The Biology and Culture of Tilapias

possibility that such a mechanism may have been responsible for at least

some of the adaptive radiation in African cichlids, as seen in the Great Lakes,

is obvious, and clearly warrants attention in the future.

The responses of tilapias during ontogeny to either natural or man-made

changes in ecological conditions are perhaps the best evidence to support our

hypothesis of heterochrony and saltatory development. Our hypothesis

predicts, for example, that as environmental conditions change (or are

changed by human intervention) towards either of the extremes described

previously, the fish should alter their ontogenetic development accordingly.

Sudden and marked alterations in environmental conditions (e.g., flooding of

river banks or lake shore lagoons, increases in adult mortality and/or decreases

in juvenile mortality) should all favor a more altricial life style. Consequently,

we would expect that any or a combination of these environmental features

(or changes) should lead to one or more of: an earlier maturation, increased

fecundity (clutch size), smaller eggs, faster growth and shorter life span in

the fish. Conversely any change(s) in the opposite direction in those environ-

mental conditions would favor a more precocial ontogeny and life style. We

would predict the fish should respond by showing some or all of: delayed

maturation, decreased fecundity, with slower, larger eggs, and increased

longevity.

The data available, both from field and aquaculture observations, appear

to support these predictions (see, in particular, extensive reports summarized

in Fryer and Iles 1972; Lowe-McConnell, this volume). The comparisons

can be made in a number of ways. Fish living (native) in habitats differing in

the above ways can be compared. Fish artificially stocked from one habitat to

another can also provide data to test these predictions, especially since the

introductions could take place either into a habitat the same as, or different

from the original one. The responses of tilapias to naturally-occurring

environmental changes can also be used here. The responses of these fish in

aquaculture facilities, particularly to different types of management pro-

cedures, can also be interpreted to test the predictions (Table 3). A brief

summary of some examples will illustrate our case.

Table 3. Size at sexual maturity (beginning of adult period), maximum size and longevity in different
stocks of tilapiine species. A smaller size at first maturation (within a species, and to alesser extent within
a guild) indicates a more altricial life history style (see text for full discussion) (adapted from Worthington
and Ricardo 1936 ; Ricardo 1939; Lowe (McConnell) 1955b, 1957,1958; Lowe-McConnell1975; Garrod
1959; Cridland 1961, 1962; P.J.P. Whitehead 1962; R.A. Whitehead 1962; Iles and Holden 1969; Hyder
1970a, 1970b; Iles 1971; Fryer and Iles 1972; Balon and Coche 1974; Ben-Tuvia 1978; Hodgkiss and
Man 1978; Silverman 1978a, 1978b; Bruton 1979; Siddiqui 1979a; De Silva and Chandrasoma 1980).
(All Tilapia species are guarders, all Sarofherodon species are bearers).

Species Locality

T. mariae
T. mariae
T. rendalli
T. zillii
T. zillii
T. zillii
T. zillii

Nigeria, Osse River

Jamieson River

Lake Kariba

L. Kinneret

Egypt, ponds

L. Naivasha

aquaria

Typical (T) or Maturation

Dwarfed (D) Stock Size (mm)

Maximum Longevity

Size (mm) (years)