Long-term marine litter monitoring program for best waste management policies in the NW Mediterranean Sea

Marine litter (ML) is accidentally removed by fishing gear every day, a fact that is being capitalized in strategies known as Fishing for Litter (FFL). In the Mediterranean Sea, this removal effect plays a particularly important role due to its enclosed nature and the reportedly high values of benthic ML abundance. In 2018, ICATMAR, the Catalan Institute of Research for the Governance of the Sea, launched a continuous fisheries monitoring program along the Catalan coast (NW Mediterranean Sea), which includes the study of ML collected from bottom trawlers. The program consists of monthly samplings on board commercial vessels from the 9 main bottom trawl ports of the area and, since 2024, it is endorsed by the UN Ocean Decade. Understanding the importance of detecting the origin of ML to provide science-based advice in waste policy, this study investigated the potential source of the ML collected by the bottom trawl fleet.

The present study analyzed data from 2018 to 2022. The ML collected was weighted and classified into six different categories including metal, plastic, rubber, textile, wood, and other waste. Benthic ML was present in 97 % of the hauls performed, and its composition varied along the coast. The overall mean density was 7.91 ± 0.96 kg km^-2. The highest densities were found in highly urbanized areas, i.e., Barcelona metropolitan area, where some hauls reached 201.94 kg km-2. Plastic represents the 63.42% of the total ML density observed in the whole area, with a mean density of 5.35 ± 0.79 kg km-2, although in some hot spots, the mean ML density was as high as 158.07 kg km-2.

Results showed that ML density was directly related to distance to river mouths, submarine outfalls, and highly populated areas. In addition, no correlation was found between ML and fishing effort, meaning that Fishing for Litter strategies can be useful sources of information, but they are not to be considered as a problem-solving strategy. The present study confirms that waste management policies must be implemented on-land, especially in highly anthropic areas with large populations.