Do Brook Trout within a Stream Network Represent a Single Population?

Brook trout (Salvelinus fontinalis) are declining across their native range and are the focus of major conservation and management initiatives. Despite considerable interest in the population dynamics of brook trout, we lack basic information on how to define brook trout populations, particularly when they occur in larger, dendritic stream networks. We used a combination of microsatellite DNA and a large-scale PIT tagging study to determine if Brook Trout in adjacent tributaries constitute a single population. Based on physical recaptures (n = 2,451) and stationary PIT antenna detections (n > 160,000), we found extremely limited movement between two adjacent tributaries, even though there were no physical movement barriers. Similarly, we found evidence for small but significant levels of divergence (pairwise F_ST: 0.011-0.013) between fish collected in the two tributaries. Program Structure strongly supported the presence of two genetic clusters within the study area, which clearly corresponded to the structure of the tributary network. We conclude that Brook Trout within the study area represent two relatively discrete populations, even though individuals sometimes co-occur in downstream reaches. These findings are particularly remarkable given the potential mobility of Brook Trout and the overall similarity of the tributaries.