Stocking Density Affects Circadian Rhythms of Locomotor Activity in African Catfish, Clarias gariepinus

Abstract

The effect of stocking density on the locomotor activity of African catfish C. gariepinus under different light regimes was investigated. C. gariepinus were stocked under different densities (1, 5, or 10 fish/tank), and their locomotor activity recorded under light-dark (LD), constant light (LL), constant darkness (DD), and LD-reversed (DL) regimens. Under the LD cycle, catfish showed a crepuscular activity pattern, irrespective of stocking density, with most of the daily activity concentrated around the light-onset and light-offset times. When fish were subjected to DD, all 4 tanks with medium (5 fish) and high (10 fish) stocking densities showed circadian rhythmicity, with an average period (τ) of 23.3 ± 0.5 and 24.6 ± 0.5 h, respectively. In contrast, only 2 low (1 fish) density tanks showed free-running rhythms. Under LL, activity levels decreased significantly in comparison with levels observed under LD and DD. Moreover, fish of 1, 2, and 3 out of the 4 tanks with low, medium, and high densities, respectively, showed free-running rhythms under these conditions. When the photocycle was reversed (DL), fish of 3, 2, and 4 out of the 4 tanks with low, medium, and high stocking densities, respectively, showed gradual resynchronization to the new phase, and transient cycles of activity were observed. These results suggest that stocking density of fish affected the display of circadian rhythmicity and the intensity of activity levels. Thus, fish kept in higher densities showed more robust rhythmicity and higher levels of daily activity, indicating that social interactions may have an influence on behavioral patterns in the African catfish. (Author correspondence: [email protected])

Keywords:

• African catfish
• Circadian rhythms
• Clara gariepinus
• Locomotor activity
• Photoperiod
• Stocking density

ACKNOWLEDGMENTS

The authors would like to thank the staff of the University of Stirling Tropical Aquarium for providing the fish and helping with the daily management and Prof. F.J. Sánchez-Vázquez for providing the system to study locomotor activity. Financial support was provided by “Fundación Séneca” and “Agencia Regional de Ciencia y Tecnología” for the postdoctoral grant awarded to L. M. Vera as well as by the Saudi Arabian Government (studentship to Sami Al-Khamees).

Declaration of Interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.