offering optimal conditions for survival of the young (protection against
predators and exploitation of the best physical sites for reproduction). The
smaller and less motivated individuals occupy marginal zones of the spawn-
ing grounds, where habitat conditions are suboptimal and the exposure to
predators is greater.
The social structure of the mouthbrooders, and their arenas comprise a
system whereby those males with the strongest reproductive drive occupy
the territories at a given time and attract the females. Less motivated and
younger individuals are pushed to the edge of the spawning ground or out-
side it (marginal individuals).
Both in the substrate-spawners and the mouthbrooders, territorial aggres-
sion is shown mainly by displays and symbolic conflicts and rarely by true
fights leading to wounds. However, the latter are known in nature, for exam-
ple in S. mossambicus where, of a total of 323 territorial individuals, Bruton
and Boltt (1975) recorded 10% carrying wounds. Individual aggression
depends mainly. on size of fish, the size of the territories (which varies with
size of male, density of the population and the nature of the substratum)
and the degree of visual isolation from the neighbors (influenced by the
nature and the variety of the bottom). One can also see specific differences;
certain species are considered aggressive (S. niloticus), others intermediate
(S. macrochir) and others feebly aggressive (S. galilaeus). The degree of
aggression is an important factor to consider in the intensive culture of
tilapias in cages or in ponds (MBlard and Philippart 1980).
The methods of reproduction and the social organization of tilapias
have practical implications for culture, which we will review in passing.
Substrate-spawners form stable territorial pairs and one can only stock a
spawning pond with the number of pairs corresponding to the space avail-
able. Any surplus will not participate in breeding, and will disturb the
spawners. For mouthbrooders, however, with breeding arenas, there is
interest in planning a spawning pond comprising three zones with distinct
uses: 1) a flat bottom, muddy or sandy, of medium depth (ca. 1 m) which
one can stock with a number of males higher than the space available for
nest making, in such a way that always the most motivated occupy the
bottom territories and ensure continuous reproduction; 2) a vegetated zone
to shelter the females separately from the males during the period of buccal
incubation and 3) a shallow zone (10 to 20 cm) to harvest the alevins before
sexing them and transferring them to growing ponds or floating cages (see
Ruwet and Voss 1974).
A precise knowledge of the breeding habitat (depth, nature of substrate,
slope, vegetation) would allow the design for each species of a model grow-
out pond limiting breeding as far as possible and, in consequence reducing
overpopulation and dwarfing. One can also envisage the possibility of har-
vesting the alevins by attracting them into traps by means of the sound
emissions (Lanzing 1974) possibly used by the parents for reassembling the
clouds of alevins, both in substrate-spawners and in mouthbrooders.