What is in our seas? Assessing anthropogenic litter on the seafloor of the central Mediterranean Sea

Abstract

Abundance, composition, and distribution of macro-litter found on the seafloor of the Strait of Sicily between 10 and 800 m depth has been studied using data collected by bottom trawl surveys MEDITS from 2015 to 2019. Three waste categories based on the items use were considered: single-use, fishing-related and generic-use. Over 600 sampling sites, just 14% of these were litter-free. The five-years average density of seafloor litter was 79.6 items/km(2) and ranged between 46.8 in 2019 and 118.1 items/km(2) in 2015. The predominant waste type was plastic (58% of all items). Regardless of material type, single-use items were a dominant (60% of items) and widespread (79% of hauls) fraction of litter with a mean density of 48.4 items/km 2 . Fishing-related items accounted for 12% of total litter items. Percentage of dirty hauls and litter density increased with depth. Analysis of the relation density-depth indicates a progressive increase of litter density beyond depth values situated within the interval 234-477 m depending on the litter category. A significant decrease in litter density by categories was observed over the period. Patterns of spatial distribution at the higher depths (200-80 Om) resulted stable over the years. Density hotspots of fishing-related items were found where the fishing activity that uses fish aggregating devices (FADs) is practised and in the proximity of rocky banks. Single-use and generic-use objects densities were greater on the seafloor along main maritime routes than other areas. Comparisons between the percentage of hauls littered with anthropic waste from the mid-1990s against those in 2018 -19 highlighted an increase of about 10.8% and 15.3% for single-use items and fishing-related items respectively, and a decrease of 18.6% for generic-use items. This study provides a snapshot of the current situation of littering in the central Mediterranean Sea and represents a solid baseline against which the effectiveness of current and future mitigation strategies of the litter impact on marine environment can be measured.