The Biology and Culture of Tilapias

inshore movement of juvenile fish continues throughout the morning and
eventually large schools of juveniles are found in shallow warm water at mid-
day. A reverse movement is noted in the late afternoon when the fish, again
responding to temperature, leave the now cooling margins to return to the
relatively warmer, homothermal deeper waters.
Such diel movements are commonly encountered in shallow gradient
shoreline areas, while fish habitually favoring deeper open waters may
occasionally show a less spectacular but nevertheless visible vertical move-
ment for a similar reason. These patterns of diel movement can however
be easily disrupted; for example, during periods of even moderate winds
the wave wash along the shore is sufficient to restrict movement into very
shallow water. Similarly, during exceptionally warm periods, or in areas
where excessive shoreline vegetation assists in insulating the warm daytime
water in the shallow margins, a nocturnal presence of juveniles is not uncom-
mon. When in areas of food paucity, fish often have to remain in the shallows
at night in order to feed. The presence of predators in deep water is also a
strong factor which will seriously disrupt any set daily pattern of movement
although avian predation along the margins during the day does not seem to
influence the movements very strongly.
Notwithstanding the possibility of such numerous disruptions, many
young tilapias are often subjected to a daily thermal variance of between
10°C and 15°C and, since fish are poikilothermic, such thermal variations
will understandably have an important bearing on almost all of the animals'
physiological functions. Of these functions, metabolism would be expected
to be most markedly affected. In broad theoretical terms, metabolic energy
demands are expected to approximately double with every 10°C rise in body
temperature. Thus a tilapia moving into the warm shallows during the day
must expend considerably more energy than it would during the night in the
relatively cooler deeper water. It may be expected that as a consequence of
this fluctuating energy demand, fish living under these conditions of thermal
fluctuation would show unnecessarily high expenditure of energy during the
day, which could otherwise have contributed to either storage or growth.
To investigate this, it is necessary first to look in some detail at the effect
temperature has on metabolism. Metabolism, as a function of aerobic respi-
ration, can be quantitatively equated to the uptake of molecular oxygen
which in turn is a simple measure that can be obtained in the laboratory.
Many extrinsic variables can modify the basic metabolic rate or oxygen
consumption of a fish in nature, but, suppressing most of the variables,
with the exception of temperature, relationships demonstrating the influence
that temperature has on metabolism can be investigated. Obviously, a labora-
tory test animal out of its natural environment is subjected to a number of
unnatural stresses which are capable of either increasing or suppressing
normal oxygen consumption but, in general, laboratory acclimated juvenile
tilapias are reasonably suited to respirometry and show few visible signs of
the stress so often encountered in the apparently more nervous cyprinids and
other fish.
Using a simple continuous flow, continuous recording respirometer as
described by Caulton (1975a), a satisfactory measure of routine metabolism
in both T. rendalli and S. mossambicus has been obtained. Routine metabolism