Title: The effects of oxygen saturation and carbon dioxide concentration on the growth and feed conversion of aquaculture fish

The aim of the study was to gain knowledge on the effect of water quality on the performance of fish in aquaculture using Arctic charr as a model. The Arctic charr (251.45±18.0g mean initial body mass±SD) were reared at two different levels of O2 availability (80% and 120% of air saturation) and three levels of CO2 concentration (LOW, MED and HIGH) for 60 days. The availability of O2 significantly affected condition factor (CF), oxygen consumption and food conversion ratio (FCR) while growth rate was not significantly affected. The final size of fish reared at 120% of air saturation did not differ significantly from (p > 0.05) groups reared at 80%, suggesting that increasing oxygen saturation from 80% to 120% does not improve the growth of Arctic charr. Carbon dioxide concentration of the rearing water significantly affected growth rate, FCR, O2 consumption and CF. The final size of fish reared at HIGH group was significantly lower (p < 0.0001) than in groups reared at MED and LOW group, an indication that CO2 limits the growth of the fish. There were no interactive effects of CO2 and O2 on growth of the fish, suggesting that hyperoxic conditions may not increase tolerance to CO2 in Arctic charr. However there was a significant interaction of CO2 and O2 on O2 consumption suggesting that the effect of CO2 is dependent on O2 saturation of the system. The O2 and CO2 concentrations affected the hematology of the fish indicating adaptations to both reduced oxygen saturation (increased hematocrit and blood hemoglobin levels) and increased CO2 concentration (reduced Cl-concentration). The current study suggests that the recommended maximum level of CO2 to maintain the welfare and maximum growth of Arctic charr is between 10-20 mg L−1 with a limited advantage of increasing the oxygen availability above 80%.