A standardized method to monitor marine litter in submarine canyons: implementation in Atlantic and Mediterranean...

Marine litter has been quantified in several deep-sea environments, including complex habitats such as submarine canyons that host Vulnerable Marine Ecosystems. However, a standardized method for its characterization and quantification is still lacking. Here, we propose a simple and replicable method based on repeated 100 m transects conducted using Remotely Operated Vehicle video surveys. The method was tested at two sites off the French coasts, in the Western Mediterranean Sea in 2023 and the North-East Atlantic in 2025. Mean litter densities reached 30,101 items km^-2 in the Lacaze-Duthiers Canyon, Gulf of Lion, and 18,333 items km^-2 in the Lampaul Canyon, Bay of Biscay. The type of litter differed according to the canyons and/or bathymetry. Fishing gear largely dominated the litter at the head of the Lacaze-Duthiers Canyon, whereas plastic bags and packaging constituted most of the litter in the Lampaul Canyon. Litter densities were particularly high in coral reefs that inhabit these canyons, lines and bags being entangled in coral branches. Our results demonstrate the efficiency, robustness, and reproducibility of this approach, as it generated comparable results across two distinct canyons. The method represents a practical and reliable tool for supporting the assessment of criterion D10C1 (seafloor litter) of Descriptor 10 (Marine Litter) of the Marine Strategy Framework Directive (MSFD). It contributes to improved monitoring of marine debris, beyond seafloor litter derived from trawling data, as well as its potential impacts on deep-sea habitats, and will support the implementation of appropriate management plans for the conservation of these vulnerable ecosystems.