The Biology and Culture of Tilapias

when the water rises. In Lake Valencia (Venezuela), again the cichlid popula-
tion is found only around the periphery where they form nesting arenas.
Diving observations indicate that much wider areas offshore have suitable
substrates, but perhaps behavioral factors are involved in the selection of
inshore sites.
In Lake Kariba (total area 5,000 km2), fish have experienced the change
from a riverine to a lacustrine environment, and for S. mortimeri and pos-
sibly some S. mossambicus this has involved a change from a situation
with relatively scarce to one with abundant breeding sites. In the fast-flowing
Zambezi, fish were able to breed only in bays and backwaters; now it has
about 2,000 km2 of lake waters of appropriate depth. With this change, the
minimum breeding size has increased from 3-400 g to 600 g. Middle Zam-
bezi fishes in the Kariba area are now called S. mortimeri although some S.
mossambicus may have been moved around and hybridization may have
occurred.
S. a. gmhami introduced to Lake Nakuru has changed the whole ecology
of the lake. For example, the population of birds has increased from a
few casual visitors to vast numbers of residents feeding on the fish. S. alca-
licus has thrived in this lake and appears to breed at a larger size than in its
more restrictive alkaline lake habitats, although the breeders are probably
still very young. This is perhaps analogous to the situation with milkfish
(Chanos chanos) in hypersaline lagoons on Christmas Island, which mature
late and at a small stunted size compared to open sea fish. S. a. gmhami,
however, may grow quickly in restricted, high temperature alkaline lakes and
breed when small and young. The fast growth could explain its low vertebral
counts. These questions will not be resolved until more data are collected on
growth rates for 'stunted' and other populations in restricted and open
habitats.
The above examples were discussed at length but there was no general
agreement on their implications for culturists. The fact that culturists
have frequently observed mouthbrooding species spawning in cages suggests
that the hypothesis concerning shortage of breeding sites in lakes may
be incorrect. A cage does, however, provide a solid substrate, with visual
cues. In the Lake George example, it was felt that there was insufficient
knowledge of the simple effects of fishing selection and the genetic make up
of the population: apparently, a few 40 cm fish are still catchable far offshore.
There were also conflicting reports on the effects of environmental
factors on maturation and spawning. For example, nest building is rarely
seen below 60 to 70 cm depth in Israeli tilapia culture but S. mossambicus
breeds normally at 2 m in Lake Sibaya. Indeed, the preferred depth for
mating pits seems to be a specific character, c.f. the Malawi species flock.
It was suggested that culturists could test the Lake George hypothesis in
ponds by adding large fish to outcompete the cultured crop for breeding
sites. The idea here is that the large fish would inhibit spawning by the
cultured crop which would therefore keep growing. This idea was not
received very enthusiastically as it would still mean accepting recruitment
from successful large breeders. An alternative suggestion was the develop-
ment of weak or non-functional males which would fail to fertilize eggs and
cause these to be rejected by the females thereby reducing the females'