From the mainland to sea and back: plastic’s path in demersal communities exploited by fishing.

Seafloor litter pollution has emerged as a significant environmental concern, posing potential threats to marine biodiversity and ecosystem functioning. The seafloor is a key habitat for numerous marine species, and the accumulation of litter disturbs the essential balance and function of this ecological system. This pollution may affect organisms at both the individual level, through ingestion or entanglement, and the community level by altering nutrient availability and modifying the physical and chemical properties of the environment. Additionally, seafloor litter pollution has been observed to have a negative impact on marine fisheries worldwide, affecting them both directly and indirectly. For instance, the presence of marine litter in fishing nets can reduce their catch efficiency, cause damage, reduce the time available for fishing, inflate repair expenses, obstruct equipment, and necessitate increased fuel consumption. Moreover, fisheries' target species, along with their prey, are at risk of both lethal and sub-lethal harm due to plastic pollution and that could reduce the fishing quality. In particular, many studies have correlated seafloor plastic pollution with reduced reproductive success, growth limitations, and reduced body mass growth in exposed organisms, which could reduce the economic value of a species and thus have a negative impact on system production. Despite the growing awareness and concern about the effects of bottom trawling and seafloor litter, there are no studies on the effects of litter pollution on fisheries and stock management, and closing the knowledge gap would be a critical task. For these reasons, the main objective of this PhD project is to evaluate the potential effects of seafloor litter within individual, community, and fishery contexts in the central Tyrrhenian Sea, with a particular emphasis on the following aspects: reconstructing the spatial distribution of seafloor litter throughout the Western Mediterranean Sea (background); evaluating the feeding habits and plastic consumption of blackmouth catsharks (individual level); investigating the impact of seafloor debris on marine communities through eDNA metabarcoding (community level); and assessing the risk of fishing grounds through an overlap analysis of seafloor plastic hotspots with commercially important species. The findings of this research indicate significant levels of seafloor litter in the western Mediterranean Sea, with multiple hotspots that could adversely affect ecosystems, specifically harming certain species. Indeed, the generalist-opportunistic feeding behavior of the blackmouth catshark leads to the incidental ingestion of plastic particles, which can be indirectly associated with the presence of macroplastics on the seafloor. Additionally, eDNA metabarcoding was utilized to examine the impact of trawling and litter on the marine community. The study revealed the presence of certain species closely associated with litter on the seafloor, while others were linked to trawling. Furthermore, it was demonstrated that seafloor plastic hotspots coincide with fishing grounds of economically significant species, resulting in adverse effects on fishermen's economic productivity. In this work, it was also possible to quantify the probability of exposure and impact of litter for each of the key trawling species. This PhD thesis provides a thorough investigation of the distribution and effects of seafloor debris on marine life in the central Tyrrhenian Sea. The methods used converge to understand the complex relationships between environmental variables, anthropogenic factors, and community composition. This study enhances our comprehension of the responses of marine ecosystems to litter accumulation and also provides essential information for designing effective conservation and management strategies to mitigate the impact of litter on marine ecosystems.