The mud crabs inhabit marine as well as brackish
water environments. Two species of mud crabs, namely
Scylla tanquebarica and Scylla
serrata are found in the inshore sea, estuaries,
backwaters, coastal lakes and mangroove swamps of all
maritime states on the main land and the creeks and bays
of Andaman and Nicobar Islands. Both the species
co-exist in the inshore sea as well as in the inland
brackish waters preferring muddy or sandy bottom.
Crab fattening is essentially a holding operation
during which post-moult or water crabs are kept for a
short period of 20 days until they 'flesh out' or
immature female crabs are held until their gonads
develop and fill the mantle cavity. This type of
activity has become very popular throughout the Asian
countries due to increasing demand for gravid females
and large size hard shelled ones in seafood restaurants.
Fattening of mud crab is undertaken in the states of
Andhra Pradesh, Tamil Nadu, Kerala, Karnataka, Orissa
and West Bengal.
Mud crabs grow to a very large size of about 22 cm in
carapace and about 2 kg in weight. The crabs belonging
to the species S.tranquebarica is free living and
grows to a large size with carapace width of 22 cm and
those of species S. serrata have borrowing habit
and grow to about 12.7 cm in carapace width. Mud crabs
are omnivorous and they feed on a wide variety of food
items such as shrimps, crabs, bivalve molluscs and fish.
The females reach sexual maturity at a size of about
12 cm in S. tranquebarica and 8.5 cm in
S.serrata in the brackish water. Both the
species are continuous breeders with peak breeding
seasons which vary from place to place. The peak seasons
of seed abundance is May to October along the southwest
coast, December to May in Madras coast and March to June
in Chilka Lake. Each crab spawns once in two months. The
number of eggs carried by S. tranquebarica
are about 1.1 to 7.0 million and by S.
serrata are 0.5 to 0.9 million. The berried
females migrate from estuarine areas to the inshore sea.
The eggs hatch out in the sea and undergo metamorphosis
and then they migrate to brackish water areas and spread
to different parts of the estuarine systems.
Crab seed are available in the nature at all sizes.
Juvenile crabs can be collected from estuaries, lakes,
backwaters, creeks, mangrooves and salt water lagoons by
using bamboo traps, lift nets or scissor nets. A
hatchery is being set up at the Central Marine Fisheries
Research Institute, Kochi for commercial production of
Mud crab fattening is done by stocking soft shelled
crabs or water crabs that are held in smaller
impoundments for 20-30 days till the shells are
hardened. The techno-economic parameters required for
crab fattening are briefly described.
The soil suitable for crab fattening is sandy or
sandy clay. A sand bottom inhibits burrowing.
There should be availability of abundant and good
quality water. Mud crabs are highly tolerant to varying
salinity conditions, so brackish water would be ideal
for crab fattening operation.
Salinity - 10 to34 ppt.
PH - 8.0 to 8.5.
Temperature - 23oC to 30oC.
Dissolved oxygen content - should be more than 3 ppm.
Size of pond and
Crab fattening is carried out in ponds, cages or
pens. Small ponds ranging from 0.1 to 0.5 ha in size
with a water depth of 1.5 m are generally selected for
the purpose. The pond should preferably have a sandy
bottom. Bunds should have a minimum of 1.0 m width at
the top to prevent crabs from escaping by borrowing
through the bunds. Crabs are capable of climbing over
the bunds, which is prevented by fixing overhanging
fences on dykes. Fencing of height between 0.5 to 1.0 m
over the dyke is done with materials like bamboo sticks,
bamboo poles and knotless nets, asbestos sheets,
fibreglass panels, etc. As the crabs are highly
cannibalistic especially on freshly moulted animals,
'refugee cages' made out of hollow bamboo pieces, cement
pipes or stones are placed inside the pond to minimise
Water supply and drainage
Water exchange is through tidal water. Sluice gates
may be used to regulate the inflow and outflow of the
tidal water. The sluice gates are fitted with bamboo
screens to prevent the escape of crabs. In regions where
tidal influence is less, sea water is pumped in.
The pond is prepared by draining out the water. The
pond is then allowed to dry or bake in sunlight before
the liming is done. The water is let in during the high
tide or sea water is pumped in to a height of 1.5 m.
Soft-shelled crabs of size 8 cm Carapace Width and
above or crabs of more than 550 gm are stocked in the
density of 1 crab/m2. The stocking density is normally 1
crab/1 to 3 m2.
Crabs are fed with bivalve meat or trash fish.
Feeding is done daily at the rate of 5 to 10% of body
weight. The duration of fattening is 20 days.
The crabs are harvested after the shell becomes
sufficiently hardened and before next moulting. The
harvesting is done by draining the pond and using scoop
nets and ring nets with baits. Harvesting should be done
in the early morning hours or evening to prevent
mortality of crabs due to overheating of water at noon
time. In a year 9 to 10 cycles of fattening can be taken
from a pond.
Production & Income
The expected production par crop for an area of 0.1
ha. is around 320 kg. The income parcrop from 0.1 ha.
has been assessed to be Rs.48000/-. (annexure II)
In India, the importance of live mud crabs as an
export commodity has opened up great opportunities for
crab farming / fattening. At present crab has good
market and in the future crab is poised to be the next
potential sea food in the world market among the edible
marine crustaceans after shrimp and lobster.
Details of the financial requirement for fattening in
0.1 ha. area have been indicated in Annexure I. The
items and cost indicated under the model are indicative
and not exhaustive. While preparing projects for
financial assistance the costs have to be assessed
taking into account actual field conditions. The
projected capital cost for 0.1 ha. unit has been
estimated to be Rs.63,750 and operational cost for one
crop to be Rs.38,450.
The following assumptions have been made for working
out the financial viability of the activity.