in fry production within the replicates of the same group: 93 to 2,981 for the
10: 5 group and 580 to 3,619 for the 6: 3 group. The spawning period in both
cases was 72 days. Two factors could have caused this big variability: some
of the females could have spawned more often than others (thus causing
a temporary higher yield) and some of the spawning groups may have
had a greater number of more fecund females than others.
- THE EFFECT OF THE SEX RATIO OF THE PARENTAL STOCK
In a field experiment (Mires, unpublished) done in two spawning ponds
(0.4 and 1.0 ha) at the Kibbutz Ein Hamifrats, two different parental sex
ratios were tested over a whole spawning season: S. niloticus B females x S.
aureus males, 1:l and 3:l. The basic assumption was that in this kind of
spawn, the females do not have a strong urge to spawn with males of a
different species and therefore a stronger "male pressure" could improve the
situation. This assumption was based on the fact that in aquarium conditions
it often happens that two or more females are ready to spawn at the same
time, while there is only one male available. In such cases it often happens
that the eggs of one of these females are not fertilized. The 1:l ratio gave
higher fry production than the 3:l (800 fry per female in the 3:l ratio
compared to 1,100 in the 1:l ratio).
The beneficial effect of the "male pressure" ratio has again been shown
recently (Lovshin 1980) when for S. niloticus Ivory Coast females x S.
hornorum males, a 1:2 sex ratio gave significantly higher fry production than
2:l or 1:l.
Differences in spawning behavior have been described for various species
of tilapias (Fryer and Iles 1972; Lee 1979). The incompatibility between the
species used in interspecific crosses have usually been studied in aquaria or in
tanks, but not in nature.
Tables 3, 4 and 5 show that S. niloticus Ghana females do not have any
incompatibility problems in crosses with S. aureus males. In the last exper-
iment (Table 3), the average spawning frequency was just as high as in
individual intraspecific spawns (Tables 1 and 2). However, S. aureus and S.
niloticus Ivory Coast females did encounter serious problems while spawning
with S. niloticus Ivory Coast and S. aureus males respectively, and therefore,
the number of spawns obtained from these crosses was very low.
In interspecific crosses in tanks between S. niloticus Ivory Coast females
and S. hornorum males, some of these incompatibility problems can be
overcome by surgical removal of the male premaxilla (Lee 1979). However,
male aggression is only one of the problems. Other factors that may cause
low fry productivity in interspecific crosses include differences in courting
behavior, in mating color display and in the form of the nest. All these
problems and others may exist separately or simultaneously and are very
hard to overcome in ponds.