Impact of the Gezhouba and Three Gorges Dams on habitat suitability of carps in the Yangtze River

摘要

The Gezhouba Dam and Three Gorges Project are the two largest hydraulic projects in the middle reach of the Yangtze River. Although these projects have yielded benefits to local communities by providing flood control, water supply, and safe navigation, they also have changed the physical habitat of the Yangtze River. As a consequence, aquatic organisms, including rare species and fishes with economic value, have been affected. The Yangtze River's four major carp species (YFMCS)-black carp (Mylopharyngodon piceus), grass carp (Ctenop haryngodon idellus), silver carp (Hypoph thal michthys molitrix), and big-head carp (Aristichyths nobilis)-have significant importance to the fresh water fishery in China. According to field surveys, the percentage of the YFMCS in the total catch of fish from Tongting Lake has gradually reduced. The percentages were 22%, 14.1%, 11.84%, and 8.5% in 1963, 1980-1982, 1997, and 2002, respectively. These reductions in species population are mainly attributed to the loss of spawning ground caused by dam construction. Considering the spawning characteristics of the YFMCS, a habitat suitability index model (HSI) has been established. A one-dimensional (1-D) mathematical model was also developed to simulate and predict aspects of the physical habitat situation for the YFMCS. By coupling the habitat suitability curves and the 1-D mathematical model, a HSI model for the YFMCS was established. The HSI model was validated by comparing measured data with predictions from the model. These comparisons show that the computed results agree well with the measured results. The HSI model for the YFMCS is used to suggest a minimum instream flow and suitable daily discharge increase during the reproduction season for the carp species. The minimum discharge needed for the YFMCS in the middle reach of the Yangtze River is 3000m~3/s. Different daily discharge increases are required for different initial discharges, too small or too large of an increase in daily discharge is not good for carps spawning. Therefore, adjusting reservoir operations to create reasonable water level fluctuations is a key method to improve the habitat suitability of carps spawning sites.