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.
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.
- 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.
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.
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.
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.
- 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.
- 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.
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