For the past few years, we have observed disease-stricken shrimp ponds (L. vannamei) that have converted to mud crab aquaculture. This is not surprising given the high price of mud crabs at the moment. However, there are still pros and cons to any given species. In this article, we will be exploring the difference between the culture of L. vannamei versus Scylla spp.
The first distinction between the L. vannamei or the Pacific White Shrimp versus Scylla spp. Or the mud crab is the prices. The Pacific White Shrimp are considered more a staple while the Scylla spp. is more of a delicacy. This is not surprising as prawns are typically consumed in your average household while crabs are occasionally consumed during the festive seasons. The average selling price for L. vannamei (50-60 count/kg) is about 5-7 USD/kg (depending on region), while the average selling price for mud crab (at 500gram/crab) is about 14-15 USD/kg. The sizes chosen for comparison are the typical size which farm operators will end their culture cycle. Bigger sized shrimps or crabs will fetch a higher price, but might not be economical to produce due to higher feed conversion and mortality.
Pricing alone is insufficient to decide between the culture species, as another factor of consideration is the crop yield. Crop yield is defined as the amount of yield (in terms of weight) that can be produced from one unit of area (typically in m2). The major factor determining crop yield in aquaculture is the space requirement needed for the particular species. This is also commonly known as stocking density which is the number of animals stocked in one unit area (m2). Here lies the significant difference between the L. vannamei versus Scylla spp. in the stocking density and crop yield. Post hatchery L. vannamei can be stocked up to 2500-3000 PL/m3 while Scylla spp. can only be stocked up to 30-50 crablets/m3. The huge difference is due to the highly cannibalistic nature of the mud crab that hinders the farm from overstocking. While the Pacific White Shrimp is also considered a cannibalistic species, they are a lot milder in comparison to the Scylla spp.
Apart from pricing and yield, another interesting factor for consideration is the risks involved in the business. For aquaculture, the risk for disease poses a grave threat to farm operators and the industry as a whole. Good biosecurity, clean water, and disease-free environment are precursors to suppress an outbreak of diseases. Nevertheless, it can be difficult to maintain or obtain due to operational and management issues. Therefore, an important aspect of disease prevention also depends huge only on the immune systems of the farmed organisms. It is generally accepted that the immune systems of the mud crabs species are much stronger than the L.vannamei. This is not surprising given the fact that crabs are known to be the carrier for diseases to shrimps ponds. The culture of L.vannamei has been impacted by multiple diseases like WSSV, EMS, Taura Virus, etc. Meanwhile, the culture of the Scylla spp. has not been known to be impacted by widespread diseases. This is not to suggest that Scylla spp. are resistant to all diseases, but diseases are not known to contribute to a global pandemic. One of the reasons for this is due to the low culturing density for mud crabs, and hence reducing the risk of disease outbreaks.
Another distinct difference between culturing both species are the feed used. For L. vannamei, commercial feeds are available for most operators. However, the same cannot be said for Scylla spp. as the industry still relies heavily on trash fish. While many research institutions have published results on commercial feed developed for Scylla spp., feed manufacturers are reluctant to commercialize due to the lower demand that other species (like L. vannamei). This problem is further aggravated by the low stocking density of the Scylla spp. Despite the higher feed conversion ratio of the mud crab versus the Pacific White Shrimp, the culture of L.vannamei has a higher percentage of operating costs associated with the feed. The amount of feed needed for a one-hectare pond production of (3-5 tons/cycle) will require 5-8 tons of feed, while the total feed needed for one production during culturing crabs (0.3-0.5 tons/cycle) is only 1-1.3 tons. Although the culture of L. vannamei is deemed to be more efficient, the risk involved rises exponentially with the age of shrimp. Shrimp farms impacted by diseases at very late stages of the culture cycles will result in severe cash flow problems.
Although pricing and yield are important aspects to any business, risk management is the key to running a profitable aquaculture venture. Understanding how diseases, climates and political changes have never been more crucial in defining your success or demise.