Comparison Studies for Surface Water Visual Survey and Surface and Water-Column Trawl for Floating and Suspended Marine Plastic Debris

A visual survey using a vessel is a representative method to assess the degree of pollution of floating plastic debris in marine environments. However, the visual survey may more easily miss plastic items on and just below water (e.g. plastic bags) than above water (e.g. PET bottles). In addition, there are very limited comparison studies for floating plastic debris on the water surface and suspended plastic debris in the water column. None of the studies quantitatively determined the difference in detection rate by visual and surface trawl surveys. The aim of this study is to evaluate what could be relatively missed and underestimated by surface water visual surveys.

Floating plastic debris was monitored by visual and trawl surveys (depth of 0.5 m) in three coastal areas (rural area, GJ; aquafarm area, JH; urban and near river mouth area, SY) of Korea over the four seasons in 2022. In addition, during the visual survey of floating plastic debris in a fishing area (GH), near the river mouth of Han River, a shrimp beam trawl was used to collect plastic debris in the water column (water depth of 10 m) except for thin surface and bottom layer over three seasons in 2022. The seasonal patterns and composition of floating plastic debris in the surface water of JH, GJ, and SY were similar between the visual and trawl surveys. But, the mean densities of most plastics obtained from trawl surveys were 3 to 7 times higher than those from visual surveys. In particular, it was hard to detect small-sized, submerged, or dark-colored fishing gear with the visual survey. Patches with small items can increase the uncertainty of the visual survey. Therefore, visual surveys may underestimate the amount of marine plastic debris above and just below the water.

Various types of floating plastic debris were observed by visual survey in the surface water of GH: plastic bags/sheets (54%), expanded polystyrene pieces (18%), plastic containers (4%), strapping (3%), plastic bottles/caps (3%), discarded fishing gear (1%), and other hard plastic pieces (14%). In the water column of GH, however, plastic bags/sheets (93%) predominated and followed by strapping (4%), discarded fishing gear (1%), and other plastics (1%). These results indicate that plastic bags/sheets and strapping would mainly submerge in the water column, but expanded polystyrene pieces, plastic containers, plastic bottles/caps, and other hard plastic pieces are more likely to float rather than sink. Thus, the application of only visual surveys for plastic pollution monitoring in water may largely miss and underestimate the plastic items transported on and below water such as plastic bags and sheets.