Capacity for thermal adaptation in Nile tilapia (Oreochromis niloticus): Effects on oxygen uptake and ventilation

Standard metabolic rate (SMR) and maximum metabolic rate (MMR) were determined for Nile tilapia acclimated to six different experimental temperatures from 18 °C to 38 °C. SMR increased exponentially with temperature, from 79.8 mg O2 kg-1 h-1 at 18 °C, to 255.1 mg O2 kg-1 h-1 at 38 °C (Q10 = 1.79). The main increase in Q10 occurred within the highest temperature range, whereas in the lower temperature from 18 °C to 22 °C, temperature did not significantly affect SMR. MMR showed a hyperbolic correlation with increasing temperature, rising from 240.5 mg O2 kg-1 h-1 at 18 °C to a peak of 373.8 mg O2 kg-1 h-1 at 30 °C, before decreasing again at higher temperatures. Absolute aerobic scope (AAS) peaked at 26.0 °C, which we conclude to be the optimal temperature for Nile tilapia. The optimal temperature range, defined as the thermal range where 80% or more of the metabolic scope (MS) can be maintained, occurred between 19.5 and 32.1 °C. The lower (TCMIN) and upper (TCMAX) critical temperatures occurred at 13.1 °C and 38.8 °C. Nile tilapia showed a 4-fold scope for increasing ventilation frequency from 24 opercular beats min-1 (OB min-1) during SMR at 18 °C, to a maximum of 100 OB min-1 which occurred during MMR at 34 °C. fV during MMR increased with temperature, but above 30 °C became uncoupled with MO2, as fish were unable to sustain their rates of oxygen consumption despite a high fV. There was a strong correlation between fV and SMR (r2 = 0.83) across all temperatures indicating that fV is a good predictor of SMR. However, the correlation between MMR and fV was weak (r2 = 0.06), due to a strong interacting effect of temperature. When selecting data from the thermal optimum range, a good correlation between fV and MO2 was obtained (r2 = 0.74).