Does light duration (photoperiod) have an effect on the mortality and welfare of cultured Oreochromis niloticus and Clarias gariepinus?


Abstract: One hundred and eighty juveniles of Oreochromis niloticus with a respective smean total length and weight of 7.30 ± 0.5 cm and 7.05 ± 0.5 g and 180 fry of Clarias gariepinus with a mean total length and weight of 14.90 ± 0.1 cm and 32.10 ± 0.5 g were separately cultured in 3 replicates in 18 plastic tanks (1 × 1 × 0.5 m; 200 L), 9 for O. niloticus and 9 for C. gariepinus, for photoperiods of 24 L: 0 D, 0 L: 24 D, and 12 L: 12 D for 13 weeks. Mortality and welfare impairment were evaluated daily. Mortality was significantly higher (P < 0.05) for the 0 L: 24 D photoperiod among O. niloticus and significantly higher (P < 0.05) for the 24 L: 0 D photoperiod among C. gariepinus. In general, mortality was significantly higher (P < 0.05) among O. niloticus than in C. gariepinus, with mortality being very high in the first 5 weeks of culture in the 2 species in all 3 photoperiods. Mortality was probably due to stress in the tanks arising from poor acclimatization to the environmental conditions, feeding inhibition, cannibalism, and maladjustment to the light/dark rhythmic cycle. The 0 L: 24 D photoperiod had a profound effect on the welfare of the juveniles of O. niloticus, and 24 L: 0 D compromised the welfare of the juveniles of C. gariepinus, as shown by the nonintrusive welfare indicators. The 2 unfavorable photoperiods manifested in stress conditions that caused injuries and affected swimming activity, behavior, coloration, and growth. If welfare challenges arising from photoperiodism are taken into account during culture of the species with photoperiodic manipulation, the species' mortality will be decreased and the farmer's profits will increase.

Keywords: Oreochromis niloticus, Clarias gariepinus, photoperiod, mortality, stress, welfare, water quality

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