The carbon dioxide concentration for the very high carbon dioxide
group was higher during the last part of the experiment compared to
the first part, and the specific growth rate was correspondingly reduced.
Concentrations above 30 mg L−1 should clearly be avoided even for
short periods. The main challenge to fish exposed to elevated CO2 is
the composition of the plasma, consisting of high bicarbonate concentration
and lowplasma chloride concentration. The change in blood plasma
composition may give rise to important questions regarding fish welfare
and to what extent changes in the plasma composition can be accepted.
It should also be noted that some fish grew well even at high concentration
(38 mg L−1) and there may be genetic and phenotypic variation in
howfish cope with this situation (Byers, 2008). Further experiments are
needed to see if CO2 tolerance is heritable.