Respiratory response of grass carp Ctenopharyngodon idellus to dissolved oxygen changes at three acclimation temperatures.

Respiratory parameters of grass carp were studied during dissolved oxygen (DO) changes from normal DO to hypoxia, then return to normal DO at 15, 25, and 30 °C acclimation, respectively. The results showed that with increases of acclimation temperature at normoxia the respiratory frequency (fR), oxygen consumption rate (VO2), respiratory stroke volume (VS.R), gill ventilation (VG), and VG/VO2 of grass carp increased significantly, but the oxygen extraction efficiency (EO2) of fish decreased significantly (P < 0.05). With declines of DO levels, the fR, VS.R, VG, and VG/VO2 of fish increased significantly at different acclimation temperatures (P < 0.05). A slight increase was found in VO2, and the EO2 of fish remained almost constant above DO levels of 3.09, 2.91, and 2.54 mg l-1 at 15, 25, and 30 °C, while the VO2 and EO2 began to decrease significantly with further reductions in DO levels (P < 0.05). After 0.5 h of recovery to normoxia from hypoxia at three acclimation, the fR, VS.R, VG, and VG/VO2 of the fish decreased sharply; meanwhile, the VO2 and EO2 increased sharply (P < 0.05). The respiratory parameters of fish gradually approached initial values with prolonged recovery time to normoxia, and reached their initial values in 2.5 h at 25 and 30 °C acclimation. The critical oxygen concentrations (Cc) of fish for VO2 were 2.42 mg l-1 at 15 °C, 2.02 mg l-1 at 25 °C, and 1.84 mg l-1 at 30 °C, respectively. The results suggest that grass carp are highly adapted to varied DO and short-term hypoxia environments.