Dr. Bowen pointed out that detritivorous fish are very selective feeders in
natural waters and suggested that a painstaking approach to defining the
essential dietary requirements of tilapias (as had been done for example for
salmonids with test diets deficient in individual amino acids and vitamins)
was probably not required in this context. Why not just characterize the
physical and chemical nature of their selected types of detritus? This ap-
proach would help to determine whether it is the microbial biomass or the
other components of detritus that are most important in detritivore nutrition
in different situations. It was stressed that the nutritional requirements and
feeding habits of juvenile fish are often different to those of adults. For
example, the juveniles of the microphagous tilapias, up to about 5 to 10 cm,
often feed on copepods. As they switch to microphagy, mucous glands
develop at the back of the buccal cavity and pharynx. The mucus captures
microorganisms for passage to the gut. This switch from zooplankton feeding
to microphagy has been observed in culture and in natural waters, e.g., Lake
George, and indicates a high requirement for animal protein in juvenile life.
The changes in digestive physiology associated with this change in diet have
yet to be investigated.
Prof. Roberts stated that the optimal protein levels, proteinlenergy ratlos
and essential amino acid requirements for S. aureus and S. mossambicus had
recently been determined by work at Stirling University for the formulation
of complete, pelleted diets. Information was also being gathered on the
nutritional value of locally available agricultural products in the tropics.
These studies show essentially that these tilapias need the same essential
amino acids as other fish but that their requirements for individual amino
acids are different. In some control feeding trials using high protein commer-
cial trout diets, tilapias showed a hyperproteinosis syndrome: loss of orienta-
tion, swollen bodies and death. The histopathology of this condition was
acute circulatory collapse and histamine release from mast cells. Conversely,
Dr. Lovshin reported that trout chow with 40% protein is fed routinely to
tilapias in ponds and cages with very satisfactory results and no deleterious
effects.
Tilapias can accept very high levels of carbohydrates in supplemental
feeds. For example, Dr. Coche described work with S. niloticus in the
Ivory Coast in which diets with up to 45% carbohydrate were accepted.
The use of growth promoters was discussed. It was generally agreed
that anabolic steroids could not be incorporated into feeds on a large scale
because of cost. Prof. Roberts repeated, however, that sex-reversed fry gain a
useful growth boost over non-treated fish from the steroids used: the treat-