Pond Boss Forums INTRO Questions & Observations Fish kill

Hi there,

A bit of background:
I have about a 3mL pond (we say dam in Australia). The dam is at least 20 years old and was stocked with Murray Cod at least 15 years ago. I have lived here for over 2 years and the dam has always been very healthy. The cod must be breeding because there are, or were at least, a small population still resident.

I stocked it with about 300 rainbow trout fingerlings about 2 months ago. This was probably a mistake because the fingerlings would probably end up being cod food.

Anyway, it is the middle of summer here in Aus and we have had some unusually hot weather for a extended period. About 3 or 4 days ago, we had quite a big thunderstorm come through with a fair amount of rain. A day or so after this I found 10-15 dead cod floating on the surface or slightly below around the banks. All had flared gills with wide open mouths. I have seen more dead each day since.

From everything I have read, it points to 'turn over' causing oxygen depletion.

The water appears to be black and smells stagnant (dead fish are probably contributing to the smell) and seems to be devoid of life. There are still insects but there seems to be no frog activity any more either. There are still plenty of aquatic plants.

There are no signs of any dead trout, so either they already all been eaten or they managed to survive...

So my questions are:

Could the introduction of the trout caused this?

Does it seem likely that this is an oxygen related kill or some pollutant?

Would a frog population be affected by low dissolved oxygen?

Is my dam dead now, will it recover?

If it is not a total catastrophe, how long will it take to recover?



Thanks for any and all help!

Welcome to the Forum! My first thought is to get a water test on that pond water. Then the next step can be discussed.
The Fish

Sounds like a classic fish kill due to a turnover.100 percent probability in my book! Especially when you mentioned a previous thunderstorm and hot weather. Testing the water would be a waste of effort and money. No offense meant Dwight.

Trout would have no bearing on it. The trout that were not eaten probably perished earlier due to the cooler deeper water being anoxic. They can sink to the bottom and not be seen.

Oxygen related but some hydrogen sulfide ( the odor). Anoxic water that was trapped in deepwater came up due to mixing from the cooler rain of the thunderstorm. It overwhelmed what little oxygen you had in the hot surface water.

Frogs most likely nof effected.

You may not have lost all your fish. The largest fish are the most susceptible to low D.O. and sometimes smaller fish survive.

Recovery depends on severity.

Only way to prevent this a bottom aeration system that prevents stratification. Either that or emergency standby surface aeration.

+1 to what Cecil said.

You mention the pond is 3 mL in size. I assume that means 3 million liters or about 2.4 acre-feet. What's the approximate surface area and depth? Do you have a lot of muck on the bottom?

When you say hot weather, how hot did the pond water get? The trout may have died do to water temp.


Welcome to PBF!

Thanks for the welcome and the replies.

Bill, yep 3 megalitres (3,000,000 litres). I just measured the dam on google maps and its surface area is around 1,430.54 m² (15,398.24 ft²). It would be around 6-7m at it's deepest. The bottom is settled sediment/rock.

I'm not sure what the temp got to, I accept that the trout are probably dead.

I agree that it is almost certainly turnover, but my biggest concern is that at night there is no longer any frog noise. It used to be loud and constant all night.

Frogs are a supposed to be a good bio-indicator for the health of your pond/dam.

Would a sudden drop in pond health due to turnover drive frogs out? Or is something more sinister going on?

Also, in 15-20 years surely this sort of thing would have happened before? How rare is would this kind of turnover be?

Thanks again!

Is it possible your fish kill has attracted scavengers that also prey on frogs? Did you have a significant air temp drop after the thunderstorms or some other change in weather? If so it may have quieted down the frogs by reducing their metabolism.

Not rare at all to have the likelyhood of a fish kill go up as a pond ages and eutrophies.

Thanks again Cecil.

I don't think there would be any more predators that normal. There are always a few snakes around.

The temp drop wasn't that significant I guess. But it was hot and cloudy and still leading up to the thunderstorm.

Another observation is that my neighbours dam (upstream of ours on the same creek) has not had the same thing happen. Our dam is significantly deeper than theirs though. There is still plenty of frog activity on the neighbours dam.

I can't seem to find any definitive info on the effects of low DO on frogs. If I could say that with certainty, I would happily accept that turnover was the cause and move on

Testing the water is cheap. Do it for your own peace of mind. Sorry Cecil!!

No apology needed Dwight.

I am not a pro and hopefully one of the aeration pros will chime in here. My understanding is that the bottom water of an older pond without adequate bottom aeration can be toxic, not just low in DO, so when it comes to the surface this water does more than just lower the DO of the overall pond. You mentioned a bad smell which could be hydrogen sulfide. I would suspect your frogs did not appreciate the reduced overall water quality so they just moved to the stream and possibly your neighbors pond. IMHO they will be back when the water gets back to normal.

I do recommend installing bottom aeration, if it is possible in your situation, to help prevent a recurrence.

I agree on the oxygen crash but agree with Dwight about the test. Get it done quickly before everything returns to normal.

The odd thing is the difference in the neighbors pond. Maybe his wasn't deep enough to have a significant thermocline.

Sounds plausible to me Bill and I'm not a pro either. As far as I know their skin is very permeable so you may be on to something.

Hopefully someone will chime in.

What's the test going to show potentially?

If you're leaning towards pesticides or herbicides that may have washed in exasperating a turnover, that testing isn't cheap and isn't always conclusive. And if they do show up in the tests what's he going to do about it?

Hey guys I just disagree. I could be wrong!

Cheers!

All your observations point toward a pond turnover and DO loss and toxic conditions from the hydrogen sulfide (H2S) (sulfur rotten egg smell) from anoxic decay in the deep water. Trout and likely cod do not tolerate even moderately low DO and both are very sensitive to H2S in low concentrations.

You mention "Our dam is significantly deeper than theirs (neighbor's) though". Shallower ponds will be able to mix from moderate to strong wind action. Depth of mixing due to wind action is dependent on amount of wind, protected vs open wind exposure, fetch (span of wind exposure), and then pond size. Evidently the neighbor's pond had significantly less thermal stratification (shallower) and no fish kill occurred due to little or no stratification. Also as ponds age they tend to be more prone to lower water quality during thermal stratification due to more accumulation of muck in the pond bottom causing more and quicker oxygen consumption (loss).

Frogs were likely stressed more by the H2S than low DO, although both were likely stressors that pushed frogs out temporarily. If you can smell H2S without aeration bubbling action the H2S is pretty concentrated in the water and highly stressing fish. Almost always H2S water has no DO unless the water has been diluted with better quality water.

Awesome replies guys. Thank you all so much. I might investigate a water test but I feel it's more than likely an oxygen crash.

One thing I didn't mention is that there are still plenty of turtles present.

I will need to come with some sort of aeration system. It would need to be a solar solution as we are completely off grid... Any hints would be appreciated...

I was thinking something like this

I suspect the turtles are still around to feed on all the easy catch dead fish.....

Interesting approach in the link you posted for solar. I am not sure how well it will work in a pond 22 feet deep pond though. FWIW There are lots of ways to go IMO when it comes to solar. Turn key setups designed for your pond like this:

http://www.vertexwaterfeatures.com/britestar-solar-aeration

or DIY where you put it together yourself. A turn key will be more expensive up front but may save you money in the long run if you're not a DIY kind of guy.

+1 to what Cecil said. My thinking is there were no problems before the storm and most probable turn over. The toxic bottom third of the water in this "old" pond came to the top when conditions were right. Proper aeration could restore the pond and make that bottom layer useable habitat.

Low dissolved oxygen won't hurt the adult frogs so they're not the best canary in the coal mine.

I much prefer DIY too, this was more of a starting point. I was thinking of anchoring a submersible pump 0.5 - 1 metre off the bottom with a floating solar panel above it that would be tethered by the cables and a flexible outlet hose. I would have enough slack in it to allow for rises and falls in water level.

I'm not sure if this will be adequate but might be a good starting point.

Water pumps are a lot less energy efficient to move water than air compressors. Evaluate the gallons of water moved per kilowattfor each system, then purchase. Your relatively small unaerated pond(0.35ac) at 6-7meter deep(19-22ft) may not have had any oxygen deeper than 2-2.5meters(6.5-8ft), and from there to the bottom was all "bad" water. Calculate that deep volume and see what that percentage is of the upper 2 meter(6.5ft) surface layer/volume. Also Once you know the volume of potential anoxic water then you have an idea of how much water needs to be moved per day to replenish the average, annual anoxic layer.

My only concern with air compressor is noise. Should only hear the splashing of water with a submersible pump.

Food for thought...If noise is your only concern then locate the compressor and solar panel a couple hundred meters from the pond in an appropriate housing. With a correctly sized air line, the frictional losses are minimal and there will be no noticeable noise at the pond other than the sound of bubbles coming up. Also, no unsightly apparatus on the surface of the pond.

Just my 1 cent

The Vertex air compressors, aren't loud, and when you get the sound deadening kit, they are even quieter.

A HOA had that same concern, with two 3/4 hp compressors in the same cabinet, you can't hear it over the wind in the cattails.

Hi again,

Fish are still continuing to die. Is it normal for a turnover related kill to continue for a week or so?

Cheers.

When fish die, sometimes they don't float right away, it takes time for the internals to build enough gasses from decomposition for them to float. Also when fish get stressed, they all don't die at the same time, some could take a few days to succumb.