Abstract: By using fixing nets and hanging nets, an investigation was conducted on the demersal fish in the middle reaches of Heilongjiang River from 2010 to 2011. A total of 47 demersal fish species were recorded, belonging to 33 genera, 12 families, and 7 orders, among which, Rutilus rutilus lacustris (Pallas) was adventive, and Acipenser schenckii (Brandt), Huso dauricus (Georgi), Lampetra japonica (Martens), and L. reissneri (Dybowski) were the fish species listed in the China Red Data Book of Endangered Animals (Pisces). Cypriniformes had the most abundant species (31), accounting for 65.96% of the total. The average density and biomass of the demersal fish were 139.00 ind·m^-3 and 344.58 g·m^-3, respectively. Fluviolacustrine species and heterophagous species were the main ecological groups, and Acheilognathus chankaensis (Dybowski), Pelteobagrus nitidus (Sauvage et Dabry), Pseudorasbora parva (Temminck et Schlegel), Leiocassis argentivittatus (Regan), and Xenocypris argentea (Gunther) were the dominant community species. The body weight of the dominant species except Silurus asotus (Linnaeus) was less than 30 g. The Margalef richness index D, Shannon diversity index H′, and Pielou evenness index J′ of the demersal fish in each section of Heilongjiang River were all lower, but had definite differences. The water quality in the middle reaches of Heilongjiang River was high, and surpassed the II class water quality during most of time. No significant relationships were observed between the fish diversity index and the environmental factors including nutrients and transparency in the River, but a positive relationship was found between the H′ and the water NH₄⁺-N and total N.

Key words: loess hilly area, organic carbon, precipitation gradient, surface soil, land use.