Nitrogen excretion and absorption efficiencies of white steenbras, Lithognathus lithognathus CuvieSparidae, under experimental culture conditions
1996
Harris, S.A. | Probyn, T.
The various components of the nitrogen budget of white steenbras,Lithognathus lithognathus Cuvier (Sparidae), were determined under experimental culture conditions. A simple closed system respirometer was used where experiments were conducted to investigate faecal prodution, nitrogen excretion rates and absorption efficiencies at 16C and 20C. Faecal loss, as a percentage of the ingested nitrogen, amounted to 3.59%. Gross, carbon and nitrogen absorption efficiencies ranged from 8 6 to 9 8 % and were not significantly affected by fish size, ration size or temperature (P < 0.05). Net protein utilization (NPU) and the biological value (BV) ranged from 78 to95%. Ammonia and urea constituted 77.93 and 10.07%, respectively, of the total exogenous nitrogen excretion, the balance being made up of other dissolved organic nitrogen (DON). Endogenous ammonia excretion rates were significantly greater in small fish compared to large fish (P < 0.05). Total non-faecal losses as a percentage of the ingested ration amounted to 21.86%. Fish size and ration size increased nitrogen excretion, but temperature had no effect. Maintenance rations were 7.55 mg g-¹ dry weight day-¹ (0.76%dry body weight) and 5.99 mgg-¹ (0.60% dry body weight) for small and large fish, respectively. The high absorption efficiencies and high nitrogen indicated that white steenbras utilize the commercial trout pellet diet very efficiently.
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