Seite - 1009 - in Book of Full Papers - Symposium Hydro Engineering

for human beings, but it provides us with precious hydropower. Dams and sluices are extensively built in the middle and lower reaches to control flooding; and tens of thousands of dams are built to meet the excessive demand for hydropower. Besides, excessive demand has given rise to overfishing in open waters. As a result, the pristine Yangtze River ecosystem was damaged. Many species are threatened with extinction and biodiversity faces an unprecedented crisis. The middle and lower reaches was once abundant in Lipotes vexillifer,Neophocaena phocaenoides, Tenualosa reevesii, Lucpobrama macrocephalus, and Longjaw grenadier ancho. Today, Lipotes vexillifer and Lucpobrama macrocephalus are hard to find. Tenualosa reevesii has dropped significantly since the 1970s, falling to a record low in the early 1980s. Few Longjaw grenadier ancho, once the main fishing target, has been spotted in recent years (Li Sifa 2001). Neophocaena phocaenoides resources are being exhausted from about 2,700 in 1991 to about 550 in 2015 (Bulletin of Fishery Ecology in the Yangtze River Basin (Trial), 2015). Compared with the 1970s, fish species of Dongting Lake and Poyang Lake decreased by 33.7% and32.6%, and the proportion of migratory fish decreased by 47.1% and 53.6%. In 1970's, 336 and 310 bird species once lived around Dongting Lake and Poyang Lake, but surveys conducted in recent years showed that only 70-80 species have survived. Since the 1990s, the number of endangered species has seen a dramatic increase. 2.2. SHRINKING POPULATION Yangtze River Basin is the most important producing area of freshwater fish in China (Yu, 1988a). The fish catches reached 450,000,000kg in 1954, accounting for 72% of the total national freshwater catches. It dropped to over 200,000 tons in the early 1980s, accounting for only about 60%. In recent years, it has maintained about 100,000,000 kg. The production of semi-migratory fish of black carps, grass carps, silver carps, and bighead carps in rivers and lakes also dropped significantly. In 1964- 1965, the annual average egg-laying amount of the Four Major Chinese Carps in the mainstream of the Yangtze River reached 115 billion. In 1981, it fell by 85% to 17 billion (Liu and He, 1992, Survey Groups of the Four Major Chinese Carps' Spawning Sites in the Yangtze River, 1982). The natural fishery yield of Dongting Lake and Poyang Lak once reached a record high of 55,000,000kg. In 2015, it dropped to 28,000 tons in Dongting Lake and to 25,000 tons in Poyang Lake. The proportion of the Four Major Chinese Carps decreased from 21.0% to 6.61% and from 12.7% to 3.1% in Dongting Lake and Poyang Lake. 2.3. MINIATURIZATION AND HIGH MORTALITY AMONG INDIVIDUALS 1009