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Stocking
The most suitable species for culture in India are the Indian white prawn Penaeus
indicus and tiger prawn P. monodon. The stocking density varies
with the type of system adopted and the species selected for the culture. As per
the directives of Supreme Court only traditional and improved traditional shrimp
farming can be undertaken within the Coastal Regulation Zone (CRZ) with a production
range of 1 to 1.5 t/ha/crop with stocking density of 40,000 to 60,000/ha/crop. Outside
CRZ extensive shrimp farming with a production range of 2.5 to 3 t/ha/crop with
stocking density of 1,00,000/ha/crop may be allowed.
Seed selection
Selection of good quality seed for stocking into a pond is the first important step
of the shrimp grow-out management. The farmer must ensure that he or she gets healthy
seed by purchasing them from reliable hatchery or hatcheries. It may not always
be possible to obtain the desired shrimp seed due to limitations in availability
and quantity.
The following parameters should be taken into consideration in purchasing shrimp
seed for stocking.
(i) Size
Seeds of PL 15-20, indicated by the appearance of 4-6 spines on the rostrum, are
recommended for stocking in a pond. The healthy PL should have the muscle-to-gut
ratio in the sixth abdominal segment of about 4:1 or the thickness of the gut should
be about the thickness of the muscle. Practically, seed from the first and second
spawning of a broodstock with uniform size can be used.
(ii) Morphology
The post larvae should have normal appearance of trunk, appendages and rostrum.
The abdominal muscle must be clear, no discoloration or erosion on any parts of
the body, the gut should be full of food, and the muscle should fill the carapace.
(iii) Color
Post larvae with the presence of pigment cells in the uropods should be used since
this indicates the stage of development. PL that will have high survival and growth
rates will be light gray, brown to dark brown and black in colour. Signs of red
or pink coloration are normally related to stress.
(iv) Behavior
Healthy seed swim straight, respond rapidly to external stimuli such as a tap on
the side of the basin, actively swim against the current when the water is stirred,
and cling to the sides rather than aggregate or be swept down into the center of
the container when the current has subsided.
(v) External Fouling
Seeds should be free from external parasites, bacteria and other fouling organisms.
The presence of these organisms indicates unhealthy conditions, which will affect
growth and survival of the PL. It is recommended that before purchasing, the farmer
should visit the hatchery to check the seed once or twice either in the early morning
or late afternoon, especially one day prior to stocking. However, healthy seed with
some fouling may be used when the animals are in good condition after treatment.
(vi) Pathogen Free
Seed should be checked for the presence of viral occlusion bodies. Seed with large
numbers of occlusions indicate stress conditions and will not so vigorous in the
pond.
Stocking Density
When a farm is ready for operation, the optimum stocking density of PL in a pond
should be determined in accordance with the production capacity of the farm and
the culture system, which include the soil and water quality, food availability,
seasonal variations, target production, and farmer's experience. It is recommended
that farmers should start a new crop with a low stocking density to access the production
capacity of the pond. If production is successful, then the stocking density could
be increased for subsequent crops. Overstocking should be avoided since it may result
in management problems and loss of entire production.
The stocking density between 10-20 PL/m2 is usually practiced in a semi-intensive
culture. In an intensive culture, a well-managed pond with consistent good water
quality can stock up to 25-30 PL/m2 at 1.2 m water depth and up to 40-50
PL/m2 at 1.5 m water depth or deeper. However, it must be emphasized
that intensive cultures involve high densities and can only be sustained in well-managed
farms under an experienced farmer.
Technique of Stocking
Proper stocking techniques will prevent unnecessary mortality of seed. The following
methods have shown excellent results.
(i) Transportation
Seed are normally transported in plastic bags. The bags are usually filled up to
1/3 with water, oxygenated and then placed inside styrofoam boxes. If the transportation
is longer than 6 hours, small bags of ice should be added into the boxes to reduce
the water temperature and maintain it at 20-22oC. The densities of PL
in a bag are 1,000-2,000 seed/l for PL 15 and 500-1,000 PL/l for PL 20. The ideal
time for transportation is in the early morning or evening to avoid excessively
high temperatures during the day, unless a covered vehicle is used.
(ii) Acclimation
To eliminate stress, the seed should be maintained in water of constant salinity
for at least 1-week prior to transfer. The adjustment of salinity by about 3 ppt
daily is advisable. Acclimation of seed to the water pH and temperature of the pond
must be rendered upon arrival. Two common techniques are used for gradual acclimation
of seed to the water conditions in the pond. The first method is accomplished by
placing the seed and water from the transported bag into a tank at the side of a
pond containing an equal volume of well-aerated pond water. The seed will be kept
for 0.5-1 hr before being siphoned into the pond. The second method, the most favorable
one, is to float the plastic bag in the pond until it has reached equilibrium. The
bags are opened one by one and pond water is added gradually to an equal volume.
After a further 30 min of acclimatization, the seed are released directly into the
pond by distributing them throughout the area of the pond or into a nursing system.
The actual numbers of seed at stocking can be estimated by counting the PL individually
in 3-5 bags with a spoon or small net to attain the average number in each bag and
multiplied by the total number of bags.
(iii) Nursing of Shrimp Post Larvae
To ensure high survival and adequate feeding of seed during the first 2-3 weeks,
some farms may stock the PL in a separate nursing pond or a small impoundment, usually
5-10 % of the total pond area, within the culture pond. The nursing system will
help in concentrating the seed in a limited area until they reach PL 30-40 and in
more accurate monitoring for survival and feeding of the PL. However, it appears
that the separate nursing pond system may lead to some unfavorable results in that
the size of the PL varies widely, (‘broken sizes'), and the seed difficult
to harvest and would experience stress during harvest and transport to the culture
pond. As a result, a farmer prefers to nurse the seed in an impoundment installed
inside the pond, rather than in a separate pond. Recently, some farmers employ a
system in which high densities of seed (100-200 PL/m2) are stocked into
a pond for 1-2 months, then approximately half of the juveniles are transferred
to another pond by large lift nets. The same acclimation process should be performed
during seed and juvenile stocking.
In a very intensive pond (stocking density greater than 30 PL/m2) where
the nursing impoundment is not available, a survival pen may be installed to estimate
the survival of the seed during the first 2 weeks after stocking to allow accurate
feeding management. The survival pen may be a small net pen or happa of approximately
1 m2 containing 100 seed or a large net pen of usually 10 m2
at 100 PL/m2 stocking density. In the small pen, the seed can be counted
accurately while the seed in the large pen may be counted by using a 1 m2
lift net placed with 10% of the feed. In this method, seed should appear in the
lift net at 3-4 days after stocking and the number of shrimp in the net should be
counted at 2 hours after feeding once daily. The survival number of shrimp can then
be estimated.
If the survival rate during the nursing period is less than 50%, the problems that
cause this initial mortality must be identified and rectified and the addition of
more seed should be considered. Seed can be added up to 30 days post-stocking without
causing a variation in size at harvest. If the survival is less than 30%, the pond
should be drained and prepared for a new crop. Some farmers release seed directly
into the pond. In this direct stocking method, the survival number of seed during
the first 2 weeks post stocking may not be accurately estimated, since the shrimp
will not approach the feeding trays during this period.
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