Effects Of Damming On Spatial Distribution Of River Fishes:Revealed By Fisheries Acoustic Method

The development of large-scale hydropower have maked far-reaching impact for the fishery resources in the Upper Yangtze River. Xiangjiaba Dam is the most downstream dam of the Jinsha River, however, the ladderlike power stations have a enormous impact on the ecological environment. The Three Gorges Dam is the world’s largest hydropower project, and the fish of upper and downstream is paid much more attention. In this paper, the acoustic surveys were conducted by using BioSonics DT-X echosounder downstream Xiangjiaba from2006to2012. And the study about Three Gorges Reservoir was conducted in2010. We explored the relationship between the spatial distribution characteristics of fish downstream Xiangjiaba in the dry season and the environmental factors, distribution characteristics of fish in the period of first impoundment of Xiangjiaba, and spatial distribution characteristics of fish when the Three Gorges Reservoir reached175m. The main results in the present study were as follows:(1) Twenty kinds of fish were gained in the reach from Xiangjiaba to Yibin of the Upper Yangtze River during dry seasons. Pelteobagrus fulvidraco, Coreius guichenoti and Coreius heterodon were the dominant species. The standard length of fishes was from10.0cm to30.0cm with the mean of21.3cm. The result of the hydroacoustic surveys showed that the mean target strength was-42.14±5.49dB and the corresponding ranged from8.44cm to130.02cm with the mean length of22.7cm, according to the empirical of the relationship between target strength and fish length. The fishes mostly distributed in the estuaries of Heng River (rkm1070.12), Min River (rkm1041.80) and Nanguang River (rkm1037.38), and the deep reaches of Datiewo (rkm1050.64) and Lijiatuo (rkm1030) in the reach of Xiangjiaba to Yibin during the dry seasons. Particularly, the junction of Jinsha River and Min river was the most concentrated area accounted for a proportion of42.9%in the fish community. The survey reach had a depth of3.40-64.90m in dry seasons, and the fish chose the locations which were deeper (mean=17.16m). There was significant difference between the depth of the gathering place and the depth of other survey reach (mean=9.94m;P<0.01). The reach had a fast velocity with the minimum value of0.14m/s and the maximum value of3.77m/s. Generally, the fish chose velocity with the range of0.82m/s to1.59m/s. There was significant difference between the velocity of fish gathering place (mean=1.11m/s) and the velocity of other survey reach (mean=1.67m/s; P<0.01). In the survey seasons, Froude number was less than1with the maximum value of0.51. The fish chose the place with Froude number in the range of0.07to0.13. The flow field of the entire survey section was complicated. The value of Froude number in the four distribution area (mean=0.09) was significantly smaller than the other places (mean=0.19; P<0.01).(2) In a short term, there were distinct variations before and after first impoundment of Xiangjiaba Dam. First, the water level and the flow, monitored at the point downstream Xiangjiaba Dam, fluctuated strongly. There were sharp decline in the first two days of impoundment. In addition, in the period of impoundment, the water level maintained at a low level until the16October. After impoundment, the level and flow rised but lower than before. Variation of the sediment concentration was intuitionistic and significant, because the natural flow regime was disturbed, large amount of sediment was retained in the reservoir along with the less sediment concentration into the downstream. The sediment concentration increased from0.55to1.0kg/m3to2.17kg/m3in the first two days of impoundment, then the sediment concentration reduced to the range of0.01to0.08kg/m3after impoundment. Transparency had a variation from the lack of30cm to more than100cm. The fish density of Xiangjiaba downstream before impoundment, the first and second day of impoundment, the fifth days of impoundment, the second day after impoundment, the seventh day after impoundment were0.30ind/km31.05ind/km3,0.10ind/km3,0.03ind/km3,0.08ind/km3, respectively. Before impoundment, the fish distributed in entire survey reach, and there were more fish in the estuary of Heng River and the reach of Datiewo, particllarly. The spatial distribution of fish had a remarkable variation at the first and second day of impoundment. There were not obvious changes in the reach from Shuifu to Baixi compared with the distribution before impoundment. However, there were abundant fish gathered in the reach of Baixi downstream, specially in the U-sharped reach (2.79ind/km3). The fish density declined distinctly at the fifth day of impoundment and there were a small amount of signals in the reach of Baixi and estuary of Heng River. Further, the number of fish declined at second day after impoundment, and there were extremely few fish near Datiewo. The number of fish increased at the seventh day after impoundment.(3) A total of8692fish signals were found in the reach from Xietan to the forequarter of Three Gorges Reservoir. TS values ranged from-70to-50dB (89.62%), and the average value of target strength was-58.33dB. According to Foote (1987) formula (TS=201og10L-71.9), the fish body length less than5cm was a proportion of 42.9%, and only0.49%greater than100cm. The maximum value of fish density was1360ind/km3and the average density was53.47ind/km3in the Three Gorges Reservoir. The fishes mostly distributed in the estuary of Xiangxi River, Yinxingtuo and the area which was near the bay with3.5km far away from the Three Gorges Reservoir. The fish densities were72.83ind/km3、135.75ind/km3、65.26ind/km3, respectively. Xiangxi River was a important tributary. There was significant difference between the depth of the reach near Yinxingtuo and the depth of other survey reach. The water layer of30to60(41.63%) had the most fish density, followed by2m to30m (27.73%), again for60to90m (21.74%). In the survey, there were a certain amount of fish in the water more than90m deep, even below150m. Statistics showed that fish density was negatively correlated with depth (P<0.01, F=-0.159)...