<![CDATA[Microplastics (<5 mm) are increasingly recognized as pervasive pollutants in urban rivers and may induce sublethal physiological stress in freshwater fish. This study assessed the impacts of microplastic contamination on cellular/hematological blood indicators in Nile tilapia (Oreochromis niloticus) from the Jagir River, Surabaya, Indonesia. Field sampling was conducted from July to August 2025 at three river segments (upstream near the sluice gate, midstream near fishing-vessel mooring and small-scale activities, and downstream approaching the estuarine outlet). Water was collected at each site for microplastic quantification, and tilapia were sampled three times at two-week intervals for blood analysis. In situ and laboratory assessments of water quality included temperature, dissolved oxygen (DO), biochemical oxygen demand (BOD), and total suspended solids (TSS). Microplastics were detected at all sites, ranging from 2,365,333.3 to 2,541,333.3 particles/m³, with site-specific dominance of film/fiber upstream, fragment/film midstream, and fiber/fragment downstream. Water quality was relatively stable in temperature (27.7–28.8°C) and DO (5.5–5.9 mg/L), while BOD indicated organic loading (3.5–4.06 mg/L) and TSS showed clear spatial differences (8–17 mg/L). Blood profiles varied across sites: erythrocytes, hemoglobin, and hematocrit were lowest at the midstream site, whereas leukocyte counts were highest downstream, suggesting spatially heterogeneous stress and immune activation. One-way ANOVA indicated significant differences in microplastic abundance among sites (p < 0.05), and Pearson correlation results showed very strong associations between microplastic abundance and most blood parameters, supporting the use of integrated microplastic metrics, water quality indicators, and blood biomarkers for urban river monitoring prior to discharge into the Madura Strait.]]>
