Mud Crab farming (Scylla spp) with Recirculating Aquaculture Systems
Mud Crabs (Scylla spp) are considered as a brackish water species, where they thrive in salinities from 10-25ppt. Traditional farms must be located near the brackish water sources, where the tidal exchange is practiced in maintaining water quality. However, due to challenges faced in the traditional pond systems and supply chains. Many have chosen to use recirculation aquaculture systems (RAS) to overcome these constraints.
The first challenge in operating a farm away from the coast is finding a reliable source of seawater. Some operators prefer transporting seawater to their storage facilities. Other operators prefer to use commercially available sea-salt and dissolving them with the municipal water supply. The ideal method will depend on logistics, price of salt, and storage cost. For our recirculation aquaculture systems (RAS) facility, we have adopted the latter approach as we are about 70-80km away from the coast with low water usage. Hence, it is optimal to use sea salt and mixing it with the municipal water supply to achieve the target salinity.
Without human intervention, the quality of culture water tends to deteriorate over time due to feeding. Like any metabolic activity, the cultured species produce wastes in the form of ammonia that accumulates in the culture water. In traditional ponds, these accumulated wastes are expelled from the ponds via tidal exchanges. Upon the rising tide, clean water is introduced into the ponds thus displacing the ammonia contaminated culture water. Hence, this restricts the culture of marine species at areas without direct access to seawater. With recent advances in wastewater treatment, recirculation aquaculture systems have enabled the recycling of the cultured water a possibility. This eliminates the need for water replacement and hence removing the location barrier. This reduces the operating cost by reducing transportation and salt usage but requires more upfront capital for the recirculation aquaculture systems (RAS).
It is important to note that recirculation aquaculture systems (RAS) are also applied for farms located in coastal regions where the quality of seawater is not consistent. Excessive rainfall might dilute brackish water that is used for culture, resulting in huge mortalities as culture species struggle to survive in freshwater condition. Some operators have incorporated a hybrid system, where the culture water is recirculated if the quality of the seawater source is less than ideal. Apart from hedging against fluctuation in water parameters, the oyster industry also uses recirculating aquaculture systems to hedge against red tides.