Assessment of stock and movement pattern for sustainable management of blue swimming crab Portunus pelagicus (Linnaeus, 1758) : case study in Kung Krabaen Bay, Chanthaburi Province, Thailand

Abstract

Assessment of stock and movement pattern in blue swimming crab Portunus pelagicus Linnaeus, 1758 at Kung Krabaen Bay was conducted from October 2008 to September 2009. This study was divided into 4 parts; 1) assessment whether the blue swimming crab population in crisis 2) populations both inside and outside the bay are the same group 3) assessment of the present blue swimming crab management in Kung Krabaen Bay and finally 4) the sustainable management of blue swimming crab was proposed based on scientific findings. Stock assessment was analysed by FiSAT program. The result indicated that the status of blue swimming crab populations in Kung Krabaen Bay was in crisis. Several key indicators have confirmed this. The fishing mortality was increasing to 4.14. The exploitation rate was 0.71 which was higher than the optimal value. Size of mature female also decreased to 7.52±1.14 cm. Low average fecudity was observed of 572,138±261,075.56 eggs. Movement pattern of blue swimming crab was carried out by tagging technique which was conducted inside the bay. In the first recaptured stage only 155 from 513 tagged crabs (30.21%) were recaptured. The result revealed that crabs living inside and outside the bay were related of the same stock. The result also showed most of the male crabs, 83.1%, traveled inside the bay especially the juveniles moved within the seagrass bed, Enhalus acoroides. Most of the non-ovigerous females of 70.3% migrated from inside to outside the bay. Moreover, ovigerous females were found both inside and outside the bay with the same proportion, 54.2% and 45.8%, respectively. Adult crabs can migrate in greater distance and in faster speed than juvenile crabs. The average migration distance of ovigerous females was highest of 2.43±1.16 km. The highest average speed of movement was also found in ovigerous females of 0.71±0.68 km/day. Movement pattern of blue swimming crab was supported by the feeding ecology study. This study showed strongly indicated migration has often been associated with an ontogenetic niche shift in habitat use. Major food items of crab were teleost fish, crustacean and organic matter. Based on the assessment of present crab management and scientific problems in Kung Krabaen Bay and scientific findings from this study, the sustainable management of blue swimming crab was proposed as followed 1) Closing season in the bay during spawning season in December, March and August 2) Conservation area proposed at seagrass beds and spawning site outside the bay 3) Restoration of seagrass bed Enhalus acoroides 4) Limited mesh size of fishing gear no less than 2.5 inch 5) Intoducing a crab bank in the mouth of the bay 6) Culturing and restocking of crab larvae in the field 7) Increase awareness on crab banks and restocking to stakeholder and 8) Networking with all stakeholders for better understanding on the ecology of blue swimming crab for sustanable fishery management in Kung Krabaen Bay.