Nitrogen budget changes of tilapia (Oreochromis niloticus) during growth

We conducted a 73-day experiment to examine the nitrogen budget changes of tilapia (Oreochromis niloticus) during its growth from fry (18 g) to adult (180 g). In the experiment, we fed the fish fully, collected feces every day in the water with dissolved oxygen from 8.0 mgL-1 to 6.0 mgL-1, pH 7.0 to 6.5, and temperature 24 ℃ to 30 ℃. Nitrogen budgets were calculated when fish weighed 50 g, 100 g and 180 g. Results show that, growth nitrogen proportion is the biggest (64%) at the initial grow-stage and the lowest (47%) at the final grow-stage, fecal nitrogen proportion gets the biggest (9%) at the middle grow-stage, and excretion nitrogen proportion increases progressively all the time. In addition, total nitrogen amount in the water increases with the growth of fish, but the increasing speed is slower in stageⅡthan stageⅠand Ⅲ.