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Cartwright-Taylor L.,Nature Society Singapore NSS | Ng H.H.,Nature Society Singapore NSS | Goh T.Y.,Nature Society Singapore NSS
Aquatic Biology | Year: 2012

Mangrove horseshoe crab Carcinoscorpius rotundicauda were tracked for 6 mo in the tropics using acoustic telemetry to determine whether they either show homing behaviour, remain near the site of capture, or travel to the open sea. Twelve adult horseshoe crabs were caught on the Mandai mudflats at Kranji and acoustic telemetry transmitters attached. Six acoustic receivers were deployed suspended from floating fish farm platforms ~1 km apart along the Strait of Johor, an estuary off the north coast of Singapore. Four individuals, 2 of each sex, were used in each of 3 tests: 4 crabs were released at the site of capture, 4 were released at a second site ~5.7 km away to determine if they exhibited homing, and 4 were caught along the muddy beach at the second site, tagged and released into deep water under one of the receivers. Although there were gaps in detection, 11 of 12 crabs were located intermittently within ~6 km of the point of release. One crab disappeared after 5 d. Homing was not exhibited, and the crabs were detected within the estuary for 6 mo. We concluded that the crabs in the tropical estuary, where there are no seasons or marked changes in water temperature, showed no tendency to travel out to sea and did not exhibit homing behaviour or any synchronized movement pattern to deeper waters. © Inter-Research 2012.

Cartwright-Taylor L.,Nature Society Singapore NSS | von Bing Y.,Nature Society Singapore NSS | von Bing Y.,National University of Singapore | Chi H.C.,Nature Society Singapore NSS | And 2 more authors.
Aquatic Biology | Year: 2011

A survey and interviews with fishermen to determine the current spatial distribution of coastal Tachypleus gigas and mangrove Carcinoscorpius rotundicauda horseshoe crabs on the main island of Singapore indicated that there are probably no sites that support a breeding population of T. gigas. The only adult T. gigas seen were trapped in nets at 1 site, no juveniles or sub-adults were found at any site, and fishermen see this species infrequently. C. rotundicauda were more abundant, and breeding populations were found on the mudflats, fringed with mangroves. These small areas may be the last sites that support a breeding population of C. rotundicauda. Population density studies of mainly surface crabs on the mudflats at 1 site gave a conservative figure of 0.5 crabs m -2 using non-randomised, longitudinal belt-transects of 5 × 50 m, set from high- to low-tide zones. Smaller randomised quadrats, searched for both buried and surface crabs, gave densities of 0.57 to 0.98 individuals m -2, equivalent to a possible abundance ranging from 29 925 to 51 450 individuals in the accessible search area of 52 500 m2. Comparisons over different months suggest that density changed little over time. Randomised quadrats and searches to depletion gave higher density figures, but they are labour intensive and difficult to set up in the terrain. Randomised, longitudinal belt- transects of 5 × 50 m are recommended for long-term monitoring of crab density. These findings provide baseline data to monitor the population at the site and to formulate conservation strategies for the 2 crab species. © Inter-Research 2011.

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