A 10-week enclosure experiment was conducted in the dry season (October to December) to assess the impact of fish stocking on water quality and phytoplankton community at three stocking levels of silver carp (Hypophthalmichthys molitrix) (0, 16 and 50 g m–3) in a shallow, eutrophic tropical reservoir (Guangdong, South China). The nanophytoplankton (< 20 μm) accounted for 60 ± 5% of total biomass and Scenedesmus quadricauda was the predominant species (size: 16.5 ± 5 µm, relative biomass 27 ± 10%). Phytoplankton biomass was markedly suppressed in high fish stocking level and the grazing effect of silver carp on the nanoplankton was significant from the sixth week to the end of the experiment. The high fish density treatment significantly increased water transparency and somewhat reduced total phosphorus. Indirect effects of silver carp on the phytoplankton community involved excretion of nitrogen that favored small species of Chlorophyta. Colonial Cyanobacteria were reduced in the high fish treatment, but enhanced in the low fish treatment.
Our results suggest that the widespread practice of harvesting fish from October to December in reservoirs of South China releases phytoplankton from consumption by filter-feeding fish, and consequently prompts algal blooms. Conversely, harvesting fish just before monsoon, around the time of restocking with small fish, would make more sense since no interruption in grazing pressure would occur. This would help to control excessive phytoplankton biomass just before monsoon. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)