Coastal Aquaculture and Mariculture (2+1)

Unit 7 - Culture of Molluscs and Seaweeds

7.1.1. Culture techniques
In nature oyster grow on hard substratum found at the bottom or column of coastal areas and estuaries. Most primitive methods of culture involve spreading clean oyster shells called cultches on the bottom. The spat settle on the empty oyster shells and grow. This is method is known as bottom culture and is still practiced in USA Canada and France.
In 1673 a Japanese culturist Gorohachi Koroshiya discovered that oyster spat also settle on bamboo poles erected in water. This observation led to the development of off-bottom culture techniques.
Four methods of off-bottom culture techniques are being followed. They are

• Raft culture
• Rack culture
• Long line culture and
• Stake culture.

Of the above four methods raft culture is the most productive and commonly used off-bottom culture technique.

Raft culture

The technique involves collection of spat for natural spat settlement areas and grow-out for market.

Raft culture of oysters

Spat collection
Various types of spat collectors are used. Most commonly used ones are 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. The time of collection is crucial. 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
Raft culture is practiced mainly in Japan. The sites selected for raft culture should have the following characteristics.

• 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 15-30^oC.
• 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.
Long line culture
This method of culture is followed in Northern Japan where wind and wave action is heavy. Unlike rafts the long lines can withstand more water agitation. They are also cheaper than rafts.
Pair of ropes 45-75m long is attached to a pair of floats on both side and anchored. Smaller floats are attached in between. 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 in oyster culture either on rafts or long lines is 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.