This visualization product displays the spatial distribution of seafloor litter density per trawl.
EMODnet Chemistry included the collection of marine litter in its 3rd phase. Since the beginning of 2018, data of seafloor litter collected by international fish-trawl surveys have been gathered and processed in the EMODnet Chemistry Marine Litter Database (MLDB).
The harmonization of all the data has been the most challenging task considering the heterogeneity of the data sources, sampling protocols (OSPAR and MEDITS protocols) and reference lists used on a European scale. Moreover, within the same protocol, different gear types are deployed during fishing bottom trawl surveys.
In cases where the wingspread and/or number of items were unknown, data could not be used because these fields are needed to calculate the density. Data collected before 2011 are affected by this filter.
When the distance reported in the data was null, it was calculated from:
- the ground speed and the haul duration using this formula: Distance (km) = Haul duration (h) * Ground speed (km/h);
- the trawl coordinates if the ground speed and the haul duration were not filled in.
The swept area is calculated from the wingspread (which depends on the fishing gear type) and the distance trawled:
Swept area (km²) = Distance (km) * Wingspread (km)
Densities have been calculated on each trawl and year using the following computation:
Density (number of items per km²) = ?Number of items / Swept area (km²)
Then a grid with 30km x 30km cells is used to calculate the weighted mean of densities in each cell from the formula :
Weighted mean (number of items per km²) = ? (Distance (km) * Density (number of items per km²)) / ? Distance (km)
Percentiles 50, 75, 95 & 99 have been calculated taking into account data for all years.
More information on data processing and calculation are detailed in the document attached.
Warning: the absence of data on the map doesn't necessarily mean that they don't exist, but that no information has been entered in the Marine Litter Database for this area.
This work is based on the work presented in the following scientific article:
O. Gerigny, M. Brun, M.C. Fabri, C. Tomasino, M. Le Moigne, A. Jadaud, F. Galgani, Seafloor litter from the continental shelf and canyons in French Mediterranean Water: Distribution, typologies and trends, Marine Pollution Bulletin, Volume 146, 2019, Pages 653-666, ISSN 0025-326X, https://doi.org/10.1016/j.marpolbul.2019.07.030.
