Working Group on Marine Litter (WGML; outputs from 2020 meeting)

The goal of the Working Group on Marine Litter (WGML) is to provide scientific guidance towards the international harmonisation of monitoring data for seafloor litter and microlitter. WGML functions as a knowledge base for other international organisations regarding seafloor litter and microlitter. WGML has mapped seafloor litter and microplastic monitoring ap-proaches, discussed relevant issues, established an overview of national and international driv-ers and linkages, and distributed key information produced by WGML (e.g. guidelines, photoguides).

WGML activities focused on reviewing and assessing the quality and potential uses of current data in the ICES DATRAS (Trawl Surveys) and DOME (Marine Environment) databases. For seafloor litter, coverage by year and country was evaluated as being sufficient for future assess-ment of presence/absence, but some data gaps and irregularities were identified. Issues with the existing seafloor litter data and methods were reviewed and used to recommend improvements for future monitoring (e.g. harmonised sampling and categorisation). Seafloor litter monitoring has been 'piggybacking' on fish stock assessment programmes taking place regularly across many regions. However, this approach has some critical drawbacks: (i) limited to soft seafloor substrates; (ii) limited depths; (iii) limited to sampling gear and survey design of the fish stock assessment surveys; (iv) differences in catchability among gears, vessel speed, mesh size, cod ends and methods used among countries and regions, programmes, observers and studies. This seriously limits intercomparison and aggregation of seafloor litter data (no. of items) based on "bycatch litter data" obtained from the fisheries surveys. Although standardisation of seafloor litter data may be achievable at the local or national scale where harmonised sampling ap-proaches are used, this is unlikely to be achieved across larger regions due to the critical draw-backs listed above.

Moreover, seafloor litter monitoring is mostly a secondary objective in fisheries surveys, therefore the interpretation, interest and attention given by observers differs from survey to survey and year to year, making the interpretation of temporal and spatial trend analyses difficult. Due to the high variability in the observed seafloor litter concentrations, WGML concludes that the current power of seafloor litter monitoring programmes is insufficient to follow temporal and spatial variations with a high degree of certainty (e.g. above 80% accuracy). Without solving these data issues, direct comparisons and harmonisation between surveys will restrict the inter-national assessments to presence/absence outputs. Alternative, less/non-destructive methods should also be investigated for the future to avoid the destructive trawling approach currently used to obtain seafloor litter data. To assist in the classification of seafloor litter in the future, and to ensure standardisation in litter categories between data collection programmes, an updated photoguide of litter items was produced. Finally, WGML generated a seafloor litter guidance document containing information about sampling, data reporting and quality assurance/quality control (QA/QC), including the definition of litter categories and subcategories.

WGML activities on microplastics started with a review of the existing microplastic data registered in the DOME database. Very few datasets were present, thus WGML created an overview of existing microplastic datasets from members. Those were assessed based on their output format and ease of use. Two distinct types of monitoring (Compliance and Investigative) were defined that are currently taking place in different member countries. WGML discussed methods for microplastic sampling, processing and analysis, providing an overview of the development status  and  needs  related  to  both  compliance monitoring vs investigative case studies.  WGML discussed microplastic presence/amounts in different marine environmental matrices (e.g. sediment/water/biota), as well as units for reporting data. WGML discussed QA/QC issues in relation to microplastics data and how these might impact data quality. The group also mapped existing proficiency schemes addressing these issues.

A critical issue for ICES WGML is the lack of data being submitted to DOME, with much of the data generated by EU member states being submitted to The European Marine Observation and Data Network (EMODNET). WGML plans to interact more closely with EMODNET going forward  to  establish  closers  links  and  to  support  each  other.  Additionally, WGML promotes  the  submission of available and future microplastic data to the ICES DOME database in case countries/institutes would like WGML to review and provide QA/QC. Guidance for submitting microplastic data has been created by the ICES Data Centre.