Determination of microplastics in the digestive content of pelagic fish of commercial interest

The study aimed to determine the amount of microplastics (MPs) found in the digestive content of some pelagic tuna fish species of commercial interest caught along the Spanish coasts. In total, 601 individuals belonging to eight tuna species, and one species of the Corypheanidae family, were examined for the presence of MPs in their stomachs. Fish were collected from 9 different locations along the Spanish coasts by commercial fishers. A total of 170 MPs were extracted from the fish stomach of 75 individuals. The number of particles present in the stomach ranged from 1 to 20 (mean 0.3± 1.2 SD MPs per individual). The species with the highest amount of MPs was Auxix rochei whereas the species that did not present any MPs was Coryphaena hippurus, the only species not belonging to the tuna group. In terms of location, the highest ratio of MPs (71%) was found in the fish collected from Azohia, and the lowest in Mazagon (7%). In general, the fish that ingested more MPs were those caught from sites that were closer to the coasts (Canarias Islands stations and Azohia). No correlation between the number of ingested MPs and the size and weight of the fish were detected nor for the whole sample nor for each species separately. Analyzing the MPs in terms of colour, green colour MPs were the most dominant (52%) followed by blue colour (25%). Analyzing the MPs in terms of size, 11 categories were considered and the most dominant size was 0-0.5 micron. Fibers were excluded from the analysis because no protocol was adopted to prevent their contamination. Furthermore, 100% of the MPs found were fragments. The data did not show a high presence of MPs in the analyzed study areas. A bias could probably be due to the method used, allowing to loose a certain quantity of very small microplastics, and also to ethological and biological factors of the species, as documented by other studies: very often MPs do not accumulate in the stomach and are expelled from the individual.