The number of eggs released from
a full spawning ranges from 300,000 to 500,000.
Partial spawnings with low hatch viability are discarded. Egg diameter is 0.3 mm. Aeration in the bowls is stopped allowing the
eggs to settle to the base. Water
exchange is given at a level of 50%.
Ovarian tissue is cleaned from the bowl sides and a 400 micron mesh net
is used to remove faeces from the water.
Strong aeration is given in the bowls to keep the eggs in
suspension.
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Figure 19 200 Litre Spawning Tanks |
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Figure 20 1350 Litre Transport Tank
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Figure 21 Maturation Tanks
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Figure 22 Nauplius 6 |
Number of eggs spawned is
calculated by taking ten random samples at different places in the spawning
bowl with a 100 ml beaker. Nauplii hatch
12-14 hours after spawning, approximately 3 pm.
Similarly, hatch rate is calculated.
Healthy nauplii exhibit a strong phototactic response. The aeration is
ceased and a grey lid is placed over the bowl with a light bulb suspended over
a one centimetre hole in the top of the lid to concentrate the nauplii at the
surface of the water. After a period of
ten minutes a hose is inserted through the hole and the nauplii are
subsequently siphoned into the nauplii catcher bucket (Figure 22). A secondary hose provides a constant flow of
saltwater to the catcher simultaneously.
A 250 micron screen in the centre of the catcher bucket enables
effective washing to occur as the nauplii are collected. This acts to prevent the vertical
transmission of viral, bacterial (Vibrio spp.), fungal, microsporidean and
other diseases from the broodstock.
Siphoning is ceased before reaching the base of the bowl to leave the
weaker nauplii and egg mass together which are then discarded and the bowls
chlorinated. The five tonne larval
rearing tanks have been prepared with 250 micron filter screens and one tonne
of seawater. If the ambient water is
< 290C the one kilowatt heaters are positioned in the tanks. The nauplii are inoculated into the tanks at
a density of 150 nauplii per litre.
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Figure 23 Zoea 1 |
There are six nauplii stages in
which no feeding occurs as nauplii are absorbing yolk supplies (Wikipedia,
2010) (Figure 23). Chaetoceros muelleri
is pumped into the larval tanks at nauplii 6 at a minimum density of 50,000
cells per mL. Larvae metamorphose to
zoea 1 the following morning and commence feeding (Figure 24). Zoeal stages 1 to 3 are fed with algae in the
ratio of 80% to 20% C.muelleri (at 80,000 cells per ml) to S.tropicum.
C.muelleri must be given at a
greater concentration to zoeal 1 and 2 stages as this algae is the appropriate
size for ingestion. The larvae are given
feed at 6 intervals throughout the day.
Supplementary feed for zoea 1 to 3 is Inve microencapsulated diet Car #1,
at a size 5 to 30 micron. At zoea 3
stage the larval tanks are at full capacity. Mysis 1 commences on day 5 and at this stage Artemia
salina at 1 individual per ml is added to the tanks (Figure 25). Initially
water exchange is at 20%, and then increased to 30-40%. Inve microencapsulated feed CD#2 (30-90
micron) is added when required. The algae
proportion is inversed for C.muelleri and S.costatum, 20% to 80%
at mysis 3 stage (Figure 26). Postlarvae
1 stage is reached at day 10 when Artemia is provided at 5 individuals per
ml. Bacteriological testing for luminescent
Vibrio and Pseudomonas spp. is routinely undertaken by the
streaking method using TCBS agar plates. Probiotic bacteria are used to control the
levels of toxic metabolites, strengthen the immune system of the cultured
animal and repress the growth of pathogenic micro-organisms. The production of
inhibitory compounds by the probiotic bacteria suppresses the metabolism of the
pathogens, in addition to inducing competition for nutrients and other
resources (Jobling, 2010). Feeding regimes are based on the specific
requirements of the various larval stages validated by frequent and detailed examination
of the feeding activity of the larvae in each tank.
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Figure 24 Nauplii Catcher
Larvae are transferred to the
Nursery Section at Postlarvae 1 stage. The
transfer is accomplished by first lowering the tank levels to the line of the
conical or in the case of the 5 tonne parabolics, to the one tonne level. The ball valve outlets of the larval tanks
are connected by a section of hose to the inlet of the 10 tonne parabolic
nursery tanks which have been filled with seawater to the level of entry
(Figure 27). The process is a moderated
gravity flow which affords minimal stress on the larvae. Postlarvae 1 enter a clean tank environment
for the duration of their development to postlarvae 15 (Figure 28). The feeding schedule consists of Artemia
maintained at 5 individuals per ml, by four feeds daily, and supplementary
nutrition of Inve Frippak PL+150 (150-200 micron) for postlarvae 1-5, and
PL+300 (200 -400 micron) for postlarvae 5-15, supplied twice daily.
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Figure 26 Mysis 3 |
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