The effects of the grass carp (Ctenopharyngodon idella Val.) on aquatic plant biomass, water quality, phytoplankton, chlorophyll a, zooplankton and benthic fauna were investigated between May and September 2000 in earthen ponds at Cifteler-Sakaryabasi Aquaculture and Research Station. Four earthen ponds with an area of 100 m(2) were used and one of them was selected as control. The other ponds were stocked at rates of 200, 400 and 600 fish per ha in May. The survival rate of harvested grass carp was 100% in September and the highest weight gain of 428 g occurred at the minimum stocking rate. Cladophora and Zygnema species of aquatic plants were consumed in June by grass carp; however, Chara was eliminated completely by August. At the end of the stocking period, Phragmites was the only plant not consumed by the grass carp. Plant biomass increased 1.4 times in the pond without grass carp but was decreased 2.5 times in the ponds stocked with 200 and 400 grass carp per ha and 4 times in the pond stocked with 600 grass carp per ha. The lowest values of nitrite-nitrogen, nitrate-nitrogen, and total phosphate were measured in the pond without grass carp (p < 0.05). The highest values of phytoplankton, zooplankton, benthic fauna abundance and chlorophyll a were found in the ponds with fish (p < 0.05).
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