Influence of Summer Water Temperatures on the Movement, Distribution, and Habitat Use of Fluvial Westslope Cutthroat Trout in the South Fork Clearwater River Basin

While many Westslope Cutthroat Trout (WCT) Oncorhynchus clarki lewisi populations in Idaho are robust and stable, some remain depressed.  In some systems, such as in the South Fork Clearwater River (SFCR) system, environmental conditions (e.g., summer temperatures) are hypothesized to limit populations of WCT.  Radiotelemetry and snorkeling methods were used to describe movement, distribution, and habitat use of WCT in the SFCR during the summer (June–August) of 2013 and 2014.  Sixty-six tags were surgically implanted into WCT (155–405 mm) and were tracked throughout the summer.  Two distinct patterns of movement by WCT were observed in the SFCR.  Twenty fish moved from the mainstem SFCR into tributaries as stream temperatures increased.  Twenty-six fish persisted in the mainstem SFCR during the summer despite high temperatures.  Snorkeling was conducted to better describe the distribution and habitat use of WCT in the mainstem SFCR.  Sixty-two sites were snorkeled from 5–14 August, 2014.  Twenty-seven WCT were observed in 13 sites and at low densities (0.0003 ± 0.0001 fish/m²).  Fish were not observed in sites where the maximum temperature exceeded 21.70°C (20.93 ± 0.14°C).  Results indicate that the mid-elevation reaches in the mainstem SFCR are important for the long-term persistence of fluvial WCT.