Fish, oxygen and toxic gases

Click the graph to see it clearly: O2 is oxygen, CO2 is carbon dioxide, H2S is hydrogen sulphide and NH3 is ammonia

For those who like graphs here’s a nice old one which still has something interesting to tell us about the causes of what is known in North America as ‘winterkill’.

Long ago, in 1957, a fish biologist called W.J. Skidmore studied the levels of oxygen, carbon dioxide, ammonia and hydrogen sulphide in some lakes in Minnesota to assess their role in fish winterkills.

It was so long ago that in those days you could draw a graph pretty much by hand and get it published in a scientific journal – not something that would be possible now, although the graphs are perfectly clear, and actually just as pleasing to the eye as the modern Microsoft-produced version would be.

Skidmore only studied six lakes, and they were quite big, between 107 acres and 2700 acres. The shallowest was 10 feet deep. So these were not ponds.

But the results were pretty clear.

All the fish kills were associated with low oxygen levels – when it was less than 0.6 milligrammes of oxygen per litre of water.

In Cannon Lake and Lake Emily distressed and dead fish were found when oxygen levels went down from about 1.4-1.6 milligrammes per litre to 0.2-0.4 milligrammes per litre. During these oxygen declines there was no appreciable change the levels of the other chemicals.

In Lake Elysian, where oxygen was low and a fish kill had started before the measurements began, levels of the other gases were not much higher than Beaver Lake and Lake Tetonka where no kills were occurring.

At Loon Lake, high conentrations of ammonia and carbon dioxide occurred, along with 5.6 milligrammes per litre of oxygen, and fish were lively.

In these lakes it looked like lack of oxygen was the problem.


One Response to “Fish, oxygen and toxic gases”

  1. Janette Crawford Says:


    I’ve been referred to this website via Staffs Wildlife Trust following my concerns when visiting a local pond on Wetley Moors to find a number of fish frozen in the ice very close to the surface and one huge one – photo attached – actually lying on the surface. The Environment Agency responded to my concerns saying this was a natural phenomenon due to the lack of oxygen as a result of the prolonged cold spell…’s very sad.
    I also have a wildlife pond of my own and was unable to break a hole in it despite numerous attempts….I am hoping that any Southern Hawker nymphs that I found in my pond during the summer will have survived…….

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