The Biology and Culture of Tilapias

MORIARTY: Going back to Dr. Chervinski's paper, one thought which occurred to me is
that the influence of temperature at the limits of distribution of tilapias, particularly
where low temperatures occur, is going to be affected by the surface area and volume
of the culture water body. Dr. Caulton's work suggests the use of a shallow pond which
will have high temperatures during the day and low temperatures at night, but extreme
low temperatures must be avoided. [Editor's note: In Transvaal, darns are stocked with
S. rnossambicus. The fishes survive winter frosts only if they can retire to a part of the
dam that is at least 2 m deep (Lombard 1959).] In summarizing the work of Drs. Caulton
and Bowen, the quantity and quality of food that tilapias eat need to be considered in
relation to environmental factors such as temperature and oxygen. The fish are all con-
sidered to be herbivorous, although one group is better termed detritivorous. The origin
of detritus is not necessarily from plants, but the role of micro-organisms is obviously
very important in the decay of materials such as grass clippings etc. The available protein
at the start of decay is not going to be very high and Dr. Bowen's work has shown that it
is the total available nitrogen that is important.
Their nitrogen, therefore, has to be in a form that is digestible by proteases, i.e., pro-
tein or polypeptides. Amino sugars and amino acids in slime layers may also be utilized,
but further work is necessary to study digestibility.
The fish have to use their digestive enzymes to digest detritus. The macrophyte feeders
have a very different task. They have thick cell walls to get through and therefore the
pharyngeal teeth of the fish are very important. We have said little about the phytoplank-
ton feeders. A lot of the fish that are used for culture feed on blue-green algae which are
the most common algae in lakes in the tropics. The work that we did in Lake George
showed that the blue-greens were a very nutritious source of food for the fish. The fish
had rapid growth rates. The actual assimilation efficiencies were between 30 and 65%:
about 45% on average. The maximum assimilation efficiency was 80%. The variability
is due to the physiology of digestion and not because there are refractory compounds
present in the food. The fish have to secrete gastric acid, and the pH has to fall to about
1.5 for effective digestion to occur. This takes five to six hours-half-way through the
daytime feeding period, which accounts for the low assimilation efficiency averaged over
a 24-hr period. Even though they assimilate only 45%, our work on the amount that the
fish ate per day and their growth rates showed that the amount that they assimilated was
more than adequate to support the growth rates obtained and their reproduction. Finally,
I would like to stress the importance of Dr. Caulton's work on dmrnal migrations be-
tween different water masses at different temperatures. Attention to this and to tem-
perature manipulation could be of direct benefit to tilapia culturists.