Crab farming serves as a natural bounty for numerous countries. Crabs are considered among the most important sea food due to their esteemed delicacy and medicinal value. In addition, there is growing demand for crab in the export market. As such, crab farming is a highly lucrative agricultural venture. Crabs generally command a higher market price and are more tolerant to varied environmental conditions. They also have a higher tolerance to diseases, hence are less prone to losses. Consequently, crab farming has increased significantly over the past few years. In fact, countries like Bangladesh have witnessed an improvement in the socio-economic status of their coastal communities due to crab farming. Note that in order for the agricultural venture to be a success, farmers should be equipped with necessary knowledge and skills to efficiently practise crab farming.
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Site for Crab Farming
The location of the crab farm is an important aspect. The site should be selected based on the method of farming. Crab farming can be practised extensively, mainly relying on photosynthetic processes and self-regulating systems. The second type of aquaculture is intensive, and so the system is artificial and relies on feeding and filtration systems. Although extensive crab farming has very little environmental impact, it should ideally be practised in large tracts of land. The opposite applies to intensive crab farming. It generally requires limited space where it is practised at greater densities. The most appropriate site should have a readily available water source. Note that if water is neither treated nor re-used onsite environmental degradation can occur. Crab farming thrives in brackish water; this is basically water that is saltier than fresh water, but not as salty as sea water. This can sometimes be achieved by mixing fresh and sea water. Examples of brackish water include tidal flats, estuarine areas, bays and lagoons. It is recommended to choose areas with low salinity as saline water inhibit the growth of crabs. Areas selected should also allow efficient fattening of crabs. The site for crab farming ought to be sheltered from strong winds and waves, particularly during adverse weather patterns. Below are the most common sites for crab farming:
The pond size should be about ½ to 1 acre; size can be altered depending on the extent of the agricultural venture. The ideal land comprises of 50% sandy soils and a similar percentage for clay soils. Note that the pond should be able to hold 3 ½ to 4 feet of water towards the inlet and approximately 4 ½ to 5 feet on the outlet. The site should allow a flow through mechanism of water exchange in order to remove leftover organic food material as well as excretory material. Crabs can sometimes escape, especially during night time and so nylon fencing can be erected as a preventive measure. In addition, fencing must be supported with split bamboos of about 1 to 5m in height so as to keep crabs from climbing over the fence. The maximum stocking density should be 1 crab per sq. meter. Several units of pens of 4 X 4 X 2.5 m can be constructed inside the pond using bamboo strips which are driven approximately 1 to 1.5 m deep into the soil to prevent the escape of the crabs by burrowing. It is recommended to build pens nearer to the dykes for easy stocking and monitoring.
Cage systems for farming crabs are usually made up of cells which are roughly 1m in length with 1m width and 20cm in height. The cells are often partitioned into 9 equal compartments. Farmers should make sure that cages have lids in order to prevent escape. In addition, a gap of 5 mm is to be provided between the canes at the top and 2.5 cm at the sides of the cages to enable free movement of water through the cages. The cage system for crab farming are usually suspended from a raft, backwaters or mangrove areas among others. Note that some crabs have cannibalistic tendencies hence placing them in cages without compartment can lead to a significant loss.
There are various types of crab breeds including the blue crabs, king crabs and hermit crabs. They mature at different sizes and have different feed requirements. For this reason, it is of the utmost important for farmers to be aware of various types of crabs and their requirements before practising crab farming. Crabs reproduce upon reaching maturity. The growth rate of abdomen width or length with respect to their carapace is generally much higher in mature females than in the immature crabs. Blue crabs reach maturity at approximately 12 to 18 months when they are about 12cm wide. . The benefit of blue crab farming is that one female can produce up to 2 million eggs. On the other hand, king crabs mature between 4 to 7 years of age. The upside of breeding king crabs is that they have a longer life expectancy of about 20 to 30 years.
Feed and Nutrition
Crab farming is centred on the health and survival of crabs hence the need for a well-balanced nutritious diet. The dietary plan should consist of good quality mixed sea food, a formulated brood stock diet that supplies balanced nutrition to the stock, or a mixture of both. Sea food that make up crab diets include squid, fish, bivalves, marine worms. It is advisable to provide them with a supplementary diet that contains a total lipid content of approximately 10%. This is done to improve larval production and quality. Another recommendation is to provide supplements of vitamins and mineral to ensure larval development. In addition, protein is necessary as it has been found to be the most important source of energy for embryo development. Larval feeds should be provided 2 to 3 times a day. Mature crabs such as hermit crabs eat once a day, however this varies according to the type of breed.
Crabs are often fattened to allow an early harvest. This is usually carried out in undeveloped ponds that are approximately 500m2. After fertilization, crabs weighing 150 to 200g are stocked during the early mornings or late afternoons. Crab fattening involves feeding 3times a day at a rate of 5 to 8% of their overall body weight. Crabs are fattened for a period of 10 to 15 days often achieving a growth increment of 110g per crab. After 15 days, they are usually fully matured hence can be harvested.
Crab harvesting is done after the shell is sufficiently hardened. The harvest of crabs can be effectively done in tide-fed ponds by letting in water through the sluice into the pond during the incoming tide (high tide). Another way to harvest crabs involves draining the pond and using scoop nets and ring nets with baits. It is advisable to harvest crabs in the early morning hours or evening to prevent mortality due to over eating of water during the afternoon. Note that that crabs can fatten at different intervals therefore ensure that they have about 50% fat prior to harvesting. Harvested crabs must be replaced with lean ones; this cycle can be maintained for up to 5 months. The expected survival rate for crabs is around 70 to 80%.