The Size Composition of Invertebrate Drift in a Rocky Mountain Stream

Abstract

Drift collections from a Rocky Mountain stream were used to examine the hypothesis that, because risk of predation by fish is greatest for large individuals during the day, aquatic insects should become increasingly nocturnal in their drift periodicity as they grow in size. Seven taxa common in the drift were analyzed by one of two approaches. Where data were extensive, a nocturnal activity index was obtained by dividing night drift density by day drift density for a number of size classes. This index showed a significant positive association with size (mean dry wt) for the two mayflies tested, Baetis bicaudatus and Cinygmula sp. The largest size classes often were exclusively nocturnal in their drift activity. For less common taxa I simply compared the size composition during the night with that during the day for each date with sufficient data. In the mayfly Drunella coloradensis, the stonefly Zapada haysi and Chironomidae, large individuals were more frequent in night than day collections. In contrast, neither Prosimulium spp. nor the mayfly Epeorus longimanus showed any size difference between night and day collections. The rarity of large individuals in day drift may be due in part to their removal by fish before reaching drift nets. However, large individuals also were absent from day drift in a section of stream from which fish were excluded, strengthening the argument that the drift behavior of insects had evolved to minimize risk of predation. I suggest that foraging behavior of aquatic insects becomes increasingly restricted to nighttime as insects grow, because the risk of dislodgement associated with foraging translates into an unacceptable risk of predation for large individuals during the day.