bution of T. busumana, T. discolor and S. multifasciatus in Lake Bosumtwi
and in the permanent and intermittent rivers flowing into it.
c. Daily changes of habitat. In Lake Magadi, Kenya, S. alcalicus grahami
feeds mainly during the day in lake zones where the temperature is 25 to
28°C and where food is abundant; it then moves toward hotter zones (35 to
40°C) which it occupies at night (Coe 1966,1967 in Caulton 197813).
In tilapias living in lakes or lagoons (S. variabilis in Lake Victoria, Wel-
comme 1964; S. mossambicus in Lake Sibaya, Bruton and Boltt 1975; T.
rendalli in Lake Kariba, Donelly 1969; T. rendalli, Caulton 1978131, the
juveniles, but above all the alevins, make daily cyclical movements between
the shallower littoral zones, occupied during the day, and the deeper zones
towards the open water, occupied by night. The juveniles of S. mossambicus
of Lake Sibaya move thus from 0.15-1.0 m to a depth of more than 1.0 m
(alevins from less than 0.15 m to 0.15-1.50 m). In the course of these
, displacements the fishes undergo a large temperature difference (up to
10°C) between the shallow water heated up by day (sometimes to more
than 35°C) and the deeper water which is colder during the night. The
change of habitat occurs at nightfall when the littoral waters become cooler
than the deeper water (Caulton 197813 for T. rendalli; Bruton and Boltt
1975 for S. mossambicus in Lake Sibaya). These daily movements are
made possible because the little fish, unlike the adults, are capable of a
very rapid adaptation to variations in pressure and depth (Caulton and Hill
1973), a phenomenon further facilitated by the high temperatures (Caulton
and Hill 1975).
Concerning the ecological significance and the survival value of these
daily changes in habitat, several interpretations have been proposed:
- They are tactics for avoiding predators, present in deep water during
the day (Fryer 1961a, 1965b; Donnelly 1969) or in shallower water
during the night (case of Clarias gariepinus, preying on S. mossambicus
alevins and juveniles of less than 15 cm in Lake Sibaya, Bruton and
Boltt 1975) although such movements also exist in the absence of
aquatic predators (Welcomme 1964; Caulton 1975a, 1975b), or in
spite of very heavy predation by day in shallow water by birds (Bru-
ton and Boltt 1975). - They reduce feeding competition by the successive exploitation in the
course of the day of abundant food in several habitats (Welcomme
1964). - They are required by physiological adaptations to give the best growth
of the juvenile population (Caulton 1978a).
Caulton (op. cit.) has supported the last hypothesis by bioenergetic study of
T. rendalli and has stressed its interest for the distribution (density and bio-
mass) of the cichlid populations in lakes (i.e., their concentration in the lit-
toral zones where temperature fluctuations are more marked, and this
despite more abundant food in open water, for example, in Lake George) as
well as certain natural examples of dwarfing apparently connected with a
homothermal regime and due, for example, to water supply from a spring.
On the practical level, it should be useful to find out if rearing tilapias would
give better results if the temperatures were varied strongly during the course
of the day (influence of pond depth).