Studies On Intensive Rearing Of Fries Of Large-mouth Catfish, Silurus Soldatovi Meridionalis (Chen) In Fixed Bed Biofilter Purgative Close Recirculation System

Fries of Silurus meridionalis were propagated and reared in recirculation system. After incubation, a compact and plump biological film was well distributed in the material on the fixed bed biofilrter. There were profound nitrofication bacteria, bacillus and photosynthetic bacteria and some other microorganisms on the film. The algal community was mainly consist of chlorophytes and euglenophytes, which made the water take a clear green-yellowish look.After treated by fixed bed biofilter, the water contained profound dissolved oxygen (DO), lower NH/(NH3)-N, NO2--N and COD. Thus the toxic impacts of NH4+(NH3)-N and NO2~-N on the fertilized eggs and fries of Silurus meridionalis was diminished. Good water quality was available for fry rearing. The effects of fixed bed biofilter on Silurus meridionalis fry rearing were evaluated on the basis of the detecting of pH, DO, COD, NH4+(NH3) -N and NO2~-N, etc and observation of incubating rates and growth character of fries.1. The good water quality was achieved in experimental pools. They had steady pH values, relatively profound DO (not less than 5 mg l-1), lower NH4+-N and NO2-N, decreased COD. All these characteristics were superior to the control pool. There was significant difference between experimental pools and control pool. The effects of fixed bed biofilter on water purgation were pronounced. The average removal rates of NH4+(NH3) -N, NO2-N and COD were 31%, 20% and 27%, respectively.2. The effects on fry rearing were obviously positive too. High fertilizing rate (averaged 89.67%) and feeding rate (averaged 82.00%) were obtained. Great growth rate of fries in experimental pools was also observed. At the end of four-week experiment, the mean body length of the fries was 42.18 mm, with an average daily growth of 2.41 mm; the mean body weight reached 317.33 mg with an average daily gaining of 20.99 mg. The growth equations are the follow: L = 1.0709 + 2.8044T(r = 0.9966, n = 15), W= 0.3892L2.4797 (r = 0.9993). The relationship between body weight and body length is W = 0.0119L2.7149 (r = 0.9956).On the contrary, the fertilizing rate (averaged 74.0%) and the feeding rate (averaged 72.0%) of the control pool were obviously lower than experimental pools. At the end of four-week experiment, the mean body length of the fries of the control pool was 33.77 mm, with an average daily growth of 1.84 mm; the mean body weight reached 317.33 mg with an average daily gaining of 17.15 mg. The growth equations are as the follow: L=2. 6432 +2.14577 (r = 0.9961, n = 15), W= 0. 3025T2.4986 (R2 = 0. 9989, n = 15). The relationship between body weight and body length is W=0. 0056L3.0212 (R2=0. 9964).3. Averagely 24863 fries were obtained in experimental pools, which was 39.29% higher than the control pool (17850 fries) .4. The propagation system can provide great fry population and consume less energy and water resource, thus greater economic benefit can be achieved.