Spatial and temporal patterns of stonefly (Plecoptera) nymph abundance were investigated with respect to predation risk in the Maligne River, Jasper National Park, Canada, from April to June in 1999 and 2000. The biomass (mg) and density (number) of nymphs on cobbles and their surface area (m 2 ) ('five-rock sampling') were measured weekly in the Maligne Lake Outlet (MLO) and in the Middle and Lower sections of the river. Harlequin ducks (Histrionicus histrionicus ) feed in the MLO and Lower Maligne from April to June but are rarely observed in the Middle Maligne. However, brook char (Salvelinus fontinalis ) and rainbow trout (Oncorhynchus mykiss ) are resident in the Middle Maligne and the MLO.

Nymph abundance was low in the Middle Maligne and high in the MLO and Lower Maligne. Abundance declined seasonally in the MLO and Lower Maligne but did not vary in the Middle Maligne. Overall, it appears that the seasonal declines may represent a behavioral response to predation risk posed by harlequin ducks. (Abstract shortened by UMI.)