This study aimed to simultaneously investigate microplastic (MP) and heavy metal contamination in two fish species, Cyprinion macrostomus and Barbus grypus, from the Kashkan River in southwestern Iran. A total of 48 fish specimens were collected from three sampling stations subjected to varying environmental pressures. Microscopic and FTIR analyses revealed that 79% of the samples were contaminated with MPs, with fibers smaller than 0.25 mm being the most dominant type. The most common polymers identified were polyethylene terephthalate (PET, 43%) and polyvinyl chloride (PVC, 23%). C. macrostomus exhibited a significantly higher number of ingested MPs (6.16 particles) compared to B. grypus (4.6 particles) (P < 0.05). ICP-OES results showed that the concentrations of heavy metals (Pb, Cr, Cd, Zn, Cu) in fish muscle tissues varied across stations and species, with Zn and Cu showing the highest levels. Regression analysis revealed a positive and significant correlation between the metal pollution index (MPI) and MP abundance in both species (R² = 0.53 for C. macrostomus and R² = 0.97 for B. grypus). Conducting risk assessment utilizing the polymer hazard index (PHI) indicated that some polymers, such as PVC and PEUR, fall into the very high-risk category. Although estimated daily intakes (EDI) of metals exceeded acceptable thresholds, target hazard quotients (THQ), hazard index (HI), and carcinogenic risk (CR) values remained below 1, suggesting no immediate health risk under standard consumption patterns. However, elevated combined exposure to these pollutants, particularly in communities consuming fish viscera, may pose long-term health hazards. These findings provide critical evidence on the co-occurrence of heavy metals and MPs, highlighting the urgent need for management interventions to control mixed pollution in freshwater ecosystems.
