Microplastic contamination in the gastrointestinal tract of Caspian kutum (Rutilus kutum) from Miankaleh fishing grounds: Implications for ecosystem and human health

Microplastic (MP) pollution has emerged as a pervasive threat to aquatic ecosystems globally, with significant implications for fish health and human food safety. The Caspian Sea, as the world's largest enclosed water body, acts as a terminal sink for contaminants, yet comprehensive assessments of MP contamination in commercially important fish species from its southeastern basin remain limited. This study investigated the occurrence, abundance, physical characteristics, and polymer composition of microplastics in the gastrointestinal tracts of Caspian kutum (Rutilus kutum) collected from three major fishing grounds, Chargoli, Shayan, and Niazabad, along the Miankaleh coastline, Golestan Province, Iran, during August–September 2024. A total of 30 fish specimens were examined using 10% potassium hydroxide digestion, density separation with saturated NaCl solution, and microscopic identification, followed by polymer confirmation via micro-Fourier transform infrared (?-FTIR) spectroscopy. Microplastics were detected in 93.7% of individuals, with a mean abundance of 2.84±1.67 items per fish, ranging from 0 to 9 items per individual. Significant spatial variation was observed, with Niazabad exhibiting the highest contamination (3.42±1.89 items fish?¹), followed by Shayan (2.76±1.54 items fish?¹) and Chargoli (2.34±1.42 items fish?¹), reflecting proximity to riverine inputs from the Gorganrud River. Fibers constituted the dominant morphology (91.3%), followed by fragments (6.2%) and films (2.5%). The predominant size class was 100-500 ?m (42.3%), indicating high bioavailability and potential for trophic transfer. Color distribution revealed distinct site-specific signatures: Niazabad was characterized by green (33.3%) and red (22.2%) MPs, while Chargoli and Shayan were dominated by black, gray, and transparent particles. Seven polymer types were identified, with cellophane (28.3%), polyamide (22.1%), and polyester (18.4%) being most prevalent, reflecting contributions from packaging materials, fishing gear degradation, and textile fibers. The predominance of high-density polyamide and polyester polymers, which have a high affinity for persistent organic pollutants, raises concerns about vector effects and co-exposure to associated contaminants. No significant correlation was found between fish length and MP abundance (p = 0.187), while a weak positive relationship with wet weight (p = 0.034) suggested moderate accumulation with increased body mass. The high occurrence and abundance of small, bioavailable MPs in R. kutum from the Miankaleh fishing grounds indicate substantial contamination pressure in this critical habitat, with direct implications for food safety, given the species' commercial importance and high consumption rates in northern Iran.