Cobia identification, breeding biology cycle, nursery, grow out farming

1. Cobia Farming
CobiaOrder:Carangiformes
Family:Rachycentridae
Genus:Rachycentron
Species: R. canadum
By:
B. Bhaskar

2. Introduction
• Aquaculture research with cobia was first reported in 1975
with the collection of wild caught cobia eggs off the coast
of North Carolina.
• Larval development was described and, after the
termination of the 131 day rearing trial, it was concluded
that cobia had good aquaculture potential because of its
rapid growth and good flesh quality.
• Additional research on cobia was conducted in the late
1980s and early 1990s in the USA and Taiwan Province of
China, with the first reported captive spawning of the
species occurring in the early 1990s in Taiwan Province of
China Research continued, and by 1997 the technology to
raise large quantities of cobia fry had been developed and
Taiwan Province of China was producing juvenile fish for
grow-out, mostly in nearshore cage systems.

3. • Cobia - Rachycentron canadum is a warm water marine
species that is widely distributed around the world especially
in the tropical and subtropical regions except for Central and
Eastern Pacific. It is the only species that belongs to
Rachycentridae family. It is considered offshore species but
found throughout the water column (0 – 1200m).
• Cobia is considered as one of the most potential species for
aquaculture due to its fast growth and good flesh quality.
• Cobia breeding was done successfully in Taiwan during 1997.
In India, Central Marine Fisheries Research Institute (CMFRI),
Mandapam, took up Cobia breeding druing 2010. At present,
Cobia seed production is being taken up at a commercial level
by the Rajiv Gandhi Centre for Aquaculture (RGCA), Sirkali,
Nagapattinam District, Tamil Nadu and CMFRI.
•

4. Identification and Biology of Cobia
• Identification: Dorsal spines (total): 7 - 9; Dorsal soft rays (total): 26-
33; Anal spines: 2-3; Anal soft rays: 22 - 28.
• This species is distinguished by the following characters: head broad and
depressed; first dorsal fin usually with VIII short but strong isolated spines,
not connected by a membrane; caudal fin lunate to emarginate in adults,
upper lobe longer than lower, rounded in young with prolonged central
rays. Colour of back and sides dark brown, with 2 sharply defined narrow
silvery bands, belly yellowish.
• Biology: Adults occur in a variety of habitats, over mud, sand and gravel
bottoms; over coral reefs, off rocky shores and in mangrove sloughs;
inshore around pilings and buoys, and offshore around drifting and
stationary objects; occasionally in estuaries.
• Form small groups and may pursue small pelagic inshore. Feed on crabs,
fishes, and squids.
• Form spawning aggregations during the warm months in the western
Atlantic; eggs and larvae planktonic.
• Caught in small quantities due to its solitary behavior. Caught mainly with
handlines, and by trolling pelagically offshore over the continental shelf.
Good food fish; marketed fresh, smoked, and frozen. Also caught for sport.

5. Production cycle of Cobia- Rachycentron canadum

6. Broodstock source, transportation and management
• Broodstock source, transportation and management
• Brood fishes are collected from the wild with the assistance of local anglers due to
their practical knowledge of species and their ability to easily find fish
aggregations.
• The collected fishes are transported to the hatchery in 500 – 1000 L tank with a
maximum stocking density of 50 kg/m3, temperature at 22 – 24OC and oxygen at 7
to 12 mg/L. The ice packs are used to lower the temperature and pure oxygen is
supplied using oxygen cylinders.
• The brood fishes are anesthetized to facilitate the handling and transportation.
Clove oil is used as an anesthetizing agent at a rate of 10 – 40 ppm; for light
sedation 10 ppm and for profound sedation 40 ppm can be used.
• After reaching the hatchery, fishes are treated with fresh water for 2 minutes and
100 ppm formalin bath for 3 minutes to remove the external parasites and lesions.
The fishes are kept under quarantine for 2 – 3 weeks for acclimatization to captive
conditions.
• Brood fishes are fed a diet of sardine and squid on an alternate basis. Three months
before the onset of spawning in Cobia, supplementary artificial diet (prepared with
use of sardine and squid, olive oil, atta, spirulina, vitamins and mineral mixes) is
provided twice daily at 3-4 percent of weight. It is continued until the end of the
spawning season.
•

7. Artificial propagation
• Male Cobia mature at the second year (size of 53 cm length) and the female mature at the third year (size of 68 cm
length).
• Cobia spawns during the month of March to September around Indian seas, but it spawns round the year under
captive condition when favourable environmental conditions are provided.
• Fishes are anesthetized for weight measurement, tagging and confirmation of its sexual maturity. A gonadal sampling is
done through a catheter (1mm dia) to differentiate the sexes. The brood fishes are inspected periodically for gonad
development by cannulation.
• Female oocytes of diameter 600 – 700 µm (700 µm is preferable) condensed with active milt of male are considered
ideal for spawning.
• Luteinizing hormone-releasing hormone (LHRH) or Human Chorionic Gonadotropin hormone (HCG) are administered
to induce the fish. The administrated LHRH is injected to the brood fishes at a dose of 20 μg/kg for female and 10 μg/kg
for the male fish or a single intramuscular injection of HCG is injected to the brood fish at a dose of 500 IU/kg of body
weight for female and 250 IU/kg of body weight for male fish.
• The fishes are anesthetized to avoid the handling stress and administer hormonal injection.
• After injection the fishes are shifted to the spawning tank of 50 – 70 m3 with sex ratio is 1:2 or 2: 3 (female: males).
Spawning usually occurs 39 hrs after administration of hormone.
• A 23 kg female brood fish releases about 2.1 million fecundity and a 35 kg female releases about 3.5 million fecundity.
• The fertilization rate recorded ranges up to 90%. The fertilized eggs are buoyant and kept afloat by large oil globule in
the egg until they hatch out. The unfertilized eggs are opaque.
• The fertilization rate is higher during onset breeding season and a little lower during offset breeding season. The
fertilized eggs are yellowish brown colour and freshly spawned egg size range between 1.0 – 1.1 mm dia.
• The spawned eggs are collected using an egg collector by airlifting method or the hand net having a mesh size of 500 µ.
• The collected eggs are disinfected using 5% iodine solution for a minute. The water should be sourced from the
breeding tank to avoid the stress.
• For spawning, the required temperature is 25 – 29OC; it can be obtained using chillers. The preferred dissolved oxygen
level is 7 – 12 mg/l. The recommended salinity is 25 – 35 ppt.
• Other water quality parameters need to be taken care of and kept at desirable limits.

8. Nursery rearing
• Tank preparation
• The Larval Rearing Tank (LRT) is prepared 4 – 5 days prior to egg stocking. The tanks should have 60 percent
water of the total water holding capacity. Add the probiotics (10%) and allow it to develop beneficial
microorganism, which provides a suitable environment to rear the larvae.
• Feeding
• Stocking of eggs @ 13 – 15 numbers per litre of water is done. A newly hatched larva nourishes itself for first
three days. At the end of the third day of post-hatchling (DPH), larval mouth opens and feeding need to be
started.
• Feeding can be taken up on the same day or next day morning depending on the percentage of the larvae with
opened mouths.
• The green water system is maintained at the proper concentration of microalgae such the Isochrysis
galbana or Nannochloropsis salina at 10000 cells/ml in LRT up to 3rd DPH.
• Initially, the larvae are fed with Rotifer at 5 ind. / ml of water and increased to a maximum of 15 ind. /ml of
water. Daily two rations of enriched Rotifer are recommended to obtain healthy larvae, and this can be done
using alga (I. galbana or N. salina) or commercial product (Algamac 3050).
• Larvae are fed the Rotifer upto 11th DPH. The larvae are fed with newly hatched Artemia nauplii on 7th DPH and
enriched Artemia can be provided on 9th DPH to 25 – 27th DPH. It depends on the artificial feed acceptance and
size of the larvae.
• The weaning feed is normally started on 14th DPH. Initially, 200 – 300µ size feed is given to the larvae then
steadily increased to 300 – 500 µ, 500 – 800 µ, 1200 µ upto 30th DPH. The suggested feeding frequency is five
when weaning feed is started, and it can be reduced to 3 times at the end of larval rearing. Excess feed may
quickly deteriorate the water quality.
• Hence Cobia larvae are to be feed until satiation using pulse feeding technique. It is a prerequisite to reduce
the cannibalism rate.
• Metamorphosis of larvae from the cutaneous mode of respiration to gill respiration takes place as early of 11 -
15 DPH depending on the water temperature. This stage is critical as it can lead to distress and subsequent
mortality. It is hence required to maintain higher dissolved oxygen @ 8 – 12 mg/l.

9. Nursery rearing of Cobia
• 1st Phase: Rearing of cobia fry(0.2) to 2-5gm size
rearing period 20-45 dph in RCc tanks, the larvae are
totally weaned to pellet feeds, size grading
recommended every week.
• 2nd phase: rearing of fingerlings from 2-5gm to 30 gm
juveniles
• Rearing period 45 to 75 dph in RCC tanks or in hapas
cages in ponds.
• Feeding fingerlings fed manually 3-4times/daily
• 3rd phase: rearing of juveniles from 30 g to 600-1000 g
• Rearing period: 75-150 dph to 180 days. Rearing can
be either inshore cages or out door ponds.

10. • Water quality
• Vigorous aeration must be avoided as it causes stress to larvae. Therefore, pure oxygen is added to the LRT to maintain
the ideal dissolved oxygen (7 – 12 mg/l). Other water quality parameters should be kept under the desirable limit. Total
ammonia nitrogen and pH to be maintained at less than 0.1 mg/l and 8 mg/l respectively. Water samples should be taken
daily to evaluate the water quality parameters in the culture system.
• Probiotics, water flow, and photoperiods
• The daily addition of probiotics is administrated at the concentration of 103 to 105/ml of CFU (5%) to the LRT. The
probiotics are mixed with water, allowed to activate (almost for 1 hour) and then are distributed slowly in the LRT. Water
flow is to be started between 15 – 20th DPH depending on water quality. Natural cycle of 13 – 14 hrs of light and 10 – 11
hrs of darkness is maintained throughout the nursery period of Cobia larvae. The bottom of larval rearing tank is cleaned
after 20th DPH depending on the degree of accumulation of debris and dead larvae. The swimming capabilities of larvae
are limited and hence care must be taken while siphoning the water. If possible, the activity must be done without
disturbing the larvae. If necessary treatment with Oxytetracycline at 5 ppm can also be resorted to.
• Larval rearing
• Cobia larvae are normally reared in cement cistern in India. A suitable size for larval rearing in a tank is 3 – 10 m3 in
volume with an average depth of 1.0 – 1.5 m.
• The stocking density ranges from 5 – 15kg/m3. There is clear indication that the growth rate decreases with increasing
stocking density. At higher stocking density, the growth and survival decrease and food conversion ratio decreases. The
larvae are fed with artificial diet at 6 – 8 percent of body weight. Daily three rations are recommended.
• The cobia larvae accomplish 25 – 43g in 90th DPH depending on the culture environment, stocking density and amount of
feed given.
• The size grading is very important to reduce the cannibalism. After harvesting from the nursery, fishes should be graded
alike small and larger size group and maintained separately within the same regime.
• In larval rearing, 100 – 200 percent of water are exchanged daily but it increases electricity cost; thus, the flow rate can
be adjusted according to the water quality.
•

11. • Nursery
• In Taiwan Province of China, cobia are reported to be raised in a series of
outdoor ponds until they reach a large enough size to be stocked into a
nearshore or offshore grow-out cage system.
• During the larval rearing stage, 'greenwater' nursery ponds <5 000 m2 in area
and 1-1.2 m deep with an adequate bloom of Chlorella, copepods, and rotifers
are utilised.
• This method typically results in larval survival of 5-10 per cent from hatch to
day 20, after which time the fish are moved to two or three pond systems
during the next 2 months, depending on the characteristics of the operation.
To reduce cannibalism and size variability cobia are graded weekly after day 45
post-hatch until they reach approximately 30 g (around day 75 post-hatch),
which is considered the minimum size for stocking in cages.
• Cobia are fed 5-6 times a day to satiation at a rate of 5 per cent body weight up
to 30 g; after this the feeding rate is reduced to 2-3 per cent body weight as the
fish approaches 200 g.
• Some producers continue raising the juvenile fish from 30 g up to 600-1 000 g in
outdoor ponds, while others use smaller (20-200 m3) nearshore cages.
• From this point onwards the overall goal, whether in ponds or cages, is to raise
the young cobia to a point where they are large enough to be stocked into a
grow-out cage system, yet small enough to be transported in large numbers
with minimal mortality.

12. Grow out
• In India, the grow-out of Cobia is practiced through open sea
cage farming rather than pond or cement cistern culture.
• The suggested stocking size is about 50 – 100 g. A greater
survival is obtained with increasing stocking size. However,
Cobia reaches about 50 g (40 – 80g) in 110th DPH and 100 g
(65 – 120 g) in 150th DPH.
• The growth rate of Cobia is reported to vary broadly and it
depends on the culture environment. It reaches about 2 – 6
kg in a year. An average weight of 6 kg has been reported with
a stocking density of 13.3 fish/m3 and 3.5 kg recorded with a
stocking density of 23.3 fish/m3.
• A lower stocking density would increase the fish production
with better FCR. The FCR value ranged from 1.3 – 2.2

13. Recommended stocking density and feeding based on
body weight of Cobia
• Recommended stocking density g/fish to no of fish/m3
• 10-50g size 30 nos/m3
• 50-500g size 18 nos/m3
• 500-2000 g size 14 nos/m3
• 2000-4000 g size 8nos/m3
• 4000-6000 g size 5nos/m3
• >6000 g size 4 nos/m3
• Feed pellet dia size(mm) based on fish body weight(g):
6.3-6.7mm- 140-243g.
• 8.3-8.7mm – 240-625gm
• 9.3-9.7mm- 620-1150gm
• 12.3-12.7mm – 1140-3350gm
• 15.6-16.2mm- > 3350 gm

15. • Ongrowing Techniques
• Early on in the cobia production cycle Taiwanese methods, which utilise outdoor
ponds for broodstock and nursery phases, tend to be more extensive when
compared to current efforts in the USA, which typically involve tank culture of
broodstock and early juveniles.
• From that point on however, grow-out methods are similar in both locations, as
they utilise net pens or cages of various sizes and types to rear the cobia to
harvestable size.
Successful grow-out of cobia has been reported in nearshore and offshore cages,
utilising both surface and submerged systems during the longest and final stage
of production.
• Taiwanese producers use 1 000-2 000 m3 cages, while some operations in the
Caribbean have used 3000 m? submersible systems successfully.
• In order to minimise grow-out time as well as disease issues, cobia produced in
cages should be located in sites that provide warm (26 ºC and above) clean water
and adequate flow rates through the cage system to provide high dissolved
oxygen levels continuously.
• Harvest numbers vary depending on the stocking rates and water temperature,
but the grow-out period for pellet fed cobia is generally about 1-1.5 years, with
fish reaching a final weight of 6-10 kg at harvest densities of 10-15 kg/m3.

17. • Feed Supply
• Cobia producers in Taiwan Province of China use both floating and sinking
pellets (42-45 per cent crude protein, and 15-16 per cent lipid), typically fed
6 days a week at a rate of 0.5-0.7 per cent BW/day towards the end of the
grow-out phase. Cobia feed conversion in Taiwan Province of China is
reported to be approximately 1.5:1. Operations in the Caribbean have used
pellets manufactured in the USA that are typically higher in crude protein
(50- 53 per cent) with 10-15 per cent lipid content.
• Harvesting Techniques
• It is reported that producers in Taiwan Province of China make selective or
partial harvests of cobia from cages allowing them to meet current market
demands, whether the fish are consumed domestically or destined for
export. Little published information is available regarding cage harvest
methods for cobia, although they would certainly be site specific and likely
to employ techniques to crowd and net or pump fish and place them into a
chilled container on a tending vessel, similar to those used in salmon
farming.

20. References
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