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Improving conservation status of arctic grayling; assessing and increasing landscape connectivity benefits of denil fishways in the Big Hole River


August 2017 - March 2019


Recovering and improving the conservation status of Arctic Grayling Thymallus arcticus populations are USFWS Region 6 priority goals. Arctic Grayling is a species of special conservation concern, and a Candidate Conservation Agreement with Assurances program was established in the Big Hole River Watershed to facilitate conservation in 2006. There is concern that barriers, such as irrigation diversions, may limit the movement of Arctic Grayling; Arctic Grayling in the Big Hole Watershed move 60-80 miles to reach habitats (spawning, refuge, feeding, etc.) throughout the year. Conservation actions, such as ensuring landscape connectivity, taken before a species is ESA listed or its habitat lost can provide simpler alternatives to expensive litigation and recovery programs. Arctic Grayling (river-dwelling) are an iconic and important component of western aquatic fauna; its mere presence means that quality riparian habitat remains intact to support other aquatic species. The Michigan population of Arctic Grayling is now extinct. The Arctic Grayling population which was widespread in the Missouri River basin above Great Falls, Montana has declined significantly. Improving the conservation status of the Arctic Grayling population that resides in the Big Hole River Watershed is important to the species overall conservation status.
Hydraulic structures such as irrigation diversions are common to the river systems in Montana. They are essential for providing water diversion for agriculture, but can be barriers to Arctic Grayling and other aquatic species. For over a decade, the United States Fish and Wildlife Service (USFWS), Montana Fish Wildlife and Parks (MTFWP), USDA-Natural Resources and Conservation Service (NRCS) Montana, Montana Department of Natural Resources and Conservation (DNRC), local ranchers, NGO’s and others have worked to balance agricultural needs, in this case namely water for ranching and farming, with the conservation of native fish, rivers, and streams. This partnership is a model for how conservation and agriculture can be blended to maintain and ideally improve both.
Denil fishways have been installed in irrigation diversions throughout the Big Hole River Watershed to provide fish passage, with more fishways planned in this watershed and others. They are either included as part of the standard design and installation (Figure 1) for new diversions or are installed as a retrofit to existing diversions. The Denils are “standard” type in their configuration in terms of baffle size, shape, and spacing. They are made of steel, are 2 ft. in cross section, and are either 6 ft. or 12 ft. in length. They are typically installed into the pin and plank portion of the diversion with a total vertical drop of 1 ft.
While designed and placed to achieve Arctic Grayling passage, the effectiveness of these fishways has not been assessed and design criteria remain untested. Conservation partners need to know the efficiency of fish passage for Arctic Grayling and other aquatic species, water usage, and economics of Denil fishways. Without assessment, conservation actions such as installing Denil fishways are at risk of being a “perceived” improvement in the conservation status of Arctic Grayling. Further, a lack of assessment limits our ability to use an adaptive management strategy that might lead to better design features; partners maybe missing opportunities for win-win conflict resolution scenarios such as developing criteria that might increase passage success while providing more economical use of water when it is scarcest. Beyond water stewardship itself, the most important action partners can take to improve Arctic Graylings conservation status is to solve landscape connectivity problems.


Current Staff

Federal Staff: 99

Masters Students: 225

Phd Students: 143

Post Docs: 53

University Staff: 247

5 Year Summary

Students graduated: 658

Scientific Publications: 1872

Presentations: 4186



  • Alexander ZalePrincipal Investigator
  • Thomas McMahonCo-Principal Investigator
  • Benjamin TrianoStudent

Funding Agencies

  • US Geological Survey


Cooperative Research Units Program Headquarters Cooperators

  1. U.S. Geological Survey