Last Update : Monday 22 January, 2001 4:52 AM
Last Update : Monday 22 January, 2001 4:52 AM |
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Research Survey Report
Survey of Distribution of Giant Clams in Mu Ko Surin Marine National Park Area
Giant clams are the largest bivalve molluscs. They have white or yellow thick shells, radial ribs, and scales lining up on the ridges. They are found in sunlit coral reefs, having Byssal attached their Umbo sides to the floor. Mostly the front of Umbo has Byssal Gap. In Giant clams, both male and female organs are present in the same individual.
Giant clams live in symbiosis with unicellular algae called zooxanthellae, which are embedded in mantle tissue. When the algae are optimum growth and reproduction, they will produce food for Giant clams.
There are 8 species of Giant clams. In Thailand are found 5 species: Tridacna crocea, Tridacna maxima, Tridacna squamosa, Tridacna gigas, and Tridacna hippopus. However, since 2 species has been extinct, there are only Tridacna crocea, Tridacna maxima, and Tridacna squamosa left.
Tridacna crocea are the smallest Giant clams. They have about 15-centimeter-wide shells with Byssal Gap, and no scales. Almost all of their bodies are embedded in corals. At present, these giant clams can be found the most.
Tridacna maxima have the largest shells at about 35 centimeters with Byssal Gap. Scales are lining close together on the ridges of their shells. They are found embedding in corals from one-third to a half of their shells. At present, they can be found the least.
Tridacna squamosa have large shells at about 45 centimeters with Byssal Gap. They have large scales at the shells, and they are not embedded in corals. At present, they are nearly extinction.
Study on the Distribution of Giant Clams in Thailand
The study of the distribution of Giant clams had been done since 1987 (Thon Thamrongnavasawat, 1987), but it was only the study on specific area of Ko Kradad, Trad Province.
The study indicated that the most Giant clams found were Tridacna crocea, but Tridacna maxima and Tridacna squamosa were found very few. Besides, it recommended that using Line Transect Intercept Technique was not suitable for a study on distribution of Giant clams because Giant clams’ population density was too low compared to quantity of area-covered corals. Therefore, the development of study method should be done for precise data.
The method of study used in this time was developed from Quadrat Method called Circle Sampling. Its advantages were the possibility to cover large area, its convenience, the possibility to do by individual, and using cheap equipment.
Objectives
- To study the distribution of Giant Clams in Mu Ko Surin Marine National Park area.
- To study the connection between Giant clams and the use of Sea Gypsies.
- To study the possibility to raise or preserve Giant clams by Sea Gypsies.
Survey Method
Since giant clams are large bivalve molluscs sticking on the sea floor, this time Giant clams’ survey method has been developed from Quadrat Method, which square-shaped equipment was used in certainly known area. But this time, the 4-meter-radius circle-shaped equipment was used by applying a 4-meter-long rope with one end tied to weight and the other end tied to stick tip.
Survey method was done by sampling survey the area that water was not too deep. The depth of water was not more than 3 meters. Drop the weight to the sea bottom, loosen the rope, and swim in 4-meter-radius circle. While swimming, count the giant clams and identify their species. When completing the circle, record the numbers of each species. Then, repeat the same process for 20 times.
From the area of circle formula , the area of circle with 4 meters radius was about 50.29 square meters.
Moreover, Several sizes of T. squamosa from the large size of 50 centimeters to the small size of 15 centimeters were removed from the mooring area in Ao Chong Kaad to the underwater natural trail in Ao Suthep.
Place & Time
The survey was done in 11 areas of Ao Mae Yaai, Ao Sai Ane, Ao Saai Daeng, Ao Jaak, Ao Mai Ngam, Ao Suthep, Ao Pakkard, Ao Tao, Ko Torinla, Ko Pajumba, and Chong Kaad as shown in picture. This time survey was done twice during December 12-18, 1999, and March 13-18, 2000.
Results
As the results, 3 species of Giant clams were found, which were Tridacna crocea, Tridacna maxima, and Tridacna squamosa. In these 3 species, Tridacna crocea were found the most, the other two species were scarcely found.
In Surin Islands area, the most diversity of Giant clams could be found at Ao Jaak, but the least diversity was at Ao Mai Ngam, as shown in the picture.
The reason why a lot of Giant clams distributing in Ao Jaak was because Ao Jaak was in distant area, so there were no disturbance of tourists and Sea Gypsies seeking the shells in the past. Also, Ao Jaak was located in the northeast of Surin Islands, so it was not too much effected by waves during the southeastern monsoon season.
Ao Mai Ngam was the area that Giant clams distributing the least. It was probably because there were a lot of tourists in the bay, and in the past, it was also the place where Morgan Sea Gypsies gathered the shells.
At present time, Sea Gypsies rarely made use of Giant clams both in making foods and selling the shells.
From the study on removing Giant clams; both large clams and small clams could be removed by SCUBA divers. After following up for 3 months (February to April), all 5 Giant clams were in good conditions, no signs of weakness or any influences of removing.
The possibility of preserving Giant clams was in several ways such as spawning and rearing Giant clams by taking juvenile clams from Aquatic Animals Spawning Development Center, Prajuab Kirikan Province; however, it might use a long time since the juvenile clams over there were breeding stocks from Ao Thai
In addition, removing Giant clams from unsuitable areas, like mooring area, to the suitable areas, such as coral reefs and tourist attraction spots, might be applied.
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