A laboratory scale recycling water unit for tilapia breeding

The technical features of a laboratory scale water recycling unit for experimental small scale tilapia breeding are described. Two units (1 and 2) were operated during a 6 month period, carrying a similar fish load (7·5 kg) and feeding rate (2% fish body weight/day). Unit 1 received natural illumination, while unit 2 was artificially illuminated (14/10 - light/dark cycle). Both units were equipped with a biological filter bed (substrate surface area, 3500 cm²). In unit 1, total ammonium and nitrite concentrations ranged from 0·05 to 0·5 mg liter^?1, while nitrate varied between 10–40 mg liter^?1. In unit 2 corresponding values were 0·15-3 mg liter^?1, 0·05–0·8 mg liter^?1 and 10–40 mg liter^?1. Temperatures ranged between 20–29°C and pH values between 7·5–6·9 in both units. Dissolved oxygen concentrations decreased gradually from 5·6 to 3·4 mg liter^?1 in unit 1 and from 5·6 to 2·6 mg liter^?1 in unit 2. Twenty-six spawnings occurred in unit 1 in March and April, while only eight spawnings occurred in unit 2, possibly because of the absence of sunlight. The significance of these results are discussed.