maximum then decrease with increasing size of fish. Thus the 'brooding
efficiency' (number of young brooded/number of eggs produced) falls as
females increase above a certain size: this aspect has been studied in most
detail in S. leucostictus by Welcomme 1967a).
The survival of the young depends on many factors. Great variations
in survival from year to year have been found in the Kafue populations,
which have obscured any effects due to the change from riverine to lacustrine
conditions as the new lake filled (Dudley 1979). Under the more climatically
stable conditions in the Great Lakes, biotic pressures including predation,
competition for suitable spawning grounds, or nursery sites, or food among
the juvenile fishes, may be particularly important.
Though there has been much discussion on the effects of predators on
cichlid speciation (see Fryer 1965; Lowe-McConnell 1975), facts concerning
differential predation are much needed. Tilapias, particularly juveniles,
are known to be eaten by Prototopterus (lungfish), Lutes, Hydrocynus,
Hepsetus, catfishes such as Clarias (see Bruton 1979), Bagrus and Eutropius
and also by piscivorous cichlids in the Great Lakes (e.g., Haplochromis
squamipinnis in Lake George) and Serranochromis spp. in the Zambezi
system. Some of these piscivores are diurnal feeders, but the catfishes are
nocturnal, taking sleeping tilapias. The reversal of the feeding rhythm in
Lake Bosumtwi tilapia was thought by Whyte (1975) to be an anti-predation
measure.
The numerous kinds of fish-eating birds that will take tilapias include
pelicans, cormorants (though these 'prefer' cylindrical fish) darters, herons,
kingfishers, gulls and waders in the shallows (see lists in Coe 1966; Vareschi
1979 and JFRO reports).
- RIVERINE AND LACUSTRINE TILAPIA POPULATIONS--
'r' AND 'K' STRATEGISTS?
Ability to breed at a dwarf size is shown by all tilapias tested, but in
natural waters we find a range of adaptations. Floodplain species tend
to have seasonally fast growth, early maturity (1 or 2 yr), high fecundity
producing numerous rather small eggs, several broods in succession; rather
generalized bottom-feeding habits. Many of these characteristics are typical
of 'r strategists' found in pioneer habitats (as indeed the floodplains are-new
habitats opened up each year). Lacustrine species in large lakes (Malawi,
Victoria), on the other hand, tend to grow more slowly, have delayed
maturity (2 to 4 yr) so the maturity/final size ratio is higher (see Iles 1971;
Fryer and Iles 1972), reduced fecundity, producing only one (rarely two)
brood a year in Malawi species, with relatively few, large ova, and they brood
the young to a correspondingly large size. They tend to be specialized
phytoplankton-feeders. (Thus they appear to have many features of 'K-stra-
tegists'). S. andersonii also has some of these features, such as brooding only