The effect of group composition on the welfare of African catfish (Clarias gariepinus)

Within fish farming grading, i.e. the process of sorting to approximate size, changes the group composition from heterogeneous to homogeneous. Although this procedure is considered an acute stressor, the long-term consequences of grading on aggression and stress levels have not yet been investigated. The objective of this study was to quantify the consequences for welfare of rearing African catfish (Clarias gariepinus) in groups of homogeneous low-, middle- and heavy-weight fish as compared with a heterogeneous group. Two thousand fish were grown for 8 weeks. Afterwards, fish were individually weighed, tagged and manually selected. Four treatments were established according to weight (mean ± S.D.): homogeneous low- (L, 83.0 ± 8.2 g), homogeneous middle- (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). Fish were sampled three times (days 13, 27 and 34) to determine aggression levels (number of skin lesions and percentage of fish bitten). Blood samples (for cortisol and glucose analysis) were taken at the end of the experiment before and after a stress test (netting). Despite all treatments exhibited high aggression levels (>50%), the H treatment exhibited the lowest aggression levels (58.34 ± 0.92% of fish bitten as compared with 74.13 ± 2.52% in L; 69.70 ± 4.38% in M and 69.94 ± 3.29% in HET, P = 0.002). Aggression significantly decreased over time in the M and H treatment but not in the L and HET treatments. In addition, heavy fish reared in heterogeneous groups exhibited higher number of skin lesions in the body (2.0 ± 0.1) than fish with similar weight reared in homogeneous groups (1.4 ± 0.1) (P = 0.002). Also, middle-weight fish showed lower cortisol values (before the stress test) in the homogeneous group (45.7 ± 7.5 ng/ml) as compared with middle-weight fish in heterogeneous groups (73.5 ± 13.3 ng/ml). The stress response in low-weight fish was also affected by the group composition. After a stress test, the glucose levels were higher for low-weight fish reared in homogeneous (7.1 ± 0.4 mmol/l) than in heterogeneous (5.1 ± 0.7 mmol/l) groups. This study suggested that the group composition affects welfare in African catfish, particularly in the L treatment where aggression levels were as high as 74%. Middle-weight and heavy fish seemed to benefit from living in homogeneous while light fish from heterogeneous groups.