Mud-crab or Scylla sp. is a type of aquatic invertebrate which lives in the water for most of the time. One of the important properties that fit them in the aquatic animal classification is the ability to breathe through gills that gets the dissolved oxygen in the water. Therefore, it is essential to know the ideal water quality to farm them to prevent the loss of profit. In this article we will look into one of the most important and the basic of farming this species, salinity. Salinity is the concentration of salt in the water, measuring in the unit of parts per thousand (ppt). The salinity of freshwater is 0 ppt and the salinity of sea water is 35ppt. The Scylla sp., is often farmed in a condition where we call brackish water, which the salinity is between freshwater and seawater, between 10-30ppt. The 4 different species of Scylla sp., even though are all mud-crabs, each has a different salinity that is ideal to farm them, due to the different areas and different zones that each of them dominates.
Generally mud-crabs could tolerate a wide range of salinities. In fact, they are able to survive in freshwater for a few hours, this enables us to be able to disinfect them in freshwater, killing any harmful bacteria that could only survives in saline water. The mud-crabs prefer salinity of 15-25 ppt. It is essential to understand the ideal salinity range to be able to optimize the growth and minimize the mortality of the specific species of the mud-crabs being farmed.
Scylla serrata, often known as the Sri-Lankan mud-crabs are the most popular species to be farmed due to its fast growing ability. They could be found in mangrove forests flooded with full salinity seawater all year round. Therefore, farming Scylla serrata would require water with higher salinity at 25-30ppt.
Scylla paramamosain, with obvious light-green claws, is very popular and has a high demand in China and Singapore. Often found abundantly in South China Sea and Java Sea, residing in mangrove forests and subtidal flats. Similar to Scylla serrata, they prefer a higher range of salinity compared to the mud-crab species.
Scylla olivicea, easily distinguished from their orange-colored claws, often found residing in subtidal flats with a lower salinity range of brackish water. They are commonly distributed in South China Sea, Indian Ocean and Pacific Ocean. The optimal salinity that suits Scylla olivicea is between 15-20ppt. Farming these crabs in suitable salinity and they will be aggressive and have good appetite, therefore lower the mortality rate and achieve higher growth rates.
Scylla transquebarica, distinguishable from its purple color claws is often associated with Scylla olivicea and can be found abundantly in South China Sea, Indian Ocean and Pacific Ocean. Their habitat is usually subtidal areas with a lower salinity.
Fattening the crabs in RAS requires a good fundamental understanding of the environment of the crabs to minimize the mortality. However, for hatchery operations it often involves a high salinity of 28-35ppt for the female crabs to be berried (carrying eggs) and have a higher hatching rate. All female crabs of Scylla sp. will migrate offshore to fully saline water when they are going to release their eggs. This explains why it is seldom for us to see or get wild-caught berried female mud-crabs. The berried female and the larvae stages of the mud-crabs are said to be very sensitive to the water quality, slight changes in the salinity or other water quality will cause mortality to the specimens.
For farms that are located near the coastal area, the most convenient way is to utilize the seawater by pumping it straight to the farm. Before using the seawater, it will go through a series of process to get rid of the solids and harmful bacteria as a biosecurity measure. The incoming seawater will pass through simple filter bag into a storage tank with or without UV light. Through sedimentation, the clear water on top will be used for farming. The disadvantage of using seawater is that the source could not be controlled. During monsoon season when it would be raining for a few months, the salinity might drop to less than 10 ppt and would cause stress and mortality in the farm.
For a farm located too far away from the coastal areas, it would be more feasible to make artificial saltwater from freshwater and salt. The advantage of doing this is being able to control the salinity as required. The down-side is the extra cost of the salt needed to raise to a specific salinity.
Changing just a simple basic parameter of the water quality could bring a lot of difference to the aquatic animals. Even though mud-crabs are hardy and have high tolerance to a wide range of salinities, it is important to run the farm according to the preferred salinity of the specific crab species. It is also important to note that acclimatization is important to make sure the crabs survive in the farm after being introduced from the wild to prevent them from being stressed out.