Effects of Large Wood and Log Jams on Eastern Slope Rocky Mountain Trout Populations

Western U.S. rivers are currently influenced by legacy effects of reduced in-stream large wood (LW) via direct and indirect mechanisms.  Direct mechanisms include clearing of LW for improved fish passage and historical tie floating.  Indirect mechanisms include reduced LW recruitment from timber harvest and reduced beaver populations.  Loss of LW results in loss of stream channel and valley complexity, changes in nutrient dynamics, and reduced aquatic insect production.  We compared trout populations in streams along a gradient of wood loading on the eastern side of the Rocky Mountains.  Brook trout population densities appear to stabilize above approximately 20 m³/ha of LW and are more variable in streams with less than 20 m³/ha LW.  Observed variability in density may be due to differences in insect productivity that appears to be higher at high LW sites.  Individual growth among populations is not greatly affected by wood loading; however, high growth rates at sites with high population density indicate that insect production is a major determinant of fish population dynamics.  Our results indicate potential losses in insect and fish production at sites historically cleared of LW and suggests a minimum level of wood loading to ensure consistent fish densities.