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Grow-out Phase
The farming of the oyster in the field comprises nursery rearing of the spat and
growing the oysters for market
Selection of farm site
For site selection several factors are to be considered, namely water depth, bottom
characteristics, protection from wave action, tidal flow and height, turbidity,
water quality including chemical parameters, predation, fouling, pollution and accessibility.
It is also important to consider the species and the method of culture. Sheltered
areas offering protection from strong wave action are preferred. From intertidal
region to areas extending up to about 5 m depth can be considered for adopting suitable
culture method. Similarly the culture technique is adopted depending upon the type
of substratum. On-bottom culture method is substrate-specific while off-bottom method
has little to do with the nature of substratum. Salinity range of 22 to 35 ppt is
suitable. The natural populations occur at a temperature range of 21 to 31oC.
Nursery rearing of spat
For nursery rearing relatively calm waters with adequate flow to bring phytoplankton
are preferred. In many cases a part of the oyster farm is used as nursery. Special
nursery ponds in the inter-tidal region are also constructed. The oyster spat taken
out of the hatchery are too small to be grown in the field without protection. They
are enclosed in velon screen bags of suitable mesh size and suspended from racks.
A string can hold six shell valves containing 80-100 spat and 3 to 4 strings are
enclosed in a bag. The bags are periodically cleaned and after 40 to 50 days they
are transferred to the farm.
Culture Management
Cultured oysters require a level of care to ensure their continued survival and
growth to marketable size. Farm management includes cleaning, thinning, sorting
or grading, pest control and predator protection.
Cleaning: Washing
the oysters, either with a pump or swishing the trays up and down in the water removes
silt and other dirt. A pump is preferable, since it can also remove newly attached
fouling organisms and is fast, saving labour. Should fouling be heavy, brushing
may be required. The frequency of cleaning depends on local conditions. If siltation
is heavy, monthly washing may be necessary. Normally, siltation is not much of a
problem during the dry season.
Thinning: Oyster spat
may be nursed at high density, but upon reaching around 3 cm length, they should
be thinned out to the final grow-out density. If the oysters are overcrowded, growth
is very slow and mortality increases rapidly. Oysters should be sorted so that each
tray contains more or less the same sizes. The sorting process continues until oysters
are marketed. Sorting also reduces labour at harvest time, as only those trays with
marketable oysters need be selected for harvest.
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