Several experiments were run on hatchery ponds to determine the effects on pond metabolism of leaving cut but untreated aquatic vegetation and the effects of treating mats of cut vegetation with 2, 4-D and ammonium nitrate. Cutting vegetation did not improve environmental conditions until cut plants decayed. Ammonium nitrate appeared to be superior to 2, 4-D for inducing decay. When mats were treated with ammonium nitrate, blooms of zooplankton were produced which persisted up to 8 months. No phytoplankton blooms developed after ammonium nitrate treatment, and the higher aquatic vegetation did not grow faster in the treated ponds. Fish kills were not observed near the untreated mats, but dissolved-oxygen conditions there were unfavorable. No fish kills were observed after ammonium nitrate treatment, and growth rates of fish in treated ponds were good.