Trout diet and the effects of headwater logging

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.