In situ study on the control of toxic Microcystis blooms using phytoplanktivorous fish in the subtropical Lake Taihu of China: A large fish pen experiment

Three large fish pens (0.36 km2 of each) stocked with silver and bighead carp were set up in Meiliang Bay for controlling toxic Microcystis blooms. The responses of plankton communities and food consumption of silver and bighead carp were studied. Crustacean zooplankton were significantly suppressed in the fish pens. Total phytoplankton biomass, Microcystis biomass and microcystin concentration were lower in the fish pens than in the surrounding lake water, but the difference was not statistically significant. The present stocking density of silver plus bighead carp (about 40 g/m3 in July) was likely too low to achieve an adequate control of Microcystis. Silver carp fed mainly on phytoplankton but bighead carp mainly on zooplankton: mean zooplankton contribution in the gut was 31.5% for silver carp and 64.7% for bighead carp. Compared with previous studies, both carp species preyed upon more zooplankton because of the abundant food resource. Daily rations of silver and bighead carp were estimated by Egger's model in the main growing season. Filtration rate was calculated from the daily ration and the density of plankton in the lake. During May–October, filtration rates of silver and bighead carp for phytoplankton were 0.22–1.53 L g− 1 h− 1 and 0.02–0.68 L g− 1 h− 1, respectively, and filtration rates for zooplankton were 0.24–0.44 L g− 1 h− 1 and 0.08–1.41 L g− 1 h− 1, respectively. Silver carp had a stronger ability of eliminating phytoplankton than bighead carp. To achieve a successful biomanipulation with a minimum effect of ichthyoeutrophication, the stocking proportion of bighead carp should be controlled in the future practice.