was fed at 4.8% of the fish biomass/day on the freshwater algae Hydrodictyon
sp. (Chlorococcales) and Cladophora sp. (Ulotrichales) (Philippart et al.
1979). The average production was about 1 kg/m3 /month and the algae gave an
FCR on a dry weight basis of 4.5 which is relatively good (Table 7). This
points out the potential of a vegetable protein diet for caged tilapia, in this
case 28% wet weight protein. The results are particularly good as T. guineensis
usually performs less well under culture conditions than S. niloticus and
S. aureus.
The comparative values of three aquatic macrophytes for growing adult S.
niloticus in cages have been determined in Indonesia (Rifai 1979, 1980):
Hydrilla verticillata, a submerged perennial growing to as much as 3 m long;
Lemna minor, which is characterized by small, free-floating thalli and Chara
sp., a coarse plant, usually coated with precipitated calcium carbonate.
Lemna minor was preferred by the fish and gave the best growth, although
with the lowest FCR (33). Table 7 summarizes the results. These were
probably adversely affected by the very small mesh size used (reduced water
exchange) and the shallow water depth in the cages.
Cage culture of T. rendalli has been proposed for rural areas in Colombia
using tropical terrestrial plants as supplemental feed. The required character-
istics of such plants are a high protein content in the leaves, edible tubers,
vegetative reproduction and good growth even in poor soils. McLarney
(1978) citing in part Prof. A.R. Patino's observations, suggests the following:
- Manihot esculenta (Euphorbiaceae) commonly known as cassava which
has edible tubers and leaves with 17.2% dry weight of protein; 2) Alocasia
macrorhiza (Araceae) which has edible tubers and leaves with 23.2% dry
weight and 6.25% wet weight of protein and 3) Colocasia spp. (Araceae)
commonly known as taro which has edible tubers, large leaves and grows
well on pond dykes.
Cnidosculus chayamansa, which has leaves with 24.2% dry weight of
protein and Xanthophyllum spp. with edible tubers have also been recom-
mended.
For tilapia culture, research seems to have concentrated on Alocasia
macrorhiza. Data from three sets of experiments in Colombia using T.
rendalli ('T. melanopleura') are summarized in Table 7 (Popma 1978). In one
of these when 10 g fish were stocked there was no growth during the first
month and for the next 14 weeks the growth was very slow. As a result, it
took 7.2 months of feeding to produce 80 to 130 g fish. Ten grams is there-
fore obviously too small a size for stocking.
The food value of Alocasia leaves is better shown by the results for
larger individuals (25 to 40 g) with an initial biomass (Bi) not exceeding
3 kg/m3. For these the average individual growth remains relatively good
at about 1 g/d and fishes weighing around 150 g can be produced in 4 to 5
months. When the Bi exceeds 4 kg/m3 the individual growth rate decreases
but the production increases. The best production (3.5 kg/m3/mo) was
obtained with a Bi of 6.6 kg/m3.
The leaves of Alocasia macrorhiza and ipil-ipil (Leucaena leucocephala,
Leguminoseae) (24.5% dry weight of protein) have also been used in com-
bination with other ingredients, e.g., wheat bran or rice bran, as a source of
relatively cheap vegetable protein for caged tilapias (see below).