Nighttime Dissolved Oxygen Dynamics and Growth Performance of Nile Tilapia Under Different Feed Loads

Abstract

Maximum feeding rate adopted in fish farming in still water in the western hemisphere ranges from 40-50 kg feed /ha /day (4-5 grams feed /m 2 /day) without water quality deterioration; while that of Egyptian water was not investigated. The study was conducted to assess the effect of feeding fish with constant areal (per area) feeding rates (8, 10, 12, 14 g diet /m 2 /day) on daily oxygen production, nighttime community respiration and pH dynamics as well as the growth performance of Nile tilapia. The control treatment received a daily feeding rate load of 6.5 g /m 2 /day. The experiment lasted 90 days during summer season. Dusk oxygen concentrations in experimental tanks were significantly higher in the 8 gram and control treatments compared to those of 10, 12 and 14 gram treatments .The 8 gram and control treatments had just the correct feeding rate (6.5-8 g/ m 2 /day) that exactly equaled the waste assimilative capacity of the water and sediment, resulting in a slightly positive oxygen budget at dawn. In the high feed loads (10 -14 g/m 2 /day) treatments, the average nighttime community respiration exceeded the daytime net oxygen production, with duration of (near zero) depleted oxygen that ranged from 01:58 to 02: 55 hours before daybreak. All other treatments resulted in positive oxygen budgets, with oxygen surplus at dawn suggesting net autotrophy. The 8 grams and control treatments did not suffer oxygen deficiency before daybreak. Feed conversion ratios and daily weight gains were significantly improved in those treatments compared to the treatments with high feed loads. The better environment of the lower feeding rates improved treatment performance in terms of daily weight gain and feed conversion ratio.