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.
Bibliographical noteFunding Information:
This work has been supported by grant 03.509 from The Research and Development Co. Ltd, Bar-Ilan University, and by grant AQ 24 from the GKSS, Gessthecht-Tesperhude, FRG. We are sincerely grateful to Dr Harald Rosenthal, of the Biologische Anstalt Helgoland, Hamburg-Bahrenfeld, FRG, for his advice and help in editing this manuscript.