Protein intake, ammonia excretion and growth of Oreochromis spilurus in sea water

The effect of protein intake on ammonia excretion and growth rates and the related effects of ammonia levels on growth rate were studied in the tilapia species Oreochromis spilurus, in sea water. The effect of protein intake for stock in the size range of 30-300g BV, was determined by manipulating dietary protein level (P:E ratio) and feeding rate. The results over a protein level range of 20-43.35% and feeding rate of 0.5-6% BV/D show that protein intake has a significant effect on both growth and ammonia excretion rates. It was possible to draw a linear relationship between growth (SQR) and ammonia excretion rate at each P:E ratio level. Variation in the P:E ratio leads to variation in the slope of this relationship which reflects mainly the different contribution of protein energy and non-protein energy to growth and ammonia output. The P:E ratio appears to be significant to growth only at low feeding rate (sub-satiation). While it is relatively unimportant at medium and high feeding rates (close to satiation). However, P:E ratio is important for ammonia output at all feeding rates. Optimum growth for small fish in terms of PCE and PER can be achieved by feeding at sub-satiation (2% BV/D) with a high protein diet (43.35%). This growth was associated with high ammonia output (21% of N intake), mainly because of the limited not-protein energy supply with this feeding regime. This suggests that high PER value obtained from nutrition studies do not necessarily mean high protein utilisation for growth and subsequently low protein catabolism and low ammonia output. It suggests, in fact, that a more efficient growth can be obtained by protein metabolism rather than carbohydrate metabolism. For low ammonia output low P:E ratio diet fed at high ration is recommended mainly to allow protein saving by the carbohydrates. It was not possible to define one particular protein intake as an optimum for growth PCR and ammonia output. However, optimum growth can be achieved with low ammonia output by feeding low P:E ratio diet at high feeding rate, but at poorer PCR and PER. The negative effect of ambient ammonia on growth rate was found to be significant. If un-ionised ammonia levels are increased from 0.00 to 0.30 mg/L, optimum growth can only be achieved at higher protein intakes. At higher ammonia levels optimum growth cannot be achieved at any protein intake. Ammonia appears to reduce food consumption, therefore, it would be advisable to provide all nutrient requirements concentrated in small rations. Furthermore, ammonia stress seems to reduce feed utilisation efficiency, therefore, it would be advisable to use a very efficient energy source, such as protein, to balance this reduction. Therefore, under conditions where ammonia output is important for controlling ambient ammonia (eg. where water supply is restricted) feeding a high protein diet at low ration seems to offer the optimum strategy.