Geomorphology, Logging Roads, and the Distribution of Bull Trout Spawning in a Forested River Basin: Implications for Management and Conservation

Abstract

The Swan Basin in Montana is considered a stronghold of regional significance for the bull trout Salvelinus confluentus, a native char whose populations are fragmented and declining throughout its range. We used correlation analysis to examine spatial and temporal variation of bull trout redd count data (1982–1995) relative to geomorphic and land-use factors among nine principal spawning tributaries of the Swan River. Bull trout redd numbers were positively correlated with the extent of alluvial valley segments bounded by knickpoints and negatively correlated with the density of logging roads in spawning tributary catchments. The density of logging roads in spawning tributary catchments was not significantly correlated with geomorphic factors. Temporal trends among the principal spawning streams were variable. In four of the nine principal spawning streams, redd numbers increased significantly during the survey period, and in the remaining streams, redd numbers showed no significant change. Changes in redd numbers with time were negatively correlated with catchment road density and positively correlated with the extent of bounded alluvial valley segments. The significance of bounded alluvial valley segments to bull trout spawning habitat may be related to groundwater–surface water exchange occurring within these segments. Our results emphasize the importance of valley geomorphology to bull trout, and our results suggest that prior land use may have adversely affected bull trout populations in the Swan Basin. Protection of critical spawning tributary catchments from additional road building and associated land-use disturbance will likely be necessary for the maintenance of viable bull trout populations in the Swan Basin. Geomorphic context and land-use status of spawning tributaries are important considerations for future monitoring and management of this species.