one brood a year (which suggests that this species may have evolved under
more lacustrine conditions).
Substrate-spawners have smaller, more numerous eggs than mouthbrooders,
and their populations can build up very quickly in the comparative absence
of predators, as, for example, did those of T. zillii in Lake Victoria, a species
adapted for life with nilotic riverine piscivores, many of which are not found
in the lake. The neotropical cichlids are nearly all substrate-spawners, includ-
ing the Cichlasoma adapted for lacustrine life in Central American lakes
(papers by McKaye 1977; Perrone 1978; Perrone and Zaret 1979). Lake
Tanganyika is the only African lake to include numbers of bothsubstrate-
spawners and mouthbrooders among its endemic species (list in Lowe-
McConnell1975); in all other African lakes most cichlids are mouthbrooders.
Why should this be so?
- POLYCULTURE AND INTRODUCTIONS
These field studies have shown that it is quite usual for several species
of tilapia to share resources in a water body (see Table 1). The main riverine
fish communities in the soudanian region and Upper Zambezi each have two
Sarotherodon species and one or two Tilapia species living together in most
of the region. The lakes too, unless they dry up (like Lakes Rukwa and
Chilwa, which each have one species) tend to have three or more tilapias
sharing resources. The experimental addition of more species to the Lake
Victoria fauna has led to the decline of the two indigenous species, but five
tilapias are still present in this lake. In Lake Malawi the unique S. squa-
mipinnis species flock (four species) appears to have evolved in response to
the increasingly openwater conditions available in this lake, in one case by
the splitting of early and late breeding populations into two species (S. saka
and S. squamipinnis). In the Cameroon crater Lake Barombi Mbo, coloniza-
tions from the river may have recurred at different times, and the resultant
tilapias have managed to coexist (again possibly by differences in breeding
seasons, but more information is needed on this).
In Lake Victoria the coincidental phenomenal rise in lake level appears
to have helped the introduced species to get established. In a number of
lakes the indigenous tilapias have thrived better than introduced species,
for example in Lake Kariba, where despite heavy stockings with S. macrochir
(albeit from the Kafue, so a riverine form, not the Mweru, lacustrine sub-
species intended) it was the indigenous S. mortimeri which underwent
a population explosion, while S. macrochir vanished for many years. How-
ever, as ecological conditions change, so may the tilapias; S. mmchir is
now said to be increasing in Lake Kariba, and S. andersonii, previously
unknown from the Middle Zambezi, has been recorded here. Again, in
the Nyumba ya Mungu reservoir on the Pangani river, it has been the indi-
genous tilapias, S. pangani and S. jipe which have grown so well (to 50 cm
TL) and dominated catches, not the introduced S. esculentus or T. ~endalli.
And in multispecific stocking of Tanzanian dams, Payne (1974) found the
indigenous S. esculentus to do better than the introduced S. macrochir; T.