Abstract: In order to understand the effects of water and sediment regulation at Xiaolangdi Dam on the fish assemblages in the midstream of Yellow River, an investigation was conducted on the fish resources from Hukou to Sanmenxia section of Yellow River during the period from June to July, 2011. Twenty six and thirty five species of fish composition were observed before and after the water and sediment regulation, respectively. After the water and sediment regulation, both the fish composition and the fish captured in Sanmenxia Reservoir declined, and the smallsized fish community, e.g., Hemiculter leucisculus, dominated. The fish kinds and catches in the section from Hukou to Tonguan after the water and sediment regulation had somewhat increase, but the smallsized fish community still occupied the main position. The adult fish resources from Hukou to Sanmenxia section reduced significantly. The analysis of the fish diversity before and after the water and sediment regulation showed that in Sanmenxia Reservoir, the Margalef richness index, Shannon diversity index, and Pielou evenness index of fish resources all changed obviously. Some advices were given in the protection of migratory fish stocks in the middle reaches of Yellow River.

Key words: soil total nitrogen, inverse distance weighting, ordinary Kriging, universal Kriging, geographically weighted regression.