EFFECT OF DIETARY PROTEIN LEVELS ON DISSOLVED OXYGEN DYNAMICS AND GROWTH PERFORMANCE OF NILE TILAPIA

Nile tilapia juveniles with initial weights range of 129.6-140.3grams /fish were distributed into 18 concrete tanks with a constant water depth of 75 cm.Two feeding rates (inputs) were employed at 8 and 9 grams diet/m 2 /day, six days a week.The commercial feed contained three levels of dietary protein (25, 30 and 35%).The rearing experiment lasted 86 days and took place during April -June 2014.Increasing dietary protein content did not have any effect on oxygen concentrations at sunset among treatments.Dissolved oxygen (DO) concentrations at sunset decreased sharply from 8.9 -10.8 g O 2 /m 2 during the second month to 3.4 -6.9 g O 2 /m 2 during the third month, with significant differences between the two periods (p<0.05).There was a sharp decline in secchi disk readings caused by the excessive increase in algal abundance during the third month.Dissolved oxygen concentrations at sunrise and sunset were drastically reduced during the third month due to the excessive algal bloom observed in the latter period.Better environment during the second month improved treatment performance in terms of nighttime oxygen availability.However, adverse environments were observed during the third month with fish suffering oxygen deficiency.The 25% crude protein diet in both the 8 gram and 9 gram feeding rates had better oxygen budget and better environmental conditions for fish production.The better environment of the 25% crude protein treatments were caused by the lower algal blooms (secchi disc readings 10.0 -13.3 cm) observed in those treatments during the third month compared to most of the other treatments.The 25% crude protein treatments also produced better feed conversion ratios (2.42-3.44:1)and protein efficiency ratios (1.16 -1.66) compared to those of the 30 and 35% crude protein treatments.