Plastic contamination in fish digestive tracts in Amazonian rivers during a period of extreme low water

Microplastics (MPs) are frequently detected in the gastrointestinal tracts of aquatic organisms worldwide. We studied the contamination with plastic in an aquatic environment and in the gastrointestinal tracts of fish in two Amazonian rivers waters, Rio Negro and Rio Solimões. The research was conducted in November/December 2023 during the low water season in a year of drastically decreased water level. We evaluated the abundance, shape, type, size as well as polymer type of the MPs and compared the contamination between fish species and feeding types. The amounts of MPs in the water were 0.25 ± 0.05 and 0.78 ± 0.19 pcs/L in the Prato canal and Vila do Janauacá, respectively. In total, 128 fish individuals belonging to different feeding types (piscivorous, planktivorous, herbivorous, invertivorous, detritivorous, omnivorous) were analysed in this study, with 84.0% and 87.4% of fish containing MPs caught from the Prato canal and Vila do Janauacá, respectively. We found no statistical relationship between the length/weight of the fish and the amount of MPs or the sizes of the plastic parts. MPs with dimensions of 0.04-5.0 mm were found in the digestive tracts of fish. There was an average of 3.3 ± 2.9 and 2.8 ± 2.1 plastic particles per fish in the study sites of the Prato canal and Vila do Janauacá, respectively. There was a statistical difference in the size of the MPs between the river sites that was correspondingly reflected in fish. In the digestive tracts of Solimões fish, plastics were on average larger than in the Rio Negro study site. At the Vila do Janauacá study site, predatory fish had the most MPs in their digestive system, while in the Prato canal, plankton-eating fish had the most MPs. Regarding the shape of MPs ingested by fishes, most were filaments, followed by fragments. The majority of the fibres were determined to be polyamide (nylon), polyethylene terephthalate and polypropylene. Our results indicate that even the remote aquatic environments may be impacted with intensive MP infestation. At the same time, with higher rainfall and higher water levels, a higher concentration of MPs is expected and this, in turn, can lead to greater contamination of fish with MPs. It has been observed that when water has a higher plastic content, more MPs end up in fish, especially in visual foragers.