Dissolved Oxygen (DO)

Dissolved oxygen (DO) is the amount of gaseous oxygen held in solution, measured in mg/L or as percent saturation against the temperature- and salinity-dependent saturation table, and it is the single most time-critical water quality parameter in aquaculture because fish cannot store oxygen and die within minutes of a severe crash. Saturation at sea level is about 9 mg/L at 20°C fresh water but only 7 mg/L at 25°C and less in saline water; warm, high-biomass systems therefore run permanently closer to the edge. Critical thresholds: most species show feed-intake suppression below roughly 70% saturation, stress responses below 50%, and acute mortality risk below 30–40%. Daily DO follows a sawtooth in ponds — rising with photosynthesis to an afternoon peak, falling through the night to a pre-dawn minimum — while in RAS the curve tracks feeding events. Continuous optical DO probes with automated alarms are standard in intensive systems. For the farmer, DO drives feeding schedules (feed is withheld or reduced during low-DO windows), aeration runtime, and stocking limits; hypoxia events cause immediate mortality plus latent gill damage that invites secondary disease. DO links to aeration, oxygen saturation, hypoxia, and thermal stratification.