The Biology and Culture of Tilapias

'expectation' of mortality is more difficult, however, and can only be
expressed in vague terms as stress factors, e.g., oxygen stress, salinity stress,
thermal stress, density stress.
The possibility of genetic determination of age at first maturity was then
discussed. It was stated that if age at first maturity is genetically controlled,
then the problem for a culturist is how to get the maximum growth within
this period. Evidence was cited against genetic determination of first spawn-
ing age in the tilapias. The example of S. esculentus breeding at two to three
years of age in natural waters and five months in captivity was repeated, but
the prospect of genetic selection for later spawning was not dismissed.
The consensus was that gross environmental changes are the overriding
factors determining agelsize at spawning. The examples of S. mossambicus
breeding at 6 g after five months in aquarium conditions and the fished
population of S. niloticus in Lake George now spawning at 18 cm (estimated
age 1 yr) as opposed to 28 cm (estimated age 1.5 yr or older) were repeated.
Dr. Caulton also cited some personal observations on S. mossambicus which
bred two or three times over a two-year period in aquaria and were about
50 g in weight. When transferred to a more spacious environment-in this
case a Macrobmchium farm raceway--the fish stopped further breeding
until they reached about 250 g and were 3 yr old. This suggests that the
flexibility of timing maturation and spawning extends beyond the agelsize at
first maturity and effects spawning frequency as well. In Lake Sibaya,
S. mossambicus reaches first maturity in one or two years.
The experiences of experimental culturists in Israel, the Philippines
and the U.K. indicate that the age at first maturity for a given species is
relatively constant, i.e., to within a matter of a few weeks at a given location,
but that great variability occurs between different populations at different
experimental stations. S. niloticus and S. aureus are later spawners than S.
mossam bicus.
In the Philippines, the age for first spawning of S. mossambicus is about
two months and for S. niloticus about three months in pond systems.
Stunted fish of 5 to 10 g spawn after two to three months as well as larger
fish. It has also been observed that S. andersonii in Zambia, living at relatively
low temperatures, breed fairly late (at about 15 months in ponds) and at a
large size. In natural waters here they also take 3 years or more to reach first
maturity. When this species was transferred to Tanzania, maturation was
noted as early as two to five months at a very small size.
In Israel, it was of note that the necessity to overwinter fish gave very
little scope for trying to alter the age at first maturity. The greatest potential
appears to be in tropical locations where 'young of the year' are cultured in
shorter production cycles.
It was also noted that while manipulation of stocking density, salinity,
temperature and other environmental factors may be biologically feasible as
a means to influence gametogenesis and spawning, any method chosen for
commercial application must be economically feasible also. For example, it
may be possible to delay or suppress reproduction by lowering the tem-
perature to 18 to 20°C, but this would be expensive and growth rates would
be severely reduced.
The effects of light and daylength were discussed and it was pointed