Recovery of the Trichoptera Fauna Near Mt. St. Helens Five Years After the 1980 Eruption

Summary

The stream fauna of the upper Clearwater Creek drainage was almost eliminated by the lateral blast and associated deposits of tephra and organic debris from the 1980 eruption of Mt. St. Helens. Although 37 species have been recorded in benthic collections, the density of caddisflies in 1985 was only 30 m^-2.

Light-trap collections of adults for nine nights between July and September 1985 were compared for a site on Clearwater Creek, in the blast zone, and on a tributary where the riparian vegetation remained intact. More than 3,400 adults were collected and 55% of these were from the Clearwater Creek site. Of the 69 species identified, 45 were taken at Clearwater Creek and 56 at the Forested Tributary ; 33 species were common to both sites. Based on both the diversity and abundance of adults, it is apparent that low density of the benthic caddisfly fauna cannot be attributed to a lack of colonizing adults.