The culture of the Scylla spp (mud crab) is still at its infancy stage. While declining wild supply has resulted in the price increase, there has been an ever-present need to improve the yield of production. One of the major barriers that limit the aquaculture production of mud crabs is the high mortality. In this blog post, we will be discussing the top 5 reasons for mortality and how to overcome these issues.
One of the highest mortality for the mud crab trade occurs after harvest, and not during the aquaculture process. This is because mud crabs are typically transported without water to reduce the freight cost. Mortality rates will usually increase according to the duration of travel. It is not uncommon for mud crabs to be in transit for 1-2 days, as they pass through multiple supply chain, customs clearance, and grading processes. Typical post-harvest mortality throughout a 2-day supply chain will yield up to 20-30% mortality. Temperature fluctuations also play a crucial part in logistic mortality. Hence, it is important to pick airports with proper storage facilities to avoid mud crabs being exposed under the hot sun for an extended period. One of the proven methods of reducing mortality during the supply chain is to bring the farm directly to consumers. Although mud crabs can last for 1-2 days without water, it is not recommended. It is a common practice in the industry to have holding stations akin to pitstop for crabs throughout the supply chain for rejuvenation.
Mud crabs are also notorious for cannibalism due to their aggressiveness. To grow mud crabs will require moulting, this process makes them vulnerable to predation by other species. Due to the asynchronous nature of moulting, it is unavoidable for some of the crabs to be cannibalized by other species. It is important to mention here that different species have different aggression levels that will affect cannibalism. In traditional pond culture, it is common to reduce the stocking density to reduce the chances for cannibalism. During the hatchery and nursery stage, fake seagrass is sometimes utilized to provide shelter for the crabs during the moulting stage. For the grow-out stages, some operators choose to used tires and coconut leaves to provide shelters to reduce mortality. The vertical crab farming system is also made to overcome this issue faced by traditional pond methods. Placing individual crabs in boxes reduces mortality due to cannibalism. In return, the usage of vertical farming boxes reduces the land requirement needed and making it ideal for locations with a high land cost.
Another factor responsible for the high mortality faced during mud crab aquaculture is poor water quality management. This affects both farms operating in traditional ponds and recirculating aquaculture systems. While many of these issues are due to poor system design, failure to execute can also result in high mortality. Stakeholders and investors need to understand the technical challenges in designing and operating a profitable farm. One of which includes running a small scale pilot to fine-tune operational aspect. Other methods include joining courses available on water quality management, recirculating aquaculture systems, etc.
One of the most neglected aspects that contribute to mortality is due to diseases. Most commercial farm operators have a weak grasp of disease that affects mud crabs. Mud crabs are known to carry diseases like WSSV and vibrio to species like L.Vannamei. Similar to shrimps, mud crabs can also be affected by these diseases resulting in huge mortalities. Furthermore, hatcheries producing crablets at the moment do not produce Specific Pathogen Free crablets. Wild broodstock sources for hatcheries often contain a wide variety of diseases that might be passed onto their offspring. Another factor that contributes to the outbreak of diseases is often due to poor feed management. It is common for farm operators to overfeed due to the overestimation of survival rates. The accumulation of uneaten feed will often result in the build-up of pathogenic bacteria that affects the crabs as the crabs are bottom feeders. These problems can be controlled with adequate pond management strategy and disinfection procedures in recirculating aquaculture systems.
Mud crabs are known for their commercial values, fetching prices up to 20-25 USD/KG at peak periods. For the areas with poorer security, it is common for mud crabs to be stolen from the culture ponds. However, it is common for operators to write these losses off as mortality. Nevertheless, crop theft can be controlled more effectively in fully indoor facilities.
In this blog post, we have shared a couple of issues about mud crab aquaculture. The stakeholders and investors must understand the risk components of aquaculture and invest in proper mitigation techniques to reap the maximum benefits.