OF PREDATION AND ITS INFLUENCE ON THE RELATIVE COMPETITIVE ABILITIES OF 2 SPECIES OF FRESH-WATER FISHES

Experiments were conducted with fathead minnow (Pimephales promelas) a nd brook stickleback (Culaea inconstans) to determine whether their re lative 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 w ater. It was hypothesized that the armour and spines of brook stickleb ack would be of greatest benefit during a direct encounter with a pred ator. I attempted to simulate this condition with turbid water and pre dator present. Feeding rates of the two species were significantly dif ferent, with fathead minnow feeding at a greater rate than brook stick leback 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 ra tes for the two species in the presence of a predator. Results demonst rate that risk of predation can alter the relative competitive abiliti es of two species in situations where one species possesses a morpholo gical adaptation against predation. In the absence of predators, these traits may be detrimental in interspecific competition for food.