Zooplankton dynamics and appropriate management approach for blue swimming crab in Kung Krabaen Bay, Thailand.

Abstract

Abstract Zooplankton dynamics were assessed, including the relationships between crab larvae, other zooplankton and ecological factors, and an appropriate approach to the management of blue swimming crab in Kung Krabaen Bay, Chanthaburi Province was explored. Sixteen stations were selected for bimonthly sample collection during 2004 and 2005 and these revealed 40 groups of zooplankton from 15 phyla. Among these, four economically important groups were recorded, including shrimp larvae, fish larvae, bivalve larvae and brachyuran larvae (crab larvae). The distribution and density of these four groups were different in relation to the influence of the monsoon season and the specific habitat in the bay where they were found. In particular, brachyuran larvae density was highest (1.4×105 to 3.8×103 ind/100 m3) during the south-west monsoon (May- October), which had a negative correlation with fish larvae and a positive correlation with Acetes spp. The study results indicated that the highest distribution and density of brachyuran larvae were found at the seagrass habitat and the bay mouth. The density and distribution of brachyuran larvae also corresponded to the study of the gonad somatic index, which found two peaks of the spawning of berried female blue swimming crab in September and January. Spawning of female crab showed a relationship with the peak density of brachyuran larvae in November. Both crab larvae and young blue swimming crab use the seagrass bed as a refugium and nursery habitat. With regards to the specific characteristics of this bay, the blue swimming crab is a dominant economic species, which has supported the livelihood of local fishing communities. Blue swimming crab production has declined from 120 t in 2002 to 80 t in 2004 and 62 t in 2005, which is clearly related to over-crabbing in this bay using small mesh size (3.50 cm) collapsible crab traps. Average catching size has also reduced from 11.22 cm in 1999 to 7.31±0.42 cm in 2005, with pre-reproductive females representing 70% of the catch. An appropriate management approach for the blue swimming crab fishery that enhances survival would include: (i) conserving seagrass beds by prohibiting crabbing during the northeast monsoon when berried female blue swimming crab and brachyuran larvae are at their highest density; (ii) enlarging the mesh size of collapsible crab traps to not less than 6.50 cm to avoid catching young crab; (iii) banning the capture of berried females; (iv) promoting crab restocking and culture; and (v) educating and publicizing sustainable crabbing to fisher communities, as well as other stakeholders.