PREY SELECTIVITY IN WELS (Silurus glanis) AND AFRICAN CATFISH (Clarias gariepinus)

The experiments aimed at prey selectivity in two Siluriformes predators. African catfish, Clarias gariepinus, and wels, Silurus glanis, were performed under laboratory conditions. Prey fish (12-22% TL of predator) were submitted to one.year.old African catfish (~220 g) and wels (~150 g) originating from intensive culture, ie with no previous experience with live fish food. In African catfish, negative selectivity (avoidance) was shown for Nile tilapia (Oreochromis niloticus) and topmouth gudgeon (Pseudorasbora parva) whilst rudd(Scardinius erythrophthalmus) and sunbleak (Leucaspius delineatus) were preferred (positive selectivity). The intensity and efficiency of African catfish predation were quite low because its feeding strategy is based rather on prey searching than hunting. Prey fish, wounded and/or dead from its clumsy attacks, were sonsumed preferably overnight. Not one successful attack of African catfish on healthy prey fish was registered. The SGR and FCR of clarias fed live fish were 0.39%. day-1 and 4.73 respectively. In wels, strong negative selectivity (avoidance) was proved for roach (Rutilus rutilus) and topmouth gungeon, and lower avoidance for Prussian carp (Carassius auratus gibelio) and chub (Leuciscus cephalus). Asp (Aspius aspius) were found to be low preferred but high preference was shown for sunbleak, rudd and bitterling (Rhodeus sericeus). Both catfishes preferred smaller prey fish during the 10.day experimental period whilst those which remained non-consumed belonged to the mean or above-mean size categories