protein or glycogen, which together represents the energy of growth or
productivity (AB). Thus, the basic equation can be given as:, or on re-arrangement, growth potential can be derived from:
Substitution of the results given in the appropriate figures and tables into the
equation results in the derivation of a simple formula that can describe
growth in T. rendalli. For example, at lS°C a fish of M grams consuming C
joules of C. demersum per day would have a daily growth potential of:or on re-arranging the equation:Similar formulae can be derived for the other experimental temperatures
and the resultant equations obtained are listed in Table 6. (Energy of move-
ment is not included in these formulae since its omission does not affect the
following discussion).
Utilizing the equations an estimate of the food intake required for main-
tenance, i.e., when AB = 0, can be obtained. At lS°C, for example, main-
tenance energy for a 50 g T. rendalli would approximate 3.4 kJ/d (Table 6)
which, in terms of C. demersum intake, would be equivalent to 189 mg dry
mass or 2.2 g of fresh plant material. When compared to the amount of food
ingested ad lib over a 12-hour laboratory feeding period at lS°C (Table 2,
Figure 8), the level of so-called satiation is almost exactly the same as the
estimated energy required for maintenance. This similarity in the level of
food intake and maintenance energy requirement not only applies to low
temperatures but to all the temperatures investigated (Table 7). This pheno-
menon illustrates the problems associated with attempting to determine
growth rates under restricted laboratory conditions, but although field
related growth patterns cannot be simulated under restricted conditions,
some useful principles concerning growth may still be derived from laboratory
work.
T. rendalli fry, like many other tilapia fry, will grow in 80-liter aquaria to
about 30 or 4Og, when growth generally decelerates and then almost stops at
about 80 g--even under conditions of abundant and nutritious food avail-
ability. In aquaria, 10 g T. rendalli fry maintained at 18°C showed negligible
growth, but a comparable group maintained at 30°c showed a fairly good
rate of growth with the latter fry reaching 40 g in six to eight weeks.
At the beginning of this paper, it was suggested that many marginal
dwelling tilapias are, in their natural environment, seldom restricted to a