Assessing and Managing Microplastic Risks in Freshwater Fisheries: Exposure Pathways and Toxicological Evidence

Microplastics (MPs; commonly defined as plastic particles <5 mm) have become pervasive contaminants in rivers, lakes, reservoirs, wetlands, and aquaculture ponds, where they co-occur with complex chemical mixtures and biological stressors. Freshwater fish are exposed through multiple routes, including direct ingestion from the water column and sediments, trophic transfer via prey, and interactions at the gill–water interface. Field surveys increasingly report MPs in gastrointestinal tracts of diverse freshwater species and link ingestion patterns to habitat use, feeding guild, local anthropogenic pressure, and polymer/shape profiles. Beyond simple presence/absence, the central scientific concern is how environmentally realistic MP exposures translate into adverse biological effects. Laboratory evidence indicates that MPs can disrupt gut physiology, induce oxidative stress and inflammatory responses, impair immune regulation, and alter metabolic and microbiome composition; in some cases, particles or associated chemicals are implicated in tissue injury and organ-level pathology. Early-life stages may be especially sensitive due to developmental windows, high feeding rates, and limited detoxification capacity. Importantly, MPs rarely occur alone in the environment; they can sorb persistent organic pollutants and metals, carry biofilms, and co-exist with pesticides or hydrocarbons, creating mixture-toxicity scenarios that challenge classical single-stressor ecotoxicology. This review synthesizes current knowledge on MP occurrence in freshwater fish, mechanistic pathways from exposure to internal dose, and toxicological outcomes spanning molecular biomarkers to organismal performance. It also evaluates methodological constraints (sampling contamination, digestion protocols, polymer confirmation, and reporting units) that limit cross-study comparability and risk assessment. Finally, it identifies research priorities to improve environmental realism, harmonize analytical workflows, and connect biomarker shifts to population-relevant endpoints needed for robust freshwater management and food-safety decisions.