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Farming Systems
They are broadly grouped as on-bottom and off-bottom culture. Raft, rack, longline
and stake are used in the various off-bottom culture practices. The off-bottom culture
methods are advantageous over the bottom culture in the following respects.
1. Relatively rapid growth and good meat yield.
2. Facilitates three-dimentional utilization of the culture area.
3. The biological functions of the oyster such as filtration, feeding etc. are carried
out independent of the tidal How, and
4. Silting and predatory problems are minimum.
Bottom culture: The
oysters are grown either in the intertidal or subtidal area directly on hard substratum.
For intertidal culture a minimum of 16 hours submergence is suggested to ensure
adequate food supply. Oyster seed attached to the collectors are planted on the
bottom and allowed to grow for the market. The disadvantages of this method arc
increased exposure to benthic predation, siltation and low production.
Bottom culture of oyster
Rack-and-string method:
It is also called ren method. The racks are constructed in 1 to 2.5 m depth.
There are several variations in the types of racks. The single beam rack consists
of a beam placed and secured to the top of posts driven into the bottom. A series
of single beams are placed in a row. The cross beam rack is constructed by placing
cross bar on top of single posts and two long beams are secured on the end of cross
beams.
In the farm the shell strings are suspended from racks. A rack covering 80 sq. m.
area holds 90 strings. In one hactare 125 racks are constructed. At the end of 12
months each string weighs around 7 to 7.5 kg and the production of oysters is estimated
at 80 t/ha. The mortality (including those fallen) is about 45%.
Rack and string method
Rack-and-tray method:
The nursery-reared single spat (cultch free) measuring about 25 mm are transferred
to trays of size 40 x 40 x 10 cm at a density of 150 to 200 oysterlings/tray. The
tray is knitted with 2 mm synthetic twine of appropriate mesh and is suspended from
rack. Once the oysters reach 50 mm length they are segregated and transferred to
rectangular trays of size 90 x 60 x 15 cm and these trays are placed on the racks.
Each tray holds 150 to 200 oysters. Each rack occupies 25 sq. m. area. The average
growth rate of the oyster is 7 mm/month and at the end of 12 months the oysters
attain an average length of 85 mm. The production is estimated at 120 t/ha/year.
Compared to the string method, this method gives higher production but the production
cost is high.
Rack and tray method
Oyster trays
Stake culture: A stake
is driven into the substratum and on the top end one nail and on the sides two nails
are fixed. The nail holds in position a shell with spat attached. The
stakes are placed 60 cm apart. In this method, the nursery rearing of spat is earned
on the same stake. For about two months the spat on the top end of the stake are
covered by a piece of velon screen. Once the oysters attain 25-30 mm, the velon
screen is removed and in another 10 months they reach the marketable size. The growth
rate of the oysters in this method is the same as that of the oysters raised by
the string method. The production is estimated at 20 t/ha/year.
Raft culture: A four-barrel
(each 200 l) raft supports about one tonne, less the weight of wooden frame. Strings
of oysters weigh about 5 times less in water than in air. The rafts are designed
taking into consideration the expected weight of oysters at harvest. Strings, trays,
lantern nets, bag nets and sticks can be suspended from rafts. In India oyster culture
from rafts has not been attempted.
Raft culture
Longline culture:
In this system long ropes or cables are anchored at each end and are supported at
intervals by floats. Longlines of 50-100 m length are easy to manage. Double longlines
comprising one line on either side of the floats are also used. Oyster strings are
hung from longlines. Longlines withstand rough sea conditions better than rafts.
Longline culture
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