Good water quality is quintessential for the success of mud crab aquaculture. Mud crabs require water in good quality and quantity to ensure high survival rates and fast growth. Water quality in the traditional pond farms can be difficult to manipulate at times due to the large amount of water. The parameters are also highly dependent on weather patterns and the lunar cycle. However, with the adoption of recirculating aquaculture system, these water parameters can be managed more efficiently. In this article, we will be covering the top few water parameters that you need to be mindful of when it comes to mud crab farming.
#01 - Salinity
The first parameter of concern is the salinity of the culture water. Although mud crabs are known to be euryhaline, long term exposure to non-optimal salinity will impact growth. The reason for maintaining an optimal salinity range is to reduce the energy requirements for osmoregulation. You can find out more on salinity on the blogpost here. Scylla serrata and Scylla paramamosain usually prefers a higher salinity (15-20ppt), while Scylla olivacea and Scylla transquebarica prefers a lower salinity (12-17ppt). We tend to operate the farm at 15-16 ppt when housing different species in the vertical farming boxes. For those operating a hatchery, salinity greater than 28 ppt is usually required for the brood-stock. Should you exceed the required salinity for crabs, you can opt to dilute the saltwater with freshwater. To increase salinity, you can choose to add sea salt or seawater.
#02 - Temperature
Temperature is also important for mud crabs as it regulates metabolism. Unlike humans, mud crabs are cold-blooded and are unable to regulate their body temperature. Hence, their metabolic activity is dependent on the temperature (high temperature equates to high metabolic activity while low temperature equates to low metabolic activity). A tropical species, optimal temperatures for mud crabs are about 27-31 degrees celsius. It might be difficult to maintain the temperature at these optimal values for traditional ponds due to the temperature difference in the day versus night time. Typical daylight and night time temperature differences in earthen pond culture systems may sometimes exceed +/- 5-7 degrees celsius. Any fluctuation beyond +/- 7 degrees will have adverse effects on the cultured species. For recirculating aquaculture system, temperature differences are in the range of +/- 2-3 degrees celsius providing a more stable environment for mud crab farming.
#03 - Dissolved Oxygen
Oxygen is also another important element in ensuring the quality of the cultured water. Akin to temperature, metabolic activity is governed by oxygen availability. The abundance of oxygen will provide the crabs with the fuel to burn calories from the acquired feed for energy or growth. You can find out more about the importance of dissolved oxygen for mud crab here. Typical dissolved oxygen concentration should exceed 4ppm for ensuring good survival rates for crabs. Should your dissolved oxygen values reduce below 3-4ppm, you might want to consider adding aeration into your ponds or recirculating aquaculture systems. Paddlewheel systems are typically used for the traditional earthen pond cultures. Recirculating aquaculture system operators might choose to use blower aeration, venturi aerations or even an oxygen concentrator for adding oxygen.
#04 - pH
pH represents the acidity of the water with values >8 being alkaline, and <7 being acidic. Mud crabs thrive in pH values of 7 - 8. An important point is that the pH scale is a logarithmic scale instead of a linear scale (pH value of 5 is x10 times more acidic than pH value of 6). It is also important to point now that ammonia toxicity is a function of pH whereby higher pH value will result in higher toxicity. Good quality seawater usually has alkalinity values greater at 250ppm with a pH of 8-8.2 at a salinity of 30ppt. Therefore, a thorough inspection of the quality of seawater should be done during the site selection to minimize the need for intervention. Typical strategies for correcting the pH involves the usage of lime in the pond preparation stage, or quicklime and hydrated lime during the culture stage (for fast correction). For recirculating aquaculture system, sodium bicarbonate is preferred due to the rapid dissolution.
#05 - Ammonia, Nitrites and Nitrates
The last parameter to focus on is the levels of ammonia, nitrates and nitrates in the systems. These nitrogenous compounds are a by-product of the metabolic process and should be kept as low as possible. Permissible values of ammonia should be less than 0.5ppm, nitrites 5ppm, and nitrates at 200ppm. Different strategies can be employed when it comes to ammonia removal. For earthen ponds, the frequent tidal exchange is practised to discharge the waste. While in recirculating aquaculture systems, biofilters are used to process and recirculate the cultured water. You can find out more on the ammonia chemistry and removal techniques for recirculating aquaculture systems here.
Practicing good water management is a precursor in being successful at mud crab aquaculture. We have provided a rough guide on the important water parameters to look out for its remedial strategies.