Experimental Studies On Effects Of Fish Stocking On Water Quality And Plankton In Lake Erhai

The Erhai Lake is the second largest freshwater lake in Yunnan plateau and its ecological function includes irrigation, aquaculture, tourism, et al. In recent decades, due to the risk of cyanobacteria blooms increasing, local management departments have been stocking the filter feeding fish-silver carp as the main method to control algal blooms in this lake. In the present study, in situ enclosures with stocking silver carp and spring carp were set to determine their effects on water quality and plankton in Lake Erhai. We hope that our result will provide a scientific basis for the control and management of fishery resources in Lake Erhai.Main results are shown as following:1) There are obvious differences on the growth of stocking fishes between 2013 and 2014. In 2013, silver carp has the growth from the beginning body weight at 63.67 ±4.79g to the end of the experiment at 75.15±5.56g while body weight of spring carp increased from 0.74±0.10g to 2.09±0.19g. The growth rate in 2014 was significantly lower than that in 2013, which silver carp increased from the beginning body weight at 37.8±7.178g to 42.76±8.64g and those of spring carp increased from 10.25±2.41g to 13.48±1.39g.2) Based on different habitats of stocking fishes in 2013, the spring carp significantly decreased the water transparency but ammonia, orthophosphate and total phosphorus, chlorophyll a concentrations were increased. However, water quality in enclosure with stocking silver carp showed little effects. Compared with the same control group in 2014, both silver carp and spring carp significantly decreased the water transparency but the ammonia nitrogen, total nitrogen, total phosphorous and chlorophyll a concentrations were increased.3) During the study period, both silver carp and spring carp have significantly changed the phytoplankton communities. Silver carp resulted in increased the biomass of Xanthophyta, and spring carp significantly increased the biomass of Pyrrhophyta. At the same time, the dominant phytoplankton taxa was transferred to flagellar species concerning on the disturbance of stocking fishes. Meanwhile, silver carp stimulate the density of in micro-algae. As for crustacean zooplankton, silver carp showed higher grazing pressure than that of spring carp during the study period. Moreover, Bosmina became the dominant species which indicated species transferred from large to small.4) Compared to stocking with fishes or not, the transparency has been improved when stocking fishes were moved. There was a decline of nutrients found in the enclosures. In addition, the density and biomass of phytoplankton were decreased as well as chlorophyll a. Furtherly, the density and biomass of crustacean zooplankton had a little restoration to a certain extent.Spring carp in Erhai, in the case of 20g/m3, can have a significant impact on the water quality. So, it is recommended that groundfish was recreased in the Erhai restocking activities.