However, the assumption that no reproductive seasonality occurs in
equatorial areas might even be questioned in regard to the precision and
the validity of the method of appreciation, if we consider the contradictions
between Hyder (1969, 1970a) and Siddiqui (197713) concerning S. leuco-
stictus in the same equatorial lake: the first studies suow a seasonal variation
in breeding activity, while the latter denies this finding and concludes that
breeding is non-seasonal.
It seems therefore that Tilapia and Sarotherodon have a capacity for
unlimited successive breeding (at least when the whole population is con-
sidered) in equatorial regions and an increased tendency for seasonality
with increasing latitude, with maximum reproductive activity during periods
of maximum temperature and day light. The effects of seasonal rainfall
cannot, however, be excluded (Aronson 1957; Lowe (McConnell) 1958;
Lowe-McConnell 1959; Hyder 1969,1970a; Marshall 1979a, 1979b; Moreau
1979) but these remain controversial, probably because of differences
between species.
It remains difficult to assess which are the key factors to stimulate,
inhibit, or exert any regulation on the various stages of the breeding cycles
of Tilapia and Sarotherodon. This uncertainty is due in part to the absence
of any experimental study to dissociate the role of separate environmental
factors, and also part to the imprecise methods which have been used to
determine the different parameters relative to the intensity of reproductive
activity.
The Role of Social Factors in the Regulation
of Reproductive ActivitySocial interactions are known to influence some parameters of reproduc-
tion, particuiarly the timing of first sexual maturation, spawning frequency
and fecundity. In cichlids, spawning frequency is increased by different
sensory stimulations coming from conspecific fishes, e.g., visual stimuli,
sound production, lateral line contacts and probably chemical communication
(Aronson 1945,1951; Polder 1971; Marshall 1972; Chien 1973).
In some Sarotherodon species the female is able to spawn regulwly even
when isolated, but shows increased interspawning intervals compared to
non-isolated females (Aronson 1945; Marshall 1972; Silverman 1978a,
197913). In S. mossam bicus Silverman (1978a, 197813) was able to dissociate
the effects of different levels of social contact on separate parameters of the
interspawning interval. He distinguished between unlimited contact (several
fish in the same aquarium), medium contact (adjacent aquaria each contain-
ing one fish so that each fish can see into the other tank) and low ,contact
(visually isolated fish in different aquaria) and showed that visual stimuli
hastened mainly ovulation with little influence on oogenesis but that other
non-visual stimuli (e.g., tactile or chemical) in unlimited-contact females
were able, in addition, to advance yolk deposition by about 7 days. Males
and females were shown to be equally effective as stimulus animals. This
may reflect some "general conspecific effect" related to gregariousness of S.
mossambicus, a species where "presence" within a social group is important,
according to Silverman. He suggested that regular spawning in isolation