The Biology and Culture of Tilapias

the feces recovered from a given meal of C. demersum varies from 4% to 15%
less than that consumed, but this removal of minerals snows no relationship
to, or definite pattern with, either assimilatory efficiencies, meal size or
temperature. In the results presented in this paper cognizance is taken of this
fact and the results given are corrected accordingly. A further feature of note
is that the relative energy content of the feces is greater (gram for gram,
mineral corrected) than the food. This is a common feature, but such differ-
ences are often misinterpreted when investigating assimilation.

Table 3. The effect of temperature on assimilation by juvenile Tilapia rendalli when fed

ad lib on C. demersum (17.96 kJ/g). (Number of determinations (n) = 28 per temperature).

• 
  
  
  
  • Mass (mg) of
    feces recovered
    per 1,000 mg
    Temperature dry C. demersum
    OC consumed
  
  
  
  

Energy (J) of

feces recovered

per kJ C.

demersum

consumed

Mean energy (J)

assimilated per Mean energy (kJ/g)

kJ C. demersum content of feces

consumed (2 x SE)

--
An assimilation efficiency ranging from 47.8% to 58.7% may be regarded
as being very good for a primary macrophagous herbivore. This efficient
utilization of food can be attributed largely to successful primary food
trituration and efficient pre-assimilatory processing of the food (Caulton
1976). The breakdown of resilient cell wall structures is the key factor and
as such, assimilatory capabilities will vary considerably,depending on the
species and composition of the plants eaten.
The mechanical trituration of the food, like digestion, absorption and
transportation of primary nutrients, also requires an energy input by the
fish. Such energy-demanding functions can be collectively termed 'apparent
specific dynamic action' (S.D.A.) or the calorigenic cost of food processing.
The additional collective energy required for these processes can be measured
by feeding fish in a respirometer and measuring the post-feeding increase in
oxygen consumption and equating any increase in oxygen uptake to the level
of energy input through feeding. The energy costs of feeding C. demersum to
T. rendalli in a respirometer are shown in Figure 9 and Table 4.
From the data presented it is apparent that meal size and processing
costs are related in a linear fashion with greater processing costs (relative per
unit food consumed) being required to synthesize larger meals. A linear
trend of increasing processing costs with increasing meal size may be expected
and has been described for a variety of fish (mainly predatory species
Edwards et al. 1972 ;Hamacia and Ida 1973; Beamish 1974), but exponential
relationships have also been described for some species (Tandler and Beamish
1979) so there appears to be no standard function to describe this relation-
ship. A second noticeable feature of the results presented in Table 4 is that
in T. rendalli food processing costs also increase with increasing temperature