Microplastics in freshwater fishes: Occurrence, impacts and future perspectives

Microplastics (MPs) are small, plastic particles of various shapes, sizes and polymers. Although well studied in marine systems, their roles and importance in freshwater environments remain uncertain. Nevertheless, the restricted ranges and variable traits of freshwater fishes result in their communities being important receptors and strong bioindicators of MP pollution. Here, the current knowledge on MPs in freshwater fishes is synthesized, along with the development of recommendations for future research and sample processing. MPs are commonly ingested and passively taken up by numerous freshwater fishes, with ingestion patterns often related to individual traits (e.g. body size, trophic level) and environmental factors (e.g. local urbanization, habitat features). Controlled MP exposure studies highlight various effects on fish physiology, biochemistry and behaviour that are often complex, unpredictable, species-specific and nonlinear in respect of dose-response relationships. Egestion is typically rapid and effective, although particles of a particular shape and/or size may remain, or translocate across the intestinal wall to other organs via the blood. Regarding future studies, there is a need to understand the interactions of MP pollution with other anthropogenic stressors (e.g. warming, eutrophication), with a concomitant requirement to increase the complexity of studies to enable impact assessment at population, community and ecosystem levels, and to determine whether there are consequences for processes, such as parasite transmission, where MPs could vector parasites or increase infection susceptibility. This knowledge will determine the extent to which MP pollution can be considered a major anthropogenic stressor of freshwaters in this era of global environmental change.