Ship-based visual observation underestimates plastic debris in marine surface water

Visual observation surveys from ships are commonly used for monitoring floating marine debris, but their detection performance has not yet been fully verified. Here, simultaneous visual observation surveys and surface trawling were conducted in three coastal areas of South Korea, each with distinct characteristics. The extent of floating debris missed by visual observations was assessed, and the characteristics of overlooked debris were identified. The mean density of floating debris observed visually was five-fold lower than that obtained from surface trawling. Loss of buoyancy and transparent colour of debris were identified as major factors contributing to the significant difference in density between the two survey methods. Our findings suggest that visual observation can underestimate the density of floating debris, especially in areas with abundant plastic bags and sheets. Supplementary methods such as surface trawls with macro-sized mesh are recommended to accurately assess the level of contamination from floating debris.