Estimating gear efficiency of toothed fishing dredge for collecting seafloor litter

In this study, depletion experiments using a toothed fishing dredge were conducted to estimate the gear efficiency for collecting seafloor litter in a nearshore area on the north side of Tokyo Bay. Using the catch data of cod-end samples and gear-entangled samples, four models (Leslie-Davis model, DeLury model, normal patch model, and negative binomial patch model) were applied to fit the dataset. Analysis of the collected seafloor litter showed that plastics were the dominant category, accounting for 61.8–90.0 % by count (depending on whether the sample was from the cod-end or gear entanglement), with polyethylene (PE) as the predominant polymer type (50.5 %). Cross-validation identified the normal patch model as the most appropriate model for inference, yielding a gear efficiency estimate of 34.8 % and a corresponding seafloor litter abundance of 4000 pieces/km² based on cod-end samples. Compared to conventional methods that assume 100 % gear efficiency, this suggests that seafloor litter abundance is underestimated by 38.6 % and 79.4 % given current experimental conditions. When all litter samples were included, i.e., both cod-end samples and gear-entangled samples, the estimated gear efficiency was 18.7 %, with a corresponding seafloor litter abundance of 45,200 pieces/km². This suggests that excluding gear-entangled litter, mainly plastic sheet fragments from bags, can result in underestimation by as much as 91 %. These findings highlight the importance of accounting for gear efficiency and incorporating all gear-collected litter in bottom-towed surveys to more accurately assess seafloor litter pollution.