Risk of Predation and Its Influence on the Relative Competitive Abilities of Two Species of Freshwater Fishes

Abstract

Experiments were conducted with fathead minnow (Pimephales promelas) and brook stickleback (Culaea inconstans) to determine whether their relative feeding rates were affected by risk of predation. Six groups of both species were observed foraging for food provided by an automated feeder while two parameters were manipulated: the presence or absence of a predator, yellow perch (Perca flavescens), and clear or turbid water. It was hypothesized that the armour and spines of brook stickleback would be of greatest benefit during a direct encounter with a predator. I attempted to simulate this condition with turbid water and predator present. Feeding rates of the two species were significantly different, with fathead minnow feeding at a greater rate than brook stickleback in the absence of a predator. In the presence of a predator the feeding rate of fathead minnow declined significantly whereas that of brook stickleback was unaffected. This resulted in similar feeding rates for the two species in the presence of a predator. Results demonstrate that risk of predation can alter the relative competitive abilities of two species in situations where one species possesses a morphological adaptation against predation. In the absence of predators, these traits may be detrimental in interspecific competition for food.