Partial replacement of fish meal with either soybean meal, brewers yeast or tomato meal in the diets of African sharptooth catfish Clarias gariepinus

As intensively cultured fish usually require high protein feeds, and since feeds are normally the largest variable cost item in commercial production, the profitability of intensive aquaculture is closely related to the world supply and cost of feed protein. Traditionally fish meal has been the major component of all fish feeds. However, its high cost has necessitated a search for alternative protein sources, especially those that are not suitable for human consumption. This study investigated the partial replacing of fish meal with alternative protein sources (tomato waste, soybean meal, brewers yeast) in iso-nitrogenous diets of C. gariepinus. The tomato used consisted of sun-dried pips and skins from ripe tomatoes used in the production of tomato pastes. The soybean consisted of either dehulled, solvent-(hexane) extracted soybean meal (Soy-2) or the same soy meal that had undergone a further extrusion process (Soy-1). The yeast used was a waste product from a local brewery. A final diet consisting of a mixture of the various ingredients was also prepared. Twenty-five catfish (30 to 45 g live mass (LM) were randomly allocated to 24 500 l tanks and four tanks were allocated to each diet. The tanks were then connected to a recirculating system (13 000 l total volume) and each had a flow rate of 7 ± 1 l·min. The water temperature was maintained at 25 ± 1 °C. The total biomass in each tank was measured weekly and the feed adjusted accordingly. The catfish were fed at 5% of total biomass for the first 4 d and 6% for the last 3 d of the week. The experiment was terminated after a 60 d feeding period. A statistical comparison of the final mean mass showed that all the diets differed significantly from each other (p=0.05), with the exception of the yeast and mixture diet and the yeast and Soy-1 diet. The descending ranking order of the mean final body mass (LM ± standard error, g) of the various diets was as follows: fish meal (284.6 ± 5.2), tomato (261.9 ± 5.3), yeast (222.2 ± 5.7), Soy-1 (220.5 ± 5.2), mix (201.4 ± 5.51), and Soy-2 (115.3 ± 5.5). The relatively strong growth experienced by fish receiving the tomato diet could possibly be caused by the high fish meal content of this diet. The poor growth of the Soy-2 diet is attributed to a high urease activity index (1.73). Soy-1 had an index of 0.07. The growth experienced by catfish fed the various protein sources indicates that C. gariepinus are able to utilise alternative protein sources successfully