Field surveys and laboratory experiments were conducted in 2009 to 2010 to examine methods to reduce outbreaks of green tides by Ulva spp. at the north pond of Osaka Nanko bird sanctuary. The green-tide-forming seaweed varied depending on the season, and U. pertusa predominated at the bird sanctuary from early summer to autumn, while U. ohnoi was abundant there from autumn to early spring. The laboratory experiments revealed that exposure of U. ohnoi to air with low salinity inhibited their photosynthesis at 35℃. These results, along with the preceding result of experiments for U. pertusa suggest the possibility of controlling a green tide of U. ohnoi or U. pertusa by a combination of exposure to air for 3-5hours with low salinities of 15-20psu in summer.