International Journal of Aquaculture, 2013, Vol.3, No.12, 55
-
62
http://ija.sophiapublisher.com
60
the larvae rearing tanks together with the attached plastic
corrugated plates. At the same time, photographs of each
plate were taken with a calibration scale for samples. The
egg number per unit (5 cm×5 cm) on the plate was
counted and calculated. A stereo microscope
(
Olympus SZ
-
61)
was used to observe and measure
the eggs. Eggs appear to be cloudy, un-global or
developed abnormally were considered as non-viable.
A sample of 10 cm×10 cm egg collection plate was
taken out daily to estimate the normal egg rate. 60
fertilized eggs were sampled from each batch, and the
diameters were measured to the nearest 0.01 mm.
The eggs were incubated in the larval rearing tanks
(
concrete, 28.8 m
3
)
at an initial density of 3.0×10
5
eggs/tank. Constant aeration was supplied to each tank.
The salinity and temperature were maintained at 30‰,
21
℃
,
respectively. After hatching, the plastic corrugated
plates with un-hatched eggs were removed from the
rearing tanks, and the yolk sac larvae were reared in
the same tank.
2.3
Larval Rearing
The rearing density of fish larvae was maintained at
6000
larvae/m
3
~8000 larvae/m
3
,
and a total of 19
tanks were used in this study. The rearing tanks were
supplied with sand-filtered seawater. Larvae were
reared with a 24 h-light photoperiod, and the
illumination was provided by fluorescent lamps
suspended over the rearing tank, and light intensity
was 1500
-
2000
lux (measured on the surface of
water). The salinity and dissolved oxygen were
maintained at 30‰, 5 mg/L ~7.0 mg/L respectively.
The water temperature was gradually increased from
21.0
℃
(0
dph) to 26.0
℃
(16
dph), and then maintained
at 26
℃
until the experiment was finished (Figure 4).
Water exchanging in the rearing tanks was started on
4
dph, the exchange rate was gradually increased from
10% (4
dph) to 100% (20 dph). On 20 dph, all the
juveniles were collected and transferred to new
rearing tanks (concrete, 28.8 m
3
)
with a stock density
of 2000 fish/m
3
.
Figure 4 Feeding scheme and rearing temperature for
T. modestus
during the first 45 days post hatching
The feeding regime was summarized in Figure 4 from
1
dph,
Nannochloropsis
sp. was supplied to the rearing
tank every morning to achieve an initial concentration
of 3×10
5
cells/mL. Larvae were fed with fertilized egg
and trochophore-stage larvae of Pacific oyster
(
Crassostrea gigas
)
three times per day from 3 dph
to 10 dph at a density of 10 inds/mL~20 inds/mL.
From 8 dph to 24 dph, rotifers (
Brachionus
sp., small
strain) enriched with Algamac 3 050 (Aquafauna
Bio-Marine, USA) were supplied to each rearing tanks
at a density of 10 inds/mL~15 inds/mL. Starting from
19
dph,
Artemia
nauplii enriched with Algamac 3 050
(
Aquafauna Bio-Marine, USA) were added into fish
tanks, the feeding density was gradually increased
from 5 nauplii/mL (19 dph) to 20 nauplii/mL (25 dph),
from 32 dph, micro-particulate feeds (Otohime B2, C1,
C2, Marubeni Nisshin Feed Co., Ltd. Japan) were
used starting from smaller size to large size. The
amounts of micro-particulate feed were distributed by
hand and adjusted according to fish demand at 1-h
intervals from 0900 hours to 1 900 hours each day.
Weaning was completed on 36 dph. Outlet screens
were cleaned, and tank bottoms were siphoned daily
to remove dead fish, uneaten feeds and faeces.
2.4
Description of Embryonic Development During
Endogenous Feeds Period
Approximately 5 000 eggs were taken from the egg
collectors within an hour of spawning. The eggs were
incubated at ambient water temperature (21
℃
)
in a
200
L semi-conical tank supplied with 5µm filtered
seawater flowing at 0.8 L/min. Light density was