is harvesting before the fish become sexually mature (Swingle 1960). No
data are available to evaluate such methods for tilapias but Payne (1970)
reported that regular seining of ponds with S. esculentus and T. zillii reduced
the fry population. Mortalities were believed to have occurred from physical
damage or deoxygenation.The Use of Irradiation, Chemosterilants
and Reproduction InhibitorsAl-Daham (1970) observed a decrease in the gonadosomatic index and
growth rate of S. aureus fry exposed to high doses of 60~o gamma ray irra-
diation. Nelson et al. (1976, as cited by Balarin and Hatton 1979) found no
obvious effects on the germ cells of 7 to 8 week old T. zillii fry treated with
60~o gamma radiation for 35 days.
Destruction of the gonads of S. aureus fry was induced by Eckstein and
Spira (1965) using estrogens at concentrations of 50 and 100 pg per liter of
aquarium water for a period of 3 to 4 weeks. Al-Daham (1970) inhibited
brood production in tilapias with the chemosterilants metepa and tetramine
administered at concentrations of 20 ppm for 2 and 3 months and 0.8 ppm
for 2 and 3 months, respectively. Sterile male tilapia were produced with the
treatment of 'OS~ at 10-lo and Cilliter (Voronina 1974, as cited by
Balarin and Hatton 1979).
Using methallibure, a compound which blocks synthesis or release of
pituitary gonadotropins, Dadzie (1974) suppressed gonadal development in
S. aureus. The treatment enhanced the growth of female fish and delayed
spawning. A major constraint in the future development of methallibure
for controlling reproduction is the discovery of its teratogenic effect in swine
(see discussion in Balarin and Hatton 1979).
The reproduction of tilapias also appears to be affected by salinity.
Chirnits (1955) reported that S. mossambicus did not reproduce in salinities
above 30960. Similarly, Chervinski and Yashouv (1971) found that S. aureus
did not reproduce in saltwater ponds with salinities of 36.6 to 44.6%0. S.
niloticus fry were not found in brackishwater ponds in salinities of 15 to
30% (Dureza, pers. comm.). Ang (pers. comm.) observed that no hatching
of S. niloticus eggs occurred in aquaria in salinities of 18760 and higher.
Light and temperature strongly influence the spawning of tilapias. Cridland
(1962) showed in laboratory experiments that sexual maturity of T. zillii
was delayed by "strong periodic illumination for 12-hr periods" and by low
temperatures. The sexual precocity of tilapias in ponds and swamps has been
related to light (Chimits 1955; Lowe (McConnell) 1958). Mires (1974)
indicated that temperature influenced the sex ratio of S. aureus: a higher
percentage of females was associated with low temperatures.