Trapped twice: Discovering the impact of marine benthic plastic debris on small organisms caught in trawl nets

This study quantifies the marine benthic debris caught in a coastal trawl fishery and its impact on capturing small organisms and codend selectivity. From April to September in both 2023 and 2024, marine debris caught during regular fishing operations by two beam trawlers in Tachibana Bay, Nagasaki, was collected and analysed. Any marine organisms found within the debris were identified and measured. The selectivity experiment was conducted in October 2024, adding five plastic bags inside the codend for experimental tows and without plastic bags for control tows. The codend selectivity for the experimental and control tows was estimated either from logistic or contact probability models. There were 1249 pieces of marine debris weighing 45.04 kg collected, which is a small quantity in the coastal waters of Japan, with plastic exceeding 94 % by both number and weight. Among the plastic debris, 250 organisms of 23 species were found trapped, showing a positive correlation (r = 0.71, p < 0.001) between marine benthic plastic debris and bycatch. Three species, Metapenaeopsis barbata, Acropoma japonicum, and Nuchequula nuchalis, were selected for the selectivity analysis. The contact probability model fits better for experimental tows, while the logistic model fits better for control tows for all three species. The estimated contact probabilities were 0.62, 0.48, and 0.59 for the three species. The selectivity curve indicated higher retention probabilities for smaller fish in nets containing plastic bags, suggesting that marine debris adversely affects the selectivity of trawl nets by trapping small organisms.