Diversion dam located on Skalkaho Creek.

The Bitterroot River system supports populations of both the non-migratory resident and fluvial migratory life-history forms of westslope cutthroat trout (Oncorhynchus clarkii lewisi). However, the migratory form has experienced significant declines due to habitat fragmentation, migration barriers, dewatering, and irrigation canal entrainment. Despite having a healthy population of resident westslope cutthroat trout, and being one of the largest tributaries to enter the Bitterroot River from the east side, Skalkaho Creek contributes little flow and few migratory cutthroat trout to the Bitterroot River due to several irrigation diversions and severe dewatering.

The research team is examining the effect of seven lowhead dams located on lower Skalkaho Creek that divert downstream migrant westslope cutthroat trout into irrigation canals. Post-spawn adults migrating back to the Bitterroot River and juveniles emigrating downstream from nursery reaches of Skalkaho Creek can become entrained and die in the irrigation canal system, resulting in a net loss to the population. Private landowners and irrigators in the Skalkaho Creek drainage expressed concern over the entrainment losses of westslope cutthroat trout to irrigation canals and worked with Montana Fish, Wildlife and Parks to successfully request funds from the Fish Restoration and Irrigation Mitigation Act program to install fish screens and siphons at some of the diversions to preclude such losses. The screens were installed in 2003 and became operational in April 2004. Siphon installation is projected for fall 2007. Evaluations estimating entrainment losses of migratory, non-anadromous westslope cutthroat trout at screened and unscreened irrigation canals have not been conducted and published in the peer-reviewed literature to date, thereby inhibiting private landowners and agencies throughout the Northern Rockies from investing in fish screens and siphons as part of their fish habitat management and restoration efforts. The first two phases of this study began addressing these information gaps.

During the first phase of our study, estimates were obtained prior to fish screen installation to quantify the magnitude of entrainment losses at the seven irrigation diversions. Significant entrainment, particularly of age-0 juveniles, was observed at the unscreened diversions. During the second phase of the study, after installation of fish screens at three diversions, estimates of entrainment and the efficiency of the fish screens were evaluated. Entrainment of both age-0 juveniles and adults was high at unscreened diversions but negligible or absent at screened diversions. Results regarding the efficiency of fish screens were inconclusive because some fish unexpectedly swam back out of the irrigation canals through the headgates, thereby evading detection. Whereas fish screens were an effective management tool for reducing irrigation canal entrainment of adult and age-0 westslope cutthroat trout, a distinct knowledge gap existed regarding: (1) the efficiency of fish screens at preventing entrainment of age-1 to age-4 westslope cutthroat trout, (2) the effect that screening irrigation canals has on young-of-the-year westslope cutthroat trout movements, and (3) the effect of entrainment at the unscreened Hedge Canal on emigrating fluvial westslope cutthroat trout juveniles. The research team is currently addressing these information gaps.  

1. Evaluate the efficacy of the screens using an improved study design that incorporates detection of fish moving upstream through the headgates.
2. Determine the effect of screening on age-0 westslope cutthroat trout movements.
3. Evaluate the effect of the unscreened Hedge and Republican canals, which are to be siphoned, on the recruitment of fluvial westslope cutthroat trout juveniles to the Bitterroot River.

A detailed final report and a Thesis describing the methods, findings, and management implications of the study will be produced. A manuscript based on this study will be submitted for publication in a peer-reviewed journal. Results will be presented at scientific meetings.

Literature Cited:

Gale, S. B.  2005.  Entrainment losses of westslope cutthroat trout into screened and unscreened irrigation canals on Skalkaho Creek, Montana. Master’s Thesis. Montana State University, Bozeman.