Mapping coastal marine debris using aerial imagery and spatial analysis

Highlights: • This is the first comprehensive marine macro-debris survey for the main eight Hawaiian Islands, spanning over 1600 km of coastline. • We identified and categorized 20,658 individual pieces of marine debris. • Debris was concentrated primarily on northeast (windward) shorelines. • Ten putative Japanese tsunami marine debris vessels were identified in the analysis. • High resolution aerial imagery combined with GIS analysis is an effective method for quantifying and categorizing marine macro-debris across large scales and remote coastlines. - Abstract: This study is the first to systematically quantify, categorize, and map marine macro-debris across the main Hawaiian Islands (MHI), including remote areas (e.g., Niihau, Kahoolawe, and northern Molokai). Aerial surveys were conducted over each island to collect high resolution photos, which were processed into orthorectified imagery and visually analyzed in GIS. The technique provided precise measurements of the quantity, location, type, and size of macro-debris (> 0.05 m²), identifying 20,658 total debris items. Northeastern (windward) shorelines had the highest density of debris. Plastics, including nets, lines, buoys, floats, and foam, comprised 83% of the total count. In addition, the study located six vessels from the 2011 Tōhoku tsunami. These results created a baseline of the location, distribution, and composition of marine macro-debris across the MHI. Resource managers and communities may target high priority areas, particularly along remote coastlines where macro-debris counts were largely undocumented.