Abstract:

Changes of fishing structure and biodiversity before and after three years′ spring fishing off were analyzed and the effects of the spring fishing off were assessed preliminarily according the monitoring data by drift net in Yibin，Banan and Wanzhou reaches of the upper Yangtze River in 2000~2005.Altogether 51 fish species belonging to 36 genera，18 families (subfamilies) and 3 orders were collected，and Cyprinidae was the dominant，accounting for 60.8% of the total species.After three years′ spring fishing off，dominances of the main species decreased，CPUE of part reaches increased，and Margalef diverdity index，Pielou evenness index and Wilhm Modified ShannonWiener diversity index of the three reaches showed the rising tendency，which indicated the richness of species number and complication of community structure，and the positive ecological effects by the spring fishing off.The fluctuation of fish species before and after the spring fishing off might result from those species with small population size and difficulty to be caught.Furthermore，decreasing trend of average body length and body weight of the main species might be by reason of the pressure from overfishing and difficult restoration in a short period.It is suggested that longer fishing off period and controlled capture efforts after the spring fishing off season should be carried out in order to improve the effects of the spring fishing off，while other conservation measures such as reconnection of rivelake，protection of water quality in important fishery areas and ecological operation of hydrological projects should be put into practice.Restoration and conservation of fishery resources in the Yangtze River is a complex project with a long period，and longterm monitoring and more studies in the future are needed for precision assessments on effects of the spring fishing off.

Key words: fishing structure, biodiversity, the spring fishing off, the upper reaches of the Yangtze River