Inventory and survey of the brown and rainbow trout populations of the Big Hole River of southwest Montana, 2001-2005

River discharge and thermal data are summarized for the Big Hole River at the USGS Melrose Gage site for water years 2002-2004. A relatively severe episode of drought continued to dominant flow and temperature regimes over the report period. Despite extremely low water yields recorded over the period, summer minimum flows were slightly mitigated through components of a Big Hole River Drought Plan administered by the Big Hole River Watershed Committee. Rainbow trout population studies were conducted by fall population estimates in the Jerry Creek and Melrose Study Sections while brown trout population studies were conducted from spring estimates in the Maiden Rock, Melrose and Hog Back Study Sections. Rainbow trout populations demonstrated declining trends in recruitment and population density but maintained near average standing crops through relatively high densities if older, larger fish in the populations. A relatively severe point source for whirling disease was identified near the head of the Melrose section. Data strongly suggest that the whirling disease infection has been reducing the rainbow trout recruitment through the reach. Brown trout populations in the Maiden Rock, Melrose and Hog back Section exhibited declining trends in density, standing crop, recruitment, numbers of older larger fish, and spring condition factor concomitant with extremely low summer flow regimes and high summer water temperatures throughout the period. Record high winter ambient temperatures in January and February of 2005 were associated with extremely high brown trout condition factor. Numbers of Age V and older brown trout in the Maiden Rock Section declined to their lowest observed density since the inception of special angling regulations in 1981.