Five plastic bags on the seafloor prevent small marine life from escaping bottom trawl nets - Nagasaki University

A research group at Nagasaki University has revealed that just five plastic bags on the seafloor can clog bottom trawl nets and prevent small marine organisms from escaping when caught in the nets. The researchers collaborated with local fishermen to examine plastic waste caught in bottom trawl nets and the organisms trapped with it, further investigating the impact of plastic bags entering the nets. The study highlighted that not only microplastics, but also large plastic waste significantly impacts marine life.

While marine debris issues have gained attention in recent years, the focus has primarily been on floating and drifting surface debris and fragmented waste like microplastics, with few studies investigating the actual situation of debris that has sunk to the seafloor.

Therefore, a research group led by Professor Yoshiki Matsushita, who studies Fisheries Science at the Graduate School of Integrated Science and Technology at Nagasaki University collaborated with two fishing vessels from the Tachibana Fisheries Cooperative Association conducting bottom trawl fishing for the green tiger prawn (Penaeus semisulcatus) and flounder from April to September 2023-24. They collected all seafloor debris caught in the bottom trawl nets and examined its material composition and size.

The results showed that among plastic materials collected over the two-year period, plastic bags caught in the nets totaled 12.29 kilograms, containers totaled 11.02 kilograms, and sheets totaled 6.36 kilograms. Other rubber and metal materials were also found, bringing the total weight to 45.04 kilograms. Much of the debris consisted of shopping bags, food containers, and agricultural materials generated from terrestrial activities.

Detailed examination of plastic waste caught in the nets revealed not only organisms entangled in bags but also creatures using the bags as habitats. Some shellfish had become integrated with the bags, leading Matsushita to conclude that "plastic debris on the seafloor is affecting marine ecosystems."

Furthermore, experimental addition of five plastic bags to bottom trawl nets showed that these bags clogged the mesh, preventing small fish and crustaceans from escaping. In the experiment, 40% of red shrimp that were small enough to pass through the mesh remained trapped in the net.

Additionally, small specimens including lanternbellies (Acropomatidae) known as a raw ingredient for jakoten fish paste and spotnape ponyfish (Nuchequula) common in inner bays were confirmed to remain trapped in the nets, unable to pass through the mesh. This phenomenon of plastic waste hindering small organisms' escape from nets is expected to occur similarly with other small creatures, suggesting potential impacts on fisheries resource management and marine ecosystem conservation.

The Tachibana Fisheries Cooperative Association collects debris caught in fishermen's nets and disposes of it using administrative subsidies and other support. Notably, Tachibana Bay had the least amount of debris when compared with Tokyo Bay, Kagoshima Bay, and the East China Sea when they were investigated using similar methods.

Despite this, there are impacts on fisheries, prompting Matsushita to emphasize that "before plastic waste becomes microplastics that cannot be recovered, it's important to not discard plastic waste and to collect and dispose of it when noticed." Future research will examine the effects of seafloor plastic waste on immobile organisms and plants such as corals and seaweed.

The research was funded by grants from the Ministry of the Environment and the Environmental Restoration and Conservation Agency's Environment Research and Technology Development Fund. The results were published in the Dutch academic journal Marine Pollution Bulletin on May 15 and announced by Nagasaki University on May 23.