International Journal of Aquaculture, 2013, Vol.3, No.12, 55
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62
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58
started between 3 and 4 dph. Yolk sac was completed
absorbed on 2 dph, while oil globule was absorbed
completely on 4 dph (104.4 d·
℃
)
when larval size
reached to (2.54±0.06) mm (TL, Table 2).
Table 2 Yolk-sac and oil globule volumes in larvae of
T. modestus
(
means ± SD)
Days post hatching Total length (mm) Yolk volume
(
×10
-3
mm
3
)
Oil globule Volume
(
×10
-3
mm
3
)
Water temperature (
℃
)
Cumulative temperature
(
day-degrees×
℃
)
0
2.081
±0.089
18.77
±0.03
4.220
±0. 001
21.1
1
2.548
±0.057
6.86
±0.05
4.577
±0. 002
20.7
41.8
2
2.597
±0.072
0
2.674
±0.006
20.6
62.4
3
2.642
±0.135
0. 894
±0.004
20.9
83.3
4
2.544
±0.061
0
21.1
104.4
5
2.682
±0.095
21.5
125.9
Generally, egg’s diameters of fish from the family of
Monacanthid were less than one millimeter, for
instance the egg’s diameters of
Rudarius ercodes
is
about 0.52 mm (Kawase and Nakazono 1994; Kawase
and Nakazono 1995),
Oxymonacanthus longirostris
is
about 0.7 mm, and
Thamnaconus modestus
is about
0.64
mm (Nakazono and Kawase 1993). In the present
study, the egg diameter of bluefin leatherjacket was
(0.607
±0.03) mm, which was smaller than Nakazono
and Kawase (1993)’s finding. The slight difference
may cause by the genetic difference between two
populations as fish from Nakazono and Kawase
(1993)’
s study was from Japanese Sea while our
broodstocks were came from Bohai Sea. Because of
the small size of egg, total length of newly hatched
larval bluefin leatherjacket was only (2.08±0.09) mm.
The character of small size larvae potentially
increased the difficulty of larvi-culture during the first
feeding period. For instant, as the mouth size of
bluefin leatherjacket was very small (unpublished
data), the size of live feeds could be the issue affecting
the successful first feeding.
Live feeds supply during the first feeding for those
small size fish at hatching has become the major
bottleneck continually hinder the larval culture in the
marine fish hatchery. Similar like grouper larvae,
bluefin leatherjacket have very small mouth gap,
under some particular situation, common rotifers may
not fulfill the size of requirement for the first feeding
bluefin leatherjacket larvae. Therefore, the production
of live feeds with small size characteristics is an
important hatchery operation in bluefin leatherjacket
breeding. As the geographical difference, super small
strain rotifers were not available in our region.
Therefore, to explore new live feeds substrates is
essential. In the present study, by using fertilized egg
and trochophore-stage larvae of Pacific oyster
(
Crassostrea gigas
)
during the initial feeding stage,
this issue was partially solved.
1.4
Growth and Seeding Rate
The SGR from 0 dph to50 dph was 6.00% day
-1
.
The
larvae grew exponentially and their growth can be
described by the equation.
Y
=3.30394
-
0.31862
x
+0.02222
x
2
(
r
2
=0.90362,
n
=427).
Where
Y
is total length in millimeters and
x
is days
post hatching (Figure 3). In this study, two heavy
mortality periods were observed, which were found
between 5 dph~10 dph and 30 dph~60 dph. After 70
days rearing, 0.33 million juveniles (mean total
length>40 mm) were obtained and the ratio of
seedling/ newly was (5.62±0.83) %.
Figure 3 Growth of
T. modestus
during the first 50 days
From the physical aspect, sufficient nutrient supply
depends on the efficiency of the digestive system.
During the early development stage, lack of sufficient
digestive capabilities is common in this phase and
larvae mainly depend on pinocytosis and intracellular
digestion and absorption (Ma et al. 2012b). Previous
study indicates that the nutrition is presumably the