Results and Discussion
The ranges of water quality parameters recorded in the experimental ponds for the duration of the experiment were: salinity: 10-18 ppt; temperature: 26-31º C; pH: 7.5-8.7 and dissolved oxygen: 4.0 7.9 mg/l. These ranges were generally within acceptable levels for mud crab fattening (Cholik and Hanafi 1992).
In encircled earthen area, crabs started to be fattened after 14 days of stocking and at 16 days all the fattened crabs were harvested from the pond but in case of cage crabs started to be fattened from 10 days and finished at 12 days of stocking. So the fattening duration was shorter in cages than the encircled earthen area. In the present research, F1 category of fattened crabs was found 87.5% in cage followed by 73.75% in encircled earthen area (Table 1). At the end of the experiment, survival rate of crab was found 93.75 ± 6.25% in cages, whereas in earthen pond it was 86.12 ± 2.16% (Table 1). The difference of survival rate was not statistically significant. Rattanachote and Dangwatanakul (1992) observed the mud crab fattening practice in earthen pond in Surat Thani province of Thailand and reported 85.20 % of survival rate and 93.77% production rate. The lower survival rate in encircled earthen pond could be due to the cannibalistic nature of mud crab. Mortality due to cannibalism has been widely documented (Iversen 1986). DA, Region VI (1988) used bamboo cage battery (with several small compartments or cubicles to accommodate a single crab per compartment) for mud crab fattening and recorded a survival of 87% which was lower than the present study. Kuntiyo (1992) showed a weight increment of 23 to 37 g and survival of 80-100%, 35 days after stocking in net cage battery. In Vietnamese intensive farming of mud crab, the stocking density was 1-1.5/m2, achieving 1.5t/ha for each crop. After 4-6 months, the crabs achieved an average weight of 300-450g and fed with trash fish and molluscs. (Thach, 2004). Escritor (1972) concluded that cannibalism and the burrowing habitat of S. serrata should be controlled to minimize losses and to make the crab culture profitable. Survival in pond culture is generally lower as a result of cannibalism and escape (Liong 1992).