Testing the health status of marine and fresh waters by investigating microplastics in the stomach contents of different fish species

Plastic pollution, and microplastics (MPs) in particular, has become a pervasive threat to aquatic ecosystems worldwide. This Ph.D. project investigated the presence, types, and distribution of MPs in the stomach contents of various fish species sampled from four distinct environments, chosen based on the different degrees of impact of human presence and activities on them. Two sampling sites are marine, one is brackish and one lacustrine: the Port of Genoa (Northwestern Italy), a commercial port overlooked by the densely populated area of the City of Genoa; the Puyuhuapi Fjord (Northen Patagonia, Chile), a fjord surrounded by hills full of rich vegetation; the S’Ena Arrubia fishpond, a protected natural inshore pond (Western Sardinia, Italy); and the Lake Lavezze (Ligurian Apennines, NW Italy), an artificial reservoir that feeds the aqueducts of the City of Genoa. The Chilean fieldwork, made possible through a scientific collaboration with the Centro de Investigación en Ecosistemas de la Patagonia (CIEP), represented a unique opportunity to explore the occurrence of MPs in remote and ecologically rich oceanic environments far from major urban or industrial centers. The analysis focused on two commercially and ecologically important fish species—Genypterus blacodes and Merluccius australis—providing valuable insight into MP ingestion within the fjord ecosystems of the Southern Pacific, an area heavily influenced by salmon farming activities. Through optical microscopy and microRaman spectroscopy, MPs were detected in all fish stomach samples. A lower detection limit of 40 μm was consistently applied throughout the analysis, regardless of the higher sensitivity of the spectroscopic instruments, in order to ensure methodological coherence with the optical microscope screening where only particles above 40 μm were classified. The study revealed considerable interindividual and interspecies variation in both the quantity and type of MPs ingested. Synthetic fragments, granules, and fibers made of polymers such as polyester, polyethylene, and nylon were the most frequently identified. This work highlights how MPs are now ubiquitous across trophic levels and aquatic habitats, from urban ports to mountain lakes and Patagonian fjords. The findings support the use of fish stomach content analysis as a reliable bioindicator of environmental contamination, emphasizing the urgent need for comprehensive monitoring of both marine and freshwater systems. By expanding existing research to include the Southern Hemisphere and high-conservation-value areas like Patagonia, this thesis can contribute to a more global understanding of the ecological and potential human health risks associated with MP pollution.