Sargassum species grow on rocks and dead coral, forming dense seaweed beds in their growth seasons. Sargassum beds play a range of ecological roles in coastal waters, but their sustainability is threatened by pollution and reclamation within ASEAN countries with rapid economic growth. Before conservation initiatives can be implemented, it is necessary to establish their present distribution. These beds can be mapped using the noncommercial satellite, ALOS, which can provide multiband images using high spatial resolution optical sensors (sensitive to 10 m-2 plots), AVNIR-2. These images are of sufficient quality for examining coastal ecosystems and, in this study, for mapping the Sargassum beds in the waters off the coast of Sattahip, Chon Buri Province, Thailand. This coastal zone is a natural marine park reserve and protected from human activities such as fishery and reclamation. Biomass data were obtained in January, February, March, and December 2009 from quadrat sampling. The biomass of Sargassum aquifolium (Turner) C.Agardh at a bottom depth of 1 m and 1.5–2 m was determined to be 7.73 and 92.75 g dw m-2, while that of Sargassum oligocystum Montagne was found to be 44.05 and 87.97 g dw m-2, respectively. Ground truth data were obtained in February and October 2012 from serial images taken by manta tow. Supervised classification is a procedure for identifying spectrally similar areas on an image by identifying “training” sites of known targets and then extrapolating those spectral signatures to other areas of unknown targets. By applying this methodology, the Sargassum beds off Sattahip can be detected with an accuracy of about 70%. It is estimated that the error is caused by mixel effects of the bottom substrates in individual pixels, each of which covers an area of 10×10 m. Our results indicate that the images captured by ALOS AVNIR-2 are informative and useful for mapping the Sargassum beds in Southeast Asia.

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The coastline of Cambodia stretches along the Gulf of Thailand including 69 islands. It supports rich diversity of marine species. Distributions of habitats including mangroves, coral reefs and seagrass beds along the pristine Cambodian coast still remains unknown compared to neighboring countries, Thailand and Vietnam. Cambodian seagrass beds form habitats with rich biodiversity and economical value through marine ecosystem services playing as a key role against climate change by reserving large amount of carbon. However, the general status of these seagrass habitats is poorly researched and documented. Satellite image of Advance Visible and Near Infrared Radiometer type 2 (ALOS AVNIR-2) with high resolution (10x10m) provides good information for seagrass habitat mapping. Study site was selected around Rabbit (Koh Tonsay) Island with area of 2 km 2 . The objectives of this study are (1) to know spatial distribution of seagrass beds around this island by ground survey and (2) map seagrass beds using the ALOS AVNIR-2 image with ground truthing data. Ground truth survey was conducted in June 2011. Surveys along three transect lines revealed 8 species of seagrasses belonging to Hydrocharitaceae (4 species) and Cymodoceaceae (4 species) around the island. We analyzed ALOS AVNIR-2 taken on 22 December 2009 to map distribution of seagrass beds around Koh Tonsay Island. Results showed that remote sensing using ALOS AVNIR-2 data provides a practical tool for mapping seagrasses beds around the island and information for future management and conservation of seagrass beds in Cambodia.