Optimal dietary carbohydrate to lipid ratio in African catfish Clarias gariepinus (Burchell 1822)

An 8-week feeding trial was conducted in a recycling water system at 28 +/- 1 degreesC to investigate carbohydrate to lipid ratio (CHO:L ratio) in African catfish Clarias gariepinus (12.32 +/- 0.04 g). Five isonitrogenous (40% crude protein) and isoenergetic (20 kJ g(-1) gross energy (GE)) fishmeal based diets with varying carbohydrate to lipid (CHO: L g/g) ratios of 0.74, 1.13, 1.66, 2.47 and 3.42 for diets 1-5, were tested, respectively. The diets containing a fixed protein to energy ratio (P:E ratio) of 20-mg protein kJ(-1) GE were fed to triplicate groups of 20 fish (per 30-L tank). Fish were fed 5% of their body weight per day adjusted fortnightly. Diet 1, containing 14% carbohydrate and 21% lipids with a CHO: L ratio of 0.74 produced the poorest (P < 0.05) growth rates, feed and protein efficiency. Increasing carbohydrate content in the diets to 27% concomitant with a reduction in lipid content to 16% with a CHO: L ration of 1.66 of diet 3 significantly improved (P < 0.05) growth rates, feed and protein efficiency. A further increase in dietary carbohydrate up to 38% and a decrease in lipids levels to 11% with a CHO:L ratio ranging from 1.66 to 3.42 (diet 3-5) did not significantly improve the fish performance. Apparent net protein utilisation (ANPU) of fish fed diet 4 was higher (P < 0.05) than for diets 1-3 but did not differ from diet 5. Higher lipid deposition (P < 0.05) in whole body and liver were observed with decreasing dietary CHO: L ratios as increasing lipid levels. Whole body protein and liver glycogen content, digestive enzyme activities ( protease and lipase) and histological examination of intestine and liver of fish fed varying CHO: L diets did not show any discernible changes among the dietary treatments. However intestinal alpha-amylase activity increased (P < 0.05) with increasing dietary carbohydrate levels. This study revealed that African catfish can perform equally well on diets containing carbohydrate ranging from 27 to 38% of the diet, with lipid content ranging from 16 to 11% or at CHO: L g/g ratio of 1.7-3.4.