Limited information on appropriate weaning time to dry feeds and diet ary requirements has been a challenge to the commercial culture of the African bony tongue (Heterotis niloticus) and has remained a major cause of the low survival rates and growth of fry. This study was conducted to determine the optimal weaning age and the effect of high and low phospholipid composition and content in two experimental weaning diets, on the digestive enzymatic activity of H. niloticus fry. The two weaning diets were isonitrogenous and isolipidic (49%CP and 20%L) but differed in content with respect to level of phospholipids and the LC-PUFAs, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) . The two diets thus had different inclusion levels of soy lecithin (phospholipids) and Algatrium DHA70 (a glyceride product high in DHA and EPA). The diets were labelled, LD-(Low-dietary; phospholipids -5.97%, DHA-0.66%, EPA-0.41%) and HD -(High dietary; phospholipids -24.32%, DHA-3.04%, EPA-0.75%). Fry were fed Artemia nauplii until 15, 25, 35, 45, 65 or 85 days after hatching (DAH) respectively, and then co-fed with Artemia and one of two extruded dry feeds for an additional 7 days after which Artemia feeding was withdrawn . Fry were weaned to one of the two weaning diets at 22, 32, 42, 52, 72, or 92 DAH and maintained on the diets for a further 10 days. A control group was kept on Artemia nauplii throughout the experiment. The trial was performed as a 2×6 factorial experiment with each treatment in triplicate in a flow-through system, with a daily water exchange of 200%. Fry were fed the experimental diets 3 times a day to apparent satiation. The results showed that the survival rate of fish on both diets LD and HD increased significantly (p < 0.05) with age; from 0% for diets at 15 DAH to a higher survival of rate of 41.6±3.9 % and 70 .0 ±3.33 % for LD and 69.4±3.9 % and 80.0± 9.4 for HD at 65 and 85 DAH respectively. However, this was lower than the survival ( mean 80.0±9.42 and 93.3% at 65 and 85 DAH, respectively ) obtained by the control, which was maintained on Artemia. Final mean weight, weight gain and survival rate were significantly improved (p < 0.05) for fry fed the HD diet, which had a higher supplementation of dietary phospholipids and LC-PUFA´s (HD vs LD ). Moreover , the digestive enzyme activity (p < 0.05) differed across the weaning ages with a decreasing trend for all enzyme with increasing fish age. The study revealed that H. niloticus requires a long adaptation period to dry feeds as fry weaned at 85 DAH (3.77±0.07g) had a lower survival than fry fed Artemia for the entire period. Additionally, an evaluation of growth and survival demonstrated beneficial effects in the inclusion of LC-PUFAs and phospholipids in their diets.
KEY WORDS: Heterotis niloticus, Artemia nauplii, weaning diets, survival and growth.