Abstract
Monitoring the health of coral is imperative following the global coral-bleaching episode that occurred in 1998, and because climate change is ongoing. However, the usual method employed to monitor coral is visual inspection, which is unable to determine some fine-detailed changes. It is known that symbiotic zooxanthellae supply coral with many nutrients and cause some changes in the reflection spectrum of the coral. Thus, we believe it is possible to monitor coral by monitoring zooxanthellae. By performing pixel operations on two pictures, taken in blue and near-infrared light, a “normalized difference vegetation index for coral” (hereafter referred to as NDCI) can be defined. Measuring photosynthetic activity by PAM, in the same area as measured by NDCI, shows high correlation between Fv/Fm and NDCI (Saito et al. 2008). We used this system to obtain estimates of the seasonal and diurnal variations of coral and converted them into numbers using the NDCI. The measurements of coral in this investigation were performed on the Ishigaki Island Shiraho reef during 2009-2010 and 2011-2012. It was found that seasonal variation of the NDCI value shows an inverse correlation with water temperature. Furthermore, the seasonal variation in the range of the NDCI value was found to depend on the species of coral. We conclude that coral exhibits change, both seasonally and diurnally, in response to low and high temperature, stress induced by light levels, and incursions of mud. Interestingly, we can also report that we observed strong and weak bleaching and deviations of the NDCI value following the passage of a typhoon.
