The Biology and Culture of Tilapias

of T. rendalli and S. mossambicus is compared. Unlike S. mossambicus, T.
rendalli shows an unexpected three-phase energy demand in which the
distinctive plateau phase, extending from approximately 28°C to 37"C, is an
unusual characteristic. Over this temperature range the metabolic demand of
S. mossambicus increases by 62% whereas the metabolic energy demand by
T. rendalli over the same temperature range increases by only 22%. Similarly
structured, unusual metabolic curves have been reported for other fish
species (Schmein-Engberding 1953; Sullivan 1954; Job 1969a, 1969b and
Fry 1971 for S. mossarnbicus). Denzer (1968) reports a similar function in
S. nilotjcus. Exactly how important, or in fact how real, such thermal
homeostasis is, is difficult to determine and respirometry technique (static
versus flowing systems) must be considered when appraising results. Obviously
any function that can appreciably depress an expected rise in energy demand
must also benefit the overall energy balance in consequence.
The metabolic energy required to sustain routine maintenance by a 50 g
T. rendalli at, for example, 28°C is approximately 175 J/hr (Figure 3) but to
appreciate this requirement in terms of the utilization of storage tissue, it
may be pertinent to consider which catabolic fuels are responsible for the
supply of energy. Glycogen or the major carbohydrate fraction is an energy
source usually stored in the liver but, in T. rendalli at least, is not of great
significance as a sustained catabolic energy source. A 50 g T. rendalli in good
condition has a maximum glycogen content of 0.46% by mass (about 230
mg) and of this amount 144 mg are stored in the liver while the remainder is
present in the muscle. On starvation approximately 100 mg of glycogen is
utilized within the first few days, thereafter the level remains relatively
constant even to the state of near death from starvation. This 100 mg could
supply sufficient energy to maintain a 50 g fish at 28°C for less than 12
hours and thus cannot be classified as of great importance as a storage fuel to
a starving fish, which is thus reliant mainly on lipids and protein as a source
of catabolic energy. The importance of condition is immediately evident and
a close relationship does exist between condition and the type of catabolic
fuel mobilized during routine maintenance. The mobilization of lipids is
always associated with some protein mobilization (and vice versa) and
follows a pattern closely linked with condition.
Condition in the present context is described as a morphometric inter-
pretation of a fish's plumpness as compared to the population mean. As
such, condition is an extremely useful measure of assessing the physiological
state of a fish and more cognizance of this measure should be taken by the
fish farmer. The classical measurement of relative condition as described by
Le Cren (1951) using the formula Kn = 100 W/L~ where Kn is the relative
condition, W the fresh mass in grams, L the standard length in cm and 'b' the
length/mass regression exponent, provides an adequate quantitative assess-
ment of condition if the basic measurements are made carefully. In T.
rendalli close relationships do exist between condition and the proportionate
mobilization of fatlprotein during metabolism but extreme care must be
exercised when interpreting these relationships (see Caulton and Bursell,
1977, for a discussion of these problems, especially the use of percentages
when measuring or interpreting results).