The Biology and Culture of Tilapias

To describe food quality, protein levels are frequently expressed as mg

assimilable protein per kJ assimilable food energy. This reflects the fact that

appetite controls protein intake indirectly through its response to food

energy levels, and consequently the amount of protein assimilated depends

on the amount associated with assimilable food energy (Harper 1967;

Russell-Hunter 1970). A minimum of 4 mg protein per kJ is required for

maintenance (Figure 2). Growth increases with increasing protein levels up

to a maximum of about 25 mg/kJ. Protein at higher levels is in excess of the

animal's ability to utilize it anabolically and thus growth decreases pre-

sumably as a result of the energetic cost of protein catabolism.

Protein-energy ratios have been used to assess food quality in two studies

of the feeding of S. mossambicus. One study was conducted at Lake Sibaya,

South Africa (Bowen 1979). The Lake Sibaya population was especially

interesting because its juveniles grew well and were in good condition but the

adults were stunted and showed extensive marasmds indicative of malnutrition

(Bruton and Allanson 1974; Bruton and Boltt 1975). Although both juveniles

and adults fed on benthic detrital aggregate, they collected their food from

different parts of the lake. Juveniles fed near shore, frequently at depths less

than 30 cm, while adults fed in water 3 to 5 m deep. Analyses of samples

from the lake bottom showed that protein-energy ratios were high in shallow,

near-shore waters, but dropped rapidly with increasing depth. Analyses of

stomach contents confirmed that juvenile diets contained considerably more

protein that adult diets. When the protein/energy criterion was applied, these

results showed that juveniles have enough protein in their diets to produce

good growth, but the adult diet lacks adequate protein for maintenance and

the observed malnutrition would be expected (Figure 3). Reasons why

A = Growth

B = Maintenance

C = Malnutrition

0

2 3 4 5

Depth (m)

Figure 3. Relationship between proteinlenergy ratios and depth for benthic detrital aggre-
gate consumed by Sarotherodon mossambicus in Lake Sibaya, South Africa, showing ex-
pected nutritional significance of feeding at different depths. Curve fitted by parabolic
: regression (after Bowen 1979).