Feeding Trials with Duckweeds:

1. Fish.

Duckweeds can be grown separately and then provided to the fish, or produced in the same pond. Production of Lemna in the same pond is not likely to work efficiently, however. Vigorous aeration of the water, as is practiced in catfish-culture, will disturb the growth of the plant. The photosynthetic activities of the plant do not oxygenate the water, in fact the covering plants reduce gas-exchange with the atmosphere (Landolt and Kandeler, 1987, p 387).

Grass carp seem particularly adapted to feeding on Lemna, and there is a large literature devoted to this application (summarized by Landolt and Kandeler, 1987, pp. 387-388.). As reviewed by Landolt and Kandeler, channel catfish have been successfully raised on duckweeds, but no commercial application of these findings seems to have been developed.

The growth of hybrid carp were studied by Cassani and Caton (1983) to determine feeding preference and feed consumption. The hybrid was grass carp, Ctenopharyngodon idella (Val.) X bighead carp, Hypophthalmichthys (Aristichthys) nobilis Rich, 12 to 18 months old. Their conclusion was, "The most preferred plant was Lemna gibba when in combination with six other species." Moreover, at the same order of preference was exhibited at two different growth temperatures (12-15 C vs. 25-28 C). This preference was the same, based either on the time to complete consumption or the relative quantity consumed. Mean daily consumption (g) fish at 25-31 C for Lemna gibba tested separately was 178.

Gaigher, et al. (1984) compared the growth of hybrid tilapia fish (Oreochromis niloticus X O. aureus) on commercial pellets vs. duckweed. The fish were cultured at high densities in an experimental recirculating unit for 89 days with duckweed (Lemna gibba) or a combination of duckweed and commercial pellets. They conclude that a combination of pellets and Lemna gave the best performance:

When fed on duckweed alone, intake rate was low, feed conversion ratio good (1:1) and relative growth rate poor (0.67% of bodyweight daily). Sixty-five percent of the duckweed consumed was assimilated and 26% converted to fish. 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. Seventy percent 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.

Porath, et al. (1985) attempted to recycle the solid wastes of these fish as a fertilizer for Lemna. The duckweed (Lemna gibba) was grown in shallow ponds containing mineral nutrients. However, the tilapia waste was poor in free NH3 and ammonium compounds. When separated and incubated at 38 C to allow anaerobic digestion to release mineral nutrients, prolonged digestion was necessary before it supported growth of the plant.

Tilapia were given duckweed as food as young fish from the larval to the fingerling stages (Moreau, et al., 1986). A comparison was made of three types of food: Lemna minor (duckweed, produced in shallow ponds or year-round in greenhouses), Chlorella (phytoplankton) and Daphnia (zooplankton). These authors studied both the nutritional value of these foods and the resulting growth rates of the fish.

Crayfish are often released in irrigated rice fields in rice- growth areas of the United States to control weeds (often duckweeds), according to Landolt and Kandeler. It is not known if the deliberate growth of Lemnaceae would be an efficient means of production of these crustaceans.