THE ANALYSIS OF THE TROPHIC LEVELS OF THE MAJOR FISH SPECIES IN THE DANING RIVER USING STABLE ISOTOPE TECHNOLOGY

In this study, we measured the stable nitrogen isotope ratios of the major fish species in the lotic reaches and the impounded reaches of the Daning River to identify their trophic levels, and we compared the mean trophic levels of several conspecifics in lentic waters during different hydrological periods. The results showed that the 15N values of particulate organic matter (POM) fluctuated in a wide range and exhibited significant seasonal variation. The 15N values of species in the lentic downstream ranged from 4.54 to 17.51, and the trophic levels of these organisms were 1.513.88 with an average of 2.49. In the upstream, the 15N values of fish species varied between 2.25 and 10.81, and their trophic levels were in the range of 1.494.01 with an average of 2.87. The fact that the mean trophic levels were higher in the upstream than that in the downstream, which indicated that the fish tended to consume preferable invertebrate in the lotic areas probably due to their high diversity and abundant biomass. The mean trophic levels of conspecifics in the impounded waters were significantly lower in the flooding period than that in the non-flooding period. This is most likely because the hydrological disturbance affected the intermediate predators. Another possibility is that during the low-water period, the fish in the downstream need to forage for food at lower trophic levels because of the intense competition. To improve the assessment of the trophic levels, we will need to determine the appropriate trophic discrimination values and to establish an adequate isotopic baseline by repeated sampling of the baseline organisms.