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Figure
1: Giant Tiger Prawn, Penaeus monodon
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P. Monodon Taxonomy
Kingdom: Animalia
Phylum:
Arthropoda
Subphylum: Crustacea
Class: Malacostracea
Order: Decapoda
Suborder: Dendrobranchiata
Family: Penaeidae
Genus: Penaeus
Species: P. Monodon
The giant tiger
prawn, Penaeus monodon, is found in the Indian Ocean and western
Pacific (Indo-West Pacific). The prawns are distributed from east and southeast
Africa to northern and eastern Australia, Japan, Pakistan and the Malay
Archipelago. It is cultured commercially in much of its range. Western Indian
Ocean and western Pacific populations have separate evolutionary histories.
The largest of all the cultivated shrimp, it can grow to a length of 36 cm
and is farmed in Asia.
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Figure 2: Life Cycle of Tiger Shrimp
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The eggs are demersal and tend to
sink while larvae are Planktonic. Prawn larva thrives mainly offshore and
undergoes three main stages: nauplius, protozoea, and mysis. At the postlarval
and juvenile stages, the prawn migrates toward the estuary. As it grows, it
starts moving to the shallow coastal waters. The adult prawn inhabits the open
sea.
Sexes are separate and can be
easily distinguished through the external genitalia located at the ventral
side. The thelycum in females and petasma in males. During mating, the male
deposits the spermatophore inside the thelycum of the female. Mating can only
occur between newly molted females and hard-shelled males. Spawning tanks place
throughout the year. The eggs are fertilized in the water after the female
simultaneously extrudes the eggs and the spermatophore. The number of eggs
released by a single spawner varies from 248,00
to 811,000.
Eggs
The eggs are small, spherical,
and vary from 0.25 to 0.27 mm in diameter. The developing Nauplius almost fills
up the entire space inside the egg. At 28-30°C, the eggs hatch 12-17 h after
spawning.
Nauplius Stage
Stage after eggs have hatched. The
prawn Nauplius is very tiny, measuring from 0.30 to 0.58 mm in total length. It
swims intermittently upward using its appendages in a “bat-like” manner. It is
attracted to light and in aerated tanks, it will concentrate in the most
lighted areas if aeration is stopped. The Nauplius molts through each
of six sub stages for a total of about 1.5-2 days. The substrates differ from
each other mainly on the furcal spine formula. The latter indicates the number
of spines at each side of the furca.
Protozoea Stage
Its body is more elongated and
measures from 0.96 to 3.30 mm in total length. It consists of the carapace,
thorax and abdomen. It can also be distinguished by its movements, it swims
vertically and diagonally forward towards the water surface.
The protozoea undergoes three
sub-stages. The paired eyes of protozoea can be obscured as two dark spots in
the upper portion of the carapace. These eyes
become stalked at protozoea II. At protozoea III, the dorsal medain spine
at the sixth abdominal segment first appears.
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Figure 3: Eyestlak Ablation
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Eyestalk ablation
The removal of one (unilateral) or both (bilateral) eyestalks from
a crustacean. It is routinely practiced on female shrimps (or prawns)
in almost every marine shrimp maturation or reproduction facility in the world,
both research and commercial. Two ways of eyestalk ablation is by incision and
pressing the eye. The most commonly accepted theory
is that a gonad inhibitory hormone (GIH) is produced in the neurosecretory
complexes in the eyestalk. This hormone apparently occurs in nature in the
non-breeding season and is absent or present only in low levels during the
breeding season. By inference, then, the reluctance of most penaeids to
routinely develop mature ovaries in captivity is a function of elevated levels
of GIH, and eyestalk ablation lowers the high hemolymph titer of GIH. The
effect of eyestalk removal is not on a single hormone such as GIH, but rather
effects numerous physiological processes
Mysis Stage
Shrimp-like with the head
pointing downward. Its body measures from 3.28 to 4.87 mm in total length. The
telson and uropods are developed. The mysis swims in quick darts accomplished
by bending the abdomen backwards. For mysis sub-stages, the most prominent
change is the development of pleopods. The pleopods appear as buds at Mysis I,
which protrude at Mysis II and finally become segmented at Mysis III.
Post larval Stage
The post larval resembles an
adult prawn. At post larvaI the rostrum is straight and exceeds the tip of the
eye. Plumose hairs are present on the
swimming legs.
THE HATCHERY FACILITIES
Larval and Post larval Tanks
Rubberized canvas, marine plywood,
fiberglass, or concrete. These can either be circular, oval or rectangular,
depending on the operator’s preference or financial capability. The capacity of
each tank may be from 1-20 t but 10-12 t tanks are more economical and
practical. Depth should only be about 1m because tanks which are too deep are
difficult to manage.
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Figure 4:Larval Tanks
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Algal tanks
Minute plants (phytoplankton) are
needed as food for the early life stages of prawn. Algal tanks must be shallow
(ideally 0.5 m deep) to allow sufficient light prevention.
Spawning Tanks
It is advantageous to have
smaller tanks with volumes ranging from 0.25 to 1 to where egg washing is done.
Artemia Hatching Tanks
Artemia or brine shrimp is a
protein-rich live food organism given to prawn larvae starting at the Mysis
stage. Artemia is available in cyst form which has to be hydrated and incubated
in tanks for at least 18-24 h.
Reservoir
Storage tank is necessary for
chlorination and holding of filtered and treated water for daily use. This must
have a total capacity of at least 50% total larval tank volume.
Aeration System
Aeration is necessary in hatchery
operations to keep food particles and algal cells in suspension and to maintain
sufficient dissolved oxygen levels. Continuous aeration is essential during
operations. A standby generator will be very useful during power interruptions.
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Figure 5:Paddle Wheel Aerator
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Preparation of Spawning, Larval, and Nursery Tanks
To prevent disease outbreak, the
hatchery should be totally dried after several production runs. Tanks and
facilities in the hatchery must also be cleaned well prior to a hatchery run. New
tanks need to be filled with fresh or seawater for at least a week to avoid
mortalities due to toxic effects of chemicals used during construction of the
tanks.
Selection and Stocking of Spawners
Nauplius to be reared to the fry
stage can come from broodstock wild or pond-reared immature females induced
to mature by unilateral eyestalk ablation. Wild spawners female prawns caught
from the sea with developed ovaries. The number of spawners needed for
a hatchery runs is dependent of the nauplii requirement. For every million
nauplii about 4-5 wild spawners or 7-8 m female broodstock are needed. Spawner
procured as nauplii source must be carefully selected to obtain high fertilization
and hatching rates of eggs. Stage of maturity should not be used as the basis
for selection. Spawners must also be disease free. To ensure development of the
eggs, females should be mated to ensure release of sperm cells necessary for
fertilization.
Stocking of Nauplii
During stocking and throughout
the culture period, prawn must not be exposed to abrupt changes in
environmental conditions. The prawn must be given time to gradually adapt to
new conditions to avoid stress and mortalities.
Feeding
Nauplii subsits on the yolk
stored in their bodies. Larvae start to feed at the first protozoeal sub stage
( diatoms like Skeletonema or Chaetoceros) Larvae at the second protozoeal sub
stage may be fed Tetraselmis. At the Mysis Stage, some fish protein must be
present in the diet. Newly hatched artemia nauplii and microparticulate diets,
most commonly used protein source which contain about 45-50% protein. When they reach the postlarval
stage, egg custard, trash fish, mussel meat or ground dried acetes (small
shrimp or alamang) can be given to supplement the Artemia nauplii diet.
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Figure 6: Feeding the Prawns' Babies
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Seawater Quality and Quantity
Seawater with minimum seasonal
fluctuation in quality is most desirable. It should not be affected by inland
discharges containing agricultural runoff or industrial wastes. Turbidity
should be as low as possible. Adequate volume of seawater should be available
when needed. The best method to determine the suitability of seawater for
larval rearing is to conduct preliminary larval rearing experiments using pails
or small tanks on the site. The production of post larvae with reasonable
survival rate from eggs in a series of at least three runs would indicate the
likelihood of success.