Studies On The Density Distribution Of Fishes In The Section From Chenglingji To Yichang Of The Yangtze River Middle Reaches

The geographical location of Chenglingji to Yichang section is very important, with the Three Gorges Dam (Yichang), the Dongting Lake (Chenglingji). Historically, this section of the river had bountiful fishery resources. But due to overfishing and human-induced changes in the environment, the fishery resources in this river are declining, especially the four famours Chinese carps. In order to supplement the breeding groups of four famours Chinese carps, the enhancements have been done done in Jianli and Shishou for4consecutive years from2010to2013by the charge of Yangtze River Fisheries Research Institute. This study using the EY60echo sounder (200kHz transducer) has been carried out for6hydroacoustic surveies during2010～2011in this section. Experimental study on the effects of different transducer tilt-angle on the investigation results was finished.And the analysis of the density distribution characteristics of the total fish and non-demersal fish combined with the results of relevant catches, and some influence factors are discussed at last. The main results are as follows:1.Effects of different transducer tilt-angle on the result of the investigation:In order to find the most suitable tilt-angle for the middle reaches of the Yangtze River,and to predict the effects of different tilt-angles (0°,20°,45°,90°) on the result of the investigation, we compared the mean values and the stabilities of density, average range, target number, target strength (TS)4parameters,the result indicates that the horizontal beam detection (0°) in the middle reaches of the Yangtze River section shows the best stability, and also has the largest target numbers and the maximum detection ranges (p<0.05), which mean the higher accuracy on the evaluation of fish density, but according to the actual situation of this early research and investigation, the vertical (90°) beam detection can support us with the actual water depth,the correct target layer, and the practical safety guidance.2. The density distribution characteristics of total fishes:First, investigations of different periods have different fish aggregation area. In June the fishes are mainly distributed in the upper and middle section (Yichang to Jiangling), with the maximum density of108.4f/1000m3, in July are mainly in the middle and lower section (Jiangling to Chenglingji), with the maximum density of248.6f/1000m3, in October showes the uniform distribution of fish with the smaller density difference, with the maximum density of43.3f/1000m3. Second, during the spawning season the fishes have mainly distributed in some spawning grounds in the literature,such as Yichang, Yidu, Jianli, Tiaoguan, Fanzui, and the maximum of density is45.9f/1000m3during this time, which is Significantly higher than that in non-spawning season (p<0.05). Fish size shows the increasing trend from Chenglingji up to Yichang direction.3.The density distribution characteristics of non-bottom fishes:We get the distribution of large or medium-sized non-bottom fishes in survey sections by setting the analysis layer and analyzing the targets selectively bigger than-55dB. In June, fish distributes mainly in the middle and upper sections, including Yichang, Yidu, Zhijiang, Jiangling,5documented spawning grounds, with the maximum density of25.2f/1000m3. It is similar of the maximum density28.2f/1000m3in July with that in June, but the differences are the increase of the fish density in Yichang section in July, the dispersing in Yidu, Zhijiang, and the droping down of the aggregation fish from Jiangling to Shishou in the middle section. In October the fish distribution is uniform, with the maximum density of9.1f/1000m3,while the bigger concentration from Chenglingji to Sanzhou section closing to the Dongting Lake. In addition, during the spawning season,the large and medium-sized non-bottom fishes have mainly distributed in the middle (Jiangling to Jianli)and upper (Yichang to Zhijiang) sections, and the maximum of density is25.5f/1000m3during this time, which is Significantly higher than that in non-spawning season (p<0.05). And the distributions of the large and medium-sized non-bottom fish are not synchronous with the total fish, fish in different sizes aggregate in different regions at the same period. In other words different segments have the different fish species composition and the different body length structure. We discusse that the distribution characteristics are possibly caused by the reproductive migration,feeding migration, temporary distribution of enhancement resources, and some other factors.