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24 water and caused odor problems. We designed and installed (piece by piece), ten 15- by 15- by 7.5-ft plexiglass enclo- sures that were anchored into the bottom of the pond with 50-lb sandbags. Half of the enclosures were to be stocked with either fifty 6-inch silver or bighead carp. The other half were to be stocked in polycul- ture which included both fish in various combinations. Only bighead carp were stocked into the enclosures because spawn- ing problems at the hatchery made silver carp unavailable. Each fish enclosure contained different numbers of bighead in a replicated series. Four enclosures were used as controls and contained no fish. Each enclosure was sampled bimonthly to evaluate the effec- tiveness of the fish on Spirogyra. The study was a joint effort between Reclama- tion and a private contractor, Quality- Water BioSystems. Preliminary results showed some promise in using the bighead carp; although we hoped to obtain silver carp from the fish hatchery in 1992 to carry out the main objective of our research. 1992 Field Study During spring 1992, bighead and silver carp were successfully spawned at the Mis- souri fish hatchery. One thousand 2- to 3-inch silver carp and 500 6-inch bighead carp were stocked directly into a pond at Jack B. Tomlinson Park in Arvada, Colo- rado. This study is being done in cooperation with Arvada/North Jeffco Parks and Recreation Department and Quality- Water Biosystems. Arvada/North Jeffco Parks was eager to let Reclamation use their pond for research because the pond has had uncontrollable algal prob- lems in past years. The pond is usually treated during the summer months with her- bicides to control scuiumy, foul-smelling, green and blue-green algal mats. The herbi- cides work temporarily but do not take care of recurring problems. A section of the pond has been netted off to allow for a control where no silver or bighead carp ex- ist, and to create a refuge for zooplankton, microscopic fauna that help to keep the tini- est flora (nannoplankton) under controL Without a refuge, the fish could easily deci- mate the zooplankton population, enabling nannoplankton blooms. Silver and bighead carp work to control the larger algae, as the protected zooplankton help to control nannoplankton. The pond was sampled monthly during the summer for chloro- phyll, algae, zooplankton, DO, pH, water temperature, conductivity, water clarity, turbidity, and nutrients. The algal mats that were dominant at the beginning of the sum- mer disappeared by August. Chlorophyll a concentrations in the con- trol increased from 10 mg/m3 during May to 57 mg/m 3 during August, and reached a season high of 61 mg/m 3 during Septem- ber. By contrast, chlorophyll a concentrations in the middle section of the lake increased from 20 mg/m3 during May to 76 mg/m3 during August, and then de- creased to 37 mg/m3 during September. The increase and then decrease of chloro- phyll a in the middle section of the lake can be attributed to the increase in nan- noplankton, and not to the noxious green and blue-green algal mats. The nannoplank- ton accounted for 80 and 88 percent of the total chlorophyll concentration during August and September 1992, respectively. The increase in the tiniest algae, the nan- noplankton, is a natural phenomenon. This is caused by the largest algae being eaten by the silver and bighead carp, and the nannoplankton not sufficiently being con- trolled by zooplankton. As a result, the nannoplankton multiplies quickly in the pond. Arvada/North Jeffco Parks personnel and folks who visit the Tomlinson Park fre- quently have commented to us that the pond aesthetics have improved. Ar- vada/North Jeffco Parks are receiving fewer complaints about the smell and un- sightly algalmats that were once a real nuisance. The silver and bighead carp have grown in size since Spring 1992. The effec- tiveness of these fish on the control of nuisance algal in the pond will continue to be assessed. This method could prove to be -- a biologically safe alternative to chemicals in an appropriate aquatic environment such as a pond, small lake or canal, and more economically feasible in the long run . . . . --