VOLUME 12 NUMBER 1 (January to June 2019)

by Paul Caesar M. Flores*¹, Fernando P. Siringan¹, Ritzelle L. Albelda², Kevin Thomas B. Go², and Patrick C. Cabaitan²

¹Geological Oceanography Laboratory, Marine Science Institute,
     University of the Philippines Diliman, Quezon City, Philippines
²Coral Reef Ecology Laboratory, Marine Science Institute,
     University of the Philippines Diliman, Quezon City, Philippines

Recreational-grade side scan sonars (RGSSS) have become available in recent years. They provide high-resolution imagery similar to more expensive models but have limited depth coverage. RGSSS have been used in mapping of benthic communities in rivers and shallow reefs; however, their ability to identify potential upper mesophotic coral ecosystems has not been explored. In this study we use a dual frequency RGSSS operating at 86/455 kHz to generate a bathymetric chart and substrate map showing the distribution of rocky/coraline substrates and loose sediments. Ground truthing utilized drop camera images of the seafloor and limited diver observations. The survey covered a total area of 2.34 km2 with a depth range of 1.5–117 m. Rocky/coralline substrate covered 0.86 km2, while loose sediments covered 0.74 km2; the remaining area is unclassified due to the loss of sound signals at depths greater than 40 m. The accuracy in discriminating hard-bottom habitats from loose sediments is 89% with a kappa coefficient of 0.78, which shows that RGSSS can be used in mapping the upper mesophotic zone (30-40 m). However, the user must be aware of certain limitations such as (1) the lack of a manufacturer-supported georeferencing tool, means that the geometric error of the sonar image may be magnified, and (2) the lack of standard procedures for processing this type of sonar images.