Random or not? Comparing microplastic ingestion and preys in Scomber colias and Trachurus trachurus

Microplastics represent a rising threat to marine biota. As the fishing industry is a fundamental source of sustenance for human beings, microplastic ingestion by fish deserves appropriate attention. Despite previous studies suggest that the feeding behavior of different fish species may affect the occurrence and the diversity of ingested microplastic types, little is known about the selection mechanisms that determine the pathways of microplastics through the food webs. In this study we combine the characterization of ingested microplastics with the analysis of the diet composition of two pelagic fish species, namely the Atlantic chub mackerel Scomber colias and the Atlantic horse mackerel Trachurus trachurus. Samples were collected during a single fishing trip carried out in spring 2021 off Anzio coast (Tyrrhenian Sea, Western Mediterranean). After the dissection, samples were divided into two subgroups to be analyzed for diet composition and microplastic analysis, respectively. Gastrointestinal tracts to be analyzed for microplastics underwent alkaline digestion (10% KOH) to degrade the organic matter. Then, solutions were filtered on glass microfiber membranes and subsequently observed under a stereomicroscope. Microplastics were characterized by shape, color, and polymer composition. Stomach contents for diet analysis were observed under a microscope and preys were identified at the lower taxonomical level. The preliminary analyses show that T. trachurus makes an active selection of fastmoving preys (mostly Teleosts) and ingest almost only threadlike microplastics, such as fibers and filaments. In contrast, the high frequency of Thaliaceans in S. colias reveals a more planktivorous behavior, which is associated to the ingestion of a wider variety of microplastic types (including also film, fragment and foam). These results seem to suggest that the microplastic ingestion patterns observed in the two examined species could be linked to the existing differences between their prey searching modalities.