Size distribution in African catfish (Clarias gariepinus) affects feeding behaviour but not growth

The goal of this study is to evaluate the effect of size distribution on growth performance and feeding behaviour in juveniles of African catfish. Two thousand sibling fish were grown for 8 weeks until the start of the experiment. Afterwards fish were individually weighed, manually selected and tagged. Four treatments were established according to similarity in weight (n = 108 per treatment, mean ± S.D.): homogeneous low-weight (L, 83.0 ± 8.2 g), homogeneous medium-weight (M, 140.2 ± 8.7 g), homogeneous heavy-weight (H, 198.0 ± 8.1 g) and a heterogeneous group (HET, 139.7 ± 48.4 g). During 27 experimental days, fish were fed ad libitum twice per day. Fish behaviour (resting and swimming activity, waiting-in-feeding area, total feeding time, TFT and feeding rate, FR) was studied using video cameras and by direct observation on days 2, 4, 8, 11, 15, 18 and 26. Growth rate, feed intake and feed conversion ratio did not differ significantly between the treatments. There were also no significant differences between fish of similar weight reared in homogeneous and heterogeneous groups. The H treatment had significantly higher swimming activity (82.8 ± 6.4%) and waiting-in-feeding-area behaviour (51.5 ± 11.2%) than the L treatment (70.3 ± 9.3% and 26.9 ± 5.7%, respectively). The TFT (min) and FR (g min− 1) were lower in L (3.2 ± 0.2 min and 23.8 ± 1.05 g min− 1) than in M (4.7 ± 0.1 min and 29.4 ± 0.82 g min− 1) and H (5.5 ± 0.1 min and 29.7 ± 2.3 g min− 1) treatments. Overall, this study indicates that size distribution does not affect growth performance. Low-weight fish do not exhibit increased growth rates in the absence of heavier fish. The use of grading in juveniles of African catfish should therefore be re-considered. The differences in weight observed in this study seem not to be a direct consequence of social hierarchies where the larger fish suppress the growth of smaller fish. Instead, feeding behaviour seems to be an important factor, with heavier fish exhibiting feeding behaviours that may give advantage when feed is limited.