Water level changes caused from the Three Gorges Dam impact on the breeding and distribution of Oncomelania snails in the marshland of Chenqiaozhou, Hexian County, Anhui Province

Objective To explore the effect of water level changes caused by the Three Gorges Dam on the breeding and distribution of Oncomelania snails in the marshland of Chenqiaozhou of Hexian County, Anhui Province. Methods From 1990 to 2016, the daily average water level of the Yangtze River in Chenqiaozhou area, the area and density of Oncomelania snails, and data of snail control with molluscicide in the marshland of Chenqiaozhou were collected, field study was also conducted to understand the development and utilization of the marshland. By comparing the changes of various indicators before and after the operation of the project, this study tried to analyze the relationship between water lever and utilization of marshland as well as breeding of snails. Results After the completion of the Three Gorges Project, the water level of the Yangtze River in Chenqiaozhou was generally declined, the median value of annual flooding days of the marshland was reduced from 59 d to 21 d (Z=2.440, P=0.015), the reclamation area had been increased from 0.95 km² to 4.55 km², which had reached 85.0% of the total area of marshland. The coverage rate for snail controlled with molluscicide was significantly reduced, and the median value of annual coverage rate was reduced from 33.0% to 0.0% (Z=4.116, P<0.001).The snail area decreased slowly, and median value of annul snail area was reduced from 317.6 hm² to 264.6 hm² (Z=1.744, P=0.081). The average density of live snails decreased rapidly, and median value of annul snail density was reduced from 0.84 number/0.11 m² to 0.27 number/0.11 m² (Z=3.779, P<0.001). Conclusions After completion of the Three Gorges Project, the change of water level in the lower reaches of the Yangtze River in Anhui area and its influence on the exploitation and utilization of the marshland affect negatively on the breeding and spreading of snails, playing a positive role in the control of schistosomiasis.