CRAB
CULTURE
Introduction
Crabs
are decapods crustaceans, belonging to the sub-order Brachyura. Their important
characteristics are well- developed carapace and greatly reduced abdomen that
is flexed under the body.
Crabs
an important item of food from the primitive times, have ocuupied an important
position in the lives of the Indian people.
In
India, six genera, Viz., Scylla, Portuns,
Charybdis, Matuta, Varuna and
Sartoriana are commonly accepted as food crabs. Scylla serrata represents the largest known
edible and highly valued economic species of the Indo pacific regions.
Habitat:
Scylla serrata
and Scylla oceanica, the common
edible crabs in India are available in mangrove areas, estuaries, back waters,
coastal lakes of all the maritime states. They are also seen in the creeks and
bays of Andaman and Nicobar Islands.
They tolerate a wide rangeof salinities. The mud
crab Scylla serrata grows to size of about 120-130 mm in
carapace width, weighing around
700-1000g and the gaint
crab, Scylla oceanica grows up to a size of 250 mm in
carapace width or more and weighs more
than 2 kg,
Scylla
tranquibarica grows upto 130-200 mm in carapace Scylla serrata
width and weighs 0.5 to 1 kg.
Food
and feeding:
Crabs are omnivorous in feeding, eat a variety of organic
materials. The total protein content of the muscle of crab is higher than that
of prawn of comparable parameters.
The culture techniques on a large scale are developed in
this line, the adult crabs can be processed and exported to foreign markets to
earn more money.
Pond
design:
Sandy or sandy clay soils are generally
preferred for pond construction or pond used for fin fish culture like milk
fish are used for crab culture. Coastal mud flats in brackish water areas,
where water salinity ranged from 15-30 ppt are selected as the farm site.
Simple
low cost, dug out earthen ponds with sizes of 0.3 to 0.5 ha and water depth of
1.5 metre are used for crab culture. The culture ponds are meant for stocking
and growing yound ones to the marketable size of 500 gms and above.
The
central portion of the pond is a shallow, often exposed, so as to provide a
natural habitat for the crabs to burrow and live. Water intake can be made
either by tidal influx or pumping depending upon the nature of the pond site,
through a simple sluice gate. Water exchange is done normally during the spring
tides.
Crabs
are capable of climbing the bunds. So fencing has to be provided all along the
length of the bunds to prevent their escape. Fencing is generally done with the
material like bamboo sticks, bamboo poles and knotless nets, asbestos sheets or
plastic sheets or polythene sheets lined gunny bags which are economically
viable are to be provided on the bund to prevent escape of the crabs during
night time. A minimum length of 2' is to be provided for fencing.
Most
important aspect is the provision of earthen stoneware pipes of the dimensions
4", 6", 8" to be provided in the floor through out the inside of
the bund. Stoneware pipes are necessary for the moulting crabs of different and
these can reduce the chances of cannibalism during ecdysis.
The
design of nursery pond differs with culture pond. Here about 100 cages of the
dimensions 3'x 2'x 2' length x width x height, made of bamboo are to be
provided into the pond for every acre of water spread area. There is an inlet
on top of each cage for introduction of breeders and also food. The cage is
usually made up of bamboo sticks.
Pond
management:
Stocking:
Source
of seed:
Crab seeds can be collected by two methods. Firstly the
juvenile crabs or small crabs not useful for consumption are available
throughout the year, weighing only 8- 10 gms, and these can be used for
culture.
In some places like West Bengal, where the traditional
methods are following, the crab seeds are entered with the tidal ingress of
water into brackish water bodies.
Usually in all the culture practices, farmers use to
maintain a stock of about 4000 – 5000 crab per hectare. Stocking density may
range from 20,000 – 25,000 crabs/hectare in case of semi-intensive farming,
using a supplementary feed.
Another method involved in sourcing for seed is
production of crab seeds in hatcheries.
The crab farmingpond of water parameters
Should
be maintained within the following
ranges:
Temperature: 28-300C
Salinity : 28-34 ‰
DO : 7 -13 ml/L
pH : 8.0-8.5
Feeding:
Scylla is an opportunistic feeder on slow moving benthic
invertebrates and can readily be fed with shelled snails or trash fish. In
culture ponds, external feeding is compulsory because crab are cannibalistic
under unfit conditions. Feeding rate is about 7-10 percent of body weight.
Juvenile crabs attain marketable size of 200-300 gms or
10-12 cm carapace width within a period of 6 month. (Scylla serrata 120-130 cm of carapace, weighing about 700-1000 gms
and Scylla oceanica 150 mm of
carapace, weighing about 2 kgs)
Mud crab farming can be carried out by two types namely
monoculture and polyculture. In monoculture the baby crabs are stocked at the
densities up to 2-3 pcs/m2. In polyculture the ponds are stocked
along with Chanos, shrimps and
seaweed (Gracillaria) etc.
Harvesting
and Marketing:
When the crabs
attain up to marketable size, they are harvested by draining the ponds. When
water in the ponds are drained crabs generally take shelter in the trenches.
They are captured using scoop nets. Hooks are employed to bring the crab from
their burrow.
Mud crabs are generally sold it live condition. Major
portion of the catch is usually packed in bamboo baskets along with soaked sea
weeds with a view for keeping the crab in cool and moist condition. The
mortality rate varies from 5-10% during the transportation.
Cage culture of crabs:
Cage culture of crabs can be defined
as the raising crab from the juvenile stage to commercial size in a volume of
water enclosed on all sides, including the bottom. While permitting the free
circulation of water through cage. This method has increased the potential
development of aquaculture through out the world.
The
cage culture methods are generally practiced in the countries like Cambodia,
Japan, India, Africa and many other countries. In India, it is a recent
practice but with great promise.
In
the traditional culture the material usually utilized are lattice of bamboo and
wood or planks as used in Kampuchea,
Indonesia and Thailand. In intensive culture, fish culturists prefer synthetic
netting or rigid metal or plastic meshed materials. When a risid farme is used
its construction material is either wood or metal.
The
major disadvantage of wood is its weight, which increase as water is absorbed.
Netting with twice of dark colour is preferred.
The
cages are generally installed in open waters like seas, estuaries, lagoons etc.
Crabs grow much faster than in still water ponds because they spend less energy
for movement leading to better food conversion. Crabs in cage are fed with
properly balanced diet.
The
size of the cage used for crab culture varies from 5 m2 area to 50 m2
area with depth of 1.0 to 1.5 m. In India experiments on cage culture of crab
was conducted in the estuaries of Kerala and Tamilnadu. The optimum stocking
density being 10-12 individuals/m3 clam meat, trash fish and salted
fish were used as feed at the rate of 5-15% of body weight.
Advantages of cage culture:
- This
method can be used in areas where the sea/estuaries bed is no suitable for
traditional shell fish farming. - The
cages used in the off bottom method predators can be more easily
controlled with less loss of the stocks. - The
water is threatened by pollution, the stock many be towed out of danger;
cage culture is generally considered to be a very uncreative practice, for
this reason the number of cages operated has been increasing very rapidly
in recent years. - Cage
culture can be considered to represent advance type of aquaculture, its
productivity is 10-20 times higher that that of pond culture for
comparable inputs and area.
Pen culture of crabs:
Culture
of crabs in enclosed bodies in large extent of water bodies is termed as ‘Pen
culture’.
‘Pen’
is simply an enclosure often which utilizes the shore line as one of the sides.
The pens/enclosures were constructed with bamboo screens in the lakes of China
during eighteen forties. Now a days synthetic fibres for twine and net such as
nylon, vinylon, polyethylene etc, are used for construction of the pens.
There
materials are durable for construction of pen/enclosures. Most enclosures are
constructed with a frame work of timber, steel or concrete piles or galvanized
chain link mesh of weldmesh stretched over the bars. Enclosures are made also
with the help of concrete barrier is kept in place by heavy sinkers and
equipped with a horizontal floating net to prevent a crab escape by jumping.
Prior
to acquisition of the site, the quality of the water should be determined by
regular sampling to check on its basic constituents as well as presence of
toxic chemicals polluting the river or estuary for the pen culture. The quality
of the water at the site must be ascertained by regular sampling and
hydrographic surveys are required to determine annual variation of salinity
temperature and dissolved oxygen content.
The
pens can be enclosed with nylon net (12 mm mesh and 2 mm nylon twice diameter)
stretched over a bamboo frame work. The bottom margins of the net enclosures were
beside 60 cm into mud, while the inner
side of the upper net margin was lined with 80 cm wide polythene film to
prevent crab stock from escaping.
Canals
were dug between rows of mangrove trees with out destroying their roots. Water
depth at the bottom of the canals was therefore 80-100 cm at the lowest tides.
The dike was further enclosed with a fine-meshed nylon screen to minimize
siltation of canals.
Crab
juveniles of Scylla were collected from their natural habitat using a shallow
push net operated in mangroves swamps and nearly intertidal and sub tidal
muddy. The crabs were placed in large transport bags and can carry 500
juveniles, crabs can be initially fed @ 10% of their estimated biomass daily
when the carapace length was <6 cm, and 5% when 6 cm of more.
Pen
culture of mud crab in tidal flats with mangrove trees can therefore be
economically viable and without the need to cut down mangroves or to build
aquaculture ponds. Stocking density levels the range of 0.5 or 1.5 crabs m/m3
and regardless of the diets, salt fish by catch alone or a mixed diet of 75%
salted brown mussel flesh and 25% salted fish by catch.
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