Comparison of net cages ('hapa') and pond culture systems

Juveniles produced from rearing megalopae for a month in net cages or hapa nets may have to grown to a bigger size to meet preference of most growers. Thus, a method for extension of nursery rearing has to be developed. A comparison was made among stocking densities (1, 3 and 5 crabs/m2) and between net cages and ponds as rearing systems for growing 1.5 cm ICW crabs to the preferred size. Regardless of stocking density, the hapa net rearing system resulted in a higher mean survival (78% than the pond system (46.1%). However, juveniles harvested from the hapa nets were smaller (3-7 g BW) than those grown in ponds (10-16 g BW). Stocking densities did not significantly influence survival in either nets or ponds. The retrieval of juveniles is a problem in ponds.

 

Net cages in pond for the culture of mud crabs

Earthen ponds for the culture of mud crabs

Salinity tolerance of Scylla spp.

S. serrata is believed to prefer more saline waters than S. tranquebarica and S. olivacea and may have different responses to salinities with respect to growth. The effect of salinity on molting was investigated in crab instar (<0.5 cm ICW) and juveniles (1.5 to 2.5 cm ICW). In S. olivacea crab instar 1, the survival was similar in salinities of 12, 16, 20, 24, and 32 ppt. The mean body weight after 1.5 months was highest in 12 and 16 ppt, followed by those in 20 and 24 ppt. Poorest growth was in 32 ppt. This result was consistent with the number of molts during the duration of the test. Crabs at 32 ppt had a mean of four molts while those in other test salinities had undergone five. Test on S. serrata and S. tranquebarica are ongoing.
Juveniles previously reared for at least a week in 32 ppt were transferred abruptly to salinities of 8, 16, 20, 24, and 32 ppt. For S. serrata, survival after third molt (2.5 months after stocking) and molt intervals were similar among salinities. For S. olivacea, lower survival was obtained at 8 ppt compared to 16, 20 and 24 ppt. At 32 ppt, survival was similar with all other salinities. Molt interval and increment were not significantly different among salinities. 

Performance of wild and hatchery-reared S. serrata 

Some crab growers have the concept that mud crab juveniles from the hatchery are inferior to the wild hence; the performance of wild and hatchery-reared juveniles was evaluated. 
Equal numbers of hatchery and wild-sourced S. serrata juveniles were stocked in the same ponds while some ponds were stocked exclusively with hatchery or wild-sourced juveniles. The crabs were injected with microwire tags to determine individual growth rates. After three months, survival was higher in ponds stocked with hatchery or wild-sourced juveniles alone than those with mixed stock. Growth was significantly higher in wild than in hatchery-reared juveniles. Hatchery-reared juveniles stocked with wild juveniles had significantly lower growth than those hatchery-reared that were stocked exclusively in pond.

Wild and hatchery-reared mud crab juveniles for pond culture

Injecting a coded wire tag into the muscle of mud crab using a handheld multishot injector

Agonistic behavior of S. serrata juveniles

The agonistic behavior of intact, autotomized (chelipeds removed) and trimmed (dactylus and polex trimmed) juveniles was studied to develop strategies to minimize cannibalism. The major behavior displays can be classified into threat, attack, fight, defense, retreat and rest.  Each major display is composed of several detailed behavior (Table 1).

Major displays of agonistic behavior of mud crab

Table 1. Detailed behavior of each major display of Scylla serrata

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