Coral reef habitats mapping of Spermonde Archipelago using remote sensing compared with in situ survey of fish abundance

Abstract

Coral reefs worldwide are now facing so great threat due to various impacts that their monitoring is urgently required for conservation and management. To understand status of coral reef ecosystem and find out indicator fish species for health of ecosystem, mapping seabed habitats with remote sensing and in situ visual survey of fish assemblage by snorkeling were conducted in coral reefs in Spermonde Archipelago, Indonesia. ALOS AVNIR-2 multi-band imagery on 14 October 2010 was analyzed to map four habitats: live coral, dead coral, seagrass and sand-rubble. Groundtruth data were obtained using towed video camera and sidescan sonar in May and June 2011. Depth-Invariant indices (DI-indices) based on ratios of radiance values between bands were applied as a water column correction. Overall classification accuracy in Tau-coefficient of mapping with the DI-indices (0.66) didn’t differ significantly (p<0.05) from that with the radiance values (0.63). Concerning visual fish survey, 12 fish groups were identified and numbers of individuals belonging to each group were counted along a transect of approximately 100m at 18 sites. We calculated Spearman’s rank correlation between abundance (Ind. /100m) of every fish group along a transect and the ratio of each habitat area mapped with DI-indices inside the circle with 50m-diameter which includes the fish transect. We detected significant correlations between abundance of five fish groups and specific habitats, especially butterflyfish and live coral. This result corresponds to the past reports that butterflyfish was a good indicator of healthy corals, suggesting meaningfulness of studying relationships between fish abundance and spatial distribution of habitats in larger scale.