The Preference of the Omnivorous–Macrophagous, Tilapia zillii (Gervais), to Consume a Natural Free-floating Fern, Azolla pinnata MOHSEN ABDEL-TAWWAB 1 Fish Ecology Department, Central Laboratory for Aquaculture Research, Abbassa, Abo-Hammad, Sharqia, Egypt 44662 Abstract Free-floating macrophytes are often common in standing water bodies. Fishponds are suitable medium for Azolla growth; however, it rapidly self-propagates and doubles its biomass in a few days and could strongly influence nutrient dynamics. The preference of Tilapia zillii (Gervais) to consume the free-floating plant, Azolla pinnata, was tested in two experiments. In the first experiment, fish with different initial body weights (2–3 g, 7–10 g, 15–20 g, and 50–60 g) were randomly distributed at a rate of 150 6 5 g/100-L aquarium and fed fresh A. pinnata for 8 wk. A known weight of fresh plant was given to fish; after 24 h, Azolla remaining in aquaria was collected, dried with tissue paper, and reweighed to determine amount consumed. In the second experiment, fish were fed one of five diets containing different levels of dry Azolla meal as replacements of the control diet (20% crude protein) at levels of 0 (control), 25, 50, 75, or 100% for 13 wk. Growth retardation and weight loss were observed in all fish sizes fed fresh Azolla, and the mortality rates were 53.3–56.7%. The consumption of fresh Azolla decreased over time up to the eighth week; otherwise, Azolla consumption was fish size dependent. Whole-body contents of crude protein decreased significantly, while total lipids and ash contents increased significantly with increasing fish size. In the second experiment, fish growth was reduced when the level of dry Azolla meal in the diet exceeds 25%. The mortality rate increased significantly with increasing Azolla level in the diet. Carcass composition showed increases in moisture and ash contents and decreases in dry matter, crude protein, and total lipids with the increase of Azolla level in the diets. Amount of diet consumed decreased, while feed conversion ratio increased significantly with the increase in dry Azolla level in the diet. Moreover, protein efficiency ratio, protein productive value, and energy utilization decreased significantly with the increase in dry Azolla level in the diet. It could be concluded that the preference of T. zillii to consume Azolla plants inhabiting water bodies is limited, but it could partially consume the dry Azolla meal at low and moderate levels. The optimum inclusion level of dry Azolla meal should be no more than 25% in T. zillii diet. Azolla, an aquatic free-floating fern, is widely distributed throughout the tropical and subtrop- ical zones. It rapidly propagates and may double its biomass in 3–10 d, reaching a standing crop of 8–10 tons/ha in rice fields (Pullin and Almazan 1983). It fixes atmospheric nitrogen through Azolla–Anabaena symbiosis, which may be used to replace part or all the inorganic nitrog- enous fertilizer required for rice production (Lumpkin and Plucknett 1980, 1982; Lumpkin 1987; Kannaiyan 1993). The culture of rice, Azolla, and fish together in rice fields is one of the integrated agriculture system used to maximize the use of land resour- ces (Van Hove 1989). Moreover, Azolla plants may be transferred to other water bodies, such as fishponds. The occurrence of Azolla plants in fishponds (over 25% of pond’s surface) is consid- ered as a weed problem, causing a drastic change in water characteristics and retarding fish produc- tion (Abdel-Tawwab 2006). On the other hand, Azolla plants have been used as a fodder for swine, ducks, and chickens in Vietnam and China (Lumpkin and Plucknett 1982). Moreover, Azolla has been used as a diet for tilapia (Angeles 1984; Antoine et al. 1986, 1987; Pantastico et al. 1986; Micha et al. 1988; Santiago et al. 1988; El-Sayed 1992; Fiogbe ´ et al. 2004; Abou et al. 2007), African catfish, Clarias gariepinus (Fasakin and Balogun 1998), and common carp, Cyprinus car- pio (Ahmad 2003). 1 Corresponding author. JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 39, No. 1 February, 2008 Ó Copyright by the World Aquaculture Society 2008 104