There have been numerous aquaria or field trials on the benefits of azolla for fish culture. The literature up to 2008 has been reviewed by Hasan et al., 2009.
Preference studies carried out in aquaria indicate that cichlids (Oreochromis, Tilapia and Cichlasoma) as well as a grass carp x bighead carp hybrid tend to prefer Azolla caroliniana (Antoine et al., 1986; Lahser, 1967; Fiogbé et al., 2004; Micha et al., 1988).
Growth studies have been carried out under laboratory rearing conditions with fresh or dried azolla.
Most of azolla experiments on cichlids have been negative. Fish died or negative growth was recorded when fed exclusively with fresh azolla (Hasan et al., 2009). In Nile tilapia fingerlings (Oreochromis niloticus), feeding azolla fresh, powdered or pelleted, replacing 10 to 90% of the control diet was detrimental to all performance parameters. Adult tilapia fed azolla ad libitum, fresh or pelleted, suffered weight loss in a 30-day feeding trial (Almazan et al., 1986). Poor growth and feed utilization were also reported with Oreochromis niloticus, Cichlasoma melanurum and Tilapia rendalli fed Azolla microphylla-based diets (Antoine et al., 1986; Micha et al., 1988). Nile tilapia fingerlings and adults fed Azolla pinnata as a fish meal replacer at substitution levels of 25-100% performed extremely poorly even at the lowest level (El-Sayed, 1992). In contrast, between 30 and 42% of fish meal-based diets fed to Nile tilapia have been successfully replaced by azolla meal without negatively affecting fish performance (Santiago et al., 1988; Naegel, 1997). In Nile tilapia fed up to 50% dried Azolla filiculoides in concrete tanks or in earthen ponds, fish grown in tanks performed less well than fish grown in ponds, due to the presence of natural feeds in the ponds. The fish fed in ponds also had a more valuable fatty acid profile (lower n-3:n-6 ratio) (Abou et al., 2013). The poor growth of cichlid fish fed diets containing higher levels of azolla may be due to excesses or deficiencies of amino acids (Fiogbé et al., 2004).
In a comparison between alfalfa, duckweed (Lemna sp.) and Azolla filiculoides fed fresh to grass carp (Ctenopharyngodon idella) at 20% BW (plus a formulated diet), azolla gave the lowest growth rates (Nekoubin et al., 2013). In rohu (Labeo rohita) fed a dried mixture of Azolla microphylla and Azolla pinnata incorporated in the diet at 15%, 25% and 35%, fish fed the 25% azolla diet had the highest growth rate and resulted in a lower fat content in the fish (Datta et al., 2011).
Azolla has been increasingly used as feed and/or fertilizer in studies with rice-fish culture systems in many Asian countries. One of the most successful uses of azolla is its use as fertilizer and/or feed in an integrated rice-fish-Azolla system, which allows the simultaneous development of rice, azolla and different fish species (planktophages, macrophytophages and polyphages). Each of the partners contributes to the equilibrium of the system. The fish derive a benefit from azolla, depending on the species. Fish waste promotes the proliferation of plankton that is consumed by some of the fish and fertilizes the rice. The polyphagous fish protect rice and azolla from a number of insects and molluscan pests. It is necessary to maintain equilibrium between the population of fish and that of azolla, either by introducing a supplementary biomass of azolla collected elsewhere, or by harvesting the excess biomass (Van Hove, 1989; Hasan et al., 2009). Several comparisons between rice-fish-azolla system and rice-fish cultivation alone have shown the potential benefits in terms of fish yields (Indian carp, tilapia) of such integrated systems in India and in the Philippines (Cagauan et al., 1991; Hasan et al., 2009).
In India, finely chopped Azolla caroliniana placed in a feeding basket under pond conditions was found to be a preferred feed for grass carp fingerlings (Ctenopharyngodon idella), with the final weight gain of azolla-fed fish being significantly higher than those on the control diet (Majhi et al., 2006). In a semi-intensive carp polyculture system, diets containing 10 to 40% dried azolla had no significant affect on the water quality, except for the nitrate-nitrogen content. All fish species recorded significantly higher growth with diets containing up to 20% azolla (Dhawan et al., 2010). In a duck-fish-azolla integrated system in the Philippines, Nile tilapia, stocked in ponds fertilized with a mixture of fresh pig and duck manure, could eat all the azolla every 6 or 7 days (Gavina, 1994). Weight gain comparisons of azolla-fed fish were carried out in China using grass carp, Nile tilapia, crucian carp (Carassius carassius) and silver carp (Hypophthalmichthys molitrix). Grass carp, Nile tilapia and crucian carp gained weight but azolla was detrimental to the growth of silver carp (Cagauan et al., 1991).