rendalli was introduced here before T. zillii and the T, zillii failed to get
established.
The establishment and dominance of S. mossambicus from pond escapes
in Plover Cove Reservoir, Hong Kong, formerly an arm of the sea, stocked
with Chinese carps, shows what hardy persistent fish these tilapias are;
their ability to use many food sources was thought to have contributed
to their success here. This also emphasizes how easily pond escapes occur,
and how difficult it may be to eradicate a species once it has gained access to
a water body.
- CONCLUSIONS FROM GROWTH STUDIES
Collated information on tilapia growth from many natural waters indicates:
That within a water body, the different species tend to grow at differ-
ent rates, the faster-growing species reaching a larger size (see Figure 4).
The same species will grow at different rates in different water bodies,
suggesting that environmental differences are more potent than
genetic differences in determining maturation and maximum size.
Under natural conditions both maturation and maximum size of
a species tend to be smaller in small bodies of water than in larger ones
(see Figure 1).
Populations with low weight for length tend to have a lower maturation
size than populations where the tilapia are in better condition (see
Figure 2).
Maturation size of a species can change within a water body, e.g., rising
as lagoon fish escape to the main lake, or falling after intensive fishing
(as in Lake George), or in low-flood years (as on Kafue flats).
In riverine populations (also in lagoon and aquarium kept-fish) males
grow larger than mature females, but in lakes males and females of the
same species may mature at, and grow to, comparable sizes.
The relative growth rates of young and old fish may be different
in different water bodies.
An extended reproductive season may slow somatic growth.
Both Kafue and Kariba studies indicate that T. rendalli has a higher
production/biomass ratio than the associated Sarotherodon species.
Finally, the great plasticity of growth in natural waters shows that tilapia
growth rates are very greatly affected by environmental and behavioral con-
ditions. This suggests that concentrating research on the environmental and
behavioral factors affecting growth and the switch to reproduction is likely
to be more helpful for fish culture than a search for faster-growing genetic
strains.AcknowledgmentsMy special thanks go to Dr. E. Trewavas for introducing me to the tilapias,
encouraging the field studies through the years, and for drawing my attention
to recent papers and for innumerable stimulating discussions. I am also very
grateful to the Trustees of the British Museum (Natural History) for working
space and excellent library facilities, and to the members of the Fish Section
who have helped in so many ways.