Acoustic characterization of mesophotic coral reef ecosystems of West Hawai'i

Coral reef habitats in Hawai'i are common in shallow waters and extend into mesophotic depths (30-150 m). However, habitat monitoring efforts have been concentrated in depths constrained by safe dive limits of 0-30 m. Mesophotic coral ecosystems (MCEs), located in depths of 30-150 m, are important components of the coral reef ecosystem, but only a small number of surveys have been conducted in these depths. Data for these deep-water habitats are essential for evaluating and monitoring their health and resilience. Classifying benthic habitats in mesophotic depths is challenging due to dive safety limits and water penetration capabilities of remote sensing options, such as satellite imagery and Light Detection and Ranging (LiDAR). The results of this effort show that acoustic data can be used to provide detailed substrate and biological cover maps that include mesophotic coral ecosystems. Here we employ a combination of principal component analyses and unsupervised classification techniques to derive six substrate and five biological cover classes from multi-beam acoustic data, which are validated by optical seafloor imagery to create a complete benthic habitat map for the West Hawai'i Habitat Focus Area (WHHFA). Our results show that the overall accuracy of the benthic habitat maps is 59% for substrate classification and 61% for biological cover classification. Accuracy was higher for the following individual classes; 76% Complex Reef, 86% Sand, and 88% Coral. These habitat maps are the first within the WHHFA to incorporate mesophotic data and provide important information for evaluating and managing the coral reef ecosystem as a whole. [doi:10.7289/V5/TM-PIFSC-61 (https://doi.org/10.7289/V5/TM-PIFSC-61)]