SESSION 2: PHYSIOLOGYChairman's OverviewTilapias are mainly lacustrine fish and are well adapted to eqclosed waters.
They produce high yields and thus are an important source of protein
in many tropical countries. But what are the biological attributes which
suit them for this role? What is known of the biology of tilapias that would
improve cultural practices and what gaps are there in our knowledge which
limit successful culture? The ability of tilapias to utilize blue-green algae has
been cited as a major reason for their high yields in shallow tropical lakes,
where blue-green algae are often abundant. I think that other factors are also
involved, but food is obviously important.
Dr. Bowen examines the nature and quality of the food of tilapias and
their digestive physiology. All large juveniles and adults feed on plant material
or detritus. (Zooplankton feeding has been described for S. aureus in Lake
Kinneret; Spataru and Zorn 1978-Editors.) Some species feed on macro-
phytes, but many feed on phytoplankton oi detritus. Most of the important,
cultured tilapias feed on bacteria, especially the blue-green algae (or cyano-
bacteria as they are now commonly called). These microorganisms are lysed
by stomach acid and subsequently digested in the long intestine. ~ila~is
which feed on macrophytes make use of their especially adapted pharyngeal
teeth to break open cells, and then digest them by similar methods.
Tilapias are generally slow moving, and do not need as much energy for
movement as predatory fish. For rapid synthesis of body protein they
require a relatively high protein content in their diet. They would get this
from blue-green algae or bacteria, which have C:N ratios from about 4:l to
10:l.r/CTilapia fry feed on zooplankton and zoobenthos as well as algae and
bacteria, which they collect around shallow lake margins. It is not known
whether they can utilize animal protein more efficiently at this stage. As
they grow larger, they move into deeper water and their preferred diet
changes to phytoplankton (or macrophytes or detritus in some species).
Dr. Bowen's work with Sarotherodon mossambicus shows that the fish
select food with a high nitrogen content. The requirement for nitrogen
and its supply in the diet is one aspect of feeding biology which needs to
be investigated in more detail. Tilapias feed at or near the base of the food
web in natural systems. This fact could be exploited in tilapia culture and it
may be considered economically wastell to supply manufactured food to
tilapias. Many of the phytoplankton-feeding species may not be able to
utilize readily other forms of food.
In studying feeding, digestion and growth, temperature is an important
factor to consider. Dr. Caulton has shown how Tilapia rendalli utilizes
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