International Journal of Aquaculture, 2013, Vol.3, No.13, 63
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handling related mortality, stocking activity are
recommended to conduct at neurula-stage (after the
formation of the optic vesicles) and by stocking eggs
into the culture tanks 2 h before hatching so that the
directly handle larvae can be avoided (Lim, 1993;
Tamaru et al., 1995, Caberoy and Quninitio 1998).
Furthermore,The larvae are sensitive to light during
the early stages of their development and are generally
kept in darkened conditions.
1.2
Larvae
Newly hatched grouper larvae are generally less than
two millimeters (Table 1), with different structure,
morphology and function from adult (Figure 2) (Bone
et al., 1995). Similar with most fish larvae, special
larval structure relevant to respiration may develop to
increase the area to volume ratio for gas exchange
(
Houde, 2001). Grouper larvae are delicate and have a
large yolk sac and an undeveloped mouth, fins, and
eyes (Figure 4). By 3-4 day post hatching, the yolk
and oil globule will be absorbed completed (Table 1).
The larval stage starts from exogenous feeding and
ends with completing metamorphosis. Like most
marine fish, massive mortality normally occurs during
larval stage because of vulnerability of larvae to
predation, starvation, unfavorable environmental
conditions and prevailing pathogens (Kamler, 1992;
Moyle and Cech, 2003).
Figure 4 Development of larvae and juveniles of
Epinephelus malabaricus
Note: A: Newly hatched larvae, 1.92 mm total length (TL); B: two days after hatching, 2.74 mm TL; C: three days after hatching,
2.80
mm TL; D: nine days after hatching, 4.11 mm TL; E: 14 days after hatching, 5.38 mm TL; F: 20 days after hatching, 7.15 mm
TL; G: 30 days after hatching, 22.05 mm TL; H: 40 days after hatching, 33.16 mm TL (Leu et al., 2005)
1.2.1
Feeding and temperature
Like most marine finfish, heavy mortalities that occur
in early-stage of grouper larvae have been considered
to be related to the initial feeding stage (Blaxter and
Hempel, 1963a; Kohno et al., 1997; Kohno, 1998).
The timing to supply feed with appropriate nutritional
composition is a key consideration in marine larval
fish culture (Cahu and Infante, 2001; Koven et al., 2001).
After yolk sac is depleted, fish larvae rely on food
from exogenous sources (Shan et al., 2008). At this
point, a delay of live food supply can result in low
survival, slow development and alimentary tract
degeneration (Heming et al., 1982; Chen et al., 2007;
Yoseda et al., 2006). Furthermore, if larvae cannot
access suitable food for an extended period (defined as
Point of no return by Blaxter and Hempel (1963) PNR)
after yolk sac depletion, they may lose the ability for
food ingestion and digestion (Blaxter and Hempel, 1963b;
Kamler, 1992). During onset of exogenous feeding,
fish mortality is likely to occur if the provision of first
feeding is beyond the PNR (Blaxter and Hempel, 1963b).
Therefore, the time at first feeding and live food
provision is crucial for the growth and survival of
postal larvae.
The PNR is closely related to temperature, as low
temperature prolongs the time for larvae to reach the
PNR and high temperature shows the opposite effect
(
Dou et al., 2005; Blaxter and Hempel, 1963b; Yin
and Blaxter, 1987). Dou et al. (2005) suggested that
the high temperature shortens the period for the first
feeding larvae to learn ingesting food before the onset
of irreversible starvation is a cause for mortality.
Similar result also has been found by Ma et al.
(
unpublished), they found higher temperatures
reduced the time for yellowtail kingfish larvae to