<![CDATA[Freshwater fish populations are highly sensitive to habitat variation, yet information on how habitat characteristics shape fish diversity in Central Java rivers remains limited. The Ijo River is an important natural system for evaluating ecological factors that determine fish communities. This study aims to assess the physical and chemical characteristics of the Ijo River and evaluate their influence on fish diversity. Research was conducted using survey methods and purposive random sampling at nine stations across three zones: upstream, middle, and downstream. The results obtained are that the environmental parameters varied across river zones. The downstream zone recorded the highest temperature (29.14°C), greatest depth (2.54 m), widest channel (32.97 m), and highest free CO₂ (12.64 ppm). The upstream zone showed the greatest light penetration (74.43 cm), fastest Current Velocity (0.75 m/s), highest dissolved oxygen (5.28 ppm), and highest pH (8.31). Fish diversity also differed between zones. The downstream zone had the highest abundance (4,885 individuals) and greatest dominance (0.17), whereas the upstream zone exhibited the highest Shannon-Wiener diversity index (2.46) and evenness (0.36). These results indicate that upstream zones, with lower temperatures, faster currents, higher dissolved oxygen, and stable pH, support greater species diversity and evenness. Downstream zones, with higher temperatures, elevated free CO₂, and wider river width, show greater abundance but are dominated by a few adaptive species. This pattern, consistent with other rivers, confirms that physical and chemical water characteristics shape distribution, abundance, and dominance of freshwater fish. Thus, fish diversity is a key indicator of river ecosystem health.]]>
