Abstract: The investigation on the distribution and population density of C.fluminea and Bellamya sp.in Chaohu Lake during September 2001 and September 2002 showed that in the west region of the lake where was seriously eutrophic,the density and biomass of C.fluminea were 5.1 ind.·m^-2 and 17.87 g·m^-2 in 2001,and 8.8 ind.·m^-2 and 47.29 g·m^-2 in 2002,while those of Bellamya sp.were 13.3 ind.·m^-2 and 45.45 g·m^-2 in 2001,and 3.8 ind.·m^-2 and 12.56 g·m^-2 in 2002,respectively.In the east region of the lake where was eutrophic,the density and biomass of C.fluminea were 23.8 ind.·m^-2 and 67.86 g·m^-2 in 2001,and 29.2 ind.·m^-2 and 96.18 g·m^-2 in 2002,while those of Bellamya sp.were 10.1 ind.·m^-2 and 32.00 g·m^-2 in 2001,and 9.4 ind.·m^-2 and 31.21 g·m^-2 in 2002,respectively.The density and biomass of C.fluminea and Bellamya sp.were declined with increasing eutrophication.In hypertrophic region,C.fluminea and Bellamya sp.were absent.The density and biomass of the two species were obviously higher in littoral than in pelagic region.The distribution type of C.fluminea was core-model,while that of Bellamya sp.was random.The correlation between the density and biomass of C.fluminea and Bellamya sp.and water depth was not significant(P>0.05).The biomass of Bellamya sp.was negatively correlated with water TN(P＜0.01),NO₃-N(P＜0.05),TP(P＜0.01) and PO₄-P(P＜0.05),while that of C.fluminea only had a significantly negative correlation with PO₄-P(P＜0.05).Compared with 1981,there was fewer C.fluminea in the lake nowadays. The effects of other environmental factors on the population distribution and growth of C.fluminea and Bellamya sp.were also discussed.

Key words: Poplars, New clones, Introduction, Selection