According to Gray (2010), waters that have CaCO3 concentrations
lower than 24mg L−1 (about 0.5mM) have lowalkalinity. The alkalinity
in the present experiment was in the range 0.4–0.5 mM, an alkalinity
range that has not been used earlier in carbon dioxide experiments to
our knowledge. More severe effects on the growth of salmonids have
been found at low alkalinity compared to high alkalinity according to
Fivelstad (2012). Since the present experiment only lasted for about 5
weeks, a longer duration might have increased the effects (Colt, 2006).
Carbon dioxide may bemore toxic than observed if the oxygen concentration
is decreased (Alabaster and Lloyd, 1982); however, in the
present investigation, the oxygen saturation (above 75%) was close to
levels recommended for fish farms. The overall reduction in condition
factor and weight in the medium, high and very high carbon dioxide
group in the present investigation, can probably be related to reduced
food intake (Smart, 1981).