Maintaining the water chemistry of your recirculating aquaculture systems is no easy feat. One of the crucial parameters to monitor is dissolved oxygen. The recirculating aquaculture system needs to ensure that enough oxygen is available for the development of mud crabs as well as the biofilter. We will discuss the importance of oxygen and the equipment needed.
Mud crabs require oxygen for metabolism. Metabolism is a generic term for chemical reactions that digest food with oxygen to produce energy. The energy is stored in the form of ATP ready to be used. Therefore, recirculating aquaculture system needs to have enough oxygen for their metabolic activity. Inadequate oxygen in mud crab boxes will result in mud crabs surfacing to breathe. Other consequences include reducing feeding rate, slower growth, increased susceptibility to diseases, and even death. Mud crab uptakes air/oxygen through their gills, and they are only able to breathe on air for a certain period (Mud crabs are not amphibians). It is common to estimate the oxygen needed in recirculating aquaculture systems by estimating the intended feeding rate, followed by the associated oxygen requirement.
It is also important that the recirculating aquaculture systems (RAS) have enough oxygen for ammonia removal. Biofilters are one of the treatment equipment that removes ammonia from the culture water. Ammonia is highly soluble in water and originates from nitrogenous waste. Fish and crabs expel ammonia through gill diffusion, gill cation exchange, and urine and feces excretion. The biofilters utilize microbes to clean up the excess ammonia in the water and converting them into less harmful components (nitrite and nitrate). The microbes also require oxygen for the conversion process. In some recirculating aquaculture systems like (BFT) Biofloc Technology, oxygen requirements by the microbes can reach up to 50% of the total available oxygen. The microbes will not convert ammonia due to insufficient oxygen, which will result in mortality for the crabs. The conversion of ammonia to nitrates requires a 2-step process where microbes first convert ammonia to nitrite, and different microbes convert nitrite to nitrate. Converting nitrite to nitrates will require more oxygen. The oxygen requirement by the biofilters can be estimated by the microbe's population which depends on the total ammonia nitrogen. It is important to take note that increasing feeding rates would result in lower oxygen availability due to the respiration by the microbes.
Having understood the importance of oxygen for mud crabs and recirculating aquaculture systems, it is important not to overcrowd your facility. Should you need to do so, we strongly advise on increasing the amount of oxygen available in the system. It is now common to directly add pure oxygen into the culture with the means of an oxygen concentrator. Using pure oxygen instead of air increases the saturation points beyond normal conditions. However, this method is not quite popular for mud crab aquaculture. Other methods of aeration include venturi aerators that use a pump to mix the liquid with air. Traditional methods would also include using a blower with air-diffuser configurations for aeration. The suitable aeration will depend on a multitude of factors like aeration depth, fouling, and efficiency. Paddlewheels are commonly a good option for those operating in an earthen pond. Algae cannot substitute the need for aeration equipment, as plants also uptake oxygen during the night. Typical values of dissolved oxygen for mud crabs should be 4-5 ppm, while the biofilters would need more than 4ppm. If your current values are lower than the 4ppm, we highly encourage you to start exploring the need for additional aeration.
Oxygen is one of the most important water parameters for mud crab aquaculture, but many will tend to overlook this issue during the design stage. While many publications exist to help determine the exact oxygen requirement, it is still common to address the issue during production. This often results in unnecessary losses during the initial start-up stage.