Effect of dietary protein on the nitrogen excretion and growth of the African catfish, Clarias gariepinus

Abstract

The rates of growth and nitrogen efflux (total nitrogen and ammonia) of individual C. garicpinus (n: = 32.2 g; S.U. = 4.8 g) kept under 4 feeding regimens, following a 48 h imposed fast (phase l), were measured pcriodically. ln phase 2 (35 d), groups A, B and C wcrc fed a 49.75%, 45.55% or 41.10% protein diet respectively at a ration of 0.5% body weight (1 d-'). Group D were not fed. In phase 3 (25 d) al1 groups were fed the 41.10% diet. In phase 1, thc ammonia efflux rates were lower than any of the values found in either phase 2 and 3. In phase 2, group A (49.75%) had a higher mean ammonia efflux rates than the other groups and ammonia comprised 60-100% of the total nitrogen efflux in al1 groups. Group D showcd a direct relationship betwccn ammonia efflux rate and length of fast. In phase 3, refeeding with the 41.1% protein diet causcd the ammonia efflux rates of groups B and D to converge within 7 days to values no different from thosc of group C, but group A maintained a significantly higher mean value until day 25. During phase 2, the growth rates in B (45.55%) were greatest, but, none of the among groups differences were significant. Group D fish (unfed) lost approximately 30% of their initial weight during phase 2. Nitrogcn efflux rates, notably ammonia, showed a pattern of excretion that was directly related to the protein content of the diet, but that the source of dietary protein, dietary energy and the total available cnergy also infiuenced nitrogen metabolism. The small differences in growth found were related to between diet differences in composition. The weight loss in the unfed group of fish was probably attributable to the utilisation of lipid ancilor protein rescrves as metabolic fuels.