No cascading negative effects of piscivorous fish stocking on phytoplankton biomass in subtropical shallow mesocosms: implications for lake restoration by biomanipulation

Biomanipulation by piscivore stocking has been widely used for restoration of temperate shallow lakes, but long-term results have been mixed. In warm (i.e., subtropical/tropical) lakes where omni-benthivorous fish prevail, the effects of piscivore stocking on small fish, plankton communities, and water quality have not been well elucidated yet. Here, we conducted a mesocosm experiment in the subtropics to examine the top-down and bottom-up effects of stocking of a native piscivorous fish, mandarin fish (Siniperca scherzeri). The experiment consisted of two treatments (piscivore present/absent) in triplicate, lasting for 90 days and including two periods. Crucian carp (Carassius carassius) were stocked twice to simulate the recruitment of small fishes. Our results showed that stocking of mandarin fish significantly reduced the abundance of crucian carp but did not increase zooplankton biomass nor reduced the phytoplankton biomass, the concentrations of nutrients or suspended solids, suggesting that both top-down and bottom-up control of phytoplankton were unaffected by stocking mandarin fish. Moreover, the presence of mandarin fish did not help maintain a clear water state in the period of crucian carp recruitment. Our results indicate that piscivore stocking may not help control phytoplankton biomass in subtropical shallow mesocosms, at least on the short term.