The Biology and Culture of Tilapias

temperature variations in its lake environment to maximize growth. By
constructing careful energy budgets, he has demonstrated that faster growth
rates were achieved when fish moved into warm inshore areas during the day
(where feeding and digestion rates were faster) and then retreated to cool,
deeper areas at night where energy demands were less, than if the fish
remained at a constant temperature. The conclusion is that yields will be
much higher in shallow waters where die1 temperature changes are more pro-
nounced. This temperature effect helps explain some of the distribution of
tilapias in lakes, but it is probably not the only reason why the fish in Lake
George, Uganda, are concentrated around the edge. This shallow lake stratifies
thermally each day, so the fish could obtain more than a 10°C variation in
temperature by varying their position in the water column. Other environ-
mental factors may therefore be involved.
Temperature also limits the distributon of tilapias. Their inability to
withstand temperatures much below 16OC confines them to tropical or warm
temperate regions. For culturing tilapias near the limits of their range,
an optimum water body size and depth is needed, which ideally allows the
diurnal temperature variation that Caulton has shown and yet which does
not cool down too much in winter. Many tilapias are euryhaline, which
increases the potential areas of water available for their culture. Oxygen
requirements are also important, but we need more data relating oxygen
levels and temperature to the energetics of metabolism and the switch from
growth to reproduction. Wk ere there are dense blooms of algae, oxygen may
be depleted at night or below the photic zone, to the point where respiration
of the fish is affected. Perhaps an effect of this nature might be part of
the explanation for preference of tilapias in Lake George for the lake edges,
where phytoplankton densities are much lower than in the middle of the
lake. The combination of high temperature and low oxygen levels would be
stressful and, as Caulton has shown with Tilapia rendalli, there is likely to be
a fine balance between energy supply in the diet and energy losses for
maintenance. If metabolic losses are too high, or low oxygen availability
decreases assimilation efficiency and limits energy supply, then growth
and reproduction will be affected. There is evidence that growth is rapid
and the onset of sexual maturity is delayed in highly oxygenated water.
In addition to food, the high reproductive potential of tilapia is an impor-
tant factor governing their success in tropical lakes. This also leads to prob-
lems, however, because stunting may occur, particularly in overcrowded
conditions, with attainment of sexual maturity at an early age. Like other
fish, gametogenesis in tilapias is regulated by the influence of external and
internal events on the nervous system and by a complex interaction of
hormones from the hypothalamus, pituitary and the gonads. We need to
know a lot more about the biochemistry of the hormones and the physiology
of reproduction before practical benefits can flow to culturists. An example
of the influence of social factors on breeding may be seen in Lake George,
Uganda. As fishing pressure increased, the minimum size at which females
reached sexual maturity decreased from 28 cm to 20 cm. As breeding sites
are limited (because most of the substrate is too soft and flocculent for