The Biology and Culture of Tilapias

in schools of several hundred individuals, notably in S. variabilis (Fryer

and Iles (1972), S. lidole (Lowe (McConnell) 1957) and S. mossambicus

(Bruton and Boltt 1975). Finally, after a variable time according to the

species (22 days in S. mossambacus, Bruton and Boltt 1975; Russock and

Schein 1977), the young, totally independent, are liberated by the female

in shallow water where they live in schools and continue their growth.

In almost all mouthbrooders, guarding the young is undertaken by the

female (uniparental maternal family) though the male may participate

exceptionally, notably in S. mossambicus (Bruton and Boltt 1975). But in

S. melanotheron, it is the male alone which incubates (uniparental paternal

family) (Oppenheimer and Barlow 1968). In S. galilaeus (Iles and Holden

1969; Fishelson and Heinrich 1963) and S. multifasciatus (Fryer and Iles

1972) the two sexes form a stable couple and both practice mouthbrooding.

This seems, therefore, to be an intermediate stage between the biparental

family of substrate-spawners and the uniparental family of mouthbrooders

(Ruwet 1968).

Table 7. Principal characteristics of development of eggs and alevins in eight species of

tilapia (after Hanon 1975). (Temperature: 26 to 27OC in aquarium).

Length Hatching First day of swimming

of eggs age length age length

Species (mm) (days) (mm) (days) (mm)

Tilapia

T. guineensis 2.7 2 5.0-5.5^7 7.5

T. rendalli 1.8 2 3.9 6 6.0

T. sparrmanii 1.6 2% 3.5 5-6 5.0

T. tholloni 1.6 2 3.6 6% 6.1

Sarotherodon

S. galilaeus 2.5^4 -^11 -

S. melanotheron 3.5 6 6.5-6.8 12-13 11.0

S. niloticus 2.8 4-5 4.5 11 8.0

S. saka 2.7 5 4.7 11 9.2

Guarding the young increases considerably the survival of the alevins and
reproductive success, but mouthbrooding represents an even more remark-
able specialization which confers on Sarotherodon great independence from
the milieu. Mouthbrooders have a better capacity for adaptation (in cases
of introduction) and resistance in water bodies subjected to rapid changes
in water level (for example, the advantage of S. macrochir over T. rendalli
in the man-made Lake Mwadingusha, Shaba Zaire: Ruwet 1961-1962,1963b,
1968, and the success of the introduction of S. macrochir in the artificial
Lake Mcnwaine, Zimbabwe, Marshall 1979b).
The territoriality of substrate-spawners is a regulatory eco-ethological
mechanism which allows a certain number of spawners-generally the largest,
the best-motivated and the most aggressiveto occupy the breeding space