by definition can be described as the energy required by an unfed fish
exhibiting spontaneous rather than directed movement. Such a measure of
metabolism is particularly suited to tilapias, since movement even in their
natural habitat is very seldom vigorously directed but can more often be
described as spontaneous fin generated movements. Obviously, exceptions do
occur, e.g., during intense feeding activity, migrations or predator avoidance.
This suggestion is borne out to a certain extent by the fact that schools
of juvenile S. mossambicus in a large communal respirometer all displaying
normal swimming, schooling behavior and feeding movements (into sterile
sand) showed an increased metabolic demand 25% greater than that required
by the same fish at night, when they were almost stationary, or that obtained
from the summation of individual oxygen requirements in a normal single
chamber respirometer. This contrasts markedly with generally faster moving,
continuously swimming fish species, especially predators, which mzy require
at least twice as much energy during normal active periods than during
inactive periods.
The conversion for equating oxygen consumption to the amount of
metabolic energy liberated by herbivorous tilapias was calculated as 13.68
J/mg O2 (Caulton 1977a), and thus with the availability of such a conversion
factor, it is possible to obtain more quantitatively meaningful information
from respirometry. The routine metabolic energy demand by S. mossam-
bicus can be satisfactorily described by the equation:
R, = 0.0086 t 2.0783 M 0.652
where Rr is the energy of metabolism expressed in J/hr, 't' the temperature
in "C and M the fresh mass of the fish in grams (restricted to fish less than
150 g, the maximum size used in the trials). The full details of the derivation
of this formula are given in Caulton (1978~).
A similar relationship between temperature and metabolism, but for a
single size range of fish, is shown graphically in Figure 3, where the metabolism
I I I I I I I I I I I I I
16 20 24 28 32 36 40Temperature OC
Figure 3. The influence of temperature on metabolic energy demand in a) Tilapia rendalli
and b) Sarotherodon mossambicus (modified after Caulton 1977a, 1978~).