We evaluated the efficacy of portable PIT detectors for tracking long‐term fish movement in an open stream environment. In June and October of 2012, we PIT‐tagged a total of 190 Colorado River Cutthroat Trout Oncorhynchus clarkii pleuriticus (CRCT) in a 1.7‐km segment of a small, montane stream. In the summers of 2012–2013 (15 total occasions), we relocated PIT‐tagged trout using portable PIT detectors. The maximum detection distance of 23‐mm PIT tags ranged from 6 to 56 cm and varied with detector, detection plane, and tag orientation. Of the CRCT tagged, 38% were never detected and 43% were detected on two or more occasions. Mean detection efficiencies of PIT‐tagged trout were 34% and 45% in 2012 and 2013, respectively, and were generally lower than in evaluations of closed systems and less mobile fishes. We observed a smaller range of CRCT than has been observed by others using radiotelemetry, a difference that could be explained by the spatial and temporal limitations of portable PIT detection we encountered. We conclude that portable PIT detector surveys have value but also drawbacks for tracking the movement of relatively mobile fishes in montane streams. Received September 27, 2014; accepted January 21, 2015

Mobile species will migrate considerable distances to find habitats suitable for meeting life history requirements, and stream‐dwelling salmonids are no exception. In April–October 2014, we used radio‐telemetry to examine habitat use and movement of 36 Colorado River cutthroat trout Oncorhynchus clarkii pleuriticus (CRCT) in a 14.9‐km fragment of Milk Creek, a relatively low‐elevation stream in the Rocky Mountains (Colorado). We also used a network of data loggers to track stream temperature across time and space. Our objectives were to (1) characterize distribution and movement of CRCT, (2) evaluate seasonal differences in distribution and movement of CRCT, and (3) explore the relationship between stream temperature and distribution and movement of CRCT. During the course of our study, median range of CRCT was 4.81 km (range = 0.14–10.94) and median total movement was 5.94 km (range = 0.14–26.02). Median location of CRCT was significantly further upstream in summer than in spring, whereas range and movement of CRCT were greater in spring than in summer. Twenty‐six of the 27 CRCT tracked through mid‐June displayed a potamodromous (freshwater migratory) life history, migrating 1.8–8.0 km upstream during the spring spawning season. Four of the seven CRCT tracked through July migrated >1.4 km in summer. CRCT selected relatively cool reaches during summer months, and early‐summer movement was positively correlated with mean stream temperature. Study fish occupied stream segments in spring and fall that were thermally unsuitable, if not lethal, to the species in summer. Although transmitter loss limited the scope of inference, our findings suggest that preferred habitat is a moving target in Milk Creek, and that CRCT move to occupy that target. Because mobile organisms move among complementary habitats and exploit seasonally‐unsuitable reaches, we recommend that spatial and temporal variability be accounted for in delineations of distributional boundaries.

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