The effect of filter feeding fish on water quality in irrigation reservoirs.
The effect of filter feeding fishes, silver carp: Hypophthalmichthys molitrix, bighead carp: Aristichthys nobilis, and a hybird of these two species on water quality in irrigation reservoirs was studied by means of field surveys and enclosures experiments. The concentration of particles that are liable to clog an irrigation system was reduced in the reservoirs that were stocked with filter feeding fish. The filter feeding fish reduced the concentration of filamentous algae, prevented bluegreen algae blooms, and also reduced the concentration of zooplankton, mainly of copepoda and cladocera in the reservoirs. Experiments in enclosures indicated that the concentration of chlorophyll-a was not always reduced in the presence of the filter feeding fish, but that the alga population was shifted to a smaller sized species. Blooms of bluegreen algae occured only in the enclosures without fish. During periods of bloom, the corresponding concentration of chlorophyll-a was also significantly higher in the enclosures without filter feeding fish. Zooplankton concentration in the enclosures with no fish was much higher (up to 10 times or higher) as compared to those enclosures with filter feeding fish. In general, the presence of filter feeding fish improved the quality of irrigation water by the removal of large suspended particles.