Use of duckweeds as fish feed is by far the most widespread application (Iqbal, 1999). Because of its attractive nutritional qualities and the relative ease of production, a significant number of studies have been carried on the potential utilization of duckweed biomass as fish feed and an extensive review of the literature up to 2009 has been produced by FAO (see Hasan et al., 2009). Duckweeds can be grown separately and then provided to the fish, or produced in the same pond. Several systems of duckweed-fish polyculture systems have been implemented, notably in Asia (Azim et al., 2003; Hasan et al., 2009).
Duckweed can be fed fresh as the only feed, or in combination with other feed components in a polyculture of Chinese and Indian carp species with tilapia. Herbivorous and omnivorous fish such as grass carp (Ctenopharyngodon idella), silver barb (Puntius gonionotus) and tilapia (Oreochromis sp.) readily feed on duckweed (Iqbal, 1999). Catla (Catla catla) and common carp (Cyprinus carpio) compete aggressively for available duckweed feed and consume it directly (Ansal et al., 2010). The preference of duckweed over other aquatic plants has been reported for grass carp and other fish species (Hasan et al., 2009).
Growth responses of different fish species fed fresh duckweed have been variable. The general trend was that carp perform better than Nile tilapia and other species and that performances obtained with duckweed as the sole feed were better than with the control diets (Hasan et al., 2009). Reviews of feeding trials have shown that duckweed included in dry diets at 13.5-40% can support growth in herbivorous or omnivorous fish, such as carp and tilapia, as well as in protein-demanding carnivorous fish, such as catfish and snakeheads (Ansal et al., 2010; Hasan et al., 2009).
Feeding trials with carp receiving duckweed have been carried out since the early 1960s with generally very positive results (Hasan et al., 2009). Grass carp seem particularly adapted to feeding on Lemna (Landolt et al., 1987). The weight of grass carp could be tripled (from 100 g to 300 g) within 50 days when feeding a mixture of Lemna gibba and Lemna minor (Porath et al., 1977). The growth of hybrid carp (Ctenopharyngodon idella (Val.) X Hypophthalmichthys nobilis) was determined by feeding preference and feed consumption. The most preferred species was Lemna gibba when compared with six other species (Cassani et al., 1983).
Dry duckweed replaced up to 50% of the commercial tilapia feed without adverse effects on fish performance (Essa, 1997; Tavares et al., 2008). In Nile tilapia fingerlings (Oreochromis niloticus) fed diets including 0 to 100% solar kiln-dried duckweed (Spirodela polyrhiza), duckweed inclusion resulted in progressively reduced growth performance and nutrient utilization, but 30% inclusion rate was found to be cost-effective (Fasakin et al., 1999). In hybrid tilapia (Oreochromis niloticus x Oreochromis aureus) grown with duckweed (Lemna gibba) or a combination of duckweed and commercial pellets, this combination gave the best performance. When fed on duckweed alone, intake was low, feed conversion ratio good (1:1) and growth rate poor (0.67% of BW daily). 65% of the duckweed consumed was assimilated and 26% converted to tissue. When the fish were fed on pellets in addition to duckweed the rate of duckweed consumption decreased and growth rate of the fish doubled with feed conversion ratios between 1.2 and 1.8. 70% of the mixed diet was assimilated but only 21% converted. Fish grown on the mixed diet performed similarly to fish grown on pellets but had a better feed conversion ratio (Gaigher et al., 1984). In Nile tilapia fed fresh duckweed, Lemna perpusilla, optimal daily feeding rates of Lemna were 5, 4 and 3% of the total fish body weight on a duckweed-dry-weight basis for fish of 25 to 44 g, 45 to 74 g and 75 to 105 g in weight, respectively (Hassan et al., 1992). Tilapia fed diets with 20% to 40% duckweed contained significantly more ash, phosphorus and protein and significantly less lipid and dry matter than tilapia fed the control diet without duckweed (El-Shafai et al., 2004).
Channel catfish (Ictalurus punctatus)
Channel catfish (Ictalurus punctatus) fed a diet containing 20% dry duckweed had similar weight gains and feed conversion efficiency as catfish fed a standard feed (Robinette et al., 1980).
African catfish (Heterobranchus longifilis)
In African catfish (Heterobranchus longifilis) diets, 10% of the fish meal was replaced with dried duckweed (Lemna paucicostata) without affecting growth performance (Effiong et al., 2009b).
Striped catfish (Pangasianodon hypophthalmus)
In Vietnam, with striped catfish fingerlings (Pangasianodon hypophthalmus) the nutrient and energy digestibility of duckweed (Lemna polyrhiza) was lower than for a wide range of other plant ingredients, such as soybean meal, broken rice, maize grain, cassava leaves and sweet potato leaves, which may be partly explained by the high mineral content of the duckweed (22% DM). This low digestibility may limit the possibility of using duckweed as replacement for fish meal, despite the high digestibility of protein and essential amino acids (Da et al., 2013).
Other fish species
Snakehead (Channa striatus)
With snakehead (Channa striatus), duckweed (Lemna minor) included at 50% of the supplementary diet resulted in higher specific growth rates and weight gains as well as lowering the feed cost (Raj et al., 2001).
Milkfish (Chanos chanos)
When duckweed (Lemna spp.) grown on sugar mill waste was harvested and transferred to a milkfish (Chanos chanos) pond, it acted as a fertilizer on the pond and resulted in higher fish production (820 kg/ha in 90 days in duckweed-fertilized ponds vs. 320 kg/ha in inorganically fertilized ponds) (Ogburn et al., 1994).
Jade perch (Scortum barcoo)
In Australia, jade perch (Scortum barcoo) actively consumed and gained weight (with 100% survival) on fresh duckweed alone harvested from an effluent treatment plant (Willett et al., 2003).