Culture of Soft Shell Mangrove Crab, Scylla Spp. Production in the Southwest Region of Bangladesh

Introduction

Mud crab aquaculture has been practiced for many years in the coastal regions, particularly in southeast (Chittagong, Cox, s Bazar, Noakhali) and southwest bhola, Potuakhali, Barguna, Phirujpur, Bhagerhat, Khulna and Satkhira) Bangladesh [1]. Farmers are to be practiced soft shell mud crab cage culture in the coastal area of Stathkhira and Cox’s Bazar districts.

There are four species of mangrove crab or mud crab of the genus Scylla, namely; S. serrata, S. olivacea, S. paramamosain and S. tranquebarica [2, 3, 4] and the two first species are common in Bangladesh. Among two common mangrove crab species in Bangladesh S. serrata is the fast growing species. For fattening this species is a very suitable for culture in estuaries including brackish water ponds. Most consumers prefer large size mud crabs. Live mud crabs are always highly demand and fetch attractive price in international markets [5]. Live crabs are mainly exported to south-east Asian countries. Female crabs are highly demandable particularly Hong Kong, Taiwan, Singapore and Japan. There is another growing market for frozen soft-shelled mud crabs in the USA as a value added product. Many consumers prefer soft-shell crabs because it can be eaten whole when cooked. Among the marine crustaceans found along the Bay of Bengal coasts, crab ranks second after shrimp. The crab fishery in Bangladesh is fast developing and there is high demand for crab due to its taste and nutrient contents [6].

The crabs molt several times to grow. During molting the old exoskeleton or shell splits and the crab backs out of its exoskeleton. The newly molted crab is soft. The supply of soft-shell crabs depends on the ability of collectors to recognize crabs in the premolt stage. Production of soft-shell crabs can be continuing if a continuous supply of 80-160 g crabs is ensured. In the present study 80-180g crabs were used to know the molting duration, weight increment and the difficulties in availability of small and bigger-sized crabs in the soft- shell crab production. In Bangladesh soft-shell crabs production is running depending on wild seed stock. But the aquaculture of crab in pen and cage would not be sustainable because of shortage of crab population.

This study presents the results of the soft-shell crab production using wild stock of Scylla spp. population. There is no established hatchery in Bangladesh. Soft shell crab farmer is totally depended on wild stock of Mangrove area. A systematic plan is needed to continue soft shell crab culture by ecofriendly capturing of mud crab from coastal area until to establish crab hatchery in Bangladesh.

Materials and Methods

Culture of soft-shell mangrove crab Scylla spp. was carried out in Kaligonj chingri farm, Allarchar chingri farm and Shamnagar private crab farm in the southwest region of Bangladesh (Figure 1).

Results

In every treatment five size groups were tested to find out the net weight gain, percentage of average body weight and average total weight. In treatment T1, size group 81-100 g showed result about 13.64 kg, size group 100-120 g showed result about 15.92 kg, size group 121- 140 g showed result about 18.68 kg, size group 141-160 g showed result about 20.80 kg and size group 161-180 g showed result about 21.24 kg. Total production was recorded at 5082.51 kg/ha. In treatment T2, size group 81-100 g showed result about 13.04 kg, size group 100- 120 g showed result about 15.54 kg, size group 121-140 g showed result about 18.26 kg, size group 141-160 g showed result about 20.19 kg and size group 161-180 g showed result about 20.87 kg. Total production was recorded at 4689.63 kg/ha. In treatment T2, size group 81-100 g showed result about 13.44 kg, size group 100- 120 g showed result about 15.55 kg, size group 121-140 g showed result about 18.21 kg, size group 141-160 g showed result about 19.21 kg and size group 161-180 g showed result about 21.77 kg. Total production was recorded at 4923.48 kg/ha (Tables 6-7).

The increase in average carapace width (%) of crab in treatment T1, T2 and T3 were (10.59-12.65) %. The increase in average carapace width (%) of crab in treatment T1, T2 and T3 were (16.67-10.78)%, (15.29- 10.10)% and (15.12-10.78)% and increase in average carapace width were (1.40-1.10)cm, (1.20-1.10) cm and (1.15-1.2) cm in treatment T1, T2 and T3 (Tables 5-7). However, the increase in average carapace width and increase in average carapace width of soft shell crab in different treatments were not significantly different (P>0.05) (Table 8).

Figure in the same row having the same superscript are not significantly different (P>0.05). Figure in the parenthesis indicates the range. Table 9: Growth performance, survival and production of Scylla spp. after 180 days of rearing; mean ± S.D. with ranges in parentheses.

The highest survival rate (95.0±1.03) was also

observed in T1 and the lowest (90.0±1.76) in T2. There

was a significant variation (P<0.05) in the survival rate of

crabs among three treatments. The net production of crab

$$ \text {w a s} (5 0 8 2. 5 1 \pm 8. 0 2 \mathrm {k g}), (4 6 8 9. 6 3 \pm 6. 1 1 \mathrm {k g}) \text {a n d} $$

(4923.48±7.12kg) ha-1.days-180 in treatment T1, T2 and T3,

respectively. Highest production of soft shell crab was

recorded in treatment T1. But no significant (P<0.05) different was recorded in treatment T1 and T3. But a significant (P>0.05) different was found between treatment T2 and treatment T1 and T3 (Table 10).

Discussion

The physicochemical parameters exert an immense influence on the maintenance of a well aquatic environment and production of food organisms. Growth, feed efficacy and feed consumption of aquatic live are normally governed by a few environmental factors [16]. The physico-chemical parameters were recorded in three treatments was favorable for the growth and survival of the crabs. For experimental period, the water temperature in three treatments T1, T2 and T3 was conducive to the growth of the soft shell crab. The salinity in treatments T1 and T3 was relatively more stable; due to a good water exchange in the pond during the culture period as the pond was located close to the seawater inlet. But in case of treatment T2 the salinity range for soft shell crab culture was not favorable for December to January. Because, the pond was not located close to the seawater inlet [17]. The pH values agree well with the findings of APHA and Clesceri LS, et al. [8, 19]. The dissolved oxygen in the morning was low in ponds stocked with a high density of fish compared to ponds stocked with a low density. Similar results were observed by Boyd (1982) and APHA (1998) [18, 19].

Crustacean’s growth is to be a combination of the increase in size after a molt the frequency of molting [20, 21]. The molt interval becomes longer in bigger- sized crabs. Majority of the crabs molted during the night. The newly molted crab expands its body and limbs by talking in water before the new shell hardness [22]. The present study shows the possibility of a continuous supply of crabs for soft shell crab production farm. I this study, about (60-70)% crabs molted during night [23]. About

80-180 g crabs was used in the box for short duration of molting. The molted crabs with less than 180 g BW were restocked in new boxes to determine the number of days until the succeeding molt. It took a longer period of time for these restocked crabs. Generally new stock of Scylla spp. with size ranges from 80-180 g sourced directly from the Mangrove area molted within 26 days. Crabs weighing 80-160g is ideal for soft shell crab production since molt duration is shorter [24]. During the culture period it is found that mortality of crabs in the cage was comparatively low because the crabs were collected from the very closed to mangrove area and the crabs were not subjected to transport stress. But in case of treatment T2 crabs were collected from a long distance mangrove area and crabs were in stress position.

The stocking densities of crabs were 9880 cages/ha which is very similar to the culture of soft shell crab in Taiwan [25]. No cannibalism was found in three treatments [26, 27]. In this study, the survival was higher due to no cannibalism of mud crab because one crab was stocked in one cage [3]. Survival of mud crab for the present study was recorded at 95%, 90% and 95% in the treatment T1, T2 and T3 [28]. This experiment has shown that the crabs were able to grow in the cage, as indicated by the increase in SGR values in three treatments. A similar growth rate is recorded by Bensam P [29] who found increase in weight in a period of 3 months of culture. However the lower FCR value in the present study indicated better food utilization efficiency. Significantly higher survival was noted in treatment T1, where, the salinity of stocking pond was very optimum level. The reason for reduced survival rate in treatment T2 was due to lower salinity of the experimental ponds [29].

The mangrove crab is omnivorous and feeds on raw crushed fish, crustaceans, bivalves, molluscs, penaeids and detrital matter. The application of trash fish as feed, the presence of naturally occurring food and muddy nature favored the growth of mud crab [30, 31]. Growth of crab depended on the quality of food available. In the present investigation, the amount of trash fish given in different treatments was based on the number of crab stocked and amount of feed provided per individual was kept at the same level. The results in the present experiment are very similar to those of Chakraborty BK, Mirera Do, et al. Escritor GL; Samonto GPB, et al.; Mwaluma J and Bensam P [1, 23, 29, 30, 31, 32, 33]. In the present study, a significant higher net benefit was obtained from treatment T1 than those from the treatment T3 and T2. Overall, highest production, survival and benefits of soft shell crab culture were recorded in treatment T1.

Finally, it can be concluded that the survival, growth, production of Scylla spp. were dependent on a good salinity, feed and stress of stocking population and management of cage. Stocking of 9880 cage/ha may be advisable for rearing of soft shell crab culture for 150 days culture period. Production of adequate quality mud crab through application of our present findings might be extremely helpful for earning economic benefit of farmer.

Acknowledgements

The author wishes to thank private soft shell farmer and Department of Fisheries who help for successfully conduct the experiments.