The Biology and Culture of Tilapias

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In males, spermatogenesis is reported to occur continuously, the testis
containing cysts at all different stages, in both Sarotherodon (Dadzie 1969;
Hyder 1970a, 1970b; Moreau 1979) and Tilapia (Moreau 1979), but quan-
tification reveals that substantial variations can occur in the intensity of
spermatogenesis in marginal temperature conditions (Moreau 1979). Methods
of quantification (Billard et al. 1974) should be used more extensively to
assess spermatogenetic continuity. As in the case of females, this would
allow investigation of the occurrence of any post-spawning inhibition in
species where the male exhibits brood care behavior.

The Role of Environmental Factors in the Regulation

of Reproductive Activity

In most teleosts, spawning periods appear to be adjusted to (and by)

environmental factors (photoperiod, temperature, salinity, rainfall, etc.) so

that they are suitable for rearing offspring (de Vlaming 1974). Fish of

temperate zones, where photoperiod and temperature variations are of great

magnitude, spawn during a limited period of the year, and only once in most

cases. In equatorial and tropical regions, where these variations are more

limited, temperature is rarely a limiting factor, but considerable environ-

mental changes can occur which might inhibit or favor offspring survival,

favoring related adaptations (for example rainfall and cloud cover during the

rainy season).

Tilapia and Sarotherodon species are abundant in both equatorial regions

and subtropical regions as well as the mediterranean and have even been in-

troduced into environments which can be considered as marginal, for at least

a part of the year, especially with respect to low temperature tolerance. This

is the case in lakes at high altitude in Madagascar (Moreau 1971,1979). Here

the low temperatures encountered are very different from those in the

original habitats of tilapias and the response in breeding patterns may be

considered as the limit of a tendency. Sarotherodon and Tilapia extend their

breeding seasons for as long as temperature is favorable. During the cold

season spermatogenesis is greatly retarded but all its developmental stages

remain present in the testis. In females, on the contrary, exogenous vitello-

genesis seems to be completely inhibited by low temperatures and all yolk-

laden oocytes disappear.

In more appropriate environmental conditions, such as equatorial lakes

and ponds, S. niloticus was found to breed throughout the year, though

the number of breeding fish was slightly higher during the wet season (Lowe

(McConnell) 1958). In areas distant from the equator, the same species exhib-

its a well defined breeding season, spawning mainly during the warmest and

most sunny season (Lowe (McConnell) 1958). Among Tilapia, T. zillii in

equatorial lakes shows no reproductive seasonality and individual fish

spawn successively year-round (Siddiqui 1979a). On the other hand, the

same species in northern areas presents a definite seasonal breeding, spawning

during the period of maximum water temperature and maximum light (Ben

Tuvia 1959; Fishelson 1966a, 1966b; Siddiqui 1977b).