ABSTRACT. The study was conducted to determine the movement of mud crabs, genus Scylla, within three sites in Sematan mangrove forest, Sarawak, Malaysia. 10 crabs were tagged using ultrasonic tags attached to the mid-dorsal carapace surface of each crab. Four tagged crabs were released to the small mangrove channel, another four tagged crabs were released to the main estuary channel and the other two crabs were released to the mouth of the estuary. The hydrophone system and portable Global Positioning System (GPS) on board the tracking boat were used to track the crabs’ locations. Date of release, tracking time, duration of tracking, distance travelled by the tagged crabs, weather conditions and water current direction were among the parameters recorded during the tracking study. Three tagged crabs remained within the small mangrove channel, three tagged crabs remained within the main estuary channel and only one tagged crab remained in the estuary mouth. Tagged crabs remained in the small mangrove channel more than 173 hours but those in the main estuary channel and estuary mouth are a remained for more than 48 hours. Only one crab covered a much greater distance than the other nine tagged crabs, moving a total distance of 2,600 m within 48 hours monitoring. The mean ground speed of the tracked crabs in the small mangrove channel was slower than in the main estuary channel and estuary mouth. The mean ground speed ranged from 0.2 to 3.8 m.h-1 in the small mangrove channel and from 15.2 to 54.2 m.h-1, both in the estuary channel and estuary mouth. 390 crab movements were recorded of which 157 (40.3%) crabs moved with the current and 233 (59.7%) crabs moved against the current with 32.45 m.h-1 mean ground speed. From the half an hour and hourly interval monitoring, the crabs were mainly active at night with the mean distance travelled being 808.3 m and mean ground speed 44.7 m.h-1. During day time, the mean distance travelled was 482.8 m and ground speed 20.7 m.h-1. No continuous movement was recorded even though the crabs were active at night especially from midnight till sunrise. The direction of travel was not straight with the straightness index range from 0.2 to 0.3 except for one crab which moved nearly in a straight line with a straightness index of 0.9.
KEYWORDS. Telemetry system, ultrasonic tag, movement, mud crab, genus Scylla.