Do cannibalistic fish forage optimally ? An experimental study of prey size preference, bioenergetics of cannibalism and their ontogenetic variations in the African catfish Heterobranchus longifilis

This study relied on the day-by-day analysis of bioenergetics and prey size preference in isolated cannibals of the African catfish Heterobranchus longifilis (13-57 mm standard length, 3-500 mg dry body mass, n = 153) that were offered ad libitum conspecific prey of adequate sizes in small-sized (2-L) environments under controlled conditions (12L: 12D, 30 degrees C). In these conditions, cannibals of increasing body size selected preferentially prey of decreasing size relative to their own, and increasingly closer to the optimal prey size (producing the highest gross conversion efficiency). The role of experience in cannibalism was found of secondary importance relative to body size, both as regards food intake and prey size selectivity. These results indicate that in environments that minimize the escape capacities of their potential victims, as applies to most aquacultural contexts, fish exercising cannibalism tend to forage optimally, which has rarely been evidenced for piscivorous behaviour. The present study further highlights that H. longifilis possesses a very high capacity for growth, which originates from the combination of high food intake and very high conversion efficiency, and makes this species of utmost interest for aquaculturists wherever fast growth is desirable, but also extremely prone to cannibalism wherever feeding schedules are inadequate