Landscape Effects on the Distribution of Brook Trout at the Southern Margin of Their Natural Range

We examined and summarized existing knowledge regarding the distribution and status of self-sustaining populations of brook trout Salvelinus fontinalis at the  southern margin of their native range at four different scales: sub-basins (mean area,= 254,172 ha); watersheds (mean area,= 41,201 ha); subwatersheds (mean area = 8,879 ha) and catchments (mean area,= 237 ha). This assessment resulted in an updated and detailed range map of historical and current brook trout distribution in the study area. Differences in the reporting scale influenced the results; brook trout occupied 88% of sub-basins; 72% of watersheds; 47% of subwatersheds and 10% of catchments. The scale at which results are reported can bias impressions of the true status and distribution of the species. A classification and regression tree using five core subwatershed metrics (percent total forest, sulfate and nitrate deposition, percent mixed forest in the water corridor, percent agriculture, and road density) was a useful predictor of brook trout distribution and status, producing an overall correct classification rate of 71%. Among the intact subwatersheds, 94% had forested lands encompassing over 68% of the land base. Continued habitat loss from land use practices and the presence of naturalized exotic fishes threaten the remaining brook trout populations. The distribution of brook trout and related threshold metrics can be used for risk assessment and prioritization of conservation efforts.