1978), while others have utilized refractile indigenous components such as
cellulose for macrophyte grazers (Buddington 1979; Buddington 1980)
and ash or HRA for detritivores (Bowen 1979, 1980b, 1981). AE is cal-
culated by comparison of food:reference, and fecesreference ratios using
the formula developed by Conover (1966). The advantage of the refractile
reference technique is that it may be applied to analysis of tilapias taken
directly from their feeding habitat. Since it appears there is no assimila-
tion through the stomach, samples of stomach contents and posterior
intestinal contents from freshly caught specimens may be compared for the
purpose of estimating AE. This means determinations may be made for fish
feeding under totally natural conditions and all handling effects are eliminated.
The few data available for assimilation efficiency of tilapia feeding on
natural die are summarized in Table 4. Two important generalizations can
be drawn. Firstly, 2. AE values are high for blue-green algae and diatoms, but
are low for green algae and macrophytes. This is not surprising since the
gastric acid that lyses blue-green cell walls and denatures diatom cell mem-
branes would not be expected to haydirect effect on the cellulose cell
walls of green algae and higher plants. Secondly, wholediet AE estimates for
these four tilapias are low in comparison to the average of 85% for a variety
of carnivorous species (Winberg 1956). Again, this is not surprising when we
consider that macrophytes and detritus contain abundant refractile com-
pounds including cellulose and lignin that are not vulnerable to vertebrate
digestive enzymes. A similarly low AE is reported for Ctenopharyngodon
idella, a cyprinid macrophyte grazer (Hickling 1966). The low total AE for
S. niloticus fed suspended matter > 100 pm filtered from Lake George
appears to be the result of the long time interval between the start of feeding
and the point at which digestion reaches peak efficiency in this species
(Moriarty 1973).
Another interesting aspect of the data in Table 4 is that protein and lipid
are assimilated much more efficiently than total organic matter or food
energy. This same result was reported for tilapia fed pelleted algae that had
been dried and ground to a powder such that all natural impediments to
enzyme-substrate interaction had been removed (Kirilenko et al. 1975).
Thus, the efficiency with which macromolecular groups are digested is
related not to selective disruption of cell walls or membranes, but rather
to basic characteristics of the digestive process itself.
Food Quality and GrowthThe food quality of a given diet is directly proportional to its ability to
support growth. The dietary component most important in limiting the
growth of herbivorous and detritivorous fishes is protein. There are 10 to 13
amino acids that are essential for building new tissues (Cowey and Sargent
1972). These amino acids cannot be synthesized by fish and must be obtained
from the diet. Carnivorous fish consume prey that may be > 80% protein
by dry weight, but the diets of tilapias range from about 50% to < 1%
protein. Values below 15% are most common (Boyd and Goodyear 1971;
Caulton 1978a; Bowen 1979,1980aY 1980b).