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re: Low oxygen vs. high CO2

>I think 2.8ppm is pretty low and it showed in  _very_ rapid breathing
>the SAEs ... but they still lived.  So I pose the question:  is it
>in the ABSOLUTE level of oxygen which bothers the fish or is it a
>RELATIVE to the level their accustomed level?

It's a reduction in the absolute level of oxygen.  For fish, the %
saturation curve of oxygen with hemoglobin is dependent upon the amount
of oxygen dissolved in the water.  This graph, if it makes it through,
shows the % saturation of oxygen with hemoglobin as a function of the
amount of oxygen dissolved in water:

    100|            *  * * * * *  X ppm CO2    
       |          *           
       |        *    * * * * * *  2X ppm CO2        
       |       *  *             
 %  50 |     *  *          
       |    *  *            
       |   *  *             
       |  *  *                 
     0 |__*_*_____________________
       0          5            10
			ppm O2

CO2 can reduce the oxygen affinity of hemoglobin, which is why I drew
two curves.  The top curve shows the percent oxygen saturation at an
arbitray level of CO2, X.  Note 100% saturation occurs at about 6 ppm
oxygen, and drops off sharply below 6ppm.  Death would be expected to
occur below about 3 ppm oxygen.  At twice the level of CO2 (lower
curve), 100% saturation is never reached because of the reduced affinity
for oxygen.  Instead, about 75% saturation is the maximum at about 6
ppm, and drops off sharply below that.  This phenomenon is known as the
root effect.  Death would be expected to occur at a slightly higher
level of oxygen than the upper curve.  Also, if the peak activity of a
fish requires 80% or higher oxygen saturation, then they could go
anaerobic pretty quickly, and crash.

Note that different species of fish have different saturation curves.
Some fish have relatively flat curves, and need high levels of oxygen to
achieve maximum saturation (e.g., rainbow trout), while others have a
fairly steep curve and can achieve maximum saturation at much lower
oxygen levels (e.g., catfish).  The presence of CO2 affects them all,
and shifts their curves to the right, requiring higher levels of oxygen
to achieve the same % saturation.  At high CO2, 100% saturation never
occurs, and very high levels of CO2 can block the uptake of oxygen
altogether, regardless of how much is in the water.  Fish can suffocate,
even in oxygen-saturated water, if CO2 is high enough.

Any blood chemistry or animal physiology reference covers this is much
greater detail.  15 ppm CO2 is a safe level for our finny friends, but
definitely exacerbates low oxygen problems.  I suspect SAE's have a
fairly flat saturation curve, and require relatively high levels of
dissolved oxygen to achieve maximum saturation, and/or they may be more
sensitive to CO2 than most fish.