Monitoring dissolved oxygen

Dissolved oxygen (DO) is the most critical and limiting factor in intensive aquaculture. Oxygen enters water through photosynthesis by aquatic plants, principally phytoplankton, and by diffusion at the air-water interface. Diffusion occurs when waters are below saturation, and the greater the deficit between the oxygen concentration in the water and the saturation concentration, the greater the rate of diffusion. In ponds, diffusion is promoted by wind and wave action and by artificial aeration. Oxygen is lost from water through respiration by fish, plankton and other organisms, and by aerobic decay of organic matter. There are distinct diurnal fluctuations of oxygen, with concentrations lowest just after dawn, increasing during daylight hours. This is because of the photosynthetic production of oxygen, (there is also usually more wind during the day) to a maximum in late afternoon, before decreasing again during the night.

Silver perch can tolerate low levels (2 mg/l) for short periods (a few hours) but exposure to sub lethal levels (<3 mg/l) for extended periods will reduce growth and stress fish.

The Signs Fish Exhibit to Low DO

• loss of appetite
• lethargy
• gasping near the surface
• fish facing into current of inlet or aerator
• death of larger fish, followed by smaller fish

Effects of Low DO

• stress increased susceptibility to disease
• poor feed conversion efficiency
• poor growth
• death of fish and other pond organisms.

Causes of Low DO

• large blooms of phytoplankton, zooplankton and other pond organisms respiring during the night
• high stocking densities of fish and high feeding rates
• "crash" of phytoplankton/zooplankton booms
• excessive turbidity, i.e. no or limited oxygen production through photosynthesis
• series of cloudy, windless days
• combinations of the above conditions.

Low DO – What can I do?

• monitor DO routinely and chart diurnal fluctuations to predict periods of low DO
• maintain aeration day and night
• provide additional aeration
• place another aerator in the pond
• spray water across the pond surface
• use tractor PTO run paddlewheels or sprayers
• stop or reduce feeding
• exchange water
• remove dead plant and animal material from the water.