中文摘要：

长江八里江江段是长江江豚最为重要的栖息地之一。2005年7月至2008年5月,在八里江江段先后进行了10次截线抽样法考察,调查了该水域的江豚种群数量和分布情况。考察总航程1 460.2km,发现江豚374群次。根据动物密度,将考察区域划分为上游水域（九江-湖口）和下游水域（湖口-三号洲洲头）,分别分析。分析结果显示,上下游水域的多次考察密度平均值分别为0.67头/km2（范围：0.10～1.28头/km^2,s=0.41）和1.75头/km^2（范围：0.62～3.0头/km2,s=0.72）,数量平均值分别37头（范围：5～71头,s=23）和52头（范围：22～78头,s=17）。八里江江段江豚常年平均数量约为90头,不同考察季节数量有所波动（范围：44～121头,s=24）。与历史考察对比分析显示八里江江段江豚仍处于下降趋势,但该研究未能在3年考察期内监测到统计学意义的种群变动情况。根据动物数量波动幅度和发生时间推测,江豚在汛期至汛末的过渡时期,可能存在一次规模为40头左右的动物迁出过程。面对日益严重的人类威胁,如挖沙、非法渔业、航运、污染和水面设施建设等,建议在该江段尽快建立江豚自然保护区。

英文摘要：

Balijiang section is one of the most important habitats for Yangtze finless porpoise in the Yangtze River.The abundance and distribution of porpoises in this area was investigated by 10 line transect surveys between July,2005 and May,2008.Survey length amounts to 1 460.2 km with 374 sightings in total.Based on our findings,the mean densities over 10 surveys were 0.67 porpoises/km2（range：0.10~1.28 porpoises/km2,s=0.41） and 1.75 porpoises/km2（range：0.62~3.0 porpoises/km2,s=0.72）,respectively in the upper and lower stratifications,and their corresponding mean abundances were 37 porpoises（range：5-71 porpoise,s=23） and 52 porpoises（range：22-78 porpoises,s =17）.Overall,the mean abundance in Balijiang section was approximately 90 porpoises and seasonal estimates might fluctuate around this number（range：44~121 porpoises,s =24）.This population is still declining by comparing with the historical survey results,however,the current study failed to detect the trends of this population statistically over 3-years study period.According to the variations and occurrence times of the fluctuations,we presume that this porpoise population may be subject to an emigration（~40 porpoises） from the middle to the end of the flood season.The population status of Yangtze finless porpoise is deteriorating due to a combination of dredging,illegal fishing,transportation,pollution and water constructions etc.,consequently,we strongly recommend to establish a natural reserve in Balijiang section immediately.