| Using fish of Bio-manipulation in the repairing and controlling of eutrophication have a widely application. In the engineering application, fish age and external environment factors have important affects to the feeding behavior of fish, then affect the algae control efficiency. The writer researched the feeding behavior and algae control efficiency of hybrid tilapia (Oreochromis niloticus×O.aureus) in different ages, water nutritional level and light, provide theoretical basis in the engineering application of using fish to controlling algal blooming.(1) Put the age of 1 months(1M), 3 months(3M), 6 months(6M) and 12 months(12M) tilapia into the water of some algae concentration respectively, cultured 48 hours, collected the fish excrement when experiment ended, determined the feeding rate, digestion coefficient and protease activity of tilapia. The largest group of feeding rate is 3M, the value is (153.37±20.3)×104 cells/(g·d), the smallest group is 12M, the value is (45.299±4.6)×104 cells/(g·d). In the research age range, digestion coefficient increases as the age grows. The largest group of digestion coefficient is 12M, the value is (98.66±1)%, the smallest group is 1M, the value is (79.80±3.6)%. As the tilapia age grows, protease activity of stomach and intestine increase. The four age groups tilapia all have algae control efficiency, the algae species in water increased after tilapia was put in. 3M and 6M group algae species structure have a more diversity. The tilapia have a effect of regulating the water algae species structure.(2) Put the tilapia into the prepared water of eutrophic, mesotrophic and oligatrophic, research digestive enzyme activity, GST activity and algae control efficiency of Tilapia. The water trophic? level? index (TLI) is 66.5,45.2 and 29.2 respectively, stocking density is (2.5±0.3) kg/m3.The protease activity of stomach and intestine is respectively 47.64 unit and 77.83 unit in eutrophic water, significantly lower than those in mesotrophy and pool nutrient water(p<0.05). There is no significant differences in amylase activity of stomach and intestine in three nutritional level water(p>0.05). GST activity in eutrophic, mesotrophic and oligatrophic water is respectively 2.02, 1.5 and 1.13 unit, and there is significant differences between each other. The tilapia has a high algae control efficiency, The algae concentration of eutrophic, mesotrophy and pool nutrient water without tilapia is up to 9.49, 9.64 and 12.75 million cells/L, the algae concentration of eutrophic, mesotrophic and oligatrophic water with tilapia is down to 0.82, 0.83 and 0.81 million cells/L.(3) Put the tilapia into three different light conditions of all dark(AD), nature light(NL) and full light(FL), cultured ten days, determined feeding rate, digestion coefficient and algae control efficiency. The feeding rate in AD, NL and FL is respectively (62.37±10.6)×104 cells/(g·d), (284.1±19.4)×104 cells/(g·d) and (252.28±14.8)×104 cells/(g·d), digestion coefficient is respectively (93.27±4.3)%, (76.39±8.9)% and (61.45±5.3)%, algae concentration is respectively 0.81×106 cells/L, 1.45×106 cells/L and 1.59×106 cells/L. Put the tilapia into 100 lx,300 lx,700 lx and 1000 lx light intensity, cultured five hours. The feeding rate in 700 lx is largest, the value is (106.12±5.4)×104 cells/(g·d), the feeding rate in 100 lx is smallest, the value is (5.49±1.2)×104 cells/(g·d). The digestion coefficient in 100 lx is largest, the value is (96.59±2.3)%, the digestion coefficient in 1000 lx is smallest, the value is (84.87±6.1)%. After five hours, reduction of the algae concentration in 700 lx is the biggest, the algae concentration is 3.71×106 cells/L. The best light condition for tilapia feeding is nature light. In this research, the higher the light intensity is, the lower the tilapia digestion coefficient is.
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