Tuesday, May 19, 2009
248

Trout diet and the effects of headwater logging

Richard Van Driesche, William J. Gerth, and Judith L. Li. Department of Fisheries and Wildlife, Oregon State University, 104 Nash Hall, Corvallis, OR 97331

Our study of trout (Oncorhynchus sp.) diet examined consumption before and after clearcut logging in southwestern Oregon. Using a BACI design, we compared gut contents from sites downstream of logged and forested headwaters. From 2004-2007, gut contents were collected by gastric lavage; prey were identified, classified to source (terrestrial, aquatic, adult of aquatic), and enumerated. Estimates of prey biomass were made using invertebrate length:weight relationships. Although prey source and gut content mass were highly variable, patterns were evident. In 2004, a pre-harvest year, prey mass was almost exclusively terrestrially derived during summer, but prey sources were variable in spring and fall; across the range of fish sizes, fish ate less in fall than in spring and summer. Overall, prey biomass consumed increased with fish weight, based on log-log regressions. Preliminary pre- and post-harvest analyses indicate that effects of upstream logging on trout feeding may be subtle and transitory. In pre-harvest years, no differences in the relationships between prey consumption and fish size were apparent based on treatments headwaters were going to receive. However, post-harvest fish consumption patterns did differ between fish downstream of logged headwaters and those downstream of forested headwaters, but only in the first year after harvest.


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