3. Results
Temperature varied between 28.4 and 30.9 °C during the experiment.
Water pH was 8.4–8.5 throughout, whereas nitrite and
ammonium increased slightly in the course of the experiment (from
0.005 to 0.012 mg NO2
− L−1
, and from 0.01 to 0.02 mg NH4
+ L−1
). At
the very end of the experiment, however, the ammonium concentration
in the system soared to 0.22 mg NH4
+ L−1
, but no mass mortality
was observed by then. The oxygen level remained stable (around
7.6 mg O2 L−1
) throughout the experiment in the tanks with the
lowest prey density (10 RFL L−1
, treatment 10:1). In all other
treatments, the oxygen level declined progressively, and proportionally
to the prey density (mean levels of 7.5, 7.0, 6.3 and 5.5 mg L−1
, at
prey densities of 30, 90, 270 and 810 RFL L−1
). The lowest oxygen level
(4.4 mg O2 L−1
) was observed at 192 hah in a tank of the 90:9
treatment but no mass mortality was observed on this day