Nobuaki Arai

Field informatics on sea turtles, Mekong giant catfish and dugongs

We are performing the Southeast Asia sea turtle associative research (SEASTAR2000), the Mekong giant catfish tracking project (MCTP) and the dugong biological survey (DBS) as part of the 21st Century Centers of Excellence (COE) Program (2002-2007) and the Global COE Program (2007- 2012). All the species of the projects are heavily endangered and need sufficient degree of protection. They are listed in the Convention on International Trade in Endangered Species (CITES) Appendix I. However, very little is known of the species while the implementation to conserve them is a pressing problem in Thailand and neighbouring ASEAN countries.


We launched the Southeast Asia sea turtle associative research (SEASTAR2000) in 1999 on the request of the Thai government. At the time, the US government had notified the Thai government of a ban on shrimp exports to the USA due to the by-catch of sea turtles by shrimp trawlers. The urgent objectives of the SEASTAR2000 were to clarify migratory paths of sea turtles in the Gulf of Thailand and the Andaman Sea using a satellite tracking system and to develop a scientific strategy for the conservation. We released around 30 female green turtles, Chelonia mydas, attached with Argos platform transmitter terminals (PTT) and clearly found the sea turtle migratory paths as shown in Fig. 1. The results clarified the migratory paths of the turtles after their nesting; in the Gulf of Thailand (Fig. 1a) they migrated to various sea areas sometimes even beyond Thai waters including the South China Sea, and in the Andaman Sea (Fig. 1b) almost all the turtles migrated to the Andaman Islands within Indian territorial waters via different routes.

Fig.1. Results of the Argos tracking of adult female green turtles in the Gulf of Thailand (a) and the Andaman Sea (b).

The accuracy of the location of the conventional Argos system that we used as mentioned above can be anywhere between 100m and 4,000m. It is not enough to understand the details of wild movement and to make actual implementation plans to conserve both their nesting areas and feeding areas. If too wide a nursery area is declared without consideration of the location errors to safely conserve sea turtles, it may pose serious problems among those local people who live on coastal fisheries. Therefore, it is necessary to develop new PTTs to describe the fine scale movement in order to declare a satisfactory conservation area that is agreeable to the local people. For this purpose, we adapted the global positioning system (GPS) and developed the GPS-Argos PTTs.

We performed tracking experiments using the prototype of the GPS-Argos PTT attached to two hawksbill turtles, Eretmochelys imbricate, in a 5 ha breeding pond in Thailand. We obtained position data from both the Argos conventional positioning system and GPS, and then compared both data according to the fixed kernel home range estimation (Table 1). This result shows that the GPS-Argos has enormous potential for increasing our understanding of the home range and the fine-scale movement patterns of sea turtles.

Table 1. Summary of results of the fi xed kernel home range estimation for two hawksbill turtles, HB1 and HB2 in a 5 ha breeding pond in Mannai Island, Rayong Province, Thailand.
Home range by Argos
Home range by GPS
HB1 (80.8) 156,740 2.96
HB2 (79.0) 184,478 0.93

*CCL: Carved Carapace Length


The Mekong giant catfish, Pangasianodon gigas, is endemic to the Mekong River basin but they are deeply endangered now. The Thai government heavily restricts the catch of Mekong giant catfish, but the number of the catch has decreased drastically. Moreover, this fish is one of the most important fisheries resources for local people who perform annual pre-catch folk rituals. The Thai government requested us to investigate the migration behaviour of the Mekong giant catfish based on the successful results of the sea turtle tracking.

Fig.2. A post doctoral fellow released a Mekong giant
catfish attached with data-loggers and a time-scheduled
releaser in Mae Peum reservoir in Phayao Province, Thailand.

We launched the Mekong giant catfishtracking project (MCTP) in 2002 and performed it both in an artificial reservoir and in the Mekong River. We examined the diel and annual movement patterns in Mae Peun reservoir in Phayao Province, Thailand (Fig.2). Fish with ultrasonic transmitters surgically attached inside their body were monitored for approximately 15 months using 14 ultrasonic receivers set on the bottom of the reservoir to cover all the area. The fish showed diel horizontal movement patterns between the inshore and the offshore and displayed regular diel vertical movement patterns. During the day, fish showed active vertical movement; while at night they remained on the shallower inshore bottom, maybe to feed.

To quantify the possibility of enhancing hatchery-reared fish to the Mekong River, field experiments were conducted in the Mekong River from 2002-2004. We released 28 Mekong giant catfish attached with the transmitters and monitored fish for up to 97 days and collected the first records of upstream, downstream, and vertical movement of these fish in the Mekong River. During the day, fish swam upstream at a speed of 16.2 km/d, and downstream at a speed of 7.2 km/d and displayed vertical movement between the surface and the bottom. Unfortunately, some of the fish might have migrated beyond the border and could not be recorded by the receivers set on the Thai side. These results indicate that hatchery-reared fish can survive and may have the ability to migrate and spawn in the Mekong River, and that we need cooperation between Thailand and Laos to monitor the fish in the Mekong River.


Dugongs, Dugong dugon, are the only marine mammals that feed on benthic seagrass and they are also greatly endangered. As with the sea turtles, dugongs are sometimes caught incidentally by fishing gear due to the many human activities in the shallow waters where they live and feed.

We launched the dugong biological survey (DBS) in Trang Province, Thailand in 2002. The motivation of the DBS is different from the other two projects. The project was originally ordered by the Japanese government to conserve dugongs inhabiting around the Okinawa Islands. They say that there are less than 50 dugongs and are about to become extinct in the near future. However, little is known of dugongs so that it is keen to compile biological knowledge on dugongs. In this background, we found the suitable experimental field to study dugong biology in Thailand under the cooperation with Thai researchers.

Fig.3. Three graduate students brought an AUSOMS-D.

We attempted to solve the problem by establishing a passive acoustic monitoring technique for dugongs. The idea of the technique is to record the dugong calls and analyze them acoustically to locate positions of the sources of the vocalizations. The advantages of this method are that it has no impact on the animals at all during monitoring and that it can be performed at constant detection efficiency even in the night. The limiting condition of the technique is that the focal animal must vocalize frequently and distinctively. A feasibility study was then necessary to better understand how these limitations could be cleared. Based on the acoustic characteristics of the calls, automatic underwater sound monitoring systems for dugongs (AUSOMS-D) were developed (Fig.3).

The AUSOMS-Ds were deployed on the sea floor at depths of about 5m south of Talibong Island, Thailand. The AUSOMS-Ds recorded underwater sound in stereo at a sampling frequency of 44.1 kHz for more than 116 consecutive hours. Dugong calls were automatically detected by newly developed software with a detection rate of 36.1% and a false alarm rate of 2.9%. In total, 3453 calls were detected during the 164 hrs of recording. The autocorrelation of the call rate indicated an attendance cycle of about 24 or 25 h, and the most frequent vocalizations were observed from 03.00 to 05.00 hrs. The calculated bearings of the sound sources, i.e., dugongs, were used as an indicator to track the relative numbers of dugongs during the monitoring periods.


The results of the SEASTAR2000 and MCTP indicate that both the sea turtle and Mekong giant catfish migrate beyond the borders so that international cooperation is essential for their conservation and the prevention of their extinction. We proved that the acoustical survey is also a useful tool to investigate dugong behavior. Although our research projects proposed the effective methodology to understand the behavior and revealed some of it, our knowledge is not sufficient to understand the whole of their habitat. Moreover, we need to be considerate to the local people who live on the coastal and inland fisheries. We are now just beginning to establish coexistence between endangered species and human beings based on the scientific knowledge.

Photo of Arai

Nobuaki Arai

Born in 1957
Field of specialization: Biotelemetry study on a aquatic biological informatics
Graduated from Faculty of Agriculture, Kyoto University
D.Agr., Kyoto University
Associate Professor, Graduate School of Informatics, Kyoto University
URL : Nobuaki ARAI (External Link) (Japanese)

I am about to start research that will help to prevent feeding damage on clams by longheaded eagle rays. I also want to develop new measuring instruments.

Associate Prof. Arai was born at Kyoto University Hospital and grew up in Kyoto. He initially entered the Department of Information Engineering at Shinshu University, and later re-entered the Department of Fisheries in the Faculty of Agriculture at Kyoto University. This was not only because he wanted to study at Kyoto University but also as a result of the problems concerning the 200-nautical-mile fishing zone which were attracting much attention at that time. In 1980, he entered the Ministry of Agriculture, Forestry and Fisheries of Japan. He was involved in fishery administration for 13 years while exercising his skills in acquiring a research budget for fishery research laboratories and in regulating fisheries in the Seto Inland Sea and the northern fishery. In 1993, encouraged by his former teacher at Kyoto University, he returned to the university as an assistant professor in the Department of Fisheries in the Faculty of Agriculture. He has occupied his present position since 1998.

Associate Prof. Arai is involved in research into marine life activities using biotelemetry the method of observing the invisible which uses radio waves and ultrasonic transmitters to trace the activities of organisms. He says: “It is not good to catch fish too much, but it is possible to allow a regulated fishing if one knows the mechanisms of marine life activities.” He conducts basic research for the preservation of various species and the acknowledgement of the coexistence of local societies, striving for the protection of rare marine animals such as sea turtles, dugongs and Finless porpoises as well as the sustainable use of important species for fisheries such as red tilefish and Mekong giant catfish.

He loved to study minerals and fossils as a child, and was excited about computers in the late 70’s. The “geologist boy” with limitless curiosity realized his dream of becoming a researcher. He often travels abroad to participate in fieldwork and encourages young researchers to “be active on a global scale.”