The hatchery technology for the large scale production of the seed of A.granosa, P.malabarica, M.meretrix and M.casta has been developed by the CMFRI

Narasimham, K A

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CLAM CULTURE K.A. NARASIMHAM * Central Marine Fisheries Research Institute, Cochin - 682014. Introduction The word 'clam' is popularly used while referring to the members of several bivalve families which burrow into the substratum with the help of usually well developed foot . A few clam species are also . known to attach to hard substrates with byssus threads. Clams are the most widely distributed and abundant among all the ex-ploited bivalve resources of India. The clams form subsistence fisheries all along the-Indian coast. They occur in shallow coastal waters and are usually hand picked at low tides. Also hand-operated dredges are used from plank - built non- powered craft. On global basis, the production of clams by aquaculture was estimated at 901, 374 tonnes in 1993, forming 21.8% of the total mollusc production by farming. In India, the Central Marine Fisheries Research Institute (CMFRI) developed packages of clam farming technologies for several species. However, clam farming on scientific lines is yet to take off in the country. Clam Resources The cultivable clam species belong to the families Arcidae, Veneridae, Corbiculidae and Tridacnidae. The surveys conducted at important production centres showed that an estimated 45,000 t of clams are landed annually. The black clam Villorita cyprinoides accounts for 64%, venerid clams Katelysia opima, Paphia malabarica, Meretrix meretrix and M.casta 30.9% and the blqod clam Anadara granosa 4.4% of the catches. Of all the maritime states, Kerala leads in clam production accounting for 72.5% followed by Karnataka 12.5%. Studies conducted in India on the ecology and biology of various clam species reveal that a) They show wide fluctuations in abundance both in time and space resulting in highly productive clam beds becoming barred 'and new beds appearing b) the nature of the substratum, particularly the particle size is important in limiting the distribution c) they feed on phytoplankton and detritus by filtering the water, attain first maturity at the age of 3-4 months and the meat forms from 8 to 23% of the total weight, depending upon the species and is usually high before spawning and d) they are recruited to the fishery when 5-6 months old andniajority of the clams are harvested before they are one year old. Clam Farming In clam farming, seeds are generally collected from the natural grounds and trans-Present Address: Kakinada Research Centre of CMFRI, Kakinada - 533004 134 lIve ped sus ex-the· l at 'aft;, 374 ped lmg dae Ned rita , lca, ~ of ;ing ogy :J ·in ;iug rity 5ht, uns IllS-planted in suitable areas for growing them to market. This practice of relaying the seeds on the ground in sheltered water bodies and growing them to marketable size is simple to adopt, involving little farm maintenance work. Seed collection from wild In the commercial cultivation of clams in other countries, seed requirements are mostly met by collections from the natural beds and supplemented to a smaller extent from the hatcheries. A hand-operated scoop net or a dredge having 2-5 mm meshbag is used for seed collection. A spade or showel is also used for scrapping the top layer of the substratum and the sediment is sieved. Hatchery production of seed The hatchery technology for the large scale production Of the seed of A.granosa, P malabarica, M.meretrix and M.casta has been developed by the CMFRI. About 30 adult clams are used as brood stock. They are conditioned in unfiltered seawa.ler of about 2~-30 ppt. held at 22-24°C and fed intensively with mixed micro algae rearep. in outdoor tanks. Mter about 3 weeks of conditi~ning, the clams attain full gonad development and are subjected to thermal stimulation by slowly raising the water temperature to about 32°C. If spawning does not occur they are transferred back to 22-24°C seawater and the process is repeated every two hours. Spawning can also be induced by placing the clams in the buffer solution of 9.0 pH for 1-2 hours and later transferred to normal seawater. Several years of studies in the hatchery showed that once the clams are ripe they readily ,spawn, to smaller disturbances such as change of water, addition of live micro algal food and transfer from the spawning chamber to the conditioning room. On several occasions no special techniques were found necessary. The fertilised eggs settle at the bottom and are reared in FRP tanks. After a series of cell divisions they develop into veliger larvae, which are also called straight hinge larvae or 'D' shape larvae. The unicellular microalga, IsochrY~fLTbana is gIven as food to theTarvae from d~ In the clam hatchery, biological filtration of the seawater which allows the nannoplankters and smaller algae, measuring upto 10 microns was found to be beneficial since supplementary feed is available to the larvae and spat. Mter passing through the mnbo and pediveliger stages the larvae settle on the tank bottom as spat in 7-10 days depending upon the species. The larvae c:--- ~ are reared at a density of 5 larvae/ml of ~awater. Spat settlement at 20-30% of the initial st,ock of veliger larvae is considered as satisfactory. The freshly set spat measure about 30011 and in the next 4-6 weeks ,reach 2-5 mm length. The spat are fed with mixed mic!oala.B'ae. For Iliirseryrearing these spat are transferred to 40x40x10 cm cages, covered with fine velon screen mesh and for additional protection, againSt damage by crabs and fishes, a 10m mesh nylon fish net is stitched over the cages, These cages are suspended from racks in shallow calm waters. They are periodically cleaned to remove silt, predators and foulers. Recently nursery rearing 135 of the hatchery produced seeds of P malabarica (2-3 mm length) in 25x25 mm nylon bags of 1-2 mm mesh and stocked at a density of 1000lbag and suspended from a rack in the Tuticorin bay gave highly encouraging results. In about 6 weeks the spat attain 10-15 mm length and are ready for planting on the grow-out grounds. Selection of site. Clams are cultured on the bottom in protected coastal waters such as backwaters, bays, creeks and estuaries. The occurrence of natural clam population in the vicinity generally indicates the suitability of the site for clam farming. About 1-2 hours tidal exposure at low tide' is desirable as it helps in the management of the farm, particularly to remove the predators. Prolonged exposure of the clam farm during the tidal cycle results in poor growth due to reduced feeding and in summer there may be mortality due to desiccation. The type of substratum preferred by the species varies . For example, M.casta thrives well on sandy bottom, while Agranosa prefers mud flats with greater proportion of silt and clay. Salinity tolerance limits of the species varies. V. cyprinoides prefers near freshwater conditions which are generally prevalent in areas subjected to heavy rains and freshwater drain from the land. Clam farms are located in areas where there is little wave action. Also strong water currents dislodge the clams from the burrows. Areas prone to frequent changes ofthe contour and vulnerable to pollution are avoided. Farming in open coastal waters The ground is levelled and clear~d of predators such as boring gastropods, starfishes, cr~and skates. Bamboo~s are placed on the boundary ofthe farm as markers. The movements of the clams are limited and hence in many cases fe~~is not necessary. Synthetic fibre net pens are erected in the U.S.A. to hold the clams within the farm against strong watercu:rrents and also to keep the.-m:..ed~torsaway. In the Kakinada bay, split bambo;SCreens of 0.3m height interlaced with hemp twine proved effective in confining the clams inside the faryp.. At high t~de, seeds measuring 10-251lli!Lin length are taken in a boat and planted in the farm, taking care for even disp~rsal as -far as possible. UOOven distribution is set right during the next low tide. Mter seed!!J..K thaiarm, 10 mm_§.Y!l:thetic nettiug is laid on the bottom and is held in stretche.~J~Q.sition by stakes; this net cover offers protection against predations-aild strong water currents. The stocking density varies with the species cultured -and the size of the-seed. In the case of A granosa in Malaysia, 1000 to . 2000/m2 are ' stocked and are thinned more than once to achieve a final density of 300 seed of 20-25mmlm2 which is considered as optimum. Except for the watch and ward and eradication of predators, no other maintenance work is involved during the grow-out phase. The clams are harvested after 5-6 months either by hand-picking or by a hand operated dredge. ---In the blood clam, Agranosa culture in the Kakinada bay, seed clams of 5.53 to 136 J. nylon I a rack ~ attain waters, vicinity rs tidal ;l farm, during Ir there thrives ,portion prefers a heavy , where ,am the bllution .rfishes, larkers . g~is not s within y. In the p twine tasuring care for ;he next tom and against ! species 1000 to msityof itch and i during .-picking f 5.53 to 7.08 g(length 21.8-25,1 mm) stocked at 140-175/mi have attained 25.5 032.9 g (39.2 to 42'.7rrnn) at harvest in 5-6 months. The fe rieva.1 was 3.4 to 88,6% when pen enclosutes are used and 41.5% without pen. Production rates of abou. 40tlha/5.5 months were obtained when pen ult-ure was pra,ctised and 2Hlha/6 mon h.s when pen was not used.1hus both retrieva.1 and production rates have come down by abo t 509.2 'n blo6d clam ~.ulture if pen was not uSHd. Fi ld.!. _flludies in blo d · lam culture showed that in pen · ultur, at a stocking den ity of 3001: "t production of 70tlha is possible. In the Mulky estuary in Karnataka, in experimental cultu e; Mrterf!tri:, has grown from 23.6 to 37.5 m)l1 in 4 months and the survival rate was 7~.5% . In the Vellar estuary M.casta has grown from 7.3 to 40.6 mm in one year. In'the Ashtamudillake I and at Munambam, P. malabarica has been successfully cultured by the CMFRI by utilising the hatchery raised seed. Seed clams of 10.7 - 12.4 mm length have grown to 30.4 -34.6 mm (7.8 -9.5 g) in 3.5 to 5.5 months. The production worked out to 14.3 to 59.3t/ha with retrieval at 7 to 17.6%. . , , Farming in coastal ponds Jams are 111so cuJtureq in coastal ponds. In T!!iwan after the s hack ill shrimp cultur,e due to disea~s, several shrim culture ouds are used for arming th~ dam Meretrix lusoria~ Thailand Agra7U)sa is grown along with the shrimp in the -- - - ........... shrimp cult~ ponds. The clam seeds are planted in the central ·elevated ar as of the shrimp ponds and are harvested after 5~6 months a long wi h the 'hrimp. In India P.mala.bari a and A.granosa frer oppo 1;unities for integrating 'o]am farming with shrimp ~ture. Apart am generating additional income clams pUl'ii)r 'h water ' by reducing the sediment load. Giant clams In the tropical Indo~Pacific, several countries have embarked upon mariculture of giant clams of the family Tridacnidae . These are high value species and listed as endangered. They occur in the Andaman & Nicobar and Lakshadweep group of islands with vast potential for the development of mariculture in these islands. Development of giant clam mariculture is of particular interest because Tridacna sp are autotrophic, requiring no artificial feeding, ocean grow~out phase is simple and the hatchery technology has already been developed elsewhere. Depuration Clams like other filter feeding bivalves, accumulate pathogenic organisms in their body. By depuration the bacterial load is brought down to permissible levels. The faecal matter, sand and silt particles are also removed from the alimentary canal. They are placed in cleaning tanks under a flow of filtered seawater. About 10-20% of the seawater is continuously replaced. The process is continued for 12 hours, the 137 water in the tank is drained and the clams are cleaned by a strong jet of water to remove the accumulated debris. They are further held in filtered seawater for 12 hours and washed again. Products In several countries clams are eaten raw or steamed. The clam meat is frozen as blocks or indiv~?ually quick frozen, canned or smoked. Clam juice, clam stripes, clam streaks, stuffed, clams, clam pickle and chowder are the other products of clam. In recent times clain~l!!~js/increasingly used as shrimp feed. Byproducts The clam shell is used in the manufacture of cement, calcium carbide, sand-lime bricks and lime. The shells of several clams are used for making curios. Marketing Shucked clam meat fetches Rs. 20-351kg in the local market depending upon the size and species. The export of clam meat to the overseas markets has increased from 371t in 1989 to 900t in 1993. In terms of value, almost fivefold increase has ben recorded from Rs .63.02 lakhs in 1989 to Rs.292.25 lakhs in 1993. Paphia malabarica is the prime species in the export trade from the country. Economics: Blood clam culture The economics ofthe on-bottom blood clam (A.granosa) culture in an area of 1 hectare at the stocking density of 300/m2 is given below. Mter the culture period of six months a production of 70 tonnes can be obtained. The shell fetches a revenue of Rs. 1000 per tonne. The suitable period for culture is October-March. There is no capital expenditure. The cost of seed is based on supply from the hatchery since availability in the clam beds is scarce. Salinity range preferred is 15-30 ppt. This species can be cultured in east coast estuaries and bays where the substratum is soft and predominantly made of silt and clay. A. Operational cost .as. 1. Casuarina poles 1,000 2. Pen enclosure 2,000 3. Seed @ Rs.55/1000 1,65,000 4. Boat hiring 2,000 5. Labour 2,000 138 G er 12 as lm In me the lsed has ,h ia ~tare If six ue of is no since This 1m is )00 )00 , JOO 000 000 '" to 6. Harvesting, depuration & shucking of)meat 10,000 J. Continge;iies 2,000 8. --fi~a13e rent 10,000 9. Watch and ward for 6 months 6,000 ThtaJ 2,00,000 B. Co~t of production 2,00,000 C. Income Shell-on weight of harvested clams 70 t Wet meat weight 10 t . Shell weight 52 t Sale of 10 t meat @ Rs. 25,0001 t 2,50,000 Sale of 52 t shell @ Rs. 1,0001 t 52,000 Tbtal 3,02,000 D. Profit (Rs. 3.02,000 - Rs. 2,00,000) 1,02,000 Net profit on investment works out to 51.0% Black clam culture The black clam Villorita cyprinoides is most suitable to culture in the upper reaches of the estuaries and backwaters of Kerala and Karnataka States. Peak spawning is during June-August and seed ,clams of 15-20 mm occur from August onwards. Under favourable conditions seed density of 8000 - 12000 nos/m2 is common in the Vembanad Lake. The se~ds occurring in the natural grounds are adequate to support moderate level clam farming operations. Preferred salinity range is 2-10 ppt and single crop of 6 months duration can be raised during September-March. Studies indicated that pen enclosure is not necessary. the retrieval is 70% and at harvest the clams attain '25 g sheHon weight. A. Operational cost 1. Casuarina poles I 2. Seed @ Rs. 10/1000 3. Boat hiring 1.39 RB. 1,000 75000 3,000 4. Labour 5 . Harvesting, depuration & shuckil)g of meat 6. Watch and ward 7. Lease rent 8. Contingencies B. Cost of production C. Income Shell-on weight of harvested clams 130 t Wet meat weight 13 t Shell weight 100 t Sale of 13 t meat @ Rs. 10,000ft Sale of 100 t shell @ Rs. 600ft Total D. Profit (Rs. 1,90,000 - Rs. 1,23,000) Net profit on investment works out to 54.5% 3,000 20,000 9,000 10,000 2,000 1,23,000 1,30,000 60,000 1,90,000 67,000 

Clam culture

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Clam culture

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