Sargassum species grow on rocks and dead corals and form dense seaweed beds. Sargassum beds play ecological roles
such as CO2 uptake and O2 production through photosynthesis, spawning and nursery grounds of fish, feeding ground for
sea urchins and abalones, and substrates for attached animals and plants on leaves and holdfasts. However, increasing
human impacts and climate change decrease or degrade Sargassum beds in ASEAN countries. It is necessary to grasp
present spatial distributions of this habitat. Thailand, especially its coastal zone along the Gulf of Thailand, is facing
degradation of Sargassum beds due to increase in industries and population. JAXA launched non-commercial satellite,
ALOS, providing multiband images with ultra-high spatial resolution optical sensors (10 m), AVNIR2. Unfortunately,
ALOS has terminated its mission in April 2011. However, JAXA has archived ALOS AVNIR2 images over the world.
They are still useful for mapping coastal ecosystems. We examined capability of remote sensing with ALOS AVNIR2 to
map Sargassum beds in waters off Sattahip protected area as a natural park in Chon Buri Province, Thailand, threatened
by degradation of water quality due to above-mentioned impacts. Ground truth data were obtained in February 2012 by
using continual pictures taken by manta tow. Supervised classification could detect Sargassum beds off Sattahip at about
70% user accuracy. It is estimated that error is caused by mixel effect of bottom substrates in a pixel with 10 x 10 m. Our
results indicate that ALOS AVNIR2 images are useful for mapping Sargassum beds in Southeast Asia.