Digestibility and transit time of Azolla filiculoides Lamarck in Oreochromis aureus (Steindachner)

Azolla, an aquatic fern that is able to fix nitrogen directly from the atmosphere, could represent an interesting low-cost food for tropical fish. In the present study, the authors evaluated the apparent digestibility of Azolla filiculoides Lamarck by Oreochromis aureus (Steindachner), an omnivorous-phytoplankton-ophagous species of tilapia. Two digestibility tests (5 days) were performed in digestibility tanks on adult O. aureus fed only with dried A. filiculoides given ad libitum (2% biomass day^?1). Experiment 1 was made on fish weighing 28-49 g (duplicate) at a water temperature of 29 ± 0.5°C; experiment 2 was made on fish weighing 36-81 g (duplicate) at a water temperature of 27 ± 0.5°C. The transit time in O. aureus (weight: 30-50 g; water temperature: 27 ± 0.5°C) fed only and ad libitum with A. filiculoides was determined with methylene blue as a vital dye. The apparent faecal digestibility coefficients (%) obtained for dry matter and crude protein were 49.0 and 55.4%, respectively, in experiment 1, and 53.5 and 60.9% in experiment 2. The transit time values of 2.5-3.0 h and 3.0-3.5 h were obtained, respectively, for fresh and dried A. filiculoides. Thus, A. filiculoides shows relatively good digestibility values in O. aureus, which are comparable to those obtained with other aquatic plants in tropical fish. However, in order to meet the nutrient requirements of tilapias, Azolla has to be mixed with other foods.