Coral-spawn slicks: Reflectance spectra and detection using optical satellite data

This study provides the first report of reflectance spectra of coral-spawn slicks and their detection using optical satellite data. The reflectance spectra show that coral-spawn slicks can be discriminated from other sea-surface features such as wave foam and floating Sargassum. Analysis of Planet Dove and Sentinel-2B imagery allowed successful detection of coral-spawn slicks at Ishigaki Island, Okinawa, Japan (24°31′40″N, 124°17′31″E), by using reflectance values in the green and red wavelength regions. This detection was attributable to both their high spatial resolution (3.7 m and 10 m, respectively) and high temporal resolution (1 day and 5 days, respectively), given that coral spawning occurs within a short period (up to several days), typically once a year, and that the coral-spawn slicks have narrow widths of ~10 m. The number of the detected coral-spawn slicks was small, possibly because of coral decline around Ishigaki Island caused by the 2016 worldwide mass coral bleaching event. High-frequency satellite observations, including those from satellite constellations, should provide a powerful tool for understanding coral-reef processes and features whose detection requires high spatial and temporal resolution.