Shrimp seed recruitment in mixed shrimp and mangrove forestry farms in Ca Mau Province, Southern Vietnam

The densities of shrimp seed recruited into ponds at mixed shrimp and mangrove forestry farms in Ca Mau province, southern Vietnam, were monitored semi-continuously from 1996 to 1998 to determine whether shrimp culture in the region is recruitment-limited. The numbers of shrimp seed lost during harvests were also monitored to assess whether current management strategies contribute to low stocking densities and pond production. Shrimp seed recruitment densities were low and highly variable with farms recruiting less than 1 postlarva m-3 between August 1996 and May 1997. Recruitment varied significantly with season (P = 0.0014), with peaks in October to November and April to May, the former having the highest recruitment densities. This trend is consistent with seasonal offshore spawning events in southern Vietnam. Recruitment did not vary significantly between day and night (P = 0.8), although data suggest that recruitment may be higher at night during peak recruitment months (November, February). Postlarval densities were significantly higher on the first day (24 h) of each spring tide period (P = 0.0046). Mean total length of shrimp seed was 13.72 ± 0.35 mm, which indicates that farms are recruiting postlarvae approximately 15-20 days post-metamorphosis (PL15-20), or 35 days post-hatch, into ponds. Recruitment densities were higher at farms on rivers than at farms on small canals during March/April 1998, which corresponded with peaks in immigration of postlarvae into estuaries on tidal currents at this time. The numbers of shrimp seed lost during harvests were high, reducing net recruitment and net stocking densities to 0.15 postlarvae m-2 from a potential gross stocking density of 0.35 postlarvae m-2. Shrimp culture in Ca Mau province is recruitment-limited. This is a major factor contributing to low shrimp yields in the region. Current harvesting techniques need to be modified to reduce postlarval losses and improve overall stocking densities and pond production. (C) 2000 Elsevier Science B.V.