observations by Popper and Lichatowich (1975) showed that in seawater
ponds in Fiji. S. mossam bicus is able to reproduce at salinities of 49%0. Fry of
S. mossambicus were found to live and be in good healthy condition at
salinities of 69%0 (Potts et al. 1967). These differences may be attributed to
different races of S. mossambicus.
Tilapia zillii was found to reproduce in Lake Qarun (Egypt) in salinities
between 10 to 26%0 (El Zarka et al. 1970a). T. zillii was found in the Red Sea
at salinities of 42.7%0 (Bayoumi 1969) and in the hypersaline Bardawil
Lagoon at salinities of 41 to 45% (Chervinski 1972).
Neither S. aureus nor T. zillii reproduces in sea water. In addition no
nest building occurs and the gonadosomatic index dropped in S. aureus
(Chervinski and Yashouv 1971; Chervinski and Zorn 1974). This fact can be
used to control wild spawning of these species. The high tolerance of S.
aureus, S. mossambicus, S. galilaeus and T. zillii for seawater was attributed
by Morgan (1972) to their natural habitat in estuaries and the lower reaches
of rivers.
Other Water Quality ParametersDue to their tolerance to poor water quality, tilapias are found in habitats
which most other fish genera are unable to inhabit. Even under conditions of
heavy feeding, fertilization and manuring no mortality occurs.
The lowest short-term DO limit recorded for a tilapia is 0.1 ppm DO for
S. mossambicus (Maruyama 1958) and S. niloticus (Magid and Babiker 1975).
S. niloticus 9 x S. hornorum d hybrids tolerate 0.3 ppm DO (Lovshin et al.
1974). Experiments conducted at Texas A&M University using fresh chicken
manure to fertilize ponds at a rate of 2,760 hens per hectare of pond water
showed DO'S at dawn: 0.4 to 0.8 ppm. A correlation was found between the
low DO measurements at dawn and reduced growth in S. aureus (McGeachin
pers. comm.).
Studies by Job (1969a, 196913) showed that the respiration of S. mossam-
bicus was independent of DO at temperatures between 15 and 30°C until the
partial pressure of oxygen dropped to 50 mm Hg, equivalent to 32% satura-
tion. Below this level the metabolic rate became dependent on available
oxygen. Similar results were shown by Rappaport et al. (1976) who found
that the growth of tilapia and carp is reduced below 25% oxygen saturation.
Mortality occurs when oxygen remains below 20% saturation for more than
2 to 3 days,
It seems that tilapias are able to tolerate DO'S as low as 1 ppm. Below this
level they may utilize atmospheric oxygen. Stickney et al. (1977) reported
that S. aureus reared in ponds receiving swine wastes experienced heavy
mortality when the pond surface became covered with duckweed, possibly
because its ability to utilize atmospheric oxygen was restricted. A well-
known phenomenon in harvesting tilapia ponds is the fact that tilapias can
survive for several days in small mud puddles with little ill effect.