Monitoring of seafloor litter on the Dutch continental shelf : International Bottom Trawl Survey 2021, Dutch Beam Trawl Survey 2020

Volwater, Joey; Hal, Ralf van


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 (D10) includes the collection of data on the presence, abundance 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 GOV (Grand Ouverture Verticale) fishing net as part of the International Bottom Trawl Survey (IBTS), which is carried out yearly in the North Sea. Anthropogenic pollution of our 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 have recently been started or are currently under discussion. Despite management measures to decrease the input of litter and to remove litter from the environment, litter remains on the seafloor. This report presents the seafloor litter composition, abundance and spatial distribution based upon catches of the regular fish surveys, the International Bottom Trawl Survey (IBTS) and the Dutch Beam Trawl Survey (BTS). Only the catches on the Dutch Continental Shelf (DCS) are used. 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: 88% (BTS) and 95% (IBTS) 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 165 (IBTS) and 201 (BTS) litter items per km2 was calculated on the DCS, with mean values per ICES rectangle exceeding 200 items per km2. It should be noted that the net used during the IBTS (GOV) and BTS (beam trawl) is not designed to catch litter. For the GOV, the catchability of many benthic species is assumed to be less than 5%, 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 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 litter present. On top of that, due to the selectivity of the fishing gears used in the surveys, only a selection of the types of litter items present retain in the net. This is reflected by the fact that hardly any (small) single-use plastics were caught. However, by including the BTS survey a slightly more representative picture of the litter types present on the seafloor is given since a wider range of litter items was caught, therefore the BTS data will be included in the coming years. Yet, the abundance and density estimations have to be considered as a minimum estimation of the amount of a select part of the litter present on the DCS, rather than the actual status of it.