Monitoring of seafloor litter on the Dutch Continental Shelf : International Bottom Trawl Survey 2023, Dutch Beam Trawl Survey 2022

The Marine Strategy Framework Directive (MSFD) requires EU Member States to develop programmes of measures that aim to achieve or maintain 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 programs for MSFD descriptors and indicators have been established by the Member States. 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, threatens wildlife, hinders human activities and reduces the recreational value of our coasts. 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. Despite management measures to decrease the input of litter and to 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 upon 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: 90% (IBTS) and 89% (BTS) of the litter items found were made of plastic. Monofilaments, plastic sheets and various types of (plastic) ropes/lines were the most commonly caught litter types. A mean density of 74 litter items per km 2 over the years 2021-2023 was calculated for the IBTS on the DCS, whereby for the BTS a mean density of 185 litter items per km 2 over the last two years (2020-2022) was calculated on the DCS. It should be noted that the nets used during 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%. The fact that these items are caught despite the suggested low catchability thus indicates that it is plausible that there are many more litter items in the trawl path and that current values are a large underestimation of the actual amount of litter present. In a recent study from Wageningen University, the beam trawl appears to catch seven times more litter than the GOV net at the North Sea level (Van Rossum & Boer, 2023). This is reflected by the fact that hardly any (small) single-use plastics, such as cutleries, straws and stirrers, were caught while these items are commonly found on beaches (Boonstra & Hougee, 2021). The inclusion of the BTS data in recent years has improved the picture of the litter types present on the seafloor. The BTS catches a wider range of litter types and roughly three times more items per swept area then the IBTS according to the data presented in this report. Yet, the abundance and density estimations of seafloor litter presented 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 (Roos et al., 2023), which shows much higher seafloor litter densities.