The Biology and Culture of Tilapias

Sarotherodon spp., although their reproductive behavior has not been fully
described. Dr. Trewavas speculated that the present situation had probably
arisen from more than one source species, and included some fascinating
scenarios requiring successive introductions of local riverine species with
extreme spates and specialization/radiation/changing lake conditions in the
intervals between spates. The Malawi species flock is another interesting
example.
The difficulty of detecting modes of speciation was also apparent from
studies on Lake Jipe in which it was not clear whether the present lacustrine
species had arisen sympatrically from incursions of S. pagani from the Pagani
River or whether S. jipe was itself originally present in the river. Also in west
Africa, the lower Nile and Israel, S. niloticus and S. aureus-which are so alike
that some museum workers have confused them in the past-are sometimes
found together. Whether this is a secondary coming-together or a result of
some sympatric speciation is difficult to say. These species do have slightly
different ecological niches, for example in feeding habits, and sexual dichro-
matism and dimorphism are more marked in aureus than in niloticus.
Allopatric speciation has been the general rule for tilapias. For example, S.
mossambicus, which occurs in the lower Zambezi and Limpopo Rivers, S.
spilurus in Kenya (this species is so similar to mossambicus that they are
difficult to separate in preserved material), S. urolepis in the Rufigi River
and S. hornorum in Zanzibar, are all part of the same group. They are of
similar appearance with a long snout and long jaws especially in breeding
males. S. hornorum is particularly easy to distinguish on characters such as
fin-ray counts.
The species living in the east African alkaline lakes, such as Lake Magadi,
were also discussed. These species, e.g., S.a. alcalicus share a common prob-
lem with tilapias inhabiting river systems that are prone to severe drying,
namely, a very restricted environment. In the alkaline lakes, the waters
habitable by fish are restricted to those after the hot spring waters (about
42" C) have cooled slightly and before their salt content becomes too elevated
by evaporation. Such species have many things in common. They generally
have low number of gill rakers, vertebrae and fin-rays; they are very aggressive
and active despite their high population density and restricted space, and
they breed at an early age and small size. In Lake Natron, very small S.a.
alcalicus males set up their breeding pits adjacent to those of mature males
but in Lake Magadi small S.a. grahami interfere with larger spawning fish.
Such populations show in natural waters the early breeding characteristics
that tilapia culturists would like to avoid. They merit much fuller study. It
would be a mistake, however, to call these wild fish 'stunted' as they are
often in good condition.
The total number of species of Tilapia and Sarotherodon is a matter
of opinion. Dr. Trewavas estimated that there are approximately 30 Tilapia
and 46 Sarotherodon species. She pointed out that Thys van der Audenaerde
regards as species populations of S. melanotheron that she considers sub-
species. Dr. Trewavas has also recognized 7 subspecies of S. niloticus, some
of which have yet to be named.
It is obvious that transplantation of species into natural waters, dams and
aquaculture systems has further complicated an already complex situation.