The blue swimming crab Portunus pelagicus is one of the major marine crabs used for domestic consumption as well as for export

Arputharaj, M R

Josileen, Jose

Maheswarudu, G

CMFRI Digital Repository

BLUE SWIMM~NG CRAB HATCHERY JOSILEEN JOSE, G. MAHESWARUDU, M.R.ARPUTHARAJ INTRODUCTION Central Marine Fisheries Research Institute, Cochin - 682014 The blue swimming crab Portunus pelagicus (Herbst) is one of the ntajol' marine crabs used for domestic con Q.mption as well as for export. Pre ently, it is priced at Rs. 30-45/kg and during certain seasons, it goes upto Rs . 60-70/kg. The price is steadily increasing due to increased demand for export and decline in .the landings. A:3 the sev·er and reC1;IT1'ingdi ease outbreak in shrimp -culture has resulted in "heavy loss it is time to restrict the overemphasis given to shtimp culture and diversify the culture practices. Blue swimming crab P. peZagicus has proved to be a good candidate • species for aquaculture. The hatchery operation and rearing of this peies is easie . wh n compared to other cr.abs like Scylla serrata .. The eas aVallability of ovigerous fema1e~...oiP'pelagi us fl low prices is an important factor that makes t elate ery ope 'ation lesscumbel'som by avoiding the long procedure of brood stock maintenance. BIOLOGY P.pelag£cus grows to a maxim m size of 20 em cara ace width and ahout ~.iD weight. Jt usually occurs in lO·15m depth. Smaller ones are llvailable in halJower areas. M.Rles and females exhibit distinct exuaJ dimo hism. The CarapaCt1 of the m alecra1:;l is briUiantl coloured with irregular white..- a~hes and ~ip of helate and first walking legs are bright blue in colour. But female crabs are dull bI'Own in colour ------..<-1 Vv'ith irregular white patches on the carapace and tips ofch late and walking legs are bt'Own. They are free swimming crabs. The maJ~o·w lar[cr B d fastes than fa-maleA. The specieij is caw:iv~\ ous and feeds on fishe shrimps, crabs nd b ·v~ves. Female crab attains sexttAl maturjty at a size of abou 1QQm.in carapace "I/idt~. After spawn-ing, female crabs carry' the fertilized e~s)n the pleopods. The inou~ation period is .0-12 days during which period he embryonic develQpment is completed . 'l.'he egg hatches out as zoetJ, which metamorphoses tw'Ougb :five zoeal stages and one megalopa stage before moulting into crab instar 1. COLLECTION OF BROOD STOCK Healthy Qvigerous females of P.pelagicus with characteristic yellow coloured eggs are collected from the sea. The be~!,ied crabs are kept in 1.5tca12ruitY tanks at a salinity of 35 ppt. pH 8.2 ± 0.1 and temperature 27 ± 2° C with continuous aeration. - ~ 108 arme ~dat lHdi1y lcavy 'Y the idate ::>ther us at some :VI in ower fthe land )lour s are n fe-male 3.wn-od is l egg .lopa eggs at a tion. I ' .. • Filtered sea water is used for the entire operation and 50% of water is exchanged every day. Fish and clam meat are given as food. The berried crab shows only occa-sional swimmIng behaviour in the tank by means offifth pair oflegs, but often stands on pereopods with the egg mass held above the substratum. HATCHING OF THE ZOEAE The changes in the egg colour are observed daily and when the egg mass becomes deep grey in co.Jour, the animal is transferred into a ~arate tank. No feeding is required at thIS stage but tank must be cleaned and water exchange~y till hatching. . The eggs h9.1~h-~l!.tduring the early morning hou~s and the number of z~e hatched out varies from 2 to 1-5 lakhs depending on the size of mother crab. Generally 100% hatching occurs. Mter hatching, the mother crab is removed from the tank and the number of larvae are counted. ---LARVAL REARING The zoeae are stocked in 5t tanks (preferably rectangular in shape) at a rate of20,000-2.5,000/t. Zoeal stages I-V are reared in the same tank with a water exchange of30% daily. Bottom sediments must be removed periodically. Zoeal phase (I-V) takes 12-14 days to metamorphose to megalopa. Each zoeal stage takes a minimum period of 2-3 days to moult. Zoeae are photopositive and active swimmers and very often they aggregate into groups along the~ of the tanks near the surface. Me,ga lopa tak.~s 6-7 day.s to reach the crab stag~. T i Jess active than zoea and s\~im by means of pleo'pods.Tt is can~tic in nature. So it is better to provi~e shelters like corrugated asbestos sheets or vertically hung polypropylene nets etc:-stOCking density of larvae in the rearing tanks must be strictly managed as follows:-Larval stage Zoe a I-V Megalopa Baby crab Larval feed Stocking density 20,000 - 25,000 I t 10,000 - 12,000 I t 5,000 - 7,000 I t Zo~ can be fed with mix~~ phytop1ankton (dominated by Cha¢lcgros spp.) Chlorella, rot~r, Mo(!/,a and Artemia n;;Plli;-Use of Artemia will increase the feed cost. For megalQPa, macer~ ~.h!:!mp .!!l~at and M!!.0!!:. are found to be the best. Cla~eat and shrimp meat are suitable for feeding the crab instar. The feeding schedule for the - -~ larvae is given below: 109 Larval stag'e Feed Conc.!ml Ration I t / day Zoe a I Chlorella 14-18 mill. 1001 Rotifer 350-400 no. 1001 Zoe a II u 2501 Zoe a III « .. 2501+ Zoea IV f! " ... Zoea V r! " ,"! Megalopa Moina 10-20 no. 1001 Baby crab Moina + Egg custard ,! 1001+ 50 g Water Quality The health and survival rate of the ae are greatly influenced by the quality of water. The desirable ranges for the different parameters for the hatchery are: Salin-ity - 3:~ . 2~ Tempemture - 28.2°g; Oxygen - 4 - 6 mIll; pH - 7.8-8 .2; Ammonia - <0.1 ppm anaitra e - <0.05 ppm - , • ECONOMICS Since the culture of Portunus pelagicus is not practised in our country, a crab hatch-ery combined with an existing shrimp hatchery is adv.ised. In order to produce one million baby crabs per year a shrimp hatchery 3-40 million can be used with a few additional facilities for live feed culture. For a production of one million crab seeds seven hatchery runs have to be carried out with 1.51akhs per run (with a survival of 25% from zoea to crab stage): The operating cost for a single hatchery run is given below: Opera ting Cost Mother crab (4 nos.) : Chemicals for live feed culture (Chlorella, rotifier and Moina) Filter cloth (40~. 10011. 150~, 250~ & 300~) Fuel and Electricity charges Salary for the staff (Manager, Supervisors, technicians, helpers and watch & ward) Total Costlcrop Annual operating cost (7 crops) Annual Income (One Rupee per seed) Net annual Income , ~. 110 Hs_ 100.00 1200.00 15000,00 20000.00 35000,00 71,300.00 499,100.00 1050000.00 550,900.00 D 

Blue swimming crab hatchery

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Blue swimming crab hatchery

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