Seite - 1010 - in Book of Full Papers - Symposium Hydro Engineering

In the 1970s, the average weight of two brass gudgeon species living in the Three Gorges Reservoir was about 500g (Ding, 1987). In 2000, the average weight of the two gudgeon species dropped to 276g and 208g, respectively (Chen et al., 2002). During 2005-2006, the average weight of two gudgeon species dropped further to 131g and 171g (Wu et al., 2007). The average ages of the Four Family Carps were 2.34, 2.10, 2.13, and 1.77 years. Compared. with the data of the 1970s, fishery catches size was down significantly, and most of them were medium or small-sized young fish (Chen et al., 2002). In Poyang Lake, small-sized commercial fish are on a general rise, and individuals tend to be younger and smaller. In 2007, there were about 12 major commercial fish species in the survey, those lake-inhabiting species such as carps, crucian carps, yellow-head catfish, and catfish are the main fishing targets, accounting for 71.3% -77.26% of the catches. The majority were 1-2 years old, accounting for 76% of the total catches. 2.4. DEGRADATION OF GERMPLASM RESOURCES In freshwater aquaculture, population blend has led to the degradation of germplasm resources. As a result, over fishing, breeding parents generally tend to be small-sized. Typical examples include silver crucian carps "giving only one times birth" to multiply and germplasm degradation among Eriocheir sinensis due to germplasm blend. Moreover, these degraded germplasm resources face the ecological risk of hybridization when they meet natural populations in the Yangtze River. As is the case with invading alien species, so far no relevant mechanism for assessing ecological risk is available in the country. 3. PARTIAL LOSS OF ENVIRONMENTAL CONDITIONS IN HABITATS For many reasons, wetland degradation in the Yangtze River Basin has been severe for over half a century, and wetlands as important habitats of fish and waterfowl are gradually losing their environmental conditions. According to statistics, more than 1/3 of the lake areas, up to130km2, in the middle and lower Yangtze River have been reclaimed since1949. More specifically, Dongting Lake dropped sharply from 43.50 km2 to the current 26.25 km2; Poyang Lake decreased from 52km2 in 1949 to the current 29.33 km2. Apart from this, almost all riverine wetlands in the middle and lower Yangtze River have been reclaimed, and about 12.13 km2 of wetland has been affected. Changes have occurred in fish composition and structures after the impoundment of the Three Gorges Reservoir. In 1997-2002, 127 fish species were commonly spotted. In 2005-2006, 108 fish species were surveyed. In 2017, only 71 fish species were discovered, accounting for only 31%-33% of the endemic fish at the upper Yangtze River. Some rapid-loving fish, such as Coreius 1010