Colour vision in juvenile African catfish Clarias gariepinus

This study determined colour vision in juvenile Clarias gariepinus by the classical conditioning method. Farmed juveniles about 6 weeks old were presented a pair of green and red plates at the bottom of six aquaria (5 fish per aquarium). In aquaria G1, G2, and G3, the fish were conditioned to associate the green plate with a feed reward, and in R1, R2, and R3, the fish were conditioned to the red plate with feed. After about 100 conditioning trials, the catfish were tested for colour discrimination by presenting them with the pair of green and red plates without the feed. The catfish showed significant bias for the colour of conditioning—for green in G1, G2, and G3, and for red in R1, R2, and R3 (binomial test, P<0.018). The fish were then tested for brightness discrimination by presenting the color of conditioning together with different shades of grey. The catfish in G3 and R2 showed significant bias for the colour of conditioning regardless of the brightness of the grey plates (χ 2 test, P<0.05), that is, they clearly discriminated colours based on chromaticity. On the other hand, (larger) juveniles in aquaria G1, G2, and R1 seemed to discriminate colours based on brightness even as they had color vision.