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Population and Distribution of Horseshoe Crab Carcinoscopius Rotundicauda at the Kranji Nature Trail Estuaries, Western Johor Straits

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Population and Distribution of Horseshoe Crab Carcinoscopius Rotundicauda at the Kranji Nature Trail Estuaries, Western Johor Straits Population and distribution of Horseshoe Crab Carcinoscopius rotundicauda at the Kranji Nature Trail estuaries, Western Johor Straits Hong Ruixia, Fiona U012717Y Undergraduate Research Opportunities in Science PROJECT REPORT submitted to the Department of Biological Sciences The National University of Singapore LSM 3000 (4 MC) April, 2004 3060 words ABSTRACT There are two parts to this research project which is firstly the establishment of a baseline map for ease of potential studies in the future and the secondly the sampling proper in order to determine where C. rotundicauda occurs within the study site, and to study possible relationships between sub-habitat preference and size class. A baseline low tide map defining the Kranji study site was carried out jointly with a fellow course-mate. The sampling proper involved the establishment of a search image and subsequent usage of two main methods namely sight and touch method and a self-derived ‘poke’ method over seven samplings in five main sites and seven different sub-habitats – small pools, shallow streamlets, in streams, edge of streams, on mud and half or fully buried in mud. A total of 162 horseshoe crabs were recorded. Juveniles can be found in high numbers on river banks imperforated with shallow streamlets and small pools while adults were discovered to fully bury themselves, indicating a possibility of high numbers in mudflats. Year-round studies need to be done to better assess the population and the seasonality, if any, of C. rotundicauda (188 Words) INTRODUCTION Mangrove area estimates have dropped from 2700 ha in 1980 to 500 ha in 1990 (FAO 2003). Fragments of mangroves are still found mainly along the northeast coastline (Khatib Bongsu, Changi Creek, Pasir Ris Park and Loyang), northwest coastline (Mandai, Sungei Buloh Wetlands Reserve and Kranji), the northern off-shore islands (P. Tekong and P. Ubin) and on Pulau Semakau (Sivasothi & Tan, 1999). Understanding the biodiversity of our mangroves is an important requirement for management and conservation. There has been a relatively sound foundation of taxonomic research conducted in Singapore mangroves with many new species discovered in Singapore (Sivasothi & Tan, 1999). However few ecological studies have been conducted in the post-Murphyian era (Mok & Munro, 1991; Sodhi et. al., 1997; Sivasothi, 2000; Karns et. al., 2002). Ecological information is thus poor and even the distributions of common species in our mangroves are not well understood. All four species of horseshoe crabs are listed on the IUCN red list. Three are categorized as ‘data deficient’ (IUCN, 2003). Of the three, Carcinoscorpius rotundicauda (mangrove horseshoe crab) can be found in our mangroves. Also the smallest species among the four, C. rotundicauda is globally distributed in the coasts of India, Indonesia, Malaysia, The Philippines, Thailand and Singapore (IUCN, 2003). There is a lot of anecdotal knowledge on the species but little scientific work has been published. There is suspected to be some studies in regional or local publications and thus not easily available (Sivasothi pers. comm.). The local population of the mangrove horseshoe crab is also believed to have decreased over the past two decades (Sivasothi pers. comm.). In light of this, here a study of the population and distribution C. rotundicauda in one study site is attempted. The Kranji mangroves study site is an approximately 5 hectares of mangroves with rich topography, offering a range of different sub-habitats within a relatively small area. In this site, one can find streams, streamlets, raised platforms, mudflats, small pools and even sandbanks. It is now accessed by the Kranji Nature Trail (KNT) which was constructed only in 2003. It now links the visitor centre of Sungei Buloh Wetlands Reserve (SBWR) and Kranji mangroves. KNT provides some hope that the area will not disappear in the near future, important for future comparisons of data and information. SBWR staff conduct regular patrols into the area. With its easy accessibility from the visitor centre, the study site is thus safe, allowing the researcher to work alone. On weekdays, there are few visitors in the Kranji Nature Trail and thus the vicinity is not very much disturbed for now. The objectives of this study are thus: 1 The establishment of a baseline low tide map of the Kranji study site with a fellow course-mate Ms. Teo Yen Ling for this and future studies. 2.1 To determine where C. rotundicauda occurs within the study site 2.2 To study the relationships between sub-habitat preference and size class. MATERIALS AND METHODS Part 1: Establishing a baseline low tide map defining the Kranji study site This sub-project was conducted jointly by Teo Yen Ling and Fiona Hong over five mapping fieldtrips. Materials Measuring Tapes, 50m and 100m, Prismatic Compass, White Spray Paint, Flagging Tape, Clipboard, Pencil, A4 sheets, wooden poles. Establishing the base and start points The base and starting point are fixed points that will not disappear or be destroyed in the near future. The base point is additionally a known point that can be linked to a road map. The stone marker commemorating the launch of the Kranji Nature Trail next to the SBWR carpark is the base point. The start point is the 3rd inner pillar of Bridge Two of Kranji Nature Trail (KNTB2) Mapping in the field Mapping is conducted as a measurement of distances and bearings of a series of straight lines by a data recorder and a surveyor. Starting out together at a fixed point, the surveyor decides the next point, and walks to it with one end of the measuring tape. This is pulled taut and flushed directly with the midline of the body over the point, facing the recorder. The distance is read of in metres. The recorder uses the compass to take the bearing to the surveyor, and records both values alongside a line indicating the relative position on a sketch map. An arrow head indicates the direction of the bearing. The approximate position of surrounding geographic features such as streams, streamlets, tree lines and bund is marked into the data sheet. Precautions against confusion of data and inaccuracies during mapping include: • Numbers are called out individually, e.g. 29.2 metres is called out as two-nine- point-two metres. • The values called out by the surveyor is echoed by the recorder, and affirmed by surveyor. • Distance and bearing are written with units next to the plotted lines. • Pencil was used on paper for recordng data. • White spray paint and flagging tapes were used to mark certain trees or points. • The widths of streams are recorded at regular intervals. Meandering bends were indicated by recording the distance of the most pronounced section away from a point. • The number of points used were minimized in order to avoid human error. • Known, distinctly noticeable points were marked onto the map and served as additional reference points and tied back via mapping. • Each session was reconnected back to the Starting Point in a circular manner. • Fixed points or prominent markers were used as far as possible. • Specific terms were used between workers: o ‘Roll In’ – roll in measuring tape o ‘Release’ – release end of measuring tape o ‘Distance’ or ‘Bearing’ – to alert recorder or the surveyor to appropriate action before data is taken. Plotting the data Draft maps were prepared based on the fixed points after each mapping session using the data, a protractor and a ruler, on a scale of 1:1000. These draft maps included the bearing arrows, distances and bearings. When loop backs of straight lines did not reconnect to the start point on paper, remapping was conducted in the field. The completed draft map was drawn on A3 paper. The map was inked with waterproof and fade-proof ink on tracing paper placed above the draft sheet. Tree-line, streams, markers and other recorded features were drawn in using another sheet of tracing paper. The final draft was inked on an additional layer of tracing paper. Appropriate labelling (labels, legend, north arrow) was added, and the resultant map was photocopied onto paper and scaled to size. Part 2: Distribution studies on the mangrove horseshoe crab, C. rotundicauda Developing a search image for juveniles A preliminary visit was taken to develop a search image of the horseshoe crab, accompanied by Professor Peter Ng and Mr Sivasothi to the Buloh East Mangroves. The juveniles cannot be distinguished easily by sight only as there were seldom differences in colour between the substrate and prosoma. They mentioned that juveniles are generally found in a sub-habitat which is slightly waterlogged and contains small streams. Sight was used to search for bulges and by lightly touching the bulges. Developing a search image for adults Search image was uses the outline of the prosoma and the telson was used as a guide. To reduce the recce period, Mr Sivasothi mentioned that adults could be found under Kranji Nature Trail Bridge 2 (KNTB2) and a first attempt to search for adults was performed there. Search Techniques 1. The initial method was the sight and touch method. This was used in most of the sub-habitats. 2. The channel bed of KR4 was felt by running through with both hands. This method however did not bring any results and was subsequently abandoned. 3. Towards the end of the study, horseshoe crabs were also found to fully bury themselves which renders the sight and touch method insufficient. A ‘poking’ method was developed and involves the use of a thin but not easily breakable stick to feel for the hard prosoma beyond the surface of the mud. The use of a stick helps to quicken the area coverage, provides safety and requires less destruction as compared to by hand.
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