(e.g. man-made lakes). This transformation, however, affects not only
current velocity but also the depth per se, the thermal regime, various
physical and chemical factors and the structure of the whole ecosystem (see
Balon 1974, for Lake Kariba on the Zambezi).Table 4. Data on depth distribution of tilapias.
S. variabilis and S. esculentus
S. variabilis
S. esculenlus
S. tanganicae
S. mossambicus
S. macrochir
S. niloticus, S. galilaeus and
T. zillii
S. multifasciatus, T. discolor and
T. busumana
T. sparrmanii
T. rendallimaximum 35 to 40 m in Lake Victoria from bottom
trawling (Bergstrand and Cordone 1970, in Fryer and
Iles 1972).
present to 13 m in Lake Victoria (Gee 1968, in Fryer
and Iles 1972).
present to 30 m but most abundant at less than 13 m
in Lake Victoria (Gee 1968, in Fryer and Iles 1972).
common in Lake Tanganyika but rarely captured at
less than 10 m (Poll 1956).
adults absent in less than 12 m but juveniles (max.
15 m) and alevins capable of descending to a greater
depth in Lake Sibaya, S. Africa (Bruton and Boltt
1975).
observed by diving to 12-14 m in Lake McIlwaine,
Zimbabwe and other lakes in that area (Caulton, pers.
comm.).
captured in gillnets to 7 m in Lake Kainji, Nigeria (Ita
1978).
maximum 30 m, 10 m and 7 m respectively in Lake
Bosumtwi, Ghana (Whyte 1975).
observed in diving to 30 m in Lake Sibaya, S. Africa
(Bruton and Boltt 1975) (but maximum depth
compensation of 15 m at 22OC as experimentally
determined by Caulton (1975b).
maximum 7.5 m in Lake Kariba (in Caulton 1976a,
1975b)Tilapias are thermophilic fishes, and their geographical distributions are
closely determined by temperature, particularly by low temperatures. Thus a
natural population will be able to maintain itself if:
a. there is, during part of the year, a temperature high enough to allow
for reproduction and for sufficient growth;
b. the temperature at no time drops below values that are lethal for all
individuals.
Figure 4 summarizes the main published data on the temperature ranges to
which tilapias are exposed in nature and their thermal tolerance limits. It
should be noted, however, that the latter are influenced by acclimation
(thermal history) and were not determined under uniform conditions (see
also Chervinski, this volume).
The most northerly natural occurrence of tilapias is in Lake Huleh, Israel,
in which the T. zillii population has to withstand temperatures of 6 to 7°C
during the coldest nights (Kirk 1972). Observations reported by Hauser
(1977) in California suggest that T. zillii can survive for two weeks at 13OC,
but some began to die at ll.Z°C. Their lower tolerance limit was 6.5OC,