Work Package 5: Fisheries and Population Dynamics

VIETNAM 

Mud crab populations in natural and planted mangroves areas on opposite sides of the Tran De River in the Mekong Delta, Vietnam have been studied. Vinh Hai is an area of planted Rhizophora apiculata protected from the sea by a dyke. The area has been subdivided and leased out, with each subdivision isolated from neighboring areas by a dyke and separated from the sea by individual sluice gates. During spring high tides the gate is raised for a few hours each day to exchange water. During the evening low tide a fishery is operated where outgoing water is seine netted for fish and crustaceans. The catch is predominantly shrimp with a by catch of fish and the mud crab S. paramamosain. Long Phu is the landings site for all crabs caught on the State Farm Island, which is situated in the middle of the estuary and surrounded by natural stands of Sonneratia caseolaris.

  Vinh Hai  

Between April 2003 and April 2004, 873 crabs were caught, all were S. paramamosain with the exception of one S. tranquebarica (Keenan et al, 1998). Modal progression allows a rough estimation of growth of approximating an increase in CW of 1 cm/month, the equivalent to 0.93 g/day for crabs between 6-9cms (Figure 5). This will be determined by LFDA analysis once the data set is completed. Recruitment into these replanted and managed mangroves appears to be restricted to a period between January and March. Following growth during April to November 2003, by November all crabs exiting the mangrove had reached maturity and all females were gravid. Females were found to be significantly larger than males (89mm vs 78mm mean CW). 

The catch per unit effort (CPUE) taken as number of crabs caught per day per sluice gate fell from 29 crab/d/gate in April 2003 to 2 crabs/d/gate in December 2003 However, catches recovered by April 2004 reaching 27.5 crabs/d/gate. This suggests that the early dry season recruits are almost fished out by December. In an evaluation of management techniques for enhancing the fishery, experimental restocking trials are underway. To date more than 2000 tagged S. paramamosain have been released into the 20 hectare site.

return to top

Figure 5: Size-frequency of Scylla spp. catches from Vinh Hai, Vietnam

  Long Phu  

The entire catch of several fishermen that use different fishing methods to target different size classes of crab have been monitored since July 2003.

1) Long handled drag nets (4mm mesh) are pulled over the flexible pneumatophores of the Sonneratia at the mangrove fringe during rising and falling tides (Figure 6). Results suggest that this method targets crabs smaller than 15mm CW, with a mean of 8.5cm CW. The recruitment appears to be variable, although the size class caught may be dependant on area fished if recruitment is not uniform

2) Torch fishermen collect crabs from inside the mangrove at night using head torches to locate crabs on the mud surface in shallow pools and hidden amongst mangrove roots. The size class found on the mangrove surface at night varies between 5 - 80mm CW with a mean of CW 38.6mm.

3) Hook fishermen that use a combination of digging and a long metal hooked rods to lever crabs out of burrows in the mangroves. Crabs 50-90mm CW were most commonly encountered in burrows with monthly means of between 65-77mm CW. Most of the crabs were immature. Males formed approximately 65% of the catch and were significantly (p = 0.021) bigger (71mm vs 66mm CW). This may be due to the larger females are migrating out to the open sea to spawn.

4) Fyke net fishermen use 500m of 1.5cm mesh net that is set along the mangrove fringe the position is changed every day and the net raised at high tide to catch fish and crustaceans migrating out of the mangrove during the ebbing tide. The fyke net caught very few crabs (2-16 crabs/day) with females forming 65% of the catch. The females were significantly (p = 0.026) bigger (76mm vs 61mm CW) and 24% mature were mature compared to 6% of the males (n = 48). 

5) Gill net fishermen use 2000m of 70mm gill net that is deployed for 3 days along the coast outside the estuary. The net is lifted and cleared several times a day, fish is the main catch. Approximately 58% of the crabs were females and there was no significant difference in mean size between the sexes (Mean CW 113mm). This suggests that the majority of crabs caught in the gill nets were mature.

return to top

Figure 7 compares the size classes targeted by the different fishing methods and illustrates how habit utilization within the mangrove changes throughout the life cycle of the crab. Crabs as young as stage 1 recruit into the mangroves settling out on the pneumatophores of the Sonneratia on the mangrove fringe. As the size increases to 15mm CW they begin move into the mangrove but still live on the surface before burrowing as immature crabs at about 40mm CW where they remain as permanent residents. A proportion of them undergo migrations into the estuary at low tide at about 60mm CW, it is unclear if this movement is daily or permanent. The majority of the crabs caught in the open sea are mature adult suggesting the pattern of niche occupation for S. paramamosain is very similar to that described for S. serrata in Australia (Hill et al, 1982). 

References

Keenan, C. P., Davie, P. J. F. and Mann, D. L. (1998). A revision of the genus Scylla de Haan, 1833 (Crustacea : Decapoda : Brachyura : Portunidae). Raffles Bulletin of Zoology 46, 217-245.

Hill, B. J., Williams, M. J. and Dutton, P. (1982). Distribution of juvenile, subadult and adult Scylla serrata (Crustacea: Portunisae) on tidal flats in Australia. Marine Biology 69, 117-120.

return to top