Survival, growth, metabolism and behaviour of Clarias gariepinus (Burchell1822) 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 ill darkness (DD) and some in ligh t (LL), whereas two groups were exposed to reversed light conditions (group s DL and LD). Survival to the end of yolk absorption was 22% greater in fis h 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 rat io, both in terms of energy) was depressed, hence in the dark, energy used for locomotor activity may have been low, leading to increased investment i n growth. We hypothesised that in juveniles light exerts an indirect effect by increasing locomotor activity which in turn promotes multiple encounter s between individuals and enhances cannibalistic behaviour. During the fift h 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 r ecommended as a simple, low-cost technique for intensification of productio n of C. gariepinus stocking material. (C) 2000 Elsevier Science B.V. All ri ghts reserved.