| After the impoundment of the Three Gorges Dam, Pengxi River became an important anabranch which has largest submersed area and the most extensive riparian zone in the Three Gorges reservoir. Changes for hydrology are expected to result in impacts to fish resources and the environmental conditions in Pengxi River. Therefore, I used 200 kHz split-beam echosounder to measure the fish density in Pengxi River during 2009 to 2010 for 5 times. As the major component of fish community, Aristichthys nobilis was measured to study the regression models between target strength(TS) and the logarithm of fish size(length, weight), which may calculate the size in the fish community. This paper also investigated and described the relationship between fish spatio-temporal distribution characteristics and environmental factors. Our results indicated that:1.The relations between TS of A. nobilis and it's size was significant. Total length, body length, weight of A. nobilis ranged from 30.05cm to 80.00cm,25.1cm to 68.80cm and 0.3 kg to 6.0kg respectively. Average target strength ranged from -51.84 to -42.06dB, while maximum target strength ranged from -47.53 to -37.06dB. The target strength of A. nobilis was linearly related to the logarithm of total length, body length and weight using linear regression, whose relations were significant (r>0.9, P<0.01). The regression equations of average target strength(SE=0.55～0.61dB) was more accurate than the maximum target strength's (SE=1.09～1.17dB). Obvious day-night vertical movement of A. nobilis was detected and the average swimming speed was 0.15 m·s-1.2. Changes in fish spatio-temporal distribution of the Pengxi River were observed. Differences in fish density not only in surface layer but also in middle layer were found among different seasons (P≤O.01, P=0.03). Fish density also varied vertically, but only between the surface layer and bottom layer in Winter (P=0.05). The fish desity and aboundance also varied significantly between day and night (P<0.05, P<0.01). Target stength(TS) varied strongly by seasons(P<0.01), with highest TS value observed in Autumn and the lowest in Spring. Fish density varied between winter of 2009 and winter of 2010(P<0.01). We applied the formula of target strength of A. nobilis generated from chapter 2 to estimate the main fish body length during the spring of 2010. Fish body length was estimated to range from 8.7 cm to 17.6 cm, and fish weight ranged from 0.01kg to 0.10kg. 3. Fish spatio-temporal distribution characteristics had a close relationship with environmental factors, such as temperature, dissolved oxygen, plankton and so on. (1) Fish distribution was positively related to temperature (r=0.52, P=0.01)and negative correlation with conductivity, total solid solubility and salinity(r<-0.3, P<0.09). And the multiple regression between fish distibution and water quality factors was significant(F=5.28, P=0.009). (2) Fish distribution was positively related to beatles (r=0.63, P=0.09) and negatively related to the logarithm of phytoplankton, protozoa(r<-0.71, P<0.06). And multiple regression model for fishes distribution and plankton was also significatnt(F=9.24, P=0.04). (3) The horizontal distribution of fish in Autumn and Winter was not significantly correlated to depth of water. The relationship between horizonal distributions of fish in Spring and the depth of water was low negatively correlated(r=-0.5, P≤O.05), suggesting other ecological factors may affect the behavior and distribution of fish. |
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