Configuration of small patch reefs and population abundance of a resident reef fish in a complex coral reef landscape

Habitat use by the resident coral reef anemonefish, Amphiprion frenatus , was examined in the complex coral reef landscape of Shiraho Reef, Ishigaki Island, Okinawa, Japan, using an enlarged color aerial photograph processed using image analysis software as an accurate field map. The anemonefish inhabit assemblages of the host sea anemone, Entacmaea quadricolor (clonal type), which inhabit various patch reefs in the back reef moat. We located all patch reefs inhabited by the host in the map based on snorkel observations: 297 anemonefish were found in 93 host assemblages in the study site of 2.9 ha. These patch reefs could be recognized by the reef colors, including the shadows (blackish color) in the photograph. Using image analysis software, the colors of the patch reefs were chosen and pixels with the same color values were regarded as potential habitat patches for the fish (PHPs). PHPs were 3D patch reefs (>0.5 m in height). Total areas (TA) and total perimeters (TP) of PHPs were measured in nine quadrats in the digitized aerial photograph. Host abundance and anemonefish abundance in a quadrat showed stronger correlations with the product of TA and TP of PHPs than TA alone. A site with numerous large 3D patch reefs (≥0.75 m 2 in situ) can be a better habitat for the fish than other sites consisting of several huge 3D patch reefs of the same total area. The methodology applied here may be useful for assessing the quality of habitats for small resident animals in shallow subtidal reefs.