Carbon dioxide is another factor that may affect feed intake
but it was not measured in the experiment. Nile tilapia can
tolerate CO2 concentrations above 20 mg L−1 (Wedemeyer,
1996) and CO2 is unlikely to have an adverse effect on fish in
intensive culture systems unless its concentration reaches
100 mg L−1 (Balarin and Haller, 1982). In our experiment,
CO2 was unlikely to affect feed intake and growth of the fish
because: (1) the densities of the fish in the experiment (lower
than 12 kg m−3 initially) were much lower than the recommended
densities of Nile tilapia in the intensive systems
(20–25 kg m−3 initially for fish of 20–250 g) (Rakocy, 1989);
(2) accumulation of CO2 in the system was counteracted by
letting the water go down through the perforated trays on top of
the trickling biofilter and by diffused aeration in the sump (see
Huguenin and Colt, 2002; chapter 12); and (3) pH in the
experiment was high (around 7.6) and thus toxic gaseous CO2
must be low. The mole fraction of gaseous CO2 at pH of 7.6 is
only 5.6% (calculated based on Henderson–Hasselbach equation;
Boyd, 1990). Therefore, the contrast between the DO
Carbon dioxide is another factor that may affect feed intakebut it was not measured in the experiment. Nile tilapia cantolerate CO2 concentrations above 20 mg L−1 (Wedemeyer,1996) and CO2 is unlikely to have an adverse effect on fish inintensive culture systems unless its concentration reaches100 mg L−1 (Balarin and Haller, 1982). In our experiment,CO2 was unlikely to affect feed intake and growth of the fishbecause: (1) the densities of the fish in the experiment (lowerthan 12 kg m−3 initially) were much lower than the recommendeddensities of Nile tilapia in the intensive systems(20–25 kg m−3 initially for fish of 20–250 g) (Rakocy, 1989);(2) accumulation of CO2 in the system was counteracted byletting the water go down through the perforated trays on top ofthe trickling biofilter and by diffused aeration in the sump (seeHuguenin and Colt, 2002; chapter 12); and (3) pH in theexperiment was high (around 7.6) and thus toxic gaseous CO2must be low. The mole fraction of gaseous CO2 at pH of 7.6 isonly 5.6% (calculated based on Henderson–Hasselbach equation;Boyd, 1990). Therefore, the contrast between the DO
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