| Phytoplankton have many, sometimes antagonistic effects on aquacultural production. Although algae form the basis of the food chain and provide most of the dissolved oxygen in fish ponds, dense phytoplankton cause severe problems, e.g., insufficient dissolved oxygen, algal die-offs, and off-flavor of fish. Current methods of managing phytoplankton are generally ineffective or counter-productive. To manage phytoplankton properly, fish culturists must understand the mechanisms controlling algal dynamics. A literature review, with emphasis on limnology and freshwater ecology, suggests new views of algae-related problems and ways to solve them. For example, algal die-offs may result from severe nutrient depletion, and dense algae of any taxa, not just blue-greens, may produce off-flavor.; Modeling of phytoplankton-oxygen dynamics demonstrates that pond oxygen levels peak at intermediate algal densities where Net Primary Productivity (NPP) is highest. Thus, fish culturists could improve dissolved oxygen levels by encouraging algal growth while maintaining algal biomass at levels optimal for NPP. Biological control of phytoplankton can achieve both goals at once. However, a review of experiments with filter-feeding fish suggests that fish alone cannot control algal biomass in fish ponds. In most trials, addition of filter-feeding fish increased algal biomass.; Feeding trials with silver carp (Hypophthalmichthys molitrix) at 20, 25, and 30{dollar}spcirc{dollar}C support this conclusion. Silver carp filter more particles as temperature, particle size, and fish size increase. With particles smaller than 70 {dollar}mu{dollar}m, filtration rates declined with particle size. Silver carp do not efficiently filter particles below 8 {dollar}mu{dollar}m. Therefore, these fish will not control algal biomass.; A new technique for biological control of phytoplankton, integrated control with herbivorous zooplankton and silver carp, was tested in large tanks and experimental ponds. Zooplankton, which eat small phytoplankton, and silver carp, which eat large phytoplankton, together eat all sizes of phytoplankton. The technique maintains coexistence of zooplankton with silver carp by excluding the carp from part of the water column. In 1000 l tanks, the combination of zooplankton and silver carp reduced algal biomass by 95% and increased algal diversity. However, in experimental ponds, the refuge did not protect zooplankton and enhance their survival nor did it reduce algal biomass. |
