Hungry Horse Dam on the South Fork Flathead River, Montana, USA, has modified the natural flow regimen for power generation, flood risk management and flow augmentation for anadromous fish recovery in the Columbia River. Concern over the detrimental effects of dam operations on native resident fishes prompted research to quantify the impacts of alternative flow management strategies on threatened bull trout (Salvelinus confluentus) and westslope cutthroat trout (Oncorhynchus clarkii lewisi) habitats. Seasonal and life‐stage specific habitat suitability criteria were combined with a two‐dimensional hydrodynamic habitat model to assess discharge effects on usable habitats. Telemetry data used to construct seasonal habitat suitability curves revealed that subadult (fish that emigrated from natal streams to the river system) bull trout move to shallow, low‐velocity shoreline areas at night, which are most sensitive to flow fluctuations. Habitat time series analyses comparing the natural flow regimen (predam, 1929–1952) with five postdam flow management strategies (1953–2008) show that the natural flow conditions optimize the critical bull trout habitats and that the current strategy best resembles the natural flow conditions of all postdam periods. Late summer flow augmentation for anadromous fish recovery, however, produces higher discharges than predam conditions, which reduces the availability of usable habitat during this critical growing season. Our results suggest that past flow management policies that created sporadic streamflow fluctuations were likely detrimental to resident salmonids and that natural flow management strategies will likely improve the chances of protecting key ecosystem processes and help to maintain and restore threatened bull trout and westslope cutthroat trout populations in the upper Columbia River Basin. Copyright © 2011 John Wiley & Sons, Ltd.

Diet and growth were compared for lake whitefish Coregonus clupeaformis caught before and after the recent introduction of Mysis relicta to Flathead Lake, Montana. Most foods and seasonal feeding patterns were similar, except that lake whitefish now also eat mysids. Mysids were eaten by fish larger than 100 mm total length, and especially by fish larger than 400 mm collected in the fall. Mean length at first annulus was less in most years following the mysid introduction than before (Student's /, P < 0.05). Mean lengths at other annuli were similar between the two periods. Larger lake whitefish may be less sensitive to reduced zooplankton abundance caused by mysid predation because they feed on benthic fauna. They may also benefit from the recent availability of mysids for their diet.

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Table 3-4 continued. Comparison of the mean relative weight of bull trout in the spring gill net series in Koocanusa Reservoir. Within each 100-mm group, years with the same letter grouping (e.g., A, B, or C) were the same using a Waller-Duncan sign ratio test (P≤0.05).