Abstract
Short periods of light or no light (18D : 06L and 24D : 00L) resulted in an increased growth compared to extended periods of light (06D : 18L and 12D : 12L) in African catfish Clarias gariepinus . Fish under longer periods of light (12D : 12L and 18D : 06L) showed higher swimming activity, more aggression (injuries on the body) and higher lactate, free fatty acids and cortisol levels compared to those who were reared at shorter periods of light (24D : 00L and 18D : 06L). Feeding activity during light and dark periods in this experiment showed that C. gariepinus had both night and day feeding activities, with a preference to diurnal feeding in the 12D : 12L photoperiod. The results showed that light plays an important role in the African catfish behaviour and its well‐being. As the hours of light increased during the 24 h cycle, data suggests that the fish were more stressed and aggressive, compared to those under a reduced number of light hours.