Large-scale mapping of live corals to guide reef conservation

Abstract

Coral is the life-form that underpins the habitat of most tropical reef ecosystems, thereby supporting biological diversity throughout the marine realm. Coral reefs are undergoing rapid change from ocean warming and nearshore human activities, compromising a myriad of services provided to societies including coastal protection, fishing, and cultural practices. In the face of these challenges, large-scale operational mapping of live coral cover within and across reef ecosystems could provide more opportunities to address reef protection, resilience, and restoration at broad management- and policy-relevant scales. We developed an airborne mapping approach combining laser-guided imaging spectroscopy and deep learning models to quantify, at a large archipelago scale, the geographic distribution of live corals to 16-m water depth throughout the main Hawaiian islands. Airborne estimates of live coral cover were highly correlated with field-based estimates of live coral cover (R² = 0.94). Our maps were used to assess the relative condition of reefs based on live coral, and to identify potential coral refugia in the face of human-driven stressors, including marine heat waves. Geospatial modeling revealed that water depth, wave power, and nearshore development accounted for the majority (>60%) of live coral cover variation, but other human-driven factors were also important. Mapped interisland and intraisland variation in live coral location improves our understanding of reef geography and its human impacts, thereby guiding environmental management for reef resiliency.

Keywords: Hawaiian Islands; coral mapping; coral reef; coral refugia; reef restoration.

Fig. 1.

Percent live coral cover at 2-m spatial resolution to 16-m depth for the eight Main Hawaiian Islands.

Fig. 2.

Island-scale relationships between reef substrate area (in hectares) and (A) total live coral cover (in hectares) and (B) ratio of live coral cover to reef substrate availability.

Fig. 3.

Field verification of airborne live coral cover mapping across four Hawaiian islands (n = 1,132 transects). ASU, Arizona State University field sites; DAR, State of Hawaii Division of Aquatic Resources field sites.

Fig. 4.

Geospatial variation in live coral cover for the Main Hawaiian Islands. Semivariogram (A) range and (B) sill are shown at the Top and Bottom, respectively. (C) Island-scale semivariograms in the Inset graph.

Fig. 5.

(A) Top 20 reefs (red and yellow dots) based on live coral cover density, with the top 10 (red dots) matching SI Appendix, Table S2. (B) Zoom maps of the top five live coral cover sites in the Main Hawaiian Islands: (1) Kīholo Bay; (2) Keawaiki; (3) ʻAnaehoʻomalu; (4) Keaukaha; and (5) Pāpā Bay, all of which are on Hawaiʻi Island.

Fig. 6.

Relative importance of factors related to live coral cover throughout the eight Main Hawaiian Islands. PAR, photosynthetically active radiation; SST, sea surface temperature. Plus (+) and minus (−) symbols indicate the relationship of the driver variable with live coral cover. See SI Appendix, Table S1 for model data information.

Fig. 7.

Partial dependency plots showing relative differences in live coral cover for the top three land–sea drivers in our analysis from Fig. 6. (A) Water depth; (B) maximum wave power; and (C) an index of nearshore development index.