Spatial patterns in relations among brown trout (Salmo trutta) distribution, summer air temperature, and stream size in Rocky Mountain streams

Stream size interacted with mean July air temperature to influence the distribution of brown trout (Salmo trutta) in southeastern Wyoming streams. The geographic range of brown trout was positively associated with mean July air temperatures of 19-22°C. Within this thermal zone, brown trout were more likely to occur in large streams (>4 m wetted width) than in small streams. We used a geographic information system to examine spatial patterns in the distribution of anomalous sites (i.e., sites predicted to have brown trout but which lacked this species). Sites that lacked brown trout but contained brook trout (Salvelinus fontinalis) tended to be on small streams at the cold margin of the 19-22°C thermal window. Sites lacking both brown trout and brook trout tended to be on small streams clustered in three of the six study drainages. The spatial aggregation of these sites suggests that additional regional factors influence the occurrence of brown trout in southeastern Wyoming. It is hypothesized that these factors could involve land-use practices interacting with basin geology and geomorphology. Classification models that incorporate a few general habitat factors are useful for identifying stream reaches with the potential to support brown trout and for directing management efforts to sites where this potential is not realized.