Survival, growth, metabolism and behaviour of Clarias gariepinus (Burchell 1822) early stages under different light conditions

African catfish, Clarias gariepinus were exposed to total darkness (group D) or continuous light (L) during endogenous feeding. During external feeding some of the fish continued to be reared in darkness (DD) and some in light (LL), whereas two groups were exposed to reversed light conditions (groups DL and LD). Survival to the end of yolk absorption was 22% greater in fish exposed to darkness; during subsequent rearing survival decreased in the sequence DD>LD>DL>LL. The onset of external feeding was delayed by a few hours in the L-group as compared with the D-group. Fish reared in dark were larger than those reared in light; the size difference increased with age. In dark, the ratio of total metabolism to body growth (the R_tot/P ratio, both in terms of energy) was depressed, hence in the dark, energy used for locomotor activity may have been low, leading to increased investment in growth. We hypothesised that in juveniles light exerts an indirect effect by increasing locomotor activity which in turn promotes multiple encounters between individuals and enhances cannibalistic behaviour. During the fifth and sixth weeks post-fertilization the biomass of fish reared in the dark was about 175% of that in fish reared in light. Light restriction may be recommended as a simple, low-cost technique for intensification of production of C. gariepinus stocking material.