ASSESSING THE IMPACTS OF RIVER REGULATION ON NATIVE BULL TROUT (SALVELINUS CONFLUENTUS) AND WESTSLOPE CUTTHROAT TROUT (ONCORHYNCHUS CLARKII LEWISI) HABITATS IN THE UPPER FLATHEAD RIVER, MONTANA, USA

Abstract

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.

An ecohydraulics virtual watershed: Integrating physical and biological variables to quantify aquatic habitat quality

“…We developed habitat time series of WUA for an entire water year (i.e., 2013) based on daily stream flows at the Anderson Ranch gage station, water depth, and velocities in each grid cell and DMT (Benjankar et al, ). Habitat time series has been used in other studies as an indicator for temporal habitat status (Li, Chen, Tonina, & Cai, ; Muhlfeld et al, ). We compared habitat (WUA) and hydraulic‐based habitat (WUA) time series to analyse the effect of temperature on habitat prediction.…”

Bull Trout Trends in Abundance and Probabilities of Persistence in Idaho

“…However, for the data included in our study, 11% of the Bull Trout observed during snorkel surveys were 400 mm TL or larger. Since Bull Trout of this length are usually considered mature Muhlfeld et al 2012) and since previous population modeling exercises have assumed that about 10% of all Bull Trout in a population are adults (Rieman and McIntyre 1993;Rieman and Allendorf 2001), the proportion of spawning adults among the fish observed during snorkeling was probably similar to the proportion of spawning adults within the actual populations. For this reason, we believe that the use of snorkel data sets in PVAs is appropriate if (1) observation error is not high or (2) observation error is high but the projected extinction risk is low (since, as mentioned above, observation error can only bias the extinction risk upward).…”

“…However, an unrelated mark-recapture study conducted in Lake Pend Oreille during 2008 produced an estimate of 12,513 Bull Trout (≥400 mm TL; Hansen, unpublished data). Since 400-mm and larger fish are mostly mature in this population ; also see Muhlfeld et al 2012), we used the latter estimate for adult abundance in the present study. Even the mark-recapture estimate assumes that there is no resident or fluvial component to the population.…”

Seasonal movements of non‐native lake trout in a connected lake and river system

“…Moreover, subadult (<300 mm) bull trout and westslope cutthroat trout emigrate from their natal tributaries primarily in the spring (Fraley & Shepard 1989; Liknes and Graham 1988) and then migrate downriver into lower portions of the Flathead River and Flathead Lake in the spring and autumn (Shepard et al. 1984; Fraley & Shepard 1989; Muhlfeld & Marotz 2005; Muhlfeld et al. 2011).…”

“…1). The current flow management strategy mimics the natural spring runoff event, within flood constraints, gradually reducing dam discharge towards stable flows during summer, autumn and winter, which maximises the availability of critical bull trout and westslope cutthroat trout habitats in the dam‐influenced reaches of the mainstem Flathead River (Muhlfeld et al. 2011).…”