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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.

With the frequent outbreak of diseases like WSSV, EHP, and etc, there has been a growing concern for biosecurity. Biosecurity entails methods and procedures design to protect the culture population from the pathogen. However, this is hard to achieve with the standard open pond systems where the tidal exchange is practiced as the pathogen can easily spread. As recirculation aquaculture systems are designed to minimize water usage, it significantly reduces transmission between farms.

Apart from the advantages of using recirculation aquaculture systems (RAS), there are also some noteworthy disadvantages. The most notable disadvantage is the additional capital needed for the recirculation aquaculture systems. Besides that, recirculation aquaculture systems (RAS) relies on pumps to move the water through a series of filters instead of the traditional tidal change. This incurs additional operating cost due in terms of energy usage. Furthermore, farm operators also have to be technically competent in operating a recirculation aquaculture system to ensure an optimal condition for the culture species.

While the allure of utilizing recirculation aquaculture systems (RAS) for aquaculture are great for a technophile, it is important to examine the benefit to cost ratios of these investments. With the deterioration of the environment due to pollution, the cost of traditional farming has been increasing due to high mortality. Furthermore, with the increasing amount of farm operators adopting the technology, equipment prices are likely to drop and further justifying investment.

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