It is well known that many species of fish, especially the tilapias, will
regulate their food intake in confined captivity. Thus, it is extremely dif-
ficult to relate quantitatively the patterns of food intake shown in Figure
8 and Table 2 to wild populations. Estimates of food ingestion in some wild
populations have, however, been calculated using a most useful technique
initiated by Moriarty and Moriarty (1973a). This particular technique is suited
for use in some algal feeding tilapias, but it is a technique possibly even more
suited to macrophagous feeders, such as T. rendalli when feeding on C.
demersurn: a food with a characteristic green color. Moriarty and Moriarty
(1973a) estimated, for example, that a 200 g S. niloticus in Lake George
would consume just less than 3 g (dry mass equivalent) of algae per day
while a T. rendalli of equal proportion in Lake Kariba would consume about
3.3 g (dry mass equivalent) of Panicum repens per day (Caulton 1977b).
Similarly, a 100 g T. rendalli in Lake Kariba would consume about 2.2 g (dry
mass) of food per day, yet a laboratory maintained fish of 100 g at 28"~
(roughly equal temperatures) consumes 12 g of fresh C. demersum per day
(Table 2) or an equivalent of 1 g dry mass of food. Thus, it would appear that
laboratory satiation is equivalent to about half the daily food intake of wild,
free-living fish. A similar observation published by Moriarty and Moriarty
(1973b) shows that this function is not restricted to macrophagous tilapias,
but is also a feature of phytoplankton-feeding species.
Table 2. Linear regressions describing the amount of Cemtophyllum demersum growing
shoots ingested by young Tilapia rendalli at various temperatures when fed ad lib in
laboratory. (Feeding period = 12 hours; C is the fresh mass of food ingested and M the
mass of the Fi) (both measured in grams: after Caulton 1978b).
'18"~ = 0.0667M - 1.061 (n = 37, r = 0.975, S. E. 'b' = 0.002)
'260' = 0.1097M + 0.278 (n = 30, r = 0.974, S. E. 'b' = 0.004)'300~ = 0.1169M + 0.683 (n = 23, r = 0.970, S. E. 'b' = 0.003)'34"' = 0.1205M + 0.868 (n = 19, 5 = 0.986, S. E. 'b' a 0.005)Quantitative and accurate measurements of the assimilatory potential
of T. rendalli fed on C. demersum in the laboratory are possible (Caulton
1978a). Table 3 summarizes the results obtained for such experiments over a
temperature range 18" C to 34" C. A definite relationship between assimilation
efficiency and temperature indicates an increased efficiency of assimilation
with increasing temperature. A relative increase in efficiency of 18.6%
between 18°C and 34°C is a feature that may be expected to have some
favorable effect on the energy balance resulting in better growth. Not only is
more nutrient mass being extracted from the food at higher temperatures
but also more energy per unit mass of food is being assimilated. This feature
is reflected in the relative decline in the energy content of the feces with
increasing temperature. It is also noted that the inorganic mineral content of