The effects of logging and mass wasting on juvenile coho salmon (Oncorhynchus kisutch), steelhead trout (O. mykiss, formerly Salmo gairdneri), and Dolly Varden char (Salvelinus malma) were assessed in streams on the Queen Charlotte Islands. Fish densities and habitat characteristics of 27-33 stream reaches were measured during summer and fall. Reaches sampled included undisturbed old-growth forest streams (unlogged), logged streams not directly affected by recent mass wasting (logged), and logged streams directly affected by recent debris torrents and slides (mass wasted). Overwinter survivals and smolt yields in three mass wasted and three non-mass wasted streams (all logged) were also estimated in a down-stream spring fish trapping program, after determining the number of fish present in each stream the previous fall.
Logged reaches had less undercut bank cover than unlogged reaches, but did not differ significantly from unlogged areas in any other habitat variable measured. Mass wasted stream reaches, in contrast, had even less undercut bank cover, less large organic debris (LOD), fewer pools and glides, and more riffles. They also had shallower pools during summer, a smaller wetted stream width relative to rooted channel width, and less overwinter cover in the form of deep pools with undercut banks and abundant LOD.
With one exception, there was no relationship between summer and fall fish densities and any of the habitat parameters measured in this study. The exception was the depth of gravel scour overwinter, which appeared to determine the early summer abundance of coho fry in mass and non-mass wasted streams (all logged). Logged reaches had significantly higher coho fry densities than unlogged or mass wasted reaches in summer and fall. Fish in mass wasted reaches exhibited faster growth rates and attained larger sizes, as long as fry were not trapped in isolated pools when reaches ''dewatered''. In mass wasted streams, a combination of poor egg-to-fry survivals due to excess gravel scour, and poor juvenile overwinter survivals due to overwinter habitat loss, nullified any gains in production attributable to logging. It also nullified the high growth rates and large size achieved by fish in their first year in mass wasted streams. Juvenile overwinter survivals for all species were 2.1-3.5 times higher in non-mass wasted streams than in mass wasted streams; smolt yields were 1.5-3.3 times higher.
The overall impacts of mass wasting on juvenile fish, and coho salmon in particular, are serious enough to jeopardize the continued existence of self-sustaining populations in directly affected reaches until stream conditions improve. Four out of 11 mass wasted reaches in 1982 and 2 out of 3 mass wasted reaches in 1984 had effectively no coho fry. Impacts on Dolly Varden and steelhead trout did not appear as serious, though they too showed declines in overwinter survivals and smolt yields. Impacts on other species such as chum and pink salmon were not investigated, though presumably these species would be negatively stressed by increased gravel scour. Fish populations in otherwise normal (logged) reaches downstream of major mass wasting events may also be adversely affected by mass wasting upstream, but the problem requires further study.
Download LMR080 PDF file (1258 KB)
To view this document you need the current version of
Adobe Acrobat Reader, available free from the
Adobe Web Site.
Updated November 26, 2008