Distribution of the Stonefly Nymph Paragnetina media (Plecoptera:Perlidae): Influence of Prey, Predators, Current Speed, and Substrate Composition

Effects of prey distribution, presence of a predator, current speed, and substrate composition on distribution of the predaceous stonefly nymph Paragnetina media (Walker) were examined in a large laboratory stream. Distribution of prey (Hydropsyche sparna, H. bronta) did not affect distribution of well-fed or non-fed (48 h) nymphs. However, P. media distribution was significantly influenced by independent and interactive effects of current speed (0.33–0.41 m∙s −1 ) and substrate composition (53.0–110.0 mm in diameter). In the presence of rainbow trout (Salmo gairdneri), P. media selected more specifically for larger substrate on which risk of predation was less than on smaller size classes. Lighting condition had no effect on substrate selection. Laboratory findings were consistent with distribution of P. media observed in the Credit River, Ontario, during midsummer, late fall, and early spring.