Recently, occurrence frequency of red-tide phenomena has been increasing in the eastern Asian Waters, which is considered as a sign of degraded marine environment. Since the red-tide phenomenon sometimes damages local fisheries, its early detection has become an urgent social requirement. Ocean Color satellite remote sensing has been considered as a tool for the red-tide detection. In the present study, we demonstrate its detection capabilities in the eastern Asian Waters. Quantitative evaluation of ocean-color red-tide detection is carried out for the areas of Seto Island Sea, Japan. The reason of aerial selection is well-organized existing red-tide observing systems in the regional seas, which consists of research vessels with public wet-laboratories, reporting systems of volunteer ships, and regular airplane observations by eye detections. Using SeaWiFS ocean-color measurements, daily quality controlled 0.01-degree grided images are produced for 1998-2002. For each grid in the study area, monthly mean nLw spectra are calculated for enhancement of the red-tide phenomena, which is known as visible discoloration of surface water. We define the red-tide phenomena as spectral deviations from the monthly mean nLws, which means that, in in situ sense, the red tide is seen as color changes from the surrounding and usual water color (long-term mean). In Osaka Bay, the red-tide phenomena were detected by comprehensive ship observations. Match-up data set were generated using the classified phenomena (Red-Tide: 19 points, non Red-Tide: 120 points) and SeaWiFS observations. Classification criteria of the satellite spectra are as follows; 1) Existence of spectral peak at 555nm, 2) the spectral gradient of 490-555nm is shaper than that of 443-490nm, and 3) nLws of 443-555nm are smaller than those of monthly mean. Sixteen of 19 red-tide cases are classified as red tide by these criteria (84.2%), and 103 of 120 non Red-Tides cases are properly detected (85.8%). The new satellite detection scheme using the ocean color images is tested against the air plane observations, which can be considered wider snapshots of visual Red-Tide detection. The satellite detection agreed quite well with the air plane one. New procedure using the spectral information of satellite ocean color can be used for eastern Asian waters, which will be discussed in the workshop presentation.
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