After their liberation by the female, the alevins of many mouthbrooding
tilapias remain in schools until a stage of development more or less advanced
according to the species (see Fryer and Iles 1972). In S. mossambicus (Bruton
and Boltt 1975) the juveniles (less than 8 cm) form schools of some hun-
dreds of individuals (several thousands when they group together after an
alarm) which mix closely and disperse during the night. Observations made
on S. variabilis of Lake Victoria (Fryer 1961a) suggest that the schools of
alevins and juveniles keep a certain coherence for up to 6 to 8 months. It
seems, however, that the schools of juveniles finish by breaking up, except
in some pelagic plankton-feeding species of Lake Malawi (S. saka, S. squa-
mipinnis and above all S. lidole) where they are maintained throughout life
(Fryer and Iles 1972; Bems et al. 1978).
In the breeding season, many types of extremely structured schools are
to be found: migrating schools (with possible presence of a leader) in S.
macrochir of Lake Mweru (Carey 1965), schools of males (S. mossambicus,
Bruton and Boltt 1975) or of females (S. lidole, Lowe 1952) near the
spawning grounds, schools of mouthbrooding females (S. variabilis, Fryer
1961a and S. mossambicus, Bruton and Boltt 1975).
It is also known that several Sarotherodon species (S. galilaeus of Lake
Kinneret, Israel, Fryer and Iles 1972, and S. niloticus of Lake Edward) form
very large schools of 100 m2 or more at the lake surface. These are probably
for feeding. Echo-soundings made in Lake Kivu (Capart 1955) showed that
schools of S. niloticus which had dispersed during the night reformed at dawn.
Again, the existence is reported in certain species (e.g., S. esculentus of
Lake Victoria) of schools of small individuals (immature juveniles) living at
the surface and of schools of large individuals (adults) living on the bottom
(Fryer and Iles 1972). Finally, it is interesting that T. rendalli of Lake Malawi,
a macrophyte-feeding littoral species, does not form schools, even though
this is the rule among the plankton-feeding pelagic species like S. saka, S.
squamipinnis and S. lidole.
The ecological significance of schooling behavior in tilapias and cichlids
in general has been discussed by Fryer and Iles (1972). These authors see it
as a mechanism with multiple functions: protection against predators,
facilitation of feeding and population regulation, i.e., information on the
level of population density at the time of reproduction (see Wynne-Edwards
1962).MOVEMENTS, HOME RANGE AND HOMING BEHAVIORThere are relatively few studies on these aspects of tilapia ecology, even in
lakes. We know with certainty that the reproductive migrations of S: macro-
chir in Lake Mweru (Carey 1965) and of S. lidole in Lake Malawi (Lowe
1952, cited by Fryer and Iles 1972) involve displacements of several kilo-
meters. After the liberation of their young, the female schools of S. variabilis
in Lake Victoria (Fryer 1961a) generally disperse a little way (less than
eight km) from the place of capture and tagging on the brooding ground,