CULTURE OF MOLLUSCS AND SEAWEEDS

OYSTER CULTURE

Two groups of oyster are cultured• Flat oysters – Osrea spp• Cupped oysters – Crossostrea spp • Are filter feeders - mainly on plankton and suspended detritus• In tropics - spawn throughout the year • The larvae pass through trochophore and veliger and metamorphose to spat and settle

on solid surfaces

Flat oysters Cupped oysters

Culture techniques:• Spreading clean oyster shells called cultches on the bottom- Most primitive

method• The spat settle on the empty oyster shells and grow• This is method is known as bottom culture• In 1673 a Japanese culturist Gorohachi Koroshiya discovered that oyster spat also

settle on bamboo poles erected in - off-bottom culture techniques.

Four methods of off-bottom culture• Raft culture - most productive and commonly used

• Rack culture

• Long line culture and

• Stake culture.

Long line culture

Raft culture:• Collection of spat for natural spat settlement areas and grow-out for market

Spat collection:

• Spat collectors - empty oyster shells strung on GI wires suspended from rafts or long lines; or lime coated tiles stacked on racks

• They are set in collection areas and left over their till spat grow to 10-20 cm

• They should be set during the period of maximum spat fall

• If set out early barnacles and undesirable organisms settle. If they are set out late spat collection will be scanty

• Plankton samples should be analysed, when maximum spat are available spat collectors should be set out

Growing to market size:

Site selection:

• The farming area should be in sheltered places like bays, fjords etc. free from strong wave action

• Tides should be sufficient and frequent to change water

• Salinity should be 23-28 ppt and temperature 15-30oC

• They area should have sufficient phytoplankton as food for the growing oysters

• The area should be free from industrial, agricultural and domestic pollution

• The rafts are made of bamboo poles tied together using strong polythene ropes• They are suspended using floats made of used empty cans or Styrofoam floats

coated with cement• Rafts of size 16x25m are generally used• One month old spat attached to oyster or scallop shells from collection areas are

restrung on galvanised wires 20cm apart using bamboo/plastic spacers• 500 to 600 wires with spat can be suspended from each raft• The spat are allowed to grow to marketable size of 30-60g individual weight• Duration of culture will depend on water temperature• In Japan it takes around 6 months for the oysters grow to marketable size

Rafts Oysters on galvanised wires

Long line culture:

• In Northern Japan where wind and wave action is heavy

• Unlike rafts the long lines can withstand more water agitation; also cheaper than rafts

• Pair of ropes 45-75m long is attached to a pair of floats on both side and anchored

• Rens with empty oyster or scallop shells containing spat are attached to the long ropes

• The spat are grown to marketable size similar to those in the rafts

• Each ren on a raft or long lines yields about 6 kg in 6-8 months

• It works out to 140 tonnes of whole oysters or 20 tonnes of oyster meat

• The main problem - fouling by barnacles, mussels and algae which affect the growth of oyster by competition for food space and oxygen

• Periodic manual cleaning of rens should be carried out remove the fouling organisms

Oyster culture in India

• Rack and tray culture method for growing the cupped oyster Crossostrea madrasensis – in Tuticorin centre of CMFRI

• Trays made of metal rods and nylon net; measuring 90x60x15cm is placed on wooden platform fixed in shallow mud flats

• 2 rows of 10 trays were place on each platform• Spat collected on empty oyster shells were placed in each tray• The oysters grew to 90 mm in 12 months yielding about 120-150 tonnes/ha of

whole oysters• Commercial farming of oysters in not yet undertaken in India