Successful seed production of cobia Rachycentron canadumand its prospects for farming in India

G. Gopakumar, G. Syda Rao, A. K. Abdul Nazar, C. Kalidas, G. Tamilmani, M. Sakthivel,V. Ashok Maharshi and K. Srinivasa RaoMandapam Regional Centre of CMFRI, Mandapam

Introduction

Availability of adequate quantity of high valuemarine finfish seed is the major prerequisite forinitiation and expansion of finfish mariculture.Breeding and seed production of marine finfishes ofhigh value have been expanding in recent yearsinternationally. Large quantities of hatchery producedseeds meet the need for sea cage farming in manycountries (Hong and Zhang, 2003). It is wellunderstood that the first step towards seedproduction technology is the development ofbroodstock. Prior to 1980s, broodstock of finfisheswere grown mainly in indoor concrete tanks. Sinceearly 1980s, wild-caught broodstock have beenraised either in outdoor earthen ponds or insea cages. It has been proved that broodstockdevelopment in sea cages was highly effective inimproving gonadal development for most finfishgroups like groupers, pompano, red seabream,cobia, Japanese flounder and yellow croaker. Thedevelopment of hatchery technology for commerciallevel seed production of marine finfishes is still in itsinfancy in India, except for the Asian seabass, Latescalcarifer. Hence, research and development needto be focused in evolving technologies for the seedproduction and farming of highly priced marine foodfishes.

In recent years, seed production and farming ofcobia (Rachycentron canadum) is rapidly gainingmomentum in many Asian countries (Liao and Leano,2007). Cobia is distributed worldwide in warm marinewaters. They are found throughout the water columnand are caught in both coastal and continental shelfwaters, although they are typically considered to be anoffshore species. Wild-caught cobia does not supporta major commercial fishery and generally is consideredas incidental catch. Sexual maturity is reported in males

at 1-2 years and in females 2-3 years, with femalesgrowing larger and faster with maximum size upto60 kg (Shaffer and Nakamura, 1989).

Fast growth rate, adaptability for captivebreeding, low cost of production, good meat qualityand high market demand especially for sashimiindustry are some of the attributes that makes cobiaan excellent species for aquaculture. Under cultureconditions, cobia can reach 3-4 kg in body weight inone year and 8-10 kg in two years. The species hasa protracted spawning season and it can spawn incaptivity. The fecundity is very high. Aquacultureresearch on cobia was first reported in 1975 with thecollection of wild caught cobia eggs off the coast ofNorth Carolina. Larval development was describedand after the termination of 131 day rearing trial, itwas concluded that cobia has a good aquaculturepotential because of its rapid growth and good fleshquality. Further research on cobia was conducted inthe late 1980s and early 1990s in the USA andTaiwan Province of China. Research continued andby 1997 the technology to raise large quantities ofcobia fry was developed and Taiwan Province ofChina was producing cobia juveniles for grow-outmostly in nearshore cage systems. Cobia productionis also reported in the United States, Puerto Rico,Bahamas, Martinique, Belize, Brazil and Panama(Bennetti et al., 2008). Envisaging the prospects ofcobia farming in India, broodstock development wasinitiated at the Mandapam Regional Centre of theCentral Marine Fisheries Research Institute in seacages in 2008 and the first successful inducedbreeding and seed production was achieved duringMarch - April 2010.

Cobia broodstock development - general aspects

The capacity to produce large, dependablequantity of quality seeds is the key for establishing

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reliable and sustainable cobia aquaculture. The majorbottlenecks in the development of commercialaquaculture are the control of reproductive processesof fish in captivity and production of biosecure andquality-certified fry. Broodstock management usuallyincludes collection, selection and domestication ofbrooders as well as control of maturation, spawningand egg collection. Wherever available, cobiabroodstock can be procured from the wild during thespawning season, then transfer them to culturesystems for rather short time, and either spontaneousnatural spawning or hormone induced spawning canbe obtained.

Cobia being a very active fish which grows tolarge size, broodstock development is mostlypractised in sea cages in order to ensure good waterexchange and healthy environment. Brood fishes canbe stocked at a density of about 2 kg/m3. Trash fishes(sardines, scads etc.) are fed once in a day at therate of 5% of biomass or till satiation. The trash fishhas to be supplemented with vitamins and HUFA (fishoil, squid liver oil). Broodstock nutrition is veryimportant and there is positive correlation betweenHUFA in the broodstock diets and in the eggs andlarvae.

Cobia attains maturity when the fish is about twoyears old. It has a protracted spawning season inthe Indian seas. It spawns under captivity naturallyas well as on inducement. It has high fecundity,ranging from 0.05 to 0.25 million eggs per kg. Biggerfishes of 10-15 kg weight having normal shapewithout any deformity and with healthy behaviour areselected as broodstock. The other important criteriafor broodstock include bright colour and with anuseasily recognisable. Broodstock nutrition plays a keyrole in the quality and viability of the larvae. Besttemperature for maturation is around 27 ºC and thebest salinity range is 30-34 ppt. Separation of malesand females from the broodstock cage is requiredfor conditioning the fish for breeding. It is requiredfor controlling the breeding and planning the seedproduction. The best time for separation is one monthbefore the breeding inducement. Conditioning thebrooders ensures best maturation as well as egg andlarval quality. Cannulation can be done to assessthe maturity condition of the female. The maturationcharacteristics of female include egg with size above0.7 mm, non-adhesive, white colour and round Fig. 1. Broodstock cages for cobia at Mandapam

shape. In the case of mature males, by gentlypressing the belly, the milt oozes out. Brooders arecharacterised by big belly, chasing behavior and red,swollen anus. The selected brooders can be broughtfrom the cages and transferred to cement tanks.Usually two males and one female are introduced tothe spawning tank. Natural spawnings also can beobtained if brooders are selected properly. Inductionof spawning can be done by injecting LHRHa20 µg kg-1 for females and 10 µg kg-1 for males.Spawning occurs within 12-24 h after the injection.Egg collection can be done manually from the tankby employing 500 µm net.

Optimisation of captive broodstock managementprotocols still remains a challenge to establish reliablebio-secure hatcheries with genetic diversity andconsistent production of high quality eggs and larvae.

Broodstock development and captive breedingat Mandapam

The broodstock at Mandapam was developedin sea cages (Fig. 1 & 2) of 6 m diameter and 3.5 mdepth (Gopakumar, 2008). The wild collected cobiabrood fishes in the size ranging from 2-10 kg weightwere stocked during December 2008 to February2009. The fishes were stocked without separatingsexes. All the fishes were collected from the hooksand line commercial catches. After transporting tohatchery, the fishes were treated with 100 ppmformalin for 2-5 min and conditioned for two to threedays in 10 t FRP tanks before transferring to cages.These fishes were fed twice daily at 0900 and1530 hrs with sardines (Sardinella sp.) and other fishspecies like Pellona and Ilisha and occasionally withsquids and portunid crabs @5% of their body weight.Vitamin and mineral supplements were also given

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twice a week along with the feed in order tocomplement any possible nutritional deficiencies intheir diet. A total of 40 fishes were stocked in fourcages. The length range and corresponding weightrange of the brood fishes recorded during April 2009ranged between 80 and 127 cm, and 4 and 20 kgrespectively. The sexes were separated bycannulation using a flexible catheter (2 mm innerdiameter) in June 2009 and stocked in separatecages. Thereafter, the females were cannulated(Fig. 3) every fortnight to assess the diameter of theintra-ovarian eggs.

On 11.03.2010, one of the females withintra-ovarian eggs (Fig. 4) of around 0.7mm size wasselected for induced breeding. The size of the femalewas 120 cm in total length and 23 kg in weight. Twomales were also selected from the male cage. Thesizes of the males were 100 cm and 103 cm in totallength and weighed 11 kg and 13.5 kg, respectively.On the same day, the selected brooders were

introduced in a 100 t roofed cement tank holdingabout 60 t of seawater. At around 1300 hours, thebrooders were induced for spawning with HCG atdoses of 500 IU per kg body weight for female and250 IU per kg body weight for males (Fig. 5).Spawning (Fig. 6) was noticed at 0430 hours on13.03.2010. The total eggs spawned wereestimated as 2.1 million. About 90% fertilisationwas recorded. Fertilised eggs (Fig. 7) amountedto 1.9 million. The eggs were collected by a500 µm mesh and stocked in incubation tanks atvarying densities.

The eggs were hatched after 22 h of incubationat a temperature range of 28-30 ºC (Fig. 8 & 9). Thepercentage of hatching was 80% and the totalnumber of newly hatched larvae was estimated as1.5 million. The newly hatched larvae measured2.2-2.7 mm in total length. The mouth opening wasformed on 16.03.2010 (on 3rd day post-hatch) at alength of around 200 µm.

Fig. 2. Cobia broodstock in cage Fig. 3. Cannulation of cobia to assess the maturity

Fig. 4. Cannulated intra-ovarian eggs Fig. 5. Administration of hormone for final oocytematuration and spawning

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Larviculture and seed production - general aspects

Cobia eggs are pelagic with single oil globulewhich is resorbed completely at 7 dph (day post-hatch). The egg diameter is 1.4 mm. Hatching occurs26 h after spawning at 27 ºC. Newly hatched larvaemeasures 3.4 mm size (Holt et al., 2007). Thoughthe larvae are vigorous, they are very sensitive toenvironmental conditions. However, they are moreresistant to some stressors when compared to othertropical marine fish larvae (Liao et al., 2004). Larvalmouth opens at 2-3 dph (temperature dependent).Metamorphosis starts from 9-11 dph.

Newly hatched larvae (Fig. 10) generally startfeeding at 3 dph and they can be fed with the enrichedrotifer (Brachionus rotundiformis) @ 10-12 nos./ml,four times a day till 10 dph. From 8-10 dph, the larvaeare fed with enriched Artemia nauplii @ 1-3 nos./ ml,4-6 times per day. During the rotifer and Artemiafeeding stage, green water technique is used inthe larviculture system with the microalgae

Nanochloropsis oculata at a cell density of1x105 cells/ml. Weaning to artificial larval diets isduring 18 to 25 dph. While weaning, formulated feedhas to be fed 30 min before feeding with live feeds.Continuous water exchange is required duringweaning. Between 25-40 dph, the larvae are highlycannibalistic and hence size-grading is undertakenevery four days to one week. During this stage thefry could be weaned totally to artificial diets. Larvalrearing can be practised both intensively in tanksand extensively in ponds. The major factors affectingthe growth and survival of larvae are nutrition,environmental conditions and handling procedures.Since there is high demand for essential fatty acids(EFAs), enrichment protocols are needed for livefeeds. During the first 12 days of larviculture, waterexchange has to be gradually increased from10-100%. Surface skimmers are employed to removeoil particles from the water surface. After 18 dph,recirculation system is preferred. The environmentalconditions required during the larviculture period are

Fig. 6. A view of the spawning behaviour of cobia insidethe spawning tank

Fig. 7. Fertilized eggs collected on 500 µm mesh

Fig. 8. Development of the embryo Fig. 9. Hatching of larva

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DO: >5 mg/l, NH3: <0.1 mg/l, pH: 7.8–8.4, Salinity:25-35 ppt, water temperature : 24-33 °C (Liao et al.,2004).

Larviculture protocols developed at Mandapam

Larviculture protocols were developed byappropriate management of live feeds in suitablequantities and also taking into consideration thenutritional requirements of the larvae. The larvaewere stocked in FRP tanks of 5 t capacity forlarviculture (Fig. 11). The intensive larviculture tankswere provided with green water at a density of about1 x 105 cells/ml and rotifers enriched with DHASELCO at a density of 6-8 nos./ml from 3 to 9 dph.The critical stage for the larvae was 5 to 7 dph whenthey entirely resorted to exogenous feeding fromyolk sac feeding. During this period, large scalemortality (about 80%) was noted. Thereafter, themortality rate was moderate. From 9 to 21 dph, thelarvae were fed four times daily with enrichedArtemia nauplii by maintaining a naupliiconcentration of 2-3 nos./ml.

During this period, co-feeding with rotifers wasalso continued due to the presence of different sizegroups of larvae. Green water was also maintainedin appropriate densities in the larval tanks. From18 dph onwards, the larvae were fed with newlyhatched Artemia nauplii and weaning to larval inertfeeds was also started as per details given below:

Fig. 10. Newly hatched larvae

Fig. 11. Larvae in the rearing tank

Fig. 12. Fingerlings of cobia

From 25 dph, grading of larvae was started. Theshooters were fed exclusively with the artificial feedof size 500-800 µ and 800-1200 µ. On 30 dph, threesize groups of juveniles were noted with mean sizesof 10 cm (10%), 6 cm (25%) and 4 cm (65%). Thejuveniles measuring 10 cm length were ready forstocking in hapas and the other two size groupswould be ready for stocking in hapas after rearingfor another two to three weeks. All the fingerlings(Fig. 12) of 10 cm length and above were stocked inhapas in the sea for nursery rearing for about a monthbefore transferring them to the grow-out cages.

Stage of larvae Size of larvae Size of feed(dph) (cm) (µ)

18 - 19 2.3 - 2.6 100 - 20020 - 23 2.5 - 3.5 300 - 50023 - 30 3.5 - 8.0 500 - 80031 onwards > 4.0 800 - 1200

Cobia farming - general aspects

Nursery rearing

Nursery rearing of cobia generally comprisesthree phases. In first phase, 0.2 - 2g to 5g fry growrapidly to fingerlings of 8-10 cm (20 to 45 dph). Inthe second nursery phase, cobia fingerlings arereared from 2 -5 to 30 g (45 to 75 dph) in large pondswith green water or in hapas in the sea. Artificial feed

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are provided manually to satiation, 5 to 6 timesdaily. The size of the pellet feeds is increasedgradually as the fish grows. Even during this phase,grading should be undertaken. The third nurseryphase is from 30 to 600 - 1000 g (75-150 dph to180 dph). Nursery rearing is either in outdoorponds or inshore cages. Grading is undertakenonly once during this stage. It is not advisable tostock cobia juveniles smaller than 30 g size inoffshore cages, because of their weak resistanceto strong water currents and also due to thenecessity of occasional grading to preventcannibalism (Liao et al., 2004).

Grow-out farming

Cobia is cultured in offshore grow-out cages untilthey reach marketable size. Culture period rangesbetween 6-8 months. Small scale family owned cagefarms and commercial cage farms are employed forcobia grow-out farming. Usually most of the cagefarms integrate nursery and grow-out culture in onearea for convenient transfer of fish stock from nurseryto grow-out cages. Sinking and floating pellet feedsare used in grow-out cages. Cobia juveniles arecultured in smaller cages for 4-5 months until theyreach a size of about 800 g and then they aretransferd to larger cages (Liao and Leano, 2007).

Nutritional aspects

Nutrition is paramount to production successand the design of specific diets for all stagesincluding broodstock, larvae, juveniles and adults.Rotifers and Artemia enriched with Isochrysisgalbana or with commercial products along withgreen water culture provided best survival for cobialarvae (Faulk and Holt, 2005). Survival of cobialarvae was improved by addition of I. galbana orN. oculata in rearing tanks. Growth and survival ratecan be improved if Artemia feeding schedule isreduced, since cobia larvae could take larger foodsize by 14 dph. Recent work (Chou et al., 2001)has reported optimum dietary protein and lipid levelsin juvenile cobia as 45% and 5-15% dry weightrespectively. However, there is limited informationon amino acid and essential fatty acid requirementsof cobia. There is no information available on vitaminor mineral requirements of cobia. At present,commercial cobia feeds are based on those forAsian seabass or grouper and achieve acceptable

growth with feed conversion ratio ranging from1.5-1.8 (Chou et al., 2004). In areas such as Taiwan,where cobia culture is popular, cobia are fed oncea day at feeding rate of 0.5-0.7% body weight on apellet diet consisting of 42-45% crude protein with15-16% fish oil and achieve FCRs of around1.5 during the grow-out stage (Liao et al., 2004).

Diseases

Diseases caused by bacteria, viruses andparasites occur in all stages of culture of cobia. Duringlarval stage, Epistylis and Nitzchia infestation is verycommon. During the nursery stage, a viral diseaseviz., Lymphocystis is common, but not fatal as longas good water and feed management are employed.Amyloodinium ocellatum also cause problem whichcan result in high mortality if left uncontrolled.Trichodina infestations are also common during thenursery stage. Mixosporidian infections are alsoreported among cobia nursery phases causing massmortality (Chen et al., 2001). In grow-out stage, theectoparasite Neobenedenia sp. Is common whichtogether with secondary bacterial infection causesblindness to cobia juvenilies. Pasteurellosis causedby Photobacterium damsela is one of the mostcommon diseases during cobia juvenile phaseswhich can cause mass mortality. Vibriosis affectsfingerlings, juveniles and adults and the causativeagent is Vibrio anguillarum. Another bacterial problemin sea cages is caused by Vibrio alginolyticus whichresult in hemorrhages and subsequent massmortality.

Prospects

Cobia is recognised as a finfish with emergingglobal potential for mariculture. Following successfuldevelopment of cobia culture in Taiwan, this activityexpanded very fast in south-east Asia, the Americasand the Caribbean regions. Cobia has all the qualitiesrequired for a successful species for aquaculture.The global aquaculture production of cobia has beenincreasing from 2003 onwards and the majorcontributors are China and Taiwan. It has been notedthat rapid growth rate and good flesh quality of cobiamake it one of the best species for future expansionof production. Increasing the supplies fromaquaculture combined with effective marketing cansubstantially enhance cobia production in the nearfuture. The present success in the captive breeding

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Fig. 1. Species composition of dolnet catch at Arnala in2002

and seed production in India can be considered asa milestone towards the development of lucrativecobia farming in the country. However, this is only afirst step and standardisation of technologies for seed

production and farming of cobia to suit ourenvironmental conditions have to be further pursuedon a priority basis so that India can also emerge asa contributor for cobia production in the near future.

Emergence of oilsardine fishery as an alternative resourcefor dolnetters at Arnala

J. D. Sarang and Sujit SundaramMumbai Research Centre of CMFRI, Mumbai

and February 2007. On 19-1-07, a total of 18,360 kgof sardines landed with a catch per unit effort of270 kg boat-1 (Fig. 3). A total of 68 units wereoperated on that day. The dolnet was of 50 m lengthwith a cod end mesh size of 25 mm. The boat was14 m in length and were operated at 20 m depthtowards the north-west direction. A total of 104fishes were measured for length frequency. Thelength of sardines ranged between 77 and 178 mmwith a mode at 160-169 mm (Fig. 4). Most ofthem were lean with the head comparatively

Fig. 2. Species composition of dolnet catch at Arnala in2006

Fig. 3. Heap of oilsardine landed by dolnetters at Arnala

Arnala is one of the major dolnet landing centresin Thane District of Maharashtra. Dolnet is a gearexclusively used in Maharashtra and Gujarat. InMaharashtra, they are anchored to poles fixed to thesea bottom and are generally operated from Augustto May. At Arnala, approximately 375 dolnets areoperated and the operation is generally confined toa depth range of 18-22 m.

Of the total fish catch, 60% is sun dried and therest sold in fresh condition. The sun dried fish is soldthrough three outlets viz., petty merchants(70%), dry fish market (25%) and at retail market(5%). At Arnala in the year 2002, the most dominantfishery was of Bombayduck followed by Coiliadussumieri, non-penaeid prawns and Acetes spp.(Fig. 1).

Of late, the Indian oilsardine, Sardinellalongiceps, has started appearing in large quantitiesin the dolnet catches. The species composition duringthe year 2006 was more or less the same except forthe increase in sardine percentage (Fig. 2). Unusualand unprecedented landings of oilsardine bydolnetters were observed at Arnala during January

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