A purified diet, the raw material zein and two diets composed of complex foodstuffs (fishmeal, soy, wheat), were tested over a seven and a half (7.5) month period, to obtain the in vivo Apparent Digestibility Coefficients (ADC) for dry matter, crude protein, lipid, carbohydrates, ash, energy and the ADCs of their amino acids, for the African catfish Clarias gariepinus (Burchell 1822). Faeces were collected by sedimentation and digestibility coefficients calculated by use of chromium oxide as external indicator. Apparent digestibility coefficients ranged between 54% and 96%. Crude protein ADCs (81-90%) recorded for diets in this experiment were similar to those previously recorded. Ash digestibility (54-89%) varied significantly between diets probably as the result of the reduced bioavailability of minerals and trace elements in their constituent raw materials. ADCs for individual amino acids ranged between 82% and 99%. Zein, as an experimental feed ingredient for African catfish, showed acceptable ADCs for both gross nutrients and amino acids. When the nutrient level in the test ingredient was very different from that in the reference diet, calculation of ADCs based on relative nutrient contributions appeared necessary. Dextrin (an intermediate product of starch hydrolysis) was the prevalent carbohydrate source in the purified diet. The high carbohydrate digestibility for this diet (72%) suggests high starch digestibility in African catfish and confirms previous hypothesis of good starch utilization in the species due to elevated amylase levels in the anterior part of the intestine.
A 26-day experiment was conducted on triplicate groups of sub-adult catfish (102.2g) using two feeding frequencies, twice per day and three times per day with fish fed to appetite on a purified diet. Fish fed twice ate 1.42 % of b. w. d’1, those fed three times consumed 1.27% of b. w. d’1. Fish fed twice also exhibited better growth and food conversion. Despite the use of purified diets, performance indices for the groups fed twice per day were regarded as good compared to previously reported data.
Administration of seven diets differing in protein: energy levels over a seventy-six day period, indicated that C. gariepinus (120-233g) performed best when fed diets containing gross energy between 22-24 kJ.g’1, P:E ratio of 21.5-23 mg protein.kJ1, crude protein at 46%, crude lipid at 10-17% and carbohydrate at 26-32%. These data served to confirm the carnivorous nature of C. gariepinus compared to the less carnivorous North American catfish Ictalurus punctatus. However, performance indices for the fastest growing groups where lower than those achieved in experiments by previous researchers, possibly due to the weight range of the experimental animals used, the available tank surface and the employed stocking densities. Carbohydrate levels (26-32%) of the best performing diets during these experiments were much higher than used by previous researchers (16-18%) for the same species and still higher than those employed for other carnivores (15-25%) (salmonids, sea bass, sea bream). Taking into account the high carbohydrate digestibility of all the diets used in these experiments more comprehensive use of carbohydrates in Clarias catfish diet formulations is suggested. The increase of carcass lipid as a result of increased dietary non-protein energy demonstrated for other species has also been demonstrated for African catfish.
Blood parameters (haematocrit and total hemoglobin) proved not to be valuable performance indices in relation to general nutritional parameters associated with growth and food utilization.
Low carcass incorporation values for arginine and methionine resulted in low requirements (1.97 and 0.26 g.lOOg’1 dietary Crude Protein, respectively) as determined by the carcass deposition technique. The same technique revealed the following values, as g required Amino Acid.lOOg’1 dietary Crude Protein: Histidine 1.39, Isoleucine 1.56, Leucine 4.87, Lysine 4.49, Phenylalanine 4.56, Threonine 2.04, Tryptophan 2.59, Valine 2.08. Diets used in these experiments were characterized by lower levels of arginine, lysine, methionine, cystine, threonine and tryptophan to those proposed by most recent research.
It is suggested that the sequence of pre-experimental adaptation and starvation might have exerted an irreparable loss for some of those amino acids and consequently lower values as determined requirements. Furthermore, mediocre oxygen levels prevailing in the employed experimental system in conjunction with sampling manipulations (prolonged emmersion) might have created a shift to ureotelism with a result in low arginine tissue levels.
Results of this study could be used to optimize formulations based on the “economically optimal protein level” and the peculiarities of individual culture systems. Such formulations could be based on both conventional and non-conventional foodstuffs of a standard supply and price for the selected country-area, which would standardize production practices, production levels and costs. Use of labelled substrates coupled with further experimentation on the required dietary carbohydrate: lipid ratios and the optimum dietary fatty acid profile will clarify the underlying metabolic pathways and probably lead to better elucidation of the deposition of essential amino acids. More information on the digestibility of various raw materials in conjunction with the determined dietary requirements, will lead to the manufacture of more balanced and cost-effective diets for the African catfish C. gariepinus.