fall. High temperatures (above 32°C) appear necessary for the males to
assume breeding colors in these fish, and at high temperatures they breed
almost continuously. Females move to brooding areas, and somehow manage
to take some food while brooding; they release the brooded young in shallow
nursery areas. Here the young escape predation by adult fish, but are open to
attack from numerous kinds of birds, especially pelicans. There is also some
cannibalism of eggs.
This species was introduced into Lake Nakuru around 1960, where it is now
one of the main herbivores and has altered the whole ecology of the lake
(Vareschi 1979). Fish distribution here is very patchy. At noon they concen-
trate near the shore, at night offshore, a migration pattern probably reflecting
preference for high temperatures. They feed primarily on the very dense
cyanophyte Spirulina platensis, 80% of the tilapia living in the top 100 cm
where they feed. The total ichthyomass of S. alcalicus in this lake was
estimated to increase from 90 t (dry weight) in 1972 to 400 t in 1973
(2.1 g/m2 to 10.2 g/m2). The main impact of this introduction on this lake
has been to increase diversity by extending food chains to fish-eating birds.
Before the introduction such birds only occasionally visited the lake, now
over 50 species are recorded there, of which the great white pelican (Pele-
canus onocrotalus roseum) predominates. Adult pelicans are estimated to eat
1,330 g (fresh weight) of tilapia a day, the young some 770 glday from
hatching to fledging. Calculations suggest pelicans remove 16 to 20,000 kg
fresh weight of fish per day from the lake. Since they nest on a neighboring
lake, taking fish there to feed their young, they have been calculated to
export some 13 t phosphorus a year from the Nakuru system.
In west African coastal lagoons Sarotherodon melanotheron, a species
which can withstand high and changing salinities, has been studied as a
possible candidate for fish culture with mullet (Mugil sp.) by Pauly (1976).
This tilapia only grows to 25 cm TL, the adults feeding on bottom mud
(30% organic content, 1.2 Kcal/g calorific content). Daily food intake of a
20 g fish was estimated to be 1.5 g (dry weight) and the calorific assimilation
900 callday. Juveniles take zooplankton and phytoplankton. S. melano-
theron is a male mouthbrooder (whose behavior has been much studied in
aquaria under the name T. macrocephala or T. heudelotii, e.g., papers by
Aronson 1951). In the field this species breeds throughout the year with
equinoxial peaks. Parasitism is low, attributed to the varying salinity in the
lagoon.
One of the best-studied examples of the role of a tilapia in an exotic
fish community is that of S. mossambicus in Plover Cove Reservoir, Hong
Kong, where an arm of the sea has been turned into a freshwater storage
reservoir (Hodgkiss and Man 1977a, 1977b; Man and Hodgkiss 1977a, 1977b).