few hundred eggs to several thousands per spawning. Under pond conditions,
precocious breeding and stunting have been reported (Fryer and Iles 1972;
Cridland 1962) which assist excessive recruitment.
The various methods for controlling the reproduction and recruitment
of tilapias are reviewed in this paper with a brief discussion on the advantages
and disadvantages of each method. To be practical, a method must be easy
to apply, effective and economical.
Monosex Culture
- RATIONALE
In the tilapias, the male in general grows faster than the female (van
Someren and Whitehead 1960a, 1960b; Fryer and Iles 1972; Guerrero and
Guerrero 1975; Anon. 197913). This appears to be genetically controlled.
Monosex male culture therefore gives faster growth and eliminates reproduc-
tion. Monosex tilapias are obtained by manual or mechanical (grader)
separation of the sexes or by production of monosex broods through hybrid-
ization or sex reversal.
Manual sexing of tilapia has been suggested and te~ted by several workers
(Hickling 1963; Meschkat 1967; Shell 1967; Guerrero and Guerrero 1975).
The sexes are distinguished by examination of the urinogenital papillae. Two
orifices are present in the female papilla and one in the male (Vaas and
Hofstede 1952). Although manual sexing is laborious and requires some skill,
it is applied commercially in Israel. Sexing of 50 g or larger fish is easily
done. One man can segregate about 2,000 male tilapias in a working day
(Lovshin and Da Silva 1975). In many countries where the method has been
introduced, however, it has failed. The major disadvantages of the method
are human error in sexing and the wastage of females. Bardach et al. (1972)
reported that hand sexing is 80 to 90% accurate. The use of mechanical
graders for separating larger sized males was tried by Pruginin and Shell
(1962) and Bard et al. (1976). The accuracy of this method, however, has
been questioned by Balarin and Hatton (1979).
The production of all-male progeny from Sarotherodon mossambicus
(female) x S. hornorum (male) hybridization was first described by Hick-
ling (1960). Five other crosses have been reported to produce all-male
F1 hybrids: S. niloticus x S. hornorum (Pruginin 1967), S. niloticus x S.
macrochir (Jalabert et al. 1971), S. niloticus x S. aureus (Fishelson 1962)
and S. niloticus x S. vuriabilis and S. spilurus niger x S. hornorum (Pruginin
1967). Other crosses that have yielded 93 to 98% males are male S. niloticus
