Coral bleaching variability during the 2017 global bleaching event on a remote, uninhabited island in the western Pacific: Farallon de Medinilla, Commonwealth of the Northern Mariana Islands

Abstract

A survey conducted in Fall 2017 by US Navy scientists around the small, uninhabited island of Farallon de Medinilla (FDM) documented severe bleaching related to extended regional heat stress. Three of the dominant scleractinian genera at FDM, Pocillopora, Leptastrea, and Astreopora, were severely impacted, with more than 90% of colonies from many species exhibiting bleaching. In contrast, several species of Porites corals, another dominant genus at FDM, fared better, with less bleaching (7%–68% by species) than the island average (78%). Bleaching was somewhat higher at shallower depths (<10 m depth stratum, compared to 10–20 m depth stratum) and on the leeward of the island. Surveying FDM presented logistical challenges including a compressed time window for survey execution, periods of strong currents >1 knot that precluded diving, rare but potentially hazardous ordnance items, survey requirements for georeferenced imagery, and quantitative data collection. The survey protocol designed to accommodate these challenges is presented here, as are lessons from an unsuccessful attempt to delineate bleached coral colonies in photographs using automated object-based image analysis.