The Xiaolangdi reservoir is a key water-control project located in the middle reaches of the Yellow River in Henan Province,China.Since 2002,the water and sediment regulation operations have been conducted in this area during June or July every year.In autumn,juvenile fish is artificially released into the reservoir to prevent the fishery resources from decreasing as a result of the project.In order to know the influence of this project on the fishery resources,acoustic investigations on fishery resources were conducted in the main channel areas from the dam to the Yellow River Three Gorges in the Xiaolangdi reservoir.A split-beam scientific echo sounder (Simrad EY60,70 kHz,Norway) was used on November 25,2013 (after the water and sediment regulation) and May 22,2014 (before water and sediment regulation),respectively.The average density of fishery resources,the population size and the spatial distribution in different areas of reservoir were surveyed and estimated according to the survey on fish target strength in situ and the use of echo integration methods.As a result,the acoustic integral value:Nautical Area Scattering Coefficient (NASC) in the reservoir were 29.38 m²/nm² and 49.77 m²/nm² in November 2013 and May 2014,respectively,also the average density of fishery resources were 0.016 ind/m² and 0.290 ind/m², and the population size were 41.56 t and 606.70 t,respectively.For the horizontal distributions of fishery resources in the different investigated areas,the average density of fishery resources in the areas are as follows:areas close to the dam saw a horizontal distribution of 0.016 ind/m²; the middle reaches of the river with 0.023 ind/m²; the upper reaches with 0.024 ind/m²; and Three Gorges area with 0.009 ind/m² in November 2013.In May 2014,the horizontal distributions of fishery resources for the same locations were 0.848 ind/m²,0.248 ind/m²,0.077 ind/m²,0.083 ind/m²,respectively.The NASC in the corresponding areas were 5.61 m²/nm²,4.60 m²/nm²,5.52 m²/nm²,6.47 m²/nm² in November 2013; and 94.09 m²/nm²,53.18 m²/nm²,28.27 m²/nm²,29.38 m²/nm² in May 2014 for the respective area segments.The fishery resources were distributed mainly in the upper and middle reaches of the reservoir in November 2013,and in the lower and middle reaches in May 2014.Fish in the reservoir showed the characteristics of layered vertical distribution.In the area of the Yellow River Three Gorges,fish is mainly distributed in the 10 m to 20 m water layer.In the main channel area,fish is mainly distributed in the surface water layer and the 20 m to 30 m layer.The fish target strength (in situ) had respectively highest proportions of -59.5 dB in November 2013,and -56.5 dB to -53.5 dB in May 2014.This shows that the fish size was relatively small and the body length increased six months later.These results support the idea that the fishery acoustic survey is an effective method for the fishery resources assessment in the Xiaolangdi reservoir.