10.1117/2.1201309.005101 Monitoring the viability of coral reefs Masahiko Sasano, Motonobu Imasato, Hiroya Yamano, and Hiroyuki Oguma Fluorescence characteristics of corals are measured using a new light detection and ranging system on a glass-bottom boat. Reef-building corals are important marine organisms. They form tropical coastal topography, have a symbiotic relationship with zooxanthellae, and are the primary producers in tropical oligo- trophic (having few nutrients) seas. Coral reefs are typically monitored using the ‘coral cover’ indicator. This is defined as the ratio of the area covered by coral to that of the seafloor. Higher coral cover indicates a better conservation status. To measure coral cover accurately, however, the viability of coral tissue must also be measured. The impact of global climate change has recently become ap- parent as the warming of oceans. Corals are fragile ecosystems with high sensitivity to water temperature. Coral communities are frequently killed due to high water temperatures, typhoons, and attacks by Acanthaster planci (crown-of-thorns starfish). It is therefore becoming increasingly important to monitor regional coral distributions. We use the fluorescent characteristics of corals to monitor their viability. Peaks in the UV-excited fluorescence spectrum of a coral colony are typical of green fluorescent proteins (see Figure 1). 1 UV-excited fluorescence diving investigations, con- ducted at night, provide more accurate coral viability checks than ordinary coral monitoring, but the size of the areas that can be studied is limited. We have developed a coral observa- tion system that uses fluorescence imaging lidar (light detection and ranging) on a glass-bottom boat. 2 Our system enables us to obtain images sequentially along a single boat track. The boat platform for our system allows a long and stable sur- vey line to be maintained, which suppresses most wave and tide disturbances. Coral fluorescence lidar is an active observation technique that uses a pulsed laser, and is not significantly af- fected by solar altitude or cloud cover. As a result, observations made with our system have a good range, i.e., more than 10m of water depth when water transparency is good. Continuous coral Figure 1. Photographs taken at night of a branch type coral colony in Okinawa, Japan, using white light (top left) and UV light (top right) illumination. The UV-excited fluorescence spectrum of the same coral colony is also shown (bottom). Fluorescence typical of green fluorescent proteins (GFPs) occurs at wavelengths of 490 and 510nm. Fluorescence of chlorophyll-a (Ch-a) occurs at 680nm. a.u.: Arbitrary units. viability checks can be achieved when measurements are made with low boat cruising speeds. Our coral fluorescence lidar system consists of a 355nm wave- length UV pulsed laser, an optical filter, and a gated intensified CCD (ICCD) camera. The timing of the laser shot and the open- ing of the ICCD gate are controlled and set appropriately for a specific water depth. Because the exposure time of the ICCD camera is very short, the effects of sunlight and motion blur are negligible for observed seafloor fluorescent images. We conducted coral fluorescence lidar observations near Take- tomi Island in Okinawa, Japan, during June 2011. 3 The aver- age boat speed during the observations was about 4 knots, and Continued on next page