EMERGENCE OF STONEFLIES (PLECOPTERA) FROM THE ROSEAU RIVER, MANITOBA

Four 1 m2 box-emergence traps were positioned over four substrates (boulder, cobble, gravel and sand) in a section of riffle and an associated pool in the Roseau River. Sampling was conducted during the open water seasons of 1976 and 1977 and traps were emptied every 2nd day. Eighteen hundred and eighteen stoneflies representing seven species, and 412 stoneflies representing six species were caught in 1976 and 1977, respectively. The reduction in density and numbers of species in 1977 is postulated to be the result of a drought in late 1976. Stoneflies emerged 6-10 days earlier in 1977 compared to 1976 as a result of higher spring water temperatures in 1977. Stonefly emergence date could be predicted from the sum of degree-days in the spring of their emergence year. Most emergence occurred at night or in the morning but a few individuals emerged during the brightest part of the day. Numerical abundance for most species was highest over larger substrates (boulder and cobble) in 1976, whereas in 1977 an unexpectedly large number emerged over smaller substrates (gravel and sand).