Page 36 - World Aquaculture Magazine - March 2011

34 March 2011
were stocked for spawning.
The broodstock adapted fair-
ly well to the reduced salinity
where they were observed to
feed voraciously and actively
swim around the STs.
Spawning and Hatching
During courtship, individ-
ual males and females were
observed to pair up where
males were swimming below
the females and continuously
hitting the belly of the females
resulting in some injuries to
their heads. Courtship lasted
for almost six hours before spawning
took place.
In 2008 the females successfully
spawned either naturally or through
induction. The fish usually spawned
in the evening and eggs were collected
the following morning. The spawning
period extended for 38 days (March
25-May 01) during which the females
yielded a total of 12,206,600 eggs out
of which 61.13 percent were good
eggs. At the initiation of the spawning
trials the floating eggs were collected
in fine-meshed 400µ collection buck-
ets placed at the overflow waters from
the spawning tanks. The volumetric
estimates (approx. 425 eggs/mL) and
quality of the collected eggs were de-
termined using a graduated cylinder.
However, it was observed that out
of the first collections of 2.6 million
buoyant eggs, only 7 percent good eggs
were separated by the measuring cylin-
der. This suggested that the eggs were
damaged during the collection process
through the overflow pipe and, hence,
Fig. 2. Egg incubation baskets.
that collection method was replaced by
manual collection of the floating eggs
by trawling a fine-meshed scoop net to
and fro across the water surface of the
STs. Fertilized eggs, averaging 1.14-1.24
mm diameter, were transferred to 600 µ
mesh incubation baskets (200 L work-
ing volume) at an average density of
360 eggs/L. All hatching buckets were
placed in 5 t rectangular fiberglass tanks
with flow-through filtered and sterilized
seawater (Figure 2). About 88 percent
of the fertilized eggs produced in 2008
were incubated in 51 ppt seawater at
a water temperature of 25-28ºC. The
hatching rate attained after 32-38 hours
was 37.1 percent. On the other hand,
865,200 fertilized eggs were incubated
in 37 ppt seawater and the hatching rate
attained was 66 percent.
In 2009 all trials of cobia spawning
and egg incubation were conducted at
37 ppt. The spawning period extended
for 28 days (March 30-April 26) during
which all fish spawned naturally at 24-
28ºC yielding 19.16 million eggs out of
which 70.96 percent were fer-
tilized good eggs. Incubated
eggs hatched after a period of
36 hours at a water tempera-
ture of 26.4ºC. The average
hatching rate recorded was 70
percent (Table 1).
Larval Rearing
The larval rearing tanks
(LRTs) used during both
spawning seasons were in-
door rectangular concrete
or fiberglass tanks ranging
in volume from 4-40 t and
all were equipped with flow-
through filtered and sterilized seawater
systems. Cobia larvae ranging in size
from 2.8-3.2 mm were stocked at a rate
of 30 larvae/L.
In the first two days posthatch (ph),
larvae in all LRTs were kept under
complete darkness by covering the
tanks with green mesh sheets. During
that period water exchange was car-
ried out during night at a rate of 100-
200 percent. On day 3 ph, the green
covers were removed and algae, Nan-
nochloropsis sp., was added daily at a
rate of 10-25 percent of water volume
in each LRT. In addition to the natu-
ral photoperiod, overhead fluorescent
lighting was provided 12 hours during
day time. Green algae, artificial light-
ing and gentle aeration of the LRTs
were provided throughout the rearing
period. The water temperature in the
LRTs ranged from 22-32ºC. From day
3 to 11 ph, rotifers Brachionus rotun-
diformis, 66-146µm, (enriched for 6-8
hours with super HUFA,2 >45 percent
ω3 fatty acids, >16 percent eicosap-