THE IMPACT OF PREDATION, ESPECIALLY BY THE TIGER‐FISH (HYDROCYON VITTATUS CAST.) ON AFRICAN FRESHWATER FISHES

Abstract

Study of Hydrocyon vittatus reveals that this fish, while a fierce and voracious predator, nearly always swallows the prey fish whole. Because of this the impact of predation is upon fish less than 40 per cent of the predator's length. The size structure of Hydrocyon populations is such that the main predation pressure is upon fishes of small size, the danger of predation becoming progressively less until a critical length, usually about 18–20 cm., is reached, above which a prey fish is reasonably safe from predation. The impact of predation is upon those fish which are mostly abundantly available to the predator, though these may not necessarily be the most abundant fish in the area. The use of cover, in the forzn of submerged vegetation or of very shallow areas of water, by fish which are small when adult or by the juveniles of larger species, is extensive. It is postulated that both the absolute number of fish and number of fish species which are small when adult, in an area inhabited by Hydrocyon vittatus, and possibly also Lates miloticus subspp., is governed by the amount of vegetal cover available throughout the year and that the spawning migrations of fishes, being found only among those species or very closely related species, which are or have once been in contact with these voracious predators, is primarily undertaken in order to place the young in an environment where refuge from these predators is available. The view that the presence of thesp predators has the effect of retarding speciation is supported.