Count and mass identify different shoreline-litter priorities: Evidence from Vietnamese monitoring programmes

Marine litter is an urgent management challenge along Vietnamese beaches, fishing grounds, and protected coasts, yet shoreline monitoring often relies primarily on item counts. Because a small number of heavy objects can dominate material burden, count-only assessments may identify different priority sites and litter groups from mass-based assessments, potentially weakening prevention, collection, and fishing-gear responses. In this study, paired count and mass data from four Vietnamese shoreline-litter programmes were reanalysed while preserving source-specific protocols and units. Within-programme rank concordance, top-quartile changes, count-mass disagreement across decision thresholds from 0.30 to 0.70, and the mass excluded from category sets representing 50-95% of counted items were quantified. Although count-mass rank correlations were positive (Spearman's ρ = 0.500-0.881), all three spatial series produced different count- and mass-defined top-quartile sets; Nha Trang had no shared top-quartile beach. Its litter profile was dominated by consumer and single-use items by count but by fishing and aquaculture items by mass. Across the evaluated profiles, metric-dependent litter-group status persisted over 4.3-76.0% of the tested threshold domain, while categories representing 80% of counted items still excluded 18.1-61.1% of reported mass. These findings demonstrate that rank correlation does not establish monitoring equivalence. Counts should be retained for product occurrence and prevention, whereas mass should be added when management concerns material handling, disposal, or fishing-gear recovery. Reporting both priority rankings and material-coverage curves would provide a more decision-relevant basis for coastal-litter management, while any count-to-mass conversion should undergo local prospective validation.