The Biology and Culture of Tilapias

pH AND ALKALINITY

S. alcalicus grahami in Lake Magadi tolerates a pH of 10.5 and an alkalinity

of 80 g CaC03/1 (Coe 1969; Reite et al. 1973). S.a. alcalicus of Lake Natron

tolerates alkalinities that are even higher: up to 161 g CaCO3/1 (Morgan

1972). Reite et al. (1974) have shown experimentally that S.a. gmhami can

withstand a pH range of 5 to 11 for at least 24 h, but dies after 2 to 6 hours

at pH's < 3.5 and >12. Morgan (1972) reported mortalities at alkalinities of

1.1 to 6.2 g CaC03/1 (at a pH of 9.0 to 9.9 and 22.5"C) in S. shimnus

chilwae, and at 3.5 to 4.9 g CaC03 /1 in T. rendalli. In the Sudan, S. niloticus

tolerates pH's of 8 to 11 in ponds (George 1975). In Lake Tumba (Zaire),

where 2'. congica lives, the pH is as low as 4.5 to 5.0 (Dubois 1959).

OXYGEN AND OTHER DISSOLVED GASES

The available observations in nature and in ponds, cages and tanks suggest
that tilapias are very resistant to low levels of dissolved oxygen (DO) (see
Balarin and Hatton 1979). DO'S as low as 0.1 pprn are tolerated by S.
rnossarnbicus (Mamyama 1958) and S. niloticus (Magid and Babiker 1975;
M&rd and Philippart 1980). This enables some tilapias to li-ve and reproduce
in swamps and shallow lakes where strong deoxygenations occur from time
to time which are fatal to other fishes. The physiological basis for this
tolerance to low oxygen may be one of several types: (1) using haemoglobins
which bind oxygen at very low tensions,O.l pprn in S. macrochir (Dussart
1963) and 0.17 pprn at 24°C in S. esculentus (Fish 1956); (2) breathing just
below the water surface, flushing the gills with oxygen-rich water (Dussart
1963) and (3) withstanding anaerobiosis (Kutty 1972; Magid and Babiker
1975).
Tilapias can withstand not only low DO'S, but also very high levels of
C02, with maximum tolerance levels of 72.6 pprn in S. macrochir (Dussart
1963), and 50 pprn in S. esculentus (Fish 1956). Other gases (NH,, H2S)
which originate from the decomposition of organic matter are also well
tolerated. A recent experimental study by Redner and Stickney (1979)
demonstrated that S. aureus can tolerate 2.4 pprn of un-ionized NH3 (LDS0,
48 hr).
Massive fish kills invol;ing tilapias do occur, however, in deep lakes in
cases where the seasonal turnover of water brings the deoxygenated deep
water to the surface, and in shallow lakes where violent storms mix the
well-oxygenated surface waters with deoxygenated bottom waters, and stir
up anoxic and toxic bottom mud (containing H2S, NH3 and CO ). Such
phenomena have affected the T. rendalli populations in Lake Chilwa f~organ
1972) in the Nampongue River, Zambia (Tait 1965), and have been reported
from Lake George, Uganda (with mortalities of 1.3 million fish, predo-
minantly S. niloticus, in a few hours in 1957, Beadle 1974) and also from
Tanganyika and Lake Victoria, near Entebbe in July, 1963 (Fryer and Iles
1972).. .i
Finally it should be mentioned that life in lakes characterized by high
phytoplankton production presumes a marked tolerance of conditions of