Production of Cobia

The cobia, Rachycentron canadum is a marine fish that lives upto a water depth of 1200 m. It is a candidate species for aquaculture because of its fast growth rate, excellent meat quality and high demand in the domestic and international market. The white meat of the fish is served as raw fish, Sashimi. Male and female fish attain sexual maturity at 1-2 years and 2-3 years of age, respectively. It exhibits annual oceanodromous migration. It is carnivorous and mainly feeds on fish. Its colour is dark-brown dorsally, pale-brown laterally and white ventrally with a black lateral band, as wide as its eyes, expands from snout to the base of the caudal fin, bordered above and below by pale bands. The body is elongated and sub-cylindrical, but the head is broad and depressed. Scales are small, embedded in thick skin. Absence of air bladder is one of the peculiar characters.

Rachycentron canadum

Seed Production

Broodstock management

The sub-adult fish is collected from wild or farm for developing into broodstock. Wild fish is collected by hook and line, anaesthetised with 25 ppm clove oil (Eugenol) solution and is transported by boat in covered FRP tank with aeration facility. The biomass of the fish should not exceed 50 kg/m³ in the tank during transportation. The fish is released into cage after measuring its body weight and length. Thereafter, the gravid brood fish are selected, anaesthetised with clove oil (10-20 ppm) and transferred to quarantine tank after giving freshwater dip for 3-5 minutes. In order to remove ectoparasites and other microbes, the fish is kept under quarantine for a period of 3 weeks.

Quarantine

After quarantine, the fish having not less than 5 kg weight is transferred to the maturation tank (25-100 t capacity) having re- circulation facility. The brood fish is fed twice daily at 1-2% of the body weight with formulated fish sausage supplemented with vitamins and minerals for ensuring balanced nutrition.

Fish sausage

Water quality management

It is practiced as explained for the milkfish.

Spawning

The fish is anaesthetised and gametes collected periodically using a 40-60 cm long, transparent and flexible cannula (1 mm inner diameter and 2 mm outer diameter) guided into the urino-genital orifice of male and oviduct of female to determine gonadal development. Then suction is applied at the other end as the cannula is withdrawn and the cannulated sample is placed in a petri-dish and observed under a microscope for determining the size and stage of development of ovum of female and the density of milt of male.

Cannulating brood fish

The female fish with ova of minimum 600 um diameter and male with thick milt is selected in 1:1 ratio for induced breeding. The females are injected with LHRH-a at 20 qg/kg, and males are injected with 10 ug/kg. After hormone injection, spawning occurs within 36-40 hours at 26-27°C. Fertilisation occurs externally, and the fertilised eggs are transparent, globular and cream coloured having 1.35-1.40 mm diameter size with a single oil globule. The fertilised eggs are floating while the unfertilised eggs generally sink to the bottom. Fecundity is 2-4 lakh/kg bodyweight.

Hormone administration

Incubation

After 12 hours of spawning, the eggs are collected using a large scoop net (1x1x1 m) of 300-400 qm mesh size and kept in graduated buckets with mild aeration. The fertilised eggs are measured, disinfected with betadine (100 ppm) dip for one minute, and the rate of fertilisation is determined by taking a sample. They are incubated in cylindro- conical FRP tank (500 1) at a density of 300-500 no./l with mild aeration or directly stocked into indoor nursery rearing FRP tank (10-30 t) at a density of 10-20 no./l. The eggs hatch-out within 21-24 hours at 27- 30°C. The average hatching rate is 80%. The newly hatched larva has an average total length of 3.5 mm with yolk sac. The mouth of the hatchling appears on 3^rd dph with an opening of about 200 pm size.

Live feed production

The method is same as that explained for the milkfish.

Rearing of larvae

If the incubation is done in cylindro-conical FRP tank, the 3 dph larvae at a density of 5 no./l are transferred to indoor FRP tank (10-30 t capacity) containing filtered seawater, kept as such after removing the waste. Pure live microalgal culture of Nannochloropsis oculata is directly added to larval rearing tank to maintain the required phytoplankton concentration of 50,000 cells/ml (green water technique) with mild aeration. It helps in maintaining water quality, controlling bacteria, enriching zooplankton and easy feeding of larvae. Larval feeding can be initiated on 3^rd day for 2-3 times with enriched Brachionus plicatilis at a density of 10-15 no./ml until 12 dph. Artemia nauplii are used for feeding from 6 dph upto 10 dph. Usually 2-3 lakh nauplii hatchout from l g of high-quality cysts. Enriched artemia is fed from 8 dph to 28 dph. Weaning on artificial diet starts from 15 dph.

Artemia hatching tank & Artemia enrichment tank

Feed requirement

Metamorphosis of larvae takes place within 10-15 dph. By this time, the colour of the larva changes from dark reddish-brown to black. During metamorphosis, there is a change from cutaneous respiration to gill respiration, which can lead to mortality due to respiratory distress. During this time, addition of pure oxygen shall be provided along with aeration. Cannibalism is a serious problem in larval rearing due to its size variation, which can be controlled by proper size grading and by increasing feeding frequency. Bottom siphoning should not be done upto 25-30 dph, but the water quality and tank bottom condition should be maintained by adding probiotics. The young-one attains the size of 0.4 g within 25-30 dph.

Rearing of fry

After 30 dph, the larvae are size graded frequently, and the different size groups are reared separately in separate FRP/cement tank (30 t capacity) containing filtered seawater. The seeds of 1-1.5 cm are stocked at 1000 no./t. The fry is fed initially with a formulated micro- encapsulated feed of 800 pm size and progressively changed into an extruded larval feed of 1.2 mm in 35 dph and 2.2 mm in 40 dph. Water quality parameters like salinity, temperature, pH, DO, ammonia etc. are closely monitored. Daily water exchange at 100- 200% is done. Feeding frequency is 4-5 times depending upon the water quality and feed intake. The seed is harvested at 2-5 g size by 45 dph.

Packing and transportation

The seed is usually transported for long distance in 1-3 t tanks with oxygenation or/and aeration facility. DO level is maintained above 8 ppm using oxygen cylinders for which one tonne tank requires 340 cm³ cylinder capacity for each hour. The seed is transported in an open tank with maximum biomass of 10 kg/m3 with water. It is starved for 48 hours before transportation. Mild sedation is given with 2 ppm Euginol. To control ammonia, de-ammonifier like 5 ppm Ammonil is used.

Farming in Cage

Site selection

Being a marine fish, it can be cultured in marine waters and also in brackish waters having minimum 15 ppt salinity. A calm area having 10-30 m water depth, 30-50 cm/s water current velocity and low wave with sandy or rocky bottom is suitable for sea cage farming. Transparency should be more than 5 m. The site should also be free from any type of turbidity. Other aspects are similar to those explained for the pearlspot.

Water quality requirement

• Temperature : 23- 30°C
• Salinity : 15-35ppt
• DO : >5ppm
• Ammonia : <0.02 ppm
• TAN : <0.1 ppm
• Nitrite : <0.05 ppm

Cage design

In off-shore waters, a circular cage having 6-12 m diameter and 6 m depth is used. Cage raft with HDPE material is highly durable, resistant to direct sunlight and chemicals. HDPE cage is fabricated with 25 cm diameter pipe (10 kg/cm², 24 mm thickness) and the handrails with 11 cm diameter pipe (10 kg/cm², 10.6 mm thickness). The pipes are joined by butt fusion, a joining technique by melting the pipe (fitting) ends and holding them together. The HDPE cage raft is fabricated on a flat concrete floor or evenly levelled seashore to achieve perfect alignment and better joining of pipes. It is better to fabricate the HDPE cage raft near to the seashore for easy launching. The fabricated cages are towed to the identified location and moored using multi-point mooring system with Samson type anchors weighing 500 kg and Dyneema anchor ropes (25 mm thickness). The length of the anchor rope should not be less than 3 times than the water depth. Grid type anchoring system is preferred for the easy management with provision for allowing the individual cage to move freely.

According to the size of fish, polyester knotless inner netting with mesh size 10-36 mm are attached to the inner ring, and outer netting with mesh size 18-50 mm is attached to the outer ring of cage frame. There should be floating buoys and navigational light buoys for demarcating the cage structure.

In the protected bay and brackish water area, circular GI cage frame having 6 m diameter and 4 m depth is used. The inner ring of the frame is attached with HDPE cage net having 10-36 mm mesh size and the outer ring of the frame with a cage net having 18-50 mm mesh size according to the size of fish. The fabricated cages are towed to the identified location and moored by using multi-point mooring system with Grapnel type anchors weighing 80 kg and UV resistant PP ropes. The other aspects are the same as that prescribed for the pearlspot.

Circular GI Cage

Stocking

After acclimatisation, the seeds of 10 g size are stocked at a density of 30 no./m³ in rearing cage (10 mm mesh size). Subsequently, the cage net is replaced with large mesh size, and the stocking density is reduced to 5 no./m³ as given in Below Table.

Stocking density & mesh size

Feeding

The cobia is weaned on a floating or slow sinking pelleted feed, shrimp waste and chopped fish and fed twice (40% in the morning and 60% in the evening). Based on the body weight, the fish is fed with pelleted feed having 40-45% protein, and 10-12% fat and its details are given in Below Table.

Feed requirement

Chopped fish/prawn waste can replace feed pellets, and it is fed @ 15% of the body weight at 150 g size and is subsequently reduced to 4.5% upto 3 kg size, to facilitate faster growth rate and reduction in feed cost.

Feeding of cobia

Care and maintenance

In the case of cage farming, there is a high risk of poaching and attack by animals. Hence, a watch house with 24 x 7 watchman security and a CCTV camera has to be provided. A warehouse and a boat are provided for the storage and transportation of feed, net, etc. Any damage to the cage net, noticed through frequent inspections, is immediately repaired; otherwise, it may cause heavy crop loss. Once in a week, cleaning of cage net with a soft brush is carried out to ensure proper water exchange. To avoid bio-fouling, net exchange is done fortnightly during the initial period and gradually reduced to once in two months at the later stage. There should be one set of spare cage net for a replacement. For net exchange, the upper edge of the two sides of the clogged net is tied with the fresh net. A weight is tied at the middle part of the net for sinking into water and to render easy movement of fish to the otherside of the fresh cage net. The clogged net is slowly lifted up from the other side and lifted into a boat and brought to the shore for immediate cleaning using high pressure jet pumps of 200 psi pressure followed by sun drying.

When fish is reared in high salinity, parasitic infection of copepods may occur. Periodic dip in freshwater for 5 minutes to remove ectoparasites, keeps the fish healthier. The fish is regularly monitored for any clinical signs for the early detection of disease. Generally, good quality water, good husbandry practices and adequate nutrition keeps away diseases. Once in a month, the length and weight of the fish are measured followed by grading and thinning. As the fish grow, it is transferred into cages with larger mesh netting. Quantity of feed also is determined accordingly. Water quality parameters are checked regularly. During periods of bad weather, anchors and fastening ropes should be checked. The fish is taken-out fortnightly using hand net and health is examined. The growth pattern is given in Table 18.4.

Growth pattern

Harvesting

In the case of cage farming, harvesting mainly depends on climate. Hence, culture activities should start by September and end before the onset of monsoon. The fish usually attains 2.5 kg within 9 months. It has both domestic and export market. When growth is non uniform and a few among the stock attains marketable size, selective harvesting of larger fish from 7^th month onwards is ideal which facilitates the smaller one to grow at a faster rate. Fish is starved for one day prior to harvest to keep its gut empty. The fish is harvested by scooping out using a hand net and is placed into a chilled container, chill killed, bled and packed in ice prior to transportation. Feeding with chopped fish during the end of the culture period gives better growth of fish. A production of 16 kg/m³ can be expected from one crop. Survival rate in cage farming is 85%.

Source : Department of Fisheries, Government of Kerala