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The Role of Oxygen in Aquaculture

Our friend and renowned Aqua Culturist Leslie wrote this article about oxygen. You can mail him here if you have any questions about Aqua Culture in particular.

It is common knowledge that oxygen is an important ingredient in all forms of aquaculture, but the degree of its' importance is often poorly understood and over simplified. As such many aquaculture systems fail to perform at optimal levels, or even suffer loss of stock, due to insufficient dissolved oxygen (DO) being present in the water.

Oxygen is required by fish for normal physiological functioning at the cellular level. Oxygen is taken in at the gills by passive osmosis across the gill filaments. Blood is supplied to the fine extremities of the gills in a counter-current manner so as to enhance the efficiency of the oxygen uptake. As the heart pumps blood around the body the oxygen is taken up from the blood by the cells. If the level of DO in the water is slightly insufficient for the requirements of the fish, the healthy functioning of the body the fish is compromised: growth stagnates and natural immunity is reduced. Should the DO levels continue to fall the fish will ultimately die.

The concentration of DO present in water is affected by various factors, including altitude above sea level, water temperature and consumption rate, an increase in any of these factors reduces the amount of oxygen present in the water. A system that is designed to support a certain density of fish at sea level may not function as well if the water temperature, altitude or stocking density is increased. Furthermore, different species have different sensitivities to the DO concentration for example salmonids typically require minimum DO levels of around 6.0mg/L and tilapia require a minimum of 5mg/L. Now, the oxygen holding capacity of water at 16C (trout) is much higher than the oxygen holding capacity of water at 28C (tilapia), assuming the same altitude. Furthermore, tilapia are commonly stocked at higher densities than trout, increasing the demand for oxygen in a tilapia production facility. As roughly 250g of oxygen is consumed for every kilogram of feed eaten by the fish, the system design needs to take all these factors into consideration to ensure sufficient oxygen is available for the optimal growth and health of the fish being cultured.

Stocking densities in earth ponds and cages are often kept well below critical levels, and as such oxygen shortage is a low risk, but the downside is that the infrastructure is being underutilised. If the stocking densities are increased to optimal levels, then the risk increases correspondingly and a plan must be made to ensure DO levels remain optimal. The higher cost (capital and operational) of a recirculating system necessitates that the fish be stocked at higher densities to offer an acceptable ROI (return on investment), and DO levels again need to be actively maintained at acceptable levels.

Oxygen enters the water at the surface and through the photosynthesis by plankton and plants during the day. If the water is clear, as is the case with most cage culture and Recirculating Aquaculture System (RAS) situations, there are no algae to produce oxygen by day. Within earth ponds the algae require light to photosynthesise, and respire at night, adding to the oxygen demand within the system. Algae can therefore clearly not be relied on as a source of oxygen. At low to medium stocking densities water exchange or aeration are adequate for maintaining acceptable DO levels, and these methods are typically employed in raceways, and cages and earth ponds. Within a RAS the temperature of the water is controlled (usually heated) to optimal levels to promote rapid growth of the stock. Large water changes, such as are required to maintain DO levels, are not practical as this means the replacement water must be heated (or cooled) to the appropriate temperature, and this is an expensive process. Aeration is therefore used to maintain oxygen levels within RASs stocked at low to medium levels (<25 - 50kg/m3 - species dependant). Once densities of fish exceed the level where aeration is adequate to maintain DO levels, gaseous or liquid oxygen is added to the system to maintain appropriate DO levels within the water.

In a well designed RAS the DO in the water flowing into the tank will be high enough so that, given the exchange rate of water through the tank, the DO in the water leaving the tank will exceed the minimum acceptable level for the species being cultured. Furthermore, the biological filter needs to be properly designed and scaled such that the effluent water DO well exceeds 2mg/L at all times. A system that is properly designed will therefore ensure that the levels of DO in the water around the fish remain optimal at all times, promoting rapid growth and immunity of the fish being cultured.

Thanks Leslie!

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