Effects Of The Mandarin Fish Cage Culture On The Environment And The Carrying Capacity Of The Water Body

A serial studies on the effects of the mandarin fish (Siniperca chuatsi) cage culture on the water quality and aquatic organisms, the concentrations of nitrogen and phosphorus of the water body, the limiting factor for the growth of phytoplankton, the carrying capacity model of the water body for the mandarin fish cage culture, and measurements for decreasing the effects of cage culture on the environment were conducted in Fuqiaohe Reservoir. The effects of piscivorous fish cage culture on the environment were quantitatively studied for the first time in the present study. A dynamic model for quantitatively evaluating the carrying capacity of the water body for cage culture including water exchange rate, reservoir volume rate, mean depth, trophic status, cultured species and culture intensity was provided, and this model was applied in Fuqiaohe Reservoir. From these studies, the following conclusions can be drawn:(1) The concentrations of nitrogen and phosphorus and the biomass of phytoplankton and protozoans around the cage area increased because of the mandarin fish cage culture.(2) The affected scope of 500 m2 mandarin fish cages for the above mentioned factors was less than 50 m.(3) Cage culture of the mandarin fish also decreased the dissolved oxygen concentrations, the species number and biomass of the zoobenthos around the cage area, but had no significant effects on the water temperature, pH, transparency, species composition of phytoplankton and zoobenthos, and the species composition and biomass of rotifers, cladecerans and copepods.(4)Cage culture of the mandarin fish had the tendency to eutrophicate the water quality.(5) The phosphorus was determined as the limiting factor for the growth of phytoplankton by comparing the concentrations of nitrogen and phosphorus of the water body.(6) A dynamic model for quantitatively evaluating the carrying capacity of the water body for cage culture including water exchange rate, reservoir volume rate, mean depth, trophic status, cultured species and culture intensity was provided.(7) It was estimated with the dynamic model that the carrying capacities of Fuqiaohe Reservoir were 0.298# for cages of the mandarin fish fed the prey fish (Jian carp) cultured separately in cages, 1.81 # for cages of the mandarin fish fed the prey fish from other places, and 0.24# for cages of the Jian carp.(8) The effects of cage culture on the environment may be decreased by decreasing the input of exotic nutrients, increasing the food conversion rate, changing the place of cages to increase the water exchange rate and moving the caged to increase the retention rate of phosphorus.