The Biology and Culture of Tilapias

Excretory products, predominantly in the form of ammonia, are largely
derived as a by-product of protein dearnination and as such will reflect the
effect of temperature on assimilation. Ammonia has an energy content of
20.4 kJ g-l (Elliott and Davison 1975) and thus by determining the rate of
either protein catabolism or protein assimilation, an estimate of the loss of
excretory energy can be calculated. Table 5 gives an estimate of the rate of
excretory energy loss when T. rendalli is feeding on C. demersurn.

Table 4. Linear regressions describing the effect of temperature on the calorigenic cost of

food processing. Ceratophyllum demersutn by Tilapia rendalli (Rs = cost of food pro-

cessing expressed in joules, C = the energy of C. demersum consumed expressed in joules

and S. E. 'b' is the standard error of regression slope. (After Caulton 1978b).

Temperature Regression

18OC Rs = 0.063C - 43.41 (r = 0.94, S. E. 'b' = 0.010)

22OC Rs = 0.074C -- 26.69 (r = 0.96, S. E. 'b' = 0.008)

24OC Rs = 0.082C - 16.95 (r = 0.98, S. E. 'by = 0.006)

34OC R, = 0.142C - 10.75 (r = 0.99, S. E. 'b' = 0.007)

Table 5. An estimate of the expected loss of ammoniacal energy per unit energy of food
consumed for Tilapia rendalli feeding on Ceratophyllum demersum.

Temperature
O C 34 30 2 6 22 18

Joules of NH4
excreted per joule
C. demersum consumed 0.028 0.028 0.027 0.026 0.023

The collective data already provided, in various forms of energy gains or
losses, can now be integrated to provide some information pertaining to the
growth and optimization of productivity by tilapias. For example, a simplified
model of physiological growth can be derived from the use of so-called
balanced energy equation (Davis and Warren 1971). This equation is derived
from the balance between energy inputs and energy outputs and from such
an equation one can determine, for example, growth potential as well as
some of the ways in which temperature can affect growth. The input of
energy in the form of the amount of food energy consumed (C) can be
balanced against the various energy losses through egesta (F = feces plus U =
excretory products), as heat by routine metabolism (Rr), through the cost of
food processing (Rs), by movement (Rm) as well as that energy stored as fat,