Observations on intraspecific aggression and coeval sibling cannibalism by larval and juvenile Claias gariepinus (Clariidae: Pisces) under controlled conditions

Factors influencing the rate of sibling cannibalism in larval and juvenile sharptooth catfish, Clarias gariepinus, were investigated under controlled conditions. The circum‐oral barbels were found to be more important for the capture of prey than the eyes. Cannibalism starts at a mean total length of 8 mm, three and a half days after the start of feeding and ceases to be of significance at approximately 80 mm, 47 days after the start of feeding. Two distinct types of cannibalism were evident, both of which are primary functions of the relationships between predator mouth width and prey head width. Type I cannibalism, which occurs in the length range from 8 to 45 mm, is characterized by the prey being caught tail‐first and swallowed up to the head, which is subsequently bitten off and discarded. Type II cannibalism on the other hand is characterized by the prey being swallowed head first and whole. During this phase, the larger fish in the population have a mouth width exceeding the head width of the smaller fishes. Type I induced mortality was higher than that induced by Type II. It was concluded that the rate of cannibalism is a consequence of intraspecific aggressive behaviour, the level of which is determined by experimental conditions. Sibling cannibalism was found to be positively density dependent, significantly suppressed by shelter and negatively correkated to food availability. Food availability was shown to have the greatest effect on the behaviour of the fish and hence on the rate of cannibalism. Cannibals grow at a faster rate than non–cannibals. It was concluded that the high rate of Type 1 cannibalism enhances the natural differential growth rate of the fish which leads to Type I1 cannibalism. There is a strong positive correlation between predator and prey size. Electivity analysis showed that the larvae and the juveniles appear to select for larger prey items. The applied implications of these observations are discussed. It was concluded that sibling cannibalism is a highly specialized predation strategy which, under a given set of environmentally induced limiting factors, acts as a density dependent population regulatory mechanism.