In some natural waters which have received multiple introductions, e.g.,
Lake Naivasha, the tilapia populations have changed year by year through
hybridization and interspecific competition. The classic definition of a
species, based on reproductive isolation, is obviously of limited use for
tilapias, especially in aquaculture where so many different hybrids have been
produced. There is a need to distinguish between postulational and opera-
tional definitions of species. For aquaculturists, an operational definition is
required, i.e., a species must be recognizable either from morphology or
biochemical tests.
There was a brief discussion on whether species developed through allo-
patric speciation would be incompatible with respect to reproduction
if brought together. The consensus of opinion was that tilapias usually
hybridize relatively easily. Differences in reproductive behavior are probably
the most important factor where interbreeding is difficult. It is possible that
bringing species together in an artificial lake or culture environment may
facilitate hybridizations which would not occur in a natural environment.
Several speakers commented on the change in appearance of transplanted
species. To illustrate this, Dr. Caulton showed a picture of typical S. mos-
sambicus from the lower/middle Zambezi (Plate 1). These large deep-bodied
fish are much larger at first maturity (around 500 g for breeding males) than
is commonly seen in introduced populations in the Far East (Plate 2).
Species for AquacultureThe species currently being used for aquaculture are a good representative
group of the tilapias. The use of additional tilapia species for aquaculture is
controversial. It could be justified where, for example, a previously unoccu-
pied niche in a polyculture system could be filled or where additional species
could outperform existing cultured species in growth and/or reproductive
performance. Additional species would also open up new possibilities for
hybridization work, but the maintenance of genetically pure lines is not an
easy task. Hybridization work has concentrated largely on the production of
all-male progeny rather than on demonstration of hybrid vigor, production
of new hybrids for brackishwater culture, etc.
There are some other cases in which new species could prove useful.
For example, Prof. Roberts stated that S. spilurus spilurus performs very
well in brackish and seawater culture unlike S. spilurus niger. Small decorative
species, such as S. alcalicus gmhami, could have a future in the aquarium
trade, although sex-reversal or other sterility-inducing techniques are needed
to make this a continuing commercial proposition. Attempts to sex-reverse
S. alcalicus fry have so far been unsuccessful and it is possible that such
small, early maturing tilapias have fry whose sex is determined before release
from the parental mouth and are therefore not easily treated with sex-
reversing steroids.
T. rendalli merits more attention in culture where vegetation is available
for feeding. Although S. hornorum x S. mossambicus hybrids produce
all-male progeny, S. spilurus x S. mossam bicus crosses do not. This is prob-
ably indicative of a closer relationship between the latter two species.