When Egypt 88 strain was introduced into China, its cold tolerance was better than
Sudan 78 (Fang et al., 1988). Egypt 88 could tolerate temperatures 1–2 jC lower than
Sudan 78. However, there was no significant difference in LD50 between Egypt 88 and
Sudan 78 in the current experiments. It has been observed that Egypt 88 strain was more
active (more alert, less sluggish and had more feeding activity) than GIFT and Sudan 78
strain on farms at low temperatures. Possibly, both strains are reaching biological limits for
tilapia cold tolerance, which would explain the difference in results after a period of 10
years. The difference of cold tolerance of these two strains needs more detailed studies.
Our experimental results indicate that tilapia can adjust to gradual decreases in temperature.
When the temperature is maintained above 12 jC, the fish experience no irreversible
harm, such as losing equilibrium, cessation of respiration, cessation of food consumption.
During winter, tilapia is more easily infected by disease. For example, the water temperature
of overwintering tanks of Nile tilapia in a tilapia farm of Zhejiang province usually winters
the fish at 14 jC, and only 20% to 30% of the fish survive. The suggested water temperature
for Nile tilapia overwintering should be above 16 jC, which can assure the fish have normal
activity and high survival rate.
Our data and that of Khater (1985) suggest that genetic variation for cold tolerance
exists in Nile tilapia. Heritabilities should be determined in these strains, particularly for
GIFT, for this trait to predict the potential genetic gains that might be possible for selection for cold tolerance.