Table 1. Comparison of measured and estimated change in mass during starvation of
Tilapia rendalli at six experimental temperatures (after Caulton 1977~).Temperature "C 18 20 2 1 24 30 34
Time of starvation (d) 10 6.25 6.75 6 12 8
Mean condition
factor (kn) 2.23 2.42 2.51 2.68 2.58 2.60
Number of fish 19 19 19 8 14 18Total measured mass
lost (mg) 2020 1260 1940 3282 3641 4196
Estimated mass of
tissue used (mg) 409 268 373 607 779 802
Estimated mass of
water lost (mg) 1759 1140 1563 2398 3101 3272
Total estimated mass
lost (mg) 2168 1408 1936 3005 3880 4074Feeding and GrowthSub-adult and adult T. rendalli are essentially macrophagous plant feeders
in areas of abundant aquatic vegetation, a feature which lends this species
most favorably to quantitative feeding experiments. The ubiquitous plant
Cemtophyllum demersum heads a list of preferred food types eaten by T.
rendalli when available, and it was for this reason that this plant was chosen
for the work to be described. Like almost all species of tilapia, T. rendalli
generally restricts its feeding to daylight hours (Figure 7) and thus to maintain
some level of simulated natural conditions laboratory feeding trials were
conducted over the same time period.
C. demersum growing shoots (terminal 1-7 g) were used throughout
the trials and the following necessary facts about the plant were established.
To convert fresh mass (limited centrifugation to remove surface moisture) of
C. demersum to dry mass equivalents the following relationship was used:
D = 0.0839 W - 0.7845 (r = 0.986, n = 50, SE 'b' = 0.003) and for
converting dry mass to fresh mass, W = 11.5941D + 26.7257 (r = 0.986,
n = 50, SE 'b' = 0.478) where W is the fresh mass and D the dry mass
both expressed in milligrams. The average energy value of the dry
shoots was determined as 17.9477 kJ g-l (n = 20, SE = 0.0236).
Laboratory maintained fish fed to satiation during a prescribed daylight
period of 12 hours showed a linear relationship between food consumed
and fish mass (Table 2), a feature also reported for wild fish (Moriarty
and Moriarty 1973a; Caulton 1977a) but in reality, over the range juvenile
to large adult, it may be expected that as growth rates decline so less food
per unit mass of fish would be consumed. Using the data presented in Table
2 for consumption rates at various temperatures, the relationship between
food consumed and temperature, for a given size fish, is found to be curvi-
linear showing increasing food intake with increasing temperature (Figure 8).
This relationship holds over the range 18°C to 30°C, but between 30°C and
35" C consumption is little affected by temperature, while at temperatures in
excess of 35"C, food intake declines and ceases at about 37 or 38°C.