year, I crowded all the fish from a 1/3-1/2 ac pond (LMB, CC, BG) into an area that was 60' x 20' x 3'-4' deep and the water was so muddy that 6" of visibility was a dream. I did run the aerator, and I didn't supplemental feed during the renovation. I lost 2 BG total. The water stayed like that for 2 months and the fish were crowded for that length of time as well.

Mark, If all your grass carp died and were all near the 20 pound mark, this was a blessing in disguise. Your GC were getting too large to be of any real value at this point. Younger, smaller GC are more effective for plant control. You should be good without the GC till rooted plants become a problem.

I think it was a combination of factors. Sue Cruz is right on. I have also seen muddy water kill fish, especially when organic laden silt is suspended. It clogs fish gills and can cause death in fish, especially older, larger fish. But, yours is most likely decaying vegetation, lack of oxygen production and muddy water together.
However, it sounds like your fish kill was minor in comparison to many I see, hear of, or investigate.

Also without knowing the exact size of your pond, you said that nearly 100 lbs of fish were killed off. Simply having too many lbs of fish biomass per acre in a pond will lead to fish kills during that "15" minute timespan where pretty much I would have to agree that a combination of many factors came into play here.

Also keep in mind that even if you havent had a problem with your pond before, that it is aging, and nutrient loading, and basically filling in every day since it was born. There are steps mentioned above to take that can essentially reverse the effects of aging, and also increase the natural carrying capacity and dissolved oxygen levels throughout your pond.