Most pH values associated with tilapia growth come from pond observa-
tions. Swingle (1961) summarized the relationship between pH and fish
culture. He found that the lethal acidic limit is approximately pH 4 and the
alkaline limit is pH 11. Huet (1971) recommended pH 7 to 8 for culture.
Bardach et al. (1972) stated that tilapias did not grow in the acid waters of
West Congo. S. alcalicus grahami in Lake Magadi, Kenya was found to
tolerate pH 5 to 11 without any adverse effect. Lovshin et al. (1977) re-
ported a pH range in culture ponds in Brazil from a minimum of pH 7.7
in the morning to over pH 10 in the afternoon. Experiments conducted at
the Aquaculture Research Center of Texas A&M University, showed that
S. aureus tolerated a pH range of 7.5 to 10.2 in tanks receiving chicken
manure (McGeachin pers. comm.). Also in ponds receiving chicken manure
(100 kg/ha/day of dry matter) the pH ranged between 7.2 and 9.3 at 11:OO
AM. (Bums and Stickney 1980).
Fish excrete most of their nitrogenous waste through the gills in the form
of ammonia. Excreted ammonia exists in water in equilibrium between the
un-ionized NH3 (toxic to fish) and ammonium ions NH; which are not
toxic. The toxicity of un-ionized ammonia depends on the DO. When the DO
is low un-ionized ammonia is toxic at a lower concentration. The toxicity of
NH3 also decreases with increasing C02 ; this depresses the pH which shifts
the NH3 /NH~ equilibrium.
The influence of un-ionized NH3 on S. aureus was investigated by Redner
and Stickney (1979). The 48-hour median lethal concentration (LC50) was
2.4 ppm. When fish were acclimated to sublethal concentrations (0.43 to
0.53 ppm) for 35 days, a concentration as high as 3.4 mg/l caused no mortal-
ity within 48 hours. This pattern is important when heavy feeding, fertiliza-
tion and manuring are being applied in intensive pond culture. The maxi-
mum total ammonia tolerated by S. aureus in experimental ponds receiving
fresh chicken manure from 2,760 hens per hectare was 11 ppm at pH 8 and
27°C. The amount of theun-ionized ammonia present was 0.75 ppm (McGea-
chin pers. comm.). Bums and Stickney (1980) reported the total ammonia
level recordedwith 4,000 hens per hectare to be 2.4 ppm, which appears low.