Monitoring of seafloor litter on the Dutch continental shelf : International Bottom Trawl Survey 2024, Dutch Beam Trawl Survey 2023

The Marine Strategy Framework Directive (MSFD) requires EU Member States to develop programmes of measures that aim achieving or maintaining Good Environmental Status (GES) in European seas. In order to be able to evaluate the quality status of marine waters on a regular basis and the effects of the measures taken, monitoring programmes have been established by the Member States for MSFD descriptors and indicators. The Dutch monitoring program for Marine Litter (Descriptor 10) includes the collection of data on the abundance, composition and distribution of macro litter on the seafloor. According to the Dutch program, the data on seafloor litter must be collected during statutory task fish surveys using a standardised Grand Ouverture Verticale (GOV) fishing net as part of the International Bottom Trawl Survey (IBTS), which is carried out yearly in the North Sea. Anthropogenic pollution of the oceans, including marine litter, poses significant threats to wildlife, disrupts human activities, and reduces the recreational value of our coastlines. Marine litter affects all groups of marine wildlife through effects such as entanglement and ingestion. Various initiatives to reduce litter in the (marine) environment are running or are currently under discussion. However, despite these management measures to decrease litter input and remove litter from the environment, litter is still found in monitoring of the seafloor. This report presents the seafloor litter composition, abundance, and spatial distribution based on catches of the regular fish surveys, the IBTS and the Dutch Beam Trawl Survey (BTS). Only the catches on the Dutch Continental Shelf (DCS) are used for data analysis. To assess the status of seafloor litter on the DCS, the Dutch data are supplemented with those from international partners surveying the DCS within the IBTS. The seafloor litter catches on the DCS consisted mainly of plastic items, accounting for 83% of the litter found in the IBTS and 87% in the BTS. The most commonly caught items were monofilaments, plastic sheets, and various types of (plastic) ropes and lines. A mean density of 68 litter items per km2 over the years 2022-2024 was calculated for the IBTS on the DCS, whereby for the BTS a mean density of 176 litter items per km2 over the last three years (2021-2023) was calculated on the DCS. It should be noted that the nets used in the IBTS (GOV) and BTS (beam trawl) are not designed to catch litter. For the GOV, the catchability of many benthic species (e.g. Asteroidea) is assumed to be less than 5% compared to a beam trawl (Piet et al., 2009), therefore the chance of catching a litter item when it is present in the trawl path is likely to be even smaller than 5%. Therefore, the fact that these items are caught despite the suggested low catchability indicates that many more litter items are likely in the trawl path, meaning current estimates significantly underestimate the actual amount of litter. The inclusion of the BTS data in recent years has improved the understanding of the litter types on the seafloor. The BTS catches a wider range of litter types and roughly three times more items per swept area compared to the IBTS. However, the abundance and density estimations of seafloor litter in this report have to be considered as a very low estimation of the total amount of a selection of litter types present on the DCS, rather than its actual status. This has been demonstrated in a new Dutch study using a benthic dredge for seafloor litter sampling, which showed much higher seafloor litter densities (Roos et al., 2023).