AGRICULTURAL RESEARCH COMMUNICATION CENTREwww.arccjournals.com/www.ijaronline.in

B-3423[1-8]

*Corresponding author’s e-mail: skdaswbuafs@gmail.com1Deptt. of Aquaculture, College of Fisheries, Guru Angad Dev Veterinary and Animal Sciences University, Ludhiana-141 004, Punjab, India2 Deptt. of Aquaculture, Faculty of Fishery Sciences, West Bengal University of Animal & Fishery Sciences, Kolkata-700 037, West Bengal, India

Indian J. Anim. Res.,Print ISSN:0367-6722 / Online ISSN:0976-0555

Studies on physico-chemical environment and management of crab (Scyllaserrata) farming in 24 Parganas (West Bengal)Amit Mandal1, Dr. S. K. Das* and Amit Majumder2

Dept. of Fisheries, Govt. of West Bengal, Harirampur,Dakshin Dinajpur-733 125, West Bengal, India.Received: 19-04-2017 Accepted: 21-06-2017 DOI:10.18805/ijar.B-3423

ABSTRACTManagement practices and environmental quality of the crab (Scylla serrata) farming ponds in Gosaba and Basanti blocks,24 Parganas (South), West Bengal has been investigated. Split bamboo fenced rectangular culture units are preferred byover 70% of the farmers with highly variable stocking density. Total inorganic nitrogen and BOD increased with theincreasing biomass of the crabs in both the sites. Salinity, total alkalinity and hardness played direct roles in body weightgain and survival rate. Water salinity of 17.50 to 20.0 ppt and hardness upto 1000 mg l-1 were found to be optimal for bodyweight gain of the cultured crabs.

Key words: Hardness, Rectangular unit, Salinity, Scylla serrata, Total alkalinity.

INTRODUCTIONAquaculture comprises diverse systems of farming

plants and animals in inland, coastal and marine areas, usingand producing a wide variety of animal and plant species.Production of aquatic animals from aquaculture in 2014amounted to 73.8 million tonnes, with an estimated first-sale value of US$160.2 billion, consisting of 49.8 milliontonnes of finfish (US$99.2 billion), 16.1 million tonnes ofmolluscs (US$19 billion), 6.9 million tonnes of crustaceans(US$36.2 billion). By 2015, world aquaculture productionreached 76.6 million tonnes valued at USD 157.9 billion(farm-gate value) (FAO, 2016). World aquaculture is heavilydominated by the Asia–Pacific region, which accounts for88 percent of production in terms of quantity and about 80per cent in terms of value. India stood at second positionwith 6.3 percent share in global aquaculture production,accounting for 4.2 million tonnes of produce (Yusufzai etal., 2014). Fisheries and aquaculture supply 17 percent ofglobal animal protein in people’s diets and support thelivelihoods of some 12 percent of the world’s population(FAO, 2016). The most recent estimates indicated that 56.6million people were engaged in the primary sector of capturefisheries and aquaculture in 2014. Small-scale aquacultureoperations continue to play a critical role in supportinglivelihoods, particularly rural livelihoods, contributing tofood security and alleviating poverty (FAO, 2016).

In spite of the availability of different technologiesfor diversified farming practices of various candidate species,coastal aquaculture in India is synonymous with shrimpculture, for which hardly 10% of the potential area is

currently utilized (Ganapathi and Viswakumar, 2001).However, primarily because of disease related setback, thefarmers are opting for alternative farming that could promisecomparable profits to shrimp farming (Devaraj andAppukuttan, 2000).

The Indo-Pacific swamp crab, commonly called asmud crabs (Scylla serrata and Scylla oceanica) are nowwidely regarded as one of the most promising alternative topenaeid shrimp culture in India. These crabs are attractivefor the growing export market as they can be easily packedand shipped live (Wilfredo et al., 2006). Crab fishing andfishery were also available from Karnataka, Kerala, TamilNadu, Andhra Pradesh and Orissa (Varadharajan andSoundarapandian, 2012). The crabs like P. pelagicus, P.sanguinolentus, S. serrata and S.tranquebarica forming amajor crab fishery along southeast coast (Sanil, 2000; JohnSamuel et al., 2004; Varadharajan et al., 2009). Accordingto the Annual Report 2009-2010 released by the CentralMarine Fisheries Research Institute, the west coast remainsmore productive, contributing 56% of the total catch in thecountry, while the remaining 44 % is contributed by the eastcoast. However, the fish landing in Tamil Nadu, Gujarat andAndhra Pradesh had shown an increasing trend during thesame period (CMFRI, 2010).

In India, shrimp ponds of high saline water havebeen converted in many places for monoculture of mud craband such sites offer ample scope for the production of gravidfemale and spawners. Pond fattening of undersized and leanmarketable sized mud crabs with partial harvest (20 days

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Fig-1: Geographical location of the present study

after stocking) is economically feasible (Liong, 1992). Acrab fattening cycle typically takes from 14 to 60 days, duringwhich time their protein content can increase from 8 to 15percent. The fast turnover of stock, relatively low operatingcosts and high survival in crab fattening systems makes themeconomically attractive (Shelley and Lovatelli, 2011). 

In West Bengal, wild crablets are available roundthe year in backwater zones of Gosaba and Basanti blocksin 24 Parganas (South), within the Sunderbans biospherereserve. The present study was designed to investigate themanagement protocols and the physico-chemicalenvironments of the farming units of mud crab (Scyllaserrata) in Gosaba (22.1600° N, 88.8000° E) and Basanti(22.2128° N, 88.6864° E) in 24 Parganas (South), WestBengal as such culture is being successfully done in thoseareas (Fig. 1).MATERIALS AND METHODS

The present study was conducted during Novemberto December, 2014 and designed to collect primary data onpre and post stocking management aspects by interviewingthe crab farmers through a structured questionnaire schedulein selected areas of Gosaba and Basanti in 24-Parganas(South), West Bengal, India. The schedule was preparedafter completing a pilot study in those areas encompassingdifferent aspects of crab farming. Purposive and stratifiedsampling was done as the crab farming is primarily restrictedonly in and around the selected locales of West Bengal.Physicochemical and biological parameters of water and soilsamples were analyzed either on spot or in the laboratory ofthe Department of Aquaculture at Chakgaria, Kolkata.

The culture units encompassing 32 and 30 nos. ofponds from Gosaba and Basanti, respectively (Table 1) wereselected for the present study which represents about 71%to 68% of the total crab farmers in the study areas. Crabculture units in general was either comprised of rectangularor circular (0.02-0.3 ha) ponds or rectangular pens (0.04-0.05 ha) designed by dividing a large pond in segments by

split bamboo fencing (Fig. 2a). The rectangular pen waspartitioned again with split bamboo fencing for rearing themale and female crabs separately. Generally the area of penfor male culture was bigger (0.02-0.3 ha) than female (0.01-0.02 ha) with one meter water depth each. The boundary oftotal culture area was fenced with bamboo sticks and nylonnet (Fig. 2b) to prevent escapement. Seawater comingthrough nearby open creeks channelled through a sluice gatewas used for culture purpose. The ponds were mostlysundried, limed and fertilized with mustard oil cake, singlesuper phosphate and urea with variable dosage (Table 1).Within 7-10 days of fertilization water colour changed tobluish green  and juveniles of crab (75-90 g) were stocked@ 937-975 kg /ha during the second half of October, 2014(Table 1) and cultured for two months. Partially dried(moisture content: 60 %) and salted (Fig. 3) locally availablechopped trash fish such as Setipinna phasa (Phasa), Coiladussumieri (Anchovy), Protosus indicus (Marine eel) wereused for feeding the crabs @ 8-10 % body weight/day duringlate afternoon.

The crabs were harvested by using lure and handpicking methods (Fig. 4) after the culture period, sortedaccording to their weight and size and marketed to theexporting agencies.

For physico-chemical conditions of the farm ponds,soil and water samples were collected at ten days intervals.Water samples were analyzed for temperature, pH, dissolvedoxygen, alkalinity and hardness (APHA, 1995) on spot,whereas, one day biological oxygen demand (BOD1), ortho-phosphate, ammonia-nitrogen (NH3-N), nitrite-nitrogen(NO2-N), nitrate-nitrogen (NO3-N) of water and pH, organiccarbon and available-P of soil samples were analyzedfollowing standard methods (APHA, 1995) in the laboratory.RESULTS AND DISCUSSION

Rectangular ponds are preferred by the crab farmersin both the blocks as more than 56% of them practice theculture in ponds with rectangular shape (Table 1). Regarding

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Table 1: Frequency and percentage of the variable management practices being adopted by the crab farmers of Gosaba and Basanti blocks.

Gosaba: Total no of farmers - 32 Basanti: Total no of farmers - 30Frequency % Frequency %

Drying 26 81.25 24 80Tilling 23 71.875 18 60Liming 25 78.825 22 73.33Fertilizer 23 71.875 23 76.67Stocking density:<937.5 kg /ha Frequency % Frequency %937.5-975kg/ha>975 kg/ha 7 21.88 7 23.3

20 62.5 21 705 15.63 2 6.67

Shape of the farming unit: Frequency % Frequency %Circular 14 43.75 13 43.33Rectangular 18 56.25 177 56.67Feeding: Frequency % Frequency %1 time 12 37.5 14 46.672 times 20 62.5 16 53.33Harvesting method: Frequency % Frequency %By hand 9 28.13 8 26.67Using lure 10 31.25 11 36.67By hand and lure 13 40.63 11 36.67

Fig-2: Rectangular pond divided by bamboo sticks and fenced with bamboo pole and net.

Fig-3: Fresh and salted dried fish used for feeding.

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Fig-4: Harvesting of crab using lure (a) and hand picking (b) by V-shaped hand net.

Table 2: Range and mean (± SD) value of physico-chemical parameters of water and soil of the culture units of Gosaba and Basanti blocks.sWater Gosaba Basanti

Range Mean ± SD Range Mean ± SDTemp. (0C) 26 - 30 28 ± 2.83 27 - 30 28.5 ± 2.12pH 8.11 - 8.39 7.34 - 7.37Total alkalinity (mg l-1) 530 - 641.2 587.86 ± 40.1 458.8 - 658.2 513.11 ± 70.5Hardness (mg l-1) 930 - 1210 1093.06± 96.05 920 - 1081 1029.43 ± 57.35Salinity (ppt) 18.12-18.35 18.21 ± 0.08 17.12 - 17.41 17.24 ± 0.1DO (mg l-1) 6.74 - 10.39 8.27 ± 1.32 6.47 - 9.28 7.85 ± 1.04NH3-N (mg l-1) 0.124 - 0.219 0.18 ± 0.032 0.115 - 0.196 0.158 ± 0.029NO2-N (mg l-1) 0.004 - 0.011 0.007 ± 0.003 0 - 0.007 0.003 ± 0.002NO3-N (mg l-1) 0.132 - 0.163 0.147 ± 0.011 0.135 - 0.178 0.155 ± 0.015Ortho-P (mg l-1) 0.163 - 0.197 0.178 ± 0.012 0.155 - 0.216 0.185 ± 0.023Total inorganic nitrogen 0.267 - 0.389 0.334 ± 0.042 0.252 - 0.381 0.316 ± 0.045(TIN) (mgl-1)TIN: Ortho-P 1.64-1.97 1.87±0.114 1.62-1.76 1.71±0.047BOD1 (mg l-1) 3.40-4.72 4.12 ± 0.55 2.34-4.52 3.48±0.89Soil Gosaba Basanti

Range Mean ± SD Range Mean ± SDpH 8.04-8.24 8.14 ± 0.07 7.22-7.52 7.36 ± 0.11Org. C (g kg-1) 1.3-1.83 1.59 ± 0.21 1.36-2.05 1.69 ± 0.24Avail-P (mg g-1) 0.134-0.178 0.16 ± 0.015 0.145-0.195 0.172 ± 0.018

bottom tilling and liming, 72% and 79% of the farmersrespectively in Gosaba practice the above prestockingmanagement against 60% and 73% in Basanti block (Table1). However,  with  regards  to  fertilizer  application,  77%farmers used to apply fertilizers like SSP and urea duringpond preparation in Basanti block against 72% farmers inGosaba block (Table 1). Though the stocking density washighly variable, most of the farmers (62.5-70%) in both theblocks stocked their pond with crablets (80-100 g) once @900-950 kg/ha. Majority of the farmers (53.33-62.5%)preferred to feed their stock twice daily; 60-70% of therequired feed before sunset and rest during morning hours). 36.67-40.63 % farmers harvest their produce both by handpicking and using lure lines (Table 1).

Surface water temperature of the culture units didnot differ much spatially during the period of investigationand ranged from 26-300C. Water pH remained alkaline

throughout the period of investigation both in Gosaba (8.11-8.39) and Basanti (7.39-7.47). Salinity of water remainedsignificantly higher (t = 3.41; p < 0.05) in Gosaba (18.62ppt.) compared to Basanti (17.21 ppt.). Likewise, totalalkalinity of water remained significantly higher (t = 4.65; p< 0.01) in Gosaba than Basanti (Table 2), however, suchdifference with regards to total hardness of water remainedinsignificant (p > 0.05). Significantly higher value ofdissolved oxygen (t =3.40; p < 0.05) was observed in Gosaba(8.27 mg l-1) than in Basanti (7.85 mg l-1) (Table 2).

Though the difference of ortho-phosphate of waterin both the sites remained insignificant (p > 0.05), NH3-Nremained significantly higher (t = 9.43; p < 0.01) in Gosaba(0.180 mg l-1) than in Basanti (0.158 mg l-1). In contrast, thevalues of nitrite and nitrate-nitrogen did not differsignificantly (p > 0.05) in both the sites (Table 2). However,total inorganic nitrogen (TIN) and TIN: ortho-P of water

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remained significantly higher (t5.42; p < 0.05) in Gosabathan in Basanti areas (Table 2). Ammonia-N and TINincreased directly as the culture period progressed withincreasing biomass of crabs in both the sites (Fig. 5, 6)indicated that substantial autochthonous loading of inorganicnitrogen resulted in from the metabolites and decompositionof the uneaten feed within the system. This is in conformityof the earlier studies of Haseeb and Bright Singh (2012). Significantly higher values of inorganic nutrients of waterresulted in higher values of primary production in Gosabaas GPP, NPP and CR values remained significantly higher(t 4.57; p < 0.05) in Gosaba than in Basanti areas (Table-2).Manasrah et al. (2006) also observed direct relationshipbetween dissolved nutrients in water and primaryproductivity.

With the increasing biomass of crabs, BOD of waterin both the sites increased with high degree of significance(Fig. 7). Anderson et al. (2002) stated that BOD of waterwas a direct function of the biomass load of the aquaticsystem. Identical to the inorganic nutrients, BOD of waterremained significantly higher (t = 3.89; p < 0.05) in Gosaba(4.12 mg l-1) than Basanti (3.48 mg l-1). However, aerobicheterotrophic bacterial population did not differ significantly(p > 0.05) in Gosaba (36.94 CFUx103 ml-1) and Basanti(32.77 CFUx103 ml-1) (Table 2).

Soil pH remained highly alkaline throughout theperiod of investigation with values ranged from 8.01 to 8.23 inGosaba and 7.22-7.56 in Basanti (Table 2). Soil nutrients likeavailable phosphorus and organic carbon also did not varymuch (p > 0.05) between Gosaba and Basanti areas (Table 2). 

The average survival rate of the crablets was fairlyhigh in both the sites (92-92.29%). Though the body weightcontinued to increase, the rate of growth slowed down whichresulted in declined live weight gain (%) as the period offattening progressed in both the study areas (Fig. 8). Theabsolute growth attained in Basanti (166.77g) was 14.84 %higher than Gosaba (139.99 g).

Among the physico-chemical parameters, salinity,total alkalinity and hardness had directly influenced uponthe body weight gain of the crabs and such direct relationshipswere highly significant (R2 =70-94%) (Fig. 9, 10, 11). Thisis the conformity with the earlier studies where salinity andhardness of water directly influenced the growth of mud crabs(Mohapatra et al., 2007; Panigrahi et al., 2007; Paital andChainy, 2012). Though the salinity and hardness of watervaried widely among the farm ponds in the present study,fitted relationship indicated that salinity of 17.5-20 ppt andhardness upto 1000 mgl-1 optimally influenced the growthof crab. Pedapoli and Ramudu (2014) also stated that salinityas limiting factor for determining the survivability and growthof the crablets in the grow out culture.

Fig-5: Fitted relationship of body weight and NH3-N in Gosaba (a) and Basanti (b) blocks.

Fig-6: Fitted relationship of body weight and TIN in Gosaba (a) and Basanti (b) blocks.

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Fig-7: Fitted relationship of body weight and BOD1 in Gosaba (a) and Basanti (b) blocks.

Fig-8: Temporal variation of body weight (a) and percent live weight gain (b) of crab in Gosaba and Basanti blocks

Fig-9: Fitted relationship of body weight and salinity of water in Gosaba (a) and Basanti (b) blocks.

Fig-10: Fitted relationship of body weight of crab and total alkalinity of water in Gosaba (a) and Basanti (b) blocks.

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Fig-11: Fitted relationship of body weight of crab and hardness of water in Gosaba (a) and Basanti (b) blocks

Feed conversion ratio (FCR) continued to increasesharply as the crabs gained body weight during the cultureperiod in both the study areas and the mean value rangedfrom 3.87 to 4.11. In contrast, feed conversion efficiency(FCE %) gradually declined with 10.55 % higher mean valuein Basanti compared to Gosaba area (Fig. 12).CONCLUSION

The study indicated that there is enough scope ofimprovement particularly with respect to feedingmanagement so that organic loading is minimized. Moreover,

Fig-12: Temporal variation of FCR and FCE (%) in Gosaba (a) and Basanti (b) blocks.

periodic exchange of water and liming could be adopted toimprove salinity, total alkalinity and hardness of water in boththe sites towards better management practice in crab farming.ACKNOWLEDGEMENTS

The authors acknowledged the crab farmers ofGosaba and Basanti blocks especially to Mr. Kanai Mondalfor supporting the research programme by providing nightstay facility to his house and Head of Deptt. of Aquaculture,Faculty of Fishery Sciences, W.B.U.A.F.S., Kolkata fortechnical help during the research programme.

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