Minimum Pool and Bull Trout Prey Base Investigations at 

Beulah Reservoir—Final Report for 2008

Open-File Report 2009-1068

Prepared in cooperation with the Bureau of Reclamation

U.S. Department of the Interior

U.S. Geological Survey

Minimum Pool and Bull Trout Prey Base Investigations at 

Beulah Reservoir—Final Report for 2008

By Brien P. Rose and Matthew G. Mesa

Prepared in cooperation with the Bureau of Reclamation

Open-File Report 2009–1068

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Rose, B.P., and Mesa, M.G., 2009, Minimum pool and bull trout prey base investigations at Beulah Reservoir—Final 

report 2008: U.S. Geological Survey Open-File Report 2009–1068, 54 p.

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copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 2

Methods......................................................................................................................................................................... 3

Water-Quality Sampling ............................................................................................................................................. 3

Aquatic Insect Sampling............................................................................................................................................. 4

Fish Sampling ............................................................................................................................................................ 4

Insect and Fish Response to Reservoir Drawdown.................................................................................................... 5

Bioenergetics Modeling.............................................................................................................................................. 5

Results........................................................................................................................................................................... 7

Reservoir Volume and Streamflow............................................................................................................................. 7

Water Quality ............................................................................................................................................................. 7

Pelagic Invertebrates ................................................................................................................................................. 7

Benthic Invertebrates ................................................................................................................................................. 8

Fish Sampling Overview ............................................................................................................................................ 8

Fish Catch per Unit Effort........................................................................................................................................... 8

Fish Population Estimates........................................................................................................................................ 10

Catch of Bull Trout ................................................................................................................................................... 11

Bioenergetics Modeling............................................................................................................................................ 12

Discussion ................................................................................................................................................................... 12

Summary ..................................................................................................................................................................... 18

Acknowledgments........................................................................................................................................................ 18

References Cited......................................................................................................................................................... 19

Figures

Figure 1. Map showing general location and tributaries of Beulah Reservoir, Oregon..................................................23

Figure 2. Discharge of water entering and exiting Beulah Reservoir and reservoir volume, 2004–08 ..........................24

Figure 3. Photographs of a fish kill that occurred downstream of Agency Valley Dam on the 

North Fork of the Malheur River during the fall of 2007 after the end of irrigation season and the 

release of water .............................................................................................................................................................25

Figure 4. Mean daily temperatures at the outflow of Beulah Reservoir, 2006–08 .........................................................26

Figure 5. Water-temperature profiles in three regions of Beulah Reservoir, 2006–08...................................................27

Figure 6. Dissolved-oxygen profiles in three regions of Beulah Reservoir, 2006–08 ....................................................28

Figure 7. Mean (± SD) Secchi disk depths measured in Beulah Reservoir, 2006–08...................................................29

Figure 8. Mean (± 95-percent confidence intervals) and median (white circles) catch per m3 of

Hydracarina (upper panel) and chironomids (lower panel) collected with vertical net tows in Beulah 

Reservoir during spring, 2006–08..................................................................................................................................30

Figure 9. Mean (± 95-percent confidence intervals) and median (white circles) catch per m2 (upper panel) 

and mass (lower panel) of benthic dipterans collected from locations in Beulah Reservoir that were frequently, 

occasionally, or usually not dewatered during typical summer drawdown, 2006–07 .....................................................31
iv

Figures—Continued

Figure 10. Mean (± 95-percent confidence intervals) and median (white circles) catch per m2 of 

benthic dipterans collected during the spring in Beulah Reservoir, 2006–07.................................................................32

Figure 11. Locations of sampling with fyke nets in Beulah Reservoir, 2006–08............................................................33

Figure 12. Locations of sampling with gillnets in Beulah Reservoir, 2006–08...............................................................34

Figure 13. Mean (and 95-percent confidence interval) and median (white circles) catch per hour of

fyke nets for species of dace in Beulah Reservoir, 2006–08 ........................................................................................35

Figure 14. Catch rates of fyke nets for longnose and speckled dace of different sizes in 

Beulah Reservoir, 2006–08 ...........................................................................................................................................36

Figure 15. Mean (and 95-percent confidence interval) and median (white circles) catch per hour of 

fyke nets and gillnets for northern pikeminnow in Beulah Reservoir, 2006–08 ..............................................................37

Figure 16. Catch rates of fyke nets (upper panels) and gillnets (lower panels) for northern 

pikeminnow of different sizes in Beulah Reservoir, 2006–08.........................................................................................38

Figure 17. Mean (and 95-percent confidence interval) and median (white circles) catch per hour of 

fyke nets and gillnets for redside shiners in Beulah Reservoir, 2006–08.......................................................................39

Figure 18. Catch rates of fyke nets (upper panel) and gillnets (lower panel) for redside shiners 

of different sizes in Beulah Reservoir, 2006–08. Data were plotted at 5-mm intervals. .................................................40

Figure 19. Mean (and 95-percent confidence interval) and median (white circles) catch per hour 

of fyke nets and gillnets for species of suckers in Beulah Reservoir, 2006–08..............................................................41

Figure 20. Catch rates of fyke nets (upper panels) and gillnets (lower panel) for bridgelip and 

largescale suckers of different sizes in Beulah Reservoir, 2006–08 ..............................................................................42

Figure 21. Mean (and 95-percent confidence interval) and median (white circles) catch per hour of 

fyke nets and gillnets for white crappies in Beulah Reservoir, 2006–08 ........................................................................43

Figure 22. Catch rates of fyke nets (upper panel) and gillnets (lower panel) for white crappies of 

different sizes in Beulah Reservoir, 2006–08.................................................................................................................44

Figure 23. Population abundance estimates (± 95-percent confidence intervals) for northern 

pikeminnow <156 mm, and redside shiners and redband trout of different sizes in Beulah Reservoir, 

2006–08.........................................................................................................................................................................45

Figure 24. Length-frequency distributions of bull trout collected from Beulah Reservoir, 2007–08...............................46

Figure 25. Fulton’s condition factor of bull trout collected from Beulah Reservoir, 2007–08 .........................................47

Figure 26. Per capita daily consumption estimates of bull trout for three prey types in Beulah Reservoir 

under three different growth rates and assuming a diet comprised of 20 percent zooplankton, 60 percent 

fish, 20 percent invertebrates.........................................................................................................................................48

Figure 27. Predicted biomass of fish <156 mm remaining in Beulah Reservoir after consumption 

by hypothetical populations of 188 (black lines) or 1,000 (grey lines) bull trout with three different 

growth rates during a 210-day period from fall through spring, 2006–07.......................................................................49

Figure 28. Predicted biomass of fish <156 mm remaining in Beulah Reservoir after consumption by 

a hypothetical population of 188 (black lines) or 1,000 (grey lines) bull trout with three different 

growth rates during a 210-day period from fall through spring, 2007–08.......................................................................50

Figure 29. Number of bull trout redds counted in the North Fork Malheur River and the minimum 

pool level of Beulah Reservoir .......................................................................................................................................51
v 

Tables

Table 1. Day of simulation, reference month, temperature range in Beulah Reservoir during that 

month, and temperatures used in bioenergetics modeling of bull trout feeding and growth...........................................52

Table 2. Percent occurrence ( %O), percent of total number (%N), and percent of total mass (%M) 

of prey items consumed by bull trout in Beulah Reservoir during the spring (N=5) and fall (N=5) of 2007 ....................52

Table 3. Mean length (and SD), weight, sample size, and energy density of fish from Beulah 

Reservoir that we used in bioenergetics modeling of bull trout feeding and growth.......................................................53

Table 4. Fyke net and gillnet sampling effort and overall catch rates of fish in Beulah Reservoir, 2006–08..................53

Table 5. Bioenergetics model estimates of consumption (kg) by bull trout with a hypothetical 

population size of 188 or 1,000 fish using three growth rates and two diet scenarios....................................................54

Table 6. Number of rainbow trout that were stocked in Beulah Reservoir, 2004–08 .....................................................54
vi

Conversion Factors, Datums, and Acronyms

SI to Inch/Pound

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2) 0.0002471 acre 

Volume

liter (L) 0.2642 gallon (gal)

cubic kilometer (km3) 0.2399 cubic mile (mi3

Mass

) 

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (kg) 2.205 pound avoirdupois (lb)

Energy

joule (J) 0.0000002 kilowatthour (kWh)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32. 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F-32)/1.8. 

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or micrograms per liter (μg/L).

Acronyms

FWS United States Fish and Wildlife Service

NFMR North Fork Malheur River

RBT rainbow trout

DO dissolved oxygen

FL fork length

CPUE catch per unit effort

h hour

FPH fish per hour
1 

Minimum Pool and Bull Trout Prey Base Investigations 

at Beulah Reservoir—Final Report for 2008

By Brien P. Rose and Matthew G. Mesa

Abstract

Beulah Reservoir in southeastern Oregon provides irrigation water to nearby farms and 

supports an adfluvial population of threatened bull trout (Salvelinus confluentus). Summer 

drawdowns in the reservoir could affect forage fish production and overwintering bull trout. To 

assess the impacts of drawdown, we sampled fish, invertebrates, and water-quality variables 

seasonally during 2006–08. In 2006, the summer drawdown was about 68 percent of full pool, which 

was less than a typical drawdown of 85 percent. We detected few changes in pelagic invertebrate 

densities, and catch rates, abundance, and sizes of fish when comparing values from spring to values 

from fall. We did note that densities of benthic insects in areas that were dewatered annually were 

lower than those from areas that were not dewatered annually. In 2007, the drawdown was 100 

percent (to run-of-river level) and resulted in decreases in abundance of invertebrates as much as 96 

percent, decreases in catch rates of fish as much as 80 percent, decreases in abundance of redside 

shiners (Richardsonius balteatus) and northern pikeminnow (Ptychocheilus oregonensis) as much as 

93 percent, and decreased numbers of small fish in catches. In the fall 2007, we estimated the total 

biomass of forage fish to be 76 kilograms, or about one-quarter of total biomass of forage fish in 

2006. Bioenergetics modeling suggested that ample forage for about 1,000 bull trout would exist 

after a moderate drawdown, but that forage remaining after a complete dewatering would not be 

sufficient for a population one-fifth the size. Our results indicate that drawdowns in Beulah 

Reservoir affect the aquatic community and perhaps the health and well-being of bull trout. The 

severity of effects depends on the extent of drawdown, population size of bull trout, and perhaps 

other factors. 
2 

Introduction

Bull trout (Salvelinus confluentus) were listed by the U. S. Fish and Wildlife Service (FWS) 

as a threatened species under the Endangered Species Act in 1998. Populations of bull trout are 

decreasing throughout their range and the State of Oregon has listed the North Fork Malheur River 

(NFMR) population as one “of special concern.” As a result, managers have spent considerable time 

and effort identifying, restoring, and preserving critical habitat and various life history forms 

(Muhlfeld and others, 2003). Reasons for the decrease of bull trout in the Malheur River basin 

include habitat degradation and fragmentation, losses through unscreened diversions, historic 

chemical treatment projects (Ratliff and Howell, 1992), and entrainment through Agency Valley 

Dam on the NFMR. 

Beulah Reservoir, which was formed after Agency Valley Dam was completed in 1935, 

currently supports a lacustrine-adfluvial population of bull trout that over-winter in the reservoir 

from November through early May (Gonzalez, 1998; Schwabe and Tiley, 1999). Over-wintering 

behavior of bull trout is similar in Flathead Lake, Montana, and Lake Billy Chinook, Oregon (Fraley 

and Shepard, 1989; Beauchamp and Van Tassell, 2001). Because the reservoir provides irrigation 

water to nearby farms and ranches, bull trout can experience changes in water level, habitat, and 

forage availability due to seasonal reservoir drawdowns. Furthermore, reservoir volume occasionally 

decreases to run-of-river levels, which could severely affect forage fish populations. Decreased 

forage fish populations could negatively affect the bull trout population in the NFMR drainage by 

contributing to decreased growth and survival. 

Understanding the resilience of the aquatic community in Beulah Reservoir to drawdown and 

occasional dewatering is important for assessing the effects of reservoir operations on bull trout. 

Open systems like streams and rivers generally are considered resilient to disturbances such as 

severe drought (Bayley and Osborne, 1993) and flooding (Matthews, 1986). Such resilience 

probably is due to large numbers of individuals in nearby tributaries or reaches that can quickly recolonize disturbed areas (Mathews, 1986; Bayley and Osborne, 1993). In contrast, reservoir fish 

communities may be more confined and less resilient to environmental disturbance. Drastic or illtimed changes in the water level of reservoirs can lead to the dewatering of spawning habitat, eggs, 

and larval fishes (Lantz and others, 1967; Estes, 1972), reduced growth and survival of fishes 

(Graham and others, 1981), and changes in aquatic vegetation, water chemistry, primary production, 

and the benthic food web (Benson and Hudson, 1975; Nichols, 1975; Woods and Falter, 1982; 

Gaboury and Patalas, 1984; Furey and others, 2006). Information on the resilience of the aquatic 

community in Beulah Reservoir to water-level changes would be useful for the management of 

threatened bull trout. 
3 

The FWS and the Bureau of Reclamation recently completed a Biological Opinion that 

addresses terms and conditions to minimize the effects of operations at Beulah Reservoir on bull 

trout. These terms (along with others) establish the need for conservation pool requirements based 

on water year conditions that supply adequate habitat and forage for bull trout in the reservoir. To 

assist in this endeavor, we (1) documented species composition, abundance, and distribution of 

forage fish in Beulah Reservoir in early spring and late fall, before and after summer drawdown; (2) 

described seasonal variation in aquatic insect abundance and distribution; (3) conducted 

bioenergetics modeling exercises to evaluate the consumption and growth of bull trout under varying 

levels of prey availability; and (4) evaluated previous data for Beulah Reservoir that might provide 

additional information on current conditions. We used this information to evaluate the effects of 

typical seasonal drawdowns and complete dewatering of the reservoir on various metrics of the 

aquatic community. 

Study Site.—Agency Valley Dam was built by the Bureau of Reclamation on the NFMR 

during 1934–35 at river km (rkm) 29 and formed Beulah Reservoir (fig. 1). The impoundment 

provides irrigation water to local farms and some flood control and has no facilities for upstream or 

downstream passage of fish. The reservoir is 1,020 m above sea level at full pool and has an average 

width of 1.9 km and a length of about 4 km. The NFMR and Warm Springs Creek enter from the 

north and the NFMR exits from the south end of the reservoir. The north end of the reservoir is 

relatively shallow (<10 m deep) with a low gradient bottom. The south end of the reservoir has a 

steep decline from shore and reaches a maximum depth of about 23 m. Although summer 

temperatures exceed 20°C at all depths throughout the reservoir (Petersen and others, 2003), it cools 

rapidly in the fall and typically ices over in winter. Beulah Reservoir is eutrophic (Bureau of 

Reclamation, 2002) with high abundances and diverse size classes of redside shiners (Richardsonius 

balteatus), redband trout (Oncorhynchus mykiss gairdneri), suckers (Catostomus sp.), and northern 

pikeminnow (Ptychocheilus oregonensis) (Petersen and others, 2003). The reservoir is stocked with 

hatchery rainbow trout (RBT) each spring. 

Methods

Water-Quality Sampling

We collected data on water quality (and sampled fish and aquatic insects) in Beulah 

Reservoir during spring (late March through early June) and fall (October through mid-November) 

of 2006 and 2007, and in only spring 2008. Water temperature, dissolved oxygen (DO), and water 

transparency were measured about every 2 weeks in three areas of the reservoir: (1) the deep, 

southern end; (2) the moderately deep, middle of the reservoir (when water depths were sufficient); 

and (3) the shallow northern end. Water temperature (oC) and DO (mg/L) were measured every 

meter from the surface to the bottom with a Yellow Springs Instruments© Model 85 multimeter.

Reservoir transparency was determined with a Secchi disk. Reservoir volume, river flows, and river 

temperature data were obtained from Bureau of Reclamation gaging stations 

(www.usbr.gov/pn/hydromet, accessed February 27, 2009). 
4 

Aquatic Insect Sampling

We estimated the density and biomass of pelagic insects in the reservoir using a 0.5-m2

We also estimated the abundance and biomass of benthic insects in substrates that 

experienced different levels of dewatering. We used data from 1977 to 2005 on reservoir elevations 

after summer drawdown (

, 500-

μm mesh conical zooplankton net towed vertically from about 1-m above the bottom to the water 

surface. During spring and fall, we conducted three net tows from randomly selected sites within 

each of four geographic areas (that is, northwest, northeast, southwest, and southeast) of the 

reservoir and combined catches into four composite samples by area. The samples were preserved in 

the field with 70 percent ethanol. In the laboratory, organisms were identified to Order and number 

and total weight recorded (blotted dry and adjusted to approximate living weights; Howmiller, 

1972). Mean weights of individuals were calculated by dividing the total number of insects per 

sample by their combined weight. Samples were averaged to derive metrics for the reservoir in each 

season.

www.usbr.gov/pn/hydromet, accessed February 27, 2009) to estimate 

areas of reservoir substrates that were frequently, occasionally, or usually not dewatered. Substrates 

that were frequently dewatered were higher than 1,010-m elevation and represented about 25 percent

of reservoir depths; substrates that were occasionally dewatered ranged from 997- to 1,010-m 

elevation and represented about 62 percent of reservoir depths; and the typically inundated 

substrates were less than 997-m elevation and represented about 13 percent of reservoir depths. We 

collected two benthic samples from random locations within these strata using a Ponar©

Fish Sampling

dredge 

during the spring when the reservoir was nearly full. We repeated this process every 2 weeks and 

processed samples as described above. 

Fish were sampled about every other week with experimental gillnets and fyke nets. The 

gillnets were 36.5 m long by 3.0 m deep and contained six 6-m panels with stretch mesh sizes of 8.9, 

7.6, 6.3, 5.1, 3.8, and 2.5 cm. Gillnets generally were fished on the bottom during daylight hours for 

30 min or less. Fyke nets (91 cm height, 122 cm width, and 0.6-cm mesh size) were set with a 13-m 

center lead extending to shore and fished overnight. We used a stratified (that is, by general 

geographic region—northwest, northeast, southwest, and southeast) randomized design to establish 

sampling locations for each gear type with sample sites replaced at the start of each session.

Captured fish were anesthetized (using either 50 mg/L Finquel® MS-222 for non-game fish and bull 

trout or one tablet of Alka-Seltzer Gold© in 2.5 L of water for game fish), identified to species 

(except for sculpins), measured for fork length (FL, in mm), weighed (only a subsample to the 

nearest 0.1 g), marked (by removing a fin or placing a Floy© tag in the dorsal musculature for fish 

>150 mm), and released. Later, we estimated the weights of individuals that were not weighed using 

the length-weight relation derived from fish that we did weigh. 

We estimated the population abundance for the most common fish (including young-of-year, 

subadult, and adult fish) in the spring and fall using the Schumacher-Eschmeyer estimator for 

multiple census mark-recapture studies (Schneider, 1998). We assumed compliance with all 

assumptions associated with this estimator (see Lagler, 1956, for a summary). During spring, we 

ended population estimates of RBT before the release of hatchery fish into the reservoir. Population 

estimates during fall, however, included all RBT that were <156 mm. We derived biomass estimates 

for each species by multiplying the mean weight of fish in each cohort by their estimated population 

size. 
5 

Insect and Fish Response to Reservoir Drawdown

To examine the effects of reservoir drawdown on aquatic insects and fish in Beulah 

Reservoir, we compared several metrics between seasons (and years) from our sampling. For our 

pelagic insect data, we compared mean density of insects collected during each season and year.

Because few planktonic chironomids were captured during fall, we restricted their analysis to spring 

data only. We derived the same metrics for benthic chironomids and compared values for those 

collected from areas that were frequently, occasionally, or generally not dewatered using data from 

2006 and 2007 combined and for each sample year using pooled data from the three categories of 

benthic strata. We also compared the mean mass of benthic chironomids from each reservoir strata. 

All statistical comparisons of invertebrate data were done with a Kruskal-Wallis test (α =0.05) 

followed by the Dunn-Šidák post-test procedure to maintain a tolerable experimentwise α level

(Sokal and Rohlf, 2003). 

We compared the mean fyke net and gillnet catch rates (CPUE) of dace, northern 

pikeminnow, redside shiner, suckers, and white crappie by number and biomass between 

consecutive seasons (for example, spring versus fall 2006, fall 2006 versus spring 2007, and so on) 

with analysis of variance (ANOVA) followed by the Bonferroni post-test procedure (Sokal and 

Rohlf, 2003). All catch data were log10

Bioenergetics Modeling

(x+1) transformed prior to analysis but are presented 

untransformed here. Standardized length frequency distributions (by N per 24 h) of fish captured in 

fyke and gillnets were plotted for the more common fish species (northern pikeminnow, suckers, and 

redside shiners) by season. We excluded RBT from statistical comparisons because we could not 

determine the influence that stocked fish had on our results. We also qualitatively compared the 

Fulton condition factor (Nielsen and Johnson, 1983) of bull trout for different periods and to fish 

from other areas. 

We used a bioenergetics model parameterized for lake trout (Salvenlinus namaycush; 

Stewart and others, 1983) to estimate the seasonal food consumption of bull trout in Beulah 

Reservoir. The lake trout model has been used in previous work with bull trout (Beauchamp and 

Van Tassell, 2001), and because these two species are closely related and occupy similar ecological 

niches, the parameters of the lake trout model are reasonable surrogates for those of bull trout. 

The intent of our simulations was to determine if sufficient prey exists in Beulah Reservoir 

after summer drawdown to sustain a population of bull trout of a certain size from fall through 

spring. Consumption was estimated for two hypothetical populations: (1) the mean estimated

spawning population of bull trout in the NFMR (N = 188; Schwabe and others, 2006); and (2) a 

population of 1,000 adult bull trout, which is an abundance recovery criterion for isolated 

populations of these fish (Malheur Watershed Council and Burns Piute Tribe, 2004). We ran 

simulations from November 1 to May 29 (210 days) using three hypothetical growth rates: (1) low—

0.01 g per day, which represented a maintenance growth rate of 1 percent; (2) medium—0.64 g per 

day, which was estimated from bull trout in the NFMR (Schwabe and Tiley, 1999; B Rose and M 

Mesa, USGS, unpub. data, 2008); and (3) high—2.3 g per day, which was estimated from bull trout 

in Lake Billy Chinook, Oregon (Beauchamp and Van Tassell, 2001). The initial biomass of fish on 

day 1 (about 231 g) was derived by subtracting the observed growth rate of bull trout in the NFMR 

from the mean mass of bull trout collected in the spring from Beulah Reservoir (Schwabe and Tiley, 

1999; this report). Final biomass of bull trout (on day 210) was dependent on the growth scenario 

used in the simulation. As such, simulations using the medium growth rate approximated the mean 

size of bull trout that overwinter in the reservoir. We obtained temperature data for the simulations 
6 

from the literature (Bureau of Reclamation, 2002; Petersen and Kofoot, 2002; Peterson and others, 

2003; Rose and Mesa, 2007) and from our fieldwork (table 1). We assumed that bull trout would 

feed in reservoir areas with temperatures near their maximum feeding temperature (see Mesa and 

others, 2007) and that they would not occupy the reservoir from late spring to fall because of high 

temperatures. 

To provide input on diet composition to the model, we analyzed stomach contents from 10 

bull trout (range of FL = 263–286 mm) captured in Beulah Reservoir during spring and fall 2007. 

These fish ate a variety of organisms (table 2) and we used the mean proportions by weight of 

invertebrates and fish from the stomach samples as input to the model. For some of our simulations, 

we assumed that bull trout ate only fish during the fall (simulation days 1-30) and had a diet 

comprised of 60 percent fish, 20 percent insects, and 20 percent zooplankton in the spring (days 

121–210). The model interpolated the diet of bull trout for simulation days 31–120. Mass of 

zooplankton eaten by bull trout was estimated using the length-weight relation for Daphnia (Pechen, 

1965) assuming a mean length of 2.5 mm (Wilhelm and others, 1999). The mass of preserved 

Dipterans was adjusted to approximate living mass (Howmiller, 1972). We also conducted 

simulations using a diet comprised of only fish and assumed an indigestible fraction of 3 and 15 

percent of total biomass for fish and invertebrates (Beauchamp and Van Tassell, 2001). The energy 

density of common prey fish in Beulah Reservoir (N = 9–10 for each species) was measured with 

bomb calorimetry during spring and fall 2006 and used as input to the model. Overall, energy 

density of prey fish ranged from 5.07 to 5.63 kJ/g (table 3). Energy densities for prey items used in 

our model simulations were 5.35 kJ/g (days 1–30) and 5.37 kJ/g (days 121–210) for fish, 3.35 kJ/g 

for chironomids, and 3.56 kJ/g for zooplankton (Luecke and Brandt, 1993; Beauchamp and others, 

1995). 

Our fieldwork provided estimates of the abundance and biomass of prey fish available for 

bull trout in Beulah Reservoir during falls 2006 and 2007. We ran separate simulations for each year 

assuming a prey fish abundance of about 122,000 and a biomass of 266 kg in fall 2006 and about 

23,000 and 76 kg in fall 2007. These numbers reflect the extent of reservoir drawdown in each year, 

with 2006 being moderately dewatered and 2007 being completely dewatered. Because bull trout 

can feed on fish as much as 50 percent of their length (Beauchamp and Van Tassell, 2001); all fish 

<156 mm (mean bull trout length × 0.5) were considered vulnerable to predation by bull trout. For 

our analysis, we plotted the decrease in prey biomass during the simulation period for each set of 

assumptions (that is, population size, growth rates, and diet composition) to determine if prey 

biomass could limit the production of bull trout. 
7 

Results

Reservoir Volume and Streamflow

The volume of Beulah Reservoir and flow from the NFMR varied greatly during the study 

(fig. 2). In spring 2006, Beulah Reservoir was filled to 99 percent of its 0.07 km3

 (59,900 acre-ft) 

capacity and was drawn down from early summer through fall to 0.023 km3 (18,656 acre-ft). This 

represented a drawdown of about 68 percent, which was less than the mean drawdown of 85 percent

from 1977 to 2005. Mean (± SD) daily streamflow in the NFMR upstream of Beulah Reservoir was 

7.0 ± 8.9 m3

/s, which was about twice the 10-year (1998–2007) average of 3.6 ± 3.7 m3/s. In 2007, 

Beulah Reservoir filled to 84 percent of its capacity and was drawn down to run-of-river level on 

August 30. During this dewatering, most of the reservoir was shallow (<0.2 m) and maximum 

depths were about 1 m. Except for several suckers, large numbers of fish mortalities upstream of the 

dam were not evident. Downstream of the dam, however, a large fish kill did occur (fig. 3). Mean 

flow in the NFMR was 2.0 ± 1.4 m3

/s, or about 55 percent of average. During spring 2008, Beulah 

Reservoir was filled to 89 percent of capacity, and, as of July 30, mean flow in the NFMR was 3.9 ± 

3.7 m3

Water Quality

/s, or about 65 percent of the 10-year average.

Mean daily water temperatures at the outflow of Beulah Reservoir ranged from near 0 in 

winter to greater than 20°C in summer depending on the year (fig. 4). There was substantial 

variation in temperatures during early spring 2008 and spring and summer temperatures in 2007 

were warmer than temperatures in 2006 and 2008. Reservoir temperature and DO profiles were 

similar for each sampling site. Temperatures near the bottom of the reservoir were less than 13° C 

for all sample sites and study periods (fig. 5). Dissolved-oxygen concentrations were less than 6.5 

mg/L throughout most of the reservoir only on October 14, 2006 (fig. 6). Stratification occurred 

during spring 2006 and again during early May 2008, but anoxic conditions near the bottom did not 

develop. Water clarity varied among study periods and years (fig. 7). In all years, Secchi depths 

generally were lower in the fall than in the spring. In the spring, Secchi depths were lowest in 2006 

(mean ± SD; 1.2 ± 0.3 m), highest in 2008 (2.5 ± 0.3 m), and intermediate in 2007 (2.0 ± 0.3 m). In 

the fall, Secchi depths were higher in 2006 (1.1 ± 0.2 m) than in 2007 (0.7 ± 0.4 m). 

Pelagic Invertebrates

Pelagic invertebrates representing seven Orders were collected from 2006 to 2008. The most 

common were Hydracarina and Diptera (primarily chironomids). Others collected (in descending 

order of abundance) included Ephemeroptera, Odonata, Coleoptera, Trichoptera, and Oligochaeta. 

Mean Hydracarina densities ranged from 0 during fall 2007 to 1.55 ± 1.21 individuals/m3 during 

spring 2007 (fig. 8). Mean densities of Hydracarina in the spring 2007 were significantly higher than 

mean densities in falls 2006 and 2007 (P < 0.01). No other comparisons were statistically

significant. 
8 

Chironomid densities were highest during spring 2006 (0.57 ± 0.67 individuals/m3), lowest 

during spring 2008 (none captured), and intermediate during spring 2007 (0.15 ± 0.18 

individuals/m3

Benthic Invertebrates

; fig. 8). Only two chironomids were captured during falls 2006 and 2007. 

Differences, however, in the springtime densities of chironomids between the years were not 

significant. 

Benthic invertebrates representing three Orders were collected during our study, including 

Dipterans (primarily chironomids), Oligochaetes, and Trichopterans (in descending order of 

abundance). For all years combined, the mean densities of Dipterans (by number and biomass) were 

significantly (P < 0.01) lower in areas of the reservoir that were frequently dewatered (100 ± 190 

individuals/m3 and 0.7 ± 0.7 g/m3) when compared to areas that were occasionally dewatered (785 ± 

426 individuals/m3and 26.8 ± 11.1 g/m3) and usually not dewatered (1,311 ± 834 individuals/m3 and 

51.8 ± 36.1 g/m3

Fish Sampling Overview

; fig. 9). Dipterans collected from areas that were frequently dewatered also were 

significantly (P < 0.01) smaller (0.01 ± 0.01 g) than dipterans collected from other areas (0.04 ± 0.01 

g). Overall, the mean densities of benthic dipterans in the spring were significantly lower in 2008 

than in 2006 and 2007 (fig. 10). Compared to 2006 and 2007, the densities of benthic chironomids in 

the spring decreased from 84 to 96 percent in 2008. 

We sampled with both gear types throughout the reservoir (figs. 11 and 12) and the effort 

was relatively similar between spring and fall of each year. In total, we captured 33,175 fish in 8,678 

h of sampling with small mesh fyke nets and gillnets (table 4). Fish captured in fyke nets comprised 

98 percent of our total catch. The most common fish species by number were redside shiners (54 

percent of total catch) and northern pikeminnow (32 percent of total catch). In contrast, RBT and 

largescale suckers (Catostomus macrocheilus) dominated the biomass of our catches, comprising 32 

and 28 percent of the total. Other taxa collected were bridgelip sucker (Catostomus columbianus), 

longnose dace (Rhinichthys cataractae), speckled dace (Rhinichthys osculus), white crappie 

(Pomoxis nigromaculatus), sculpins (Cottus sp.), chiselmouth (Acrocheilus alutaceus), mountain 

whitefish (Prosopium williamsoni), and bull trout. 

Fish Catch per Unit Effort

Catch rates of fish in Beulah Reservoir varied greatly by season, year, and gear type. The 

overall CPUE of fyke nets for all species combined was highest during spring 2007 and spring 2006 

(6.0–6.5 fish/h or FPH), lowest in fall 2007 and spring 2008 (1.2–1.5 FPH), and intermediate during 

fall 2006 (5.0 FPH; table 4). When we used gillnets, overall CPUE was highest in fall 2006 (10.6 

FPH), lowest during springs 2006 and 2008 (about 1.8 FPH), and intermediate during spring (4.7 

FPH) and fall 2007 (4.5 FPH; table 4).

Catch rates of fyke nets for dace ranged from 0.25 ± 0.56 FPH (mean ± SD) during spring 

2006 to 0.01 ± 0.02 FPH during fall 2007 (fig. 13). Catch rates of dace during spring 2006 were 

5–48 times higher than other study periods and significantly higher than catch rates in fall 2006 

(P < 0.001). Pairwise comparisons of CPUE for other sample periods were not significant. Dace 

≤50 mm comprised 76–83 percent of our fyke net catch before and about one-half of our catch after 
9 

the reservoir was dewatered (fig. 14). The proportion of dace 51–70 mm collected increased by 

about 81–200 percent after the reservoir was dewatered. Dace >70 mm were more common during 

spring 2008 (7 percent of catch) than during all other periods (≤4 percent of catch). Dace were not 

captured by gillnets.

Catch rates of fyke nets for northern pikeminnow ranged from 2.90 ± 6.73 FPH during fall 

2006 to 0.20 ± 0.41 FPH during fall 2007 and did not differ between sample periods (fig. 15). Catch 

rates of small northern pikeminnow (<156 mm, the size that would be suitable prey for bull trout) 

ranged from 2.89 ± 6.72 FPH during fall 2006 to 0.19 ± 0.39 FPH during fall 2007 and again did not 

differ between samples. The CPUE of these fish were 4–15 times higher before the reservoir was 

dewatered than after the reservoir was dewatered. Northern pikeminnow ≤100 mm represented most 

of catches (87–95 percent) for all sample periods (fig. 16). During spring 2006, the modal length of 

small pikeminnow (range = 61–65 mm) was greater than modal length from other years (range = 

36–50 mm). Northern pikeminnow 101–200 mm comprised only 4–9 percent of the catches with 

fyke nets. The percentage of northern pikeminnow >200 mm in the catches was highest (about 

9 percent of annual catch) immediately following dewatering (that is, fall 2007). When the reservoir 

refilled in spring 2008, large fish comprised less than 1 percent of the total catch. The catch rates of 

gillnets for all sizes of northern pikeminnow ranged from 6.09 ± 9.20 FPH during fall 2006 to 

0.07 ± 0.36 FPH during fall 2007 (fig. 15). The catch rates of gillnets after reservoir dewatering (in 

summer 2007) were 63–99 percent lower than catch rates before dewatering. Total catch rates 

significantly increased from spring to fall 2006 (P<0.001) and decreased significantly from spring to 

fall 2007 (P<0.001). The CPUE of gillnets for smaller northern pikeminnow were highest during fall 

2006 (2.65 ± 4.39 FPH) and nil after the reservoir was dewatered. The CPUE of gillnets for small 

northern pikeminnow significantly increased from spring to fall 2006 (P <0.001) and significantly 

decreased from fall 2006 to spring 2007 and from spring to fall 2007 (P <0.01; fig. 15). Prior to 

reservoir dewatering, fish from 101 to 200 mm comprised 70–93 percent of the catch by gillnet and 

they were not collected after the reservoir was dewatered (fig. 16). Northern pikeminnow >200 mm 

comprised from 7 percent (fall 2006) to 100 percent (fall 2007) of the catch by gillnet. Northern 

pikeminnow ≤100 mm were not collected by gillnets.

The CPUE of fyke nets for redside shiners ranged from 4.40 ± 9.81 FPH during spring 2007 

to 0.74 ± 1.71 FPH during spring 2008 (fig. 17). Catch rates of redside shiners were 80–83 percent

lower after the reservoir was dewatered when compared to rates before dewatering. Mean CPUE of 

redside shiners significantly decreased from spring to fall 2007 (P<0.01); all other pairwise 

comparisons did not differ. The modal lengths of redside shiners collected by fyke nets were higher 

for spring 2007 (56–60 mm) than modal lengths for other study periods (31–50 mm; fig. 18. Redside 

shiners ≤50 mm comprised from 28 percent (springs 2007 and 2008) to 88 percent (fall 2006) of the 

catch by fyke net. Redside shiners 51–70 mm were more common during spring 2007 (57 percent of 

total catch) than for other years (7–26 percent). Prior to dewatering of the reservoir, fish >70 mm 

comprised only 4–16 percent of the catch. After the reservoir was dewatered, fish >70 mm 

comprised 30–46 percent of the catch. When we used gillnets, mean CPUE of redside shiners ranged 

from 0.33 ± 1.17 FPH during spring 2007 to 0.02 ± 0.21 during spring 2008. Although CPUE 

decreased from 55 to 62 percent after the reservoir was dewatered (that is, from spring to fall 2007), 

no seasonal comparisons significantly differed. The sizes of redside shiners also were similar 

between sample periods (fig. 18). 
10

Mean CPUE of fyke nets for suckers of all sizes ranged from 0.53 ± 1.22 FPH during spring 

2007 to 0.06 ± 0.12 FPH during spring 2008 (fig. 19). Catch rates were 76–88 percent lower after 

the reservoir was dewatered when compared to rates before dewatering. Mean CPUE of all suckers 

significantly decreased from spring to fall 2007 (P<0.001). The catch rates of fyke nets for smaller 

suckers (<156 mm) ranged from 0.36 ± 1.19 FPH during spring 2007 to 0.02 ± 0.06 FPH during fall 

2007. Mean CPUE for these smaller suckers significantly decreased from spring to fall 2006 

(P<0.05; a 58-percent decrease) and from spring to fall 2007 (P<0.001; a 95- percent decrease).

Immediately after the reservoir was dewatered, suckers ≤100 mm collected by fyke nets comprised 

only 14 percent of the catch compared to 47–67 percent of the catch during other periods (fig. 20).

Suckers from 101 to 200 mm comprised 9 and 14 percent of the catch for fall 2006 and spring 2007, 

and 21–29 percent of the catch for other sample periods. The percentages of suckers >200 mm in the 

catches were highest immediately following dewatering (65 percent), lowest for spring 2006 

(4 percent), and intermediate for other sample periods (24–36 percent). The CPUE of gillnets for all 

suckers ranged from 2.85 ± 3.41 FPH during fall 2007 to 0.20 ± 0.78 FPH during spring 2006 

(fig. 19). Mean CPUE of gillnets for all suckers significantly differed (P<0.001) for all seasonal 

comparisons. Mean CPUE of gillnets for small suckers were low (<0.05 FPH) and did not differ 

between sample periods (fig. 19). Suckers ≤100 mm were collected only during spring 2006 

(8 percent of their catch; fig. 20). Suckers 101–200 mm comprised 12–27 percent of the catch by 

gillnet before the reservoir was dewatered and only 0–3 percent of the catch after reservoir 

dewatering. Suckers >200 mm comprised most of the catch (73–100 percent) by gillnet and suckers

were somewhat more common after the reservoir was dewatered (97–100 percent) compared to 

before reservoir dewatering (73–88 percent).

Mean CPUE of fyke nets for white crappie were highest during fall 2006 (0.14 ± 0.23 FPH) 

and spring 2007 (0.12 ± 0.24 FPH) and white crappie were not collected after the reservoir was 

dewatered (fig. 21). Mean CPUE of fyke nets for white crappie significantly increased from spring 

to fall 2006 (P<0.001) and significantly decreased from spring to fall 2007 (P<0.001). Prior to 

dewatering, white crappies 50–150 mm comprised 75–100 percent of the catch (fig. 22). Crappies 

151–250 mm comprised 4 and 25 percent of the catch for spring 2007 and fall 2006, and they were 

not collected during spring 2006. Fish >250 mm were collected by fyke nets only during fall 2006 

(1 percent of the catch). When we used gillnets, mean CPUE of white crappie was highest during 

fall 2006 (1.49 ± 3.39 FPH), nil during fall 2007 and spring 2008, and intermediate during spring 

2007 (fig. 21). Mean CPUE of gillnets for white crappie significantly increased from spring to fall 

2006 (P<0.001) and significantly decreased from fall 2006 to spring 2007 (P<0.001) and from 

spring to fall 2007 (P<0.01). White crappies from 50 to 150 mm comprised 93–100 percent of the 

catch by gillnet (fig. 22). Crappies from 151 to 250 mm were collected by gillnets only during fall 

2006 (7 percent of catch) and fish >250 mm were not collected by this gear.

Fish Population Estimates

We estimated population abundance of different size cohorts of redside shiners, northern 

pikeminnow, and RBT in Beulah Reservoir during springs and falls 2006 and 2007 and spring 2008. 

Redside shiners and northern pikeminnow were the most abundant fish during all sample periods 

and their population estimates varied considerably between sample periods. All fish showed a 

substantial decrease in abundance following the reservoir dewatering in 2007. 

Population estimates for all sizes of redside shiners were highest during spring sample 

periods (except for 2008) and lowest during the fall (fig. 23). Abundance was highest in the spring 

2007 (198,313 ± 48,100) and lowest in the fall of the same year (14,777 ± 3,172). After a moderate 
11

reservoir drawdown during summer 2006, the population estimate for all sizes of redside shiners 

decreased by 44 percent from spring to fall (fig. 23). The magnitude of decrease was similar for 

juvenile (<68 mm; 43 percent) and adult fish (51 percent). After the reservoir was dewatered in 

summer 2007, the abundance of redside shiners in the fall was 93 percent lower than the abundance

in the spring. Again, the magnitude of the decrease was similar for juvenile and adult fish. The 

population estimate for redside shiners of all sizes was 70 percent higher in spring 2008 (about 

6 months after the reservoir was dewatered) than in fall 2007 (immediately after dewatering).

Abundances of juvenile redside shiners were similar between fall 2007 and spring 2008 whereas the 

abundance of adult fish increased fourfold. Adult redside shiners represented about one-half of the 

total population during spring 2008, which was considerably higher than other periods where adults 

comprised 6–24 percent of the total. 

Population estimates of northern pikeminnow that would be suitable prey for bull trout (that 

is, fish <156 mm) were highest during spring 2006 (80,881 ± 32,884) and lowest during fall 2007 

(3,940 ± 2,497; fig. 23). Population sizes of all fish in spring 2006 were 2 to 20 times higher than 

other sample periods. After the moderate drawdown of Beulah Reservoir in summer 2006, the 

abundance of prey-sized northern pikeminnow in fall 2006 was 52 percent lower than the abundance 

from the previous spring and was about 15 percent lower in spring 2007 than fall 2006 (fig. 23). The 

population estimate for all sizes of northern pikeminnow after the reservoir was dewatered (fall 

2007) was 88 percent lower than the estimate from spring 2007 (fig. 23). In spring 2008, the 

abundance of all sizes of northern pikeminnow was 66 percent higher than the abundance from fall 

2007. 

Population estimates for smaller RBT that would be suitable prey for bull trout were less

precise because of low sample sizes and recapture rates (no estimate was derived for spring 2006). 

Estimates ranged from 6,321 ± 6,272 fish in spring 2008 to 1,920 ± 1,435 fish in spring 2007 (fig.

23). The estimate from fall 2006 was twice that from spring 2007, but the 95-percent CI almost 

entirely overlapped. The population size of RBT more than doubled after the reservoir was 

dewatered and increased by 53 percent from fall 2007 to spring 2008 (fig. 23). 

Catch of Bull Trout

We did not catch any bull trout in 2006, but caught 5 fish during spring 2007, 3 in fall 2007, 

and 14 in spring 2008. In spring 2007, we caught 3 bull trout in the northwest and 2 in the southern 

region of the reservoir, soon after the start of irrigation withdrawal. Of the 3 fish we captured in fall 

2007, 2 were recaptured once during our sampling and the third fish was originally tagged in spring 

2007. Of the 14 bull trout we captured during spring 2008, we recaptured 3 of these fish once. All 

bull trout in 2008 were captured in the northern half of the reservoir. Bull trout collected during 

spring 2007 were <300 mm, whereas bull trout collected from fall 2007 were >300 mm (fig. 24).

Bull trout collected during spring 2008 ranged in length from 261 to 476 mm. Mean (± SD) 

condition factors for bull trout were highest in fall 2007 (1.13 ± 0.08), lowest in spring 2008 

(1.00 ± 0.09), and intermediate in spring 2007 (1.08 ± 0.10; fig. 25). All bull trout in 2007 had 

values of K greater than 1.00, but only 35 percent of fish collected in 2008 had K values exceeding 

1. All bull trout that were recaptured in 2007 showed an increase in mass between captures (range 

0.4–0.8 g/day), whereas fish that were recaptured during spring 2008 lost mass (range 1.2–2.0 

g/day). Bull trout fed on invertebrates and fish during springs 2007 and 2008, and fed only on fish 

during fall 2007. Bull trout captured during 2007 (10 percent) and 2008 (70 percent) had less than 

1 g of food in their stomachs. 
12

Bioenergetics Modeling

Our simulations indicated that the per capita consumption of fish and invertebrates by bull 

trout was highest in spring, lowest in winter, and correlated with reservoir temperature (fig. 26). The 

growth rates used in our simulations had obvious effects on the total consumption by populations of 

188 and 1,000 bull trout (table 5). For example, the predatory impact of a population of 188 bull 

trout with a diet comprised of only fish and a high growth rate (2.3 g/day) was over twice that of fish 

with a medium growth rate (0.64 g/day) and over five times greater than fish with a low growth rate 

(0.01 g/day; table 5). 

The total consumption by a hypothetical population of 188 bull trout during our simulation 

period exceeded the available prey biomass in the fall 2006 (which we estimated was about 266 kg) 

when they consumed only fish and grew at a high rate (fig. 27). When their diet was comprised of 

fish and invertebrates, bull trout consumed from 19 to 96 percent of the available prey fish and from 

14 to 120 percent of the prey fish when their diet was comprised of fish only. When we conducted 

simulations using a population of 1,000 bull trout, total consumption exceeded the available prey 

fish, depending on growth rate and regardless of diet composition (fig. 27). A population of 1,000 

bull trout with a medium growth rate (0.64 g/day) eliminated their prey fish base on days 145–180 

and those with a high growth rate (2.3 g/day) exhausted their prey on days 70–81. In contrast, bull 

trout with a low growth rate (0.01 g/day) exhausted their forage base when their diet was comprised 

of only fish (day 192). 

In fall 2007, we estimated the biomass of available prey fish in Beulah Reservoir to be just 

76 kg, or less than one-quarter of the value in 2006. The estimated total consumption by a 

population of 188 bull trout exceeded the available forage, depending on diet composition and 

growth rate (fig. 28). For a population of 188 bull trout, only those with a low growth rate (0.01 

g/day) did not exhaust their prey fish base regardless of their diet composition. The number of days 

for a population of 188 bull trout to eliminate their prey for other diet and growth scenarios ranged 

from 113 to 181 days. When we conducted simulations using a population of 1,000 bull trout, total 

consumption exceeded the available prey fish base in every scenario we modeled (fig. 28). The 

number of days it required for 1,000 bull trout to eliminate their prey fish base ranged from 17 to 86 

days, depending on diet composition and daily growth rate. 

Discussion

The purpose of our research at Beulah Reservoir was twofold. First, we wanted to determine 

whether typical water-level management of Beulah Reservoir adversely affected the prey base (fish 

and invertebrates) for bull trout. Second, we wanted to determine whether changes to prey 

populations could adversely affect consumption and production by overwintering bull trout within 

the reservoir. Our results indicate that the effects of a summer drawdown on the aquatic community 

within Beulah Reservoir depended on the magnitude of the event. In 2006, where the drawdown was 

relatively mild compared to other years, we detected few changes in the fish community when 

comparing metrics from fish caught in the spring to fish caught in the fall. However, we did note that 

the densities of benthic insects in areas that were frequently dewatered were lower than those in 

areas that remain inundated, which could have implications for bull trout foraging. In 2007, Beulah 

Reservoir was drawn down to run-of-river levels and the amount of habitat available for fish and 

invertebrates after dewatering was <1 percent of habitat available at full pool and all deep-water 

habitat was eliminated. As such, the densities of pelagic and benthic invertebrates decreased, the 

abundance of some fish species decreased, the sizes of fish in the catches changed, and we estimated 
13

the biomass of prey fish in Beulah Reservoir in fall 2007 to be just 76 kg, or less than one-quarter of 

the value in 2006. Our bioenergetics modeling exercises suggested that the moderate drawdown of 

2006 left enough prey fish in the fall to provide even a large (≤1,000) population of bull trout with, 

at the least, a maintenance ration of forage for overwintering. In contrast, the extreme drawdown of 

2007 severely reduced the prey fish biomass available in the fall and our simulations indicated that 

consumption by even a modest population of bull trout could exceed the available forage. 

Collectively, our results indicate that current water-level management of Beulah Reservoir does 

affect the aquatic community and may affect the health and well-being of overwintering bull trout.

As we mentioned earlier, the severity of effects would be dependent on the extent of drawdown, 

population size of bull trout, their diet composition, and perhaps other unknown factors. Because we 

only evaluated the effects of two levels of drawdown—one moderate and one extreme—we can not 

predict the effects of drawdowns that occur between the levels we studied. At the least, our results 

provide information that could help establish minimum pool levels in Beulah Reservoir for the 

maintenance of forage fish and bull trout. 

Water-level management in Beulah Reservoir is similar to other reservoirs in the Western 

United States in that a summer drawdown typically begins in April and May and continues into 

September and early October. However, water-level fluctuations in Beulah Reservoir seem to be 

more severe than most other reservoirs (see www.usbr.gov/pn/hydromet, accessed February 27, 

2009). Although this management strategy may facilitate providing irrigation water, it contrasts with 

a water management strategy for the benefit of fish, in which reservoirs would be full during spring 

and summer for growth and spawning of fish and reduced during winter (Hulsey, 1957). The wet 

winter of 2005–06 allowed Beulah Reservoir to fill earlier and stay full longer when compared to 

other years. This resulted in an early start to withdrawal (March) and water releases over the 

spillway, which may increase entrainment of fish through Agency Valley Dam (Schwabe and Tiley, 

1999). The drier winter and spring 2007 led to a reduced reservoir volume and drafting that caused 

the reservoir to reach run-of-river levels on August 30. During this extreme drawdown, mortalities 

of fish upstream of the dam were not common (except for several suckers) but downstream of the 

dam, a large fish kill occurred. Because of the large number of fish involved in the die-off, we 

surmise that most of these fish were entrained and that some of the changes we saw in our fish 

population metrics after drawdown were due to entrainment. Capturing and moving some of these 

fish back to the reservoir, or installing a fish barrier at Agency Valley Dam, may offset some of 

these losses. 

Water-quality variables differed between the spring and fall. The magnitude of drawdown in 

Beulah Reservoir had minor effects on water quality when comparing metrics in the spring to those 

in the fall. As such, changes in water quality after drawdown probably had little effect on fish health 

or well being. For example, we rarely recorded temperatures >15oC or DO concentrations 

<6.5 mg/L, which are threshold values considered suitable for bull trout (Rieman and McIntyre, 

1993; Bureau of Reclamation, 2002). Most of the time, water temperatures in the reservoir and in the 

NFMR were less than the tolerance limits of other species, except on occasion for mountain 

whitefish and perhaps sculpin (Black, 1953; Scott and Crossman, 1973; Symons and others, 1976; 

Kaya, 1978; Castleberry and Cech, 1986; Eaton and others, 1995; Hillman and others, 1999; Selong 

and others, 2001). Despite the high productivity of Beulah Reservoir (Bureau of Reclamation, 2002), 

anaerobic conditions were not evident in the hypolimnion. 

We observed considerable changes in water clarity between seasons and years at Beulah 

Reservoir. The decreases in mean Secchi depths from spring to fall were likely related to increases

in primary production (Bureau of Reclamation, 2002) and to low reservoir volumes mixing with 
14

incoming and re-suspended sediments (Fox and others, 1977; Popp and Hoagland, 1995). The 

relatively low water clarity during spring 2006 may have resulted from increased sediment inputs 

from tributary streams (Popp and Hoagland, 1995) and from water being released over the spillway, 

which facilitates nutrient exchange from deeper, nutrient-rich waters and increases primary 

production (Murphy, 1962). The effects of changes in water turbidity on the behavior and foraging 

success of bull trout are unknown. 

The drawdowns we studied in Beulah Reservoir resulted in lower densities of pelagic 

invertebrates, lower densities of chironomids in sediments that were frequently dewatered, and a 

smaller size of individuals in areas of frequent dewatering. All these changes could affect bull trout 

in the reservoir because invertebrates can be an important forage item for these fish at certain times 

of the year. For example, Wilhelm and others (1999) noted that the diet of bull trout (>250 mm) in 

high alpine lakes of the Canadian Rocky Mountains (where other fishes were not present) contained 

more than 90 percent chironomids following ice out. Beauchamp and Van Tassel (2001) reported 

that 69–88 percent of the diet of bull trout (200–400 mm) consisted of invertebrates from January to 

May in Lake Billy Chinook, Oregon. Our results showed that bull trout could consume substantial 

quantities of invertebrates, indicating that more detailed study of the effects of reservoir 

management on invertebrate populations is warranted. A detailed description of the aquatic 

invertebrates in Beulah Reservoir and the effects of drawdown on them were beyond the scope of 

our work. 

Although the most common pelagic invertebrates we captured were Hydracarina and 

chironomids, our highest mean catch during any sample period was only 1.5 Hydracarina per m3

. 

We only caught relatively high numbers of Hydracarina in spring 2007—at all other times our catch 

was essentially nil. Our catches were on the low side of the range reported by other studies, which 

spanned from an “occasional event” to more than 100 individuals per m3 (Gliwicz and Biesiadka, 

1975; Riessen, 1982; Cassano and others, 2002). Because water mites are considered unpalatable for 

fish (Kerfoot, 1982), we do not know the influence that changes in their populations would have on 

overwintering bull trout in Beulah Reservoir. Our results, however, suggest that Hydracarina 

densities were severely reduced after the complete dewatering of 2007. For pelagic chironomids, we 

only captured them in relatively high numbers in springs 2006 and 2007—catches were essentially 

nil during other periods. Even so, our highest mean catch was only 0.5 individuals per m3 

We observed fewer and smaller chironomids in sediments of the reservoir that were 

frequently dewatered compared to areas that typically were inundated, which is similar to the results 

of Furey and others (2006). This may be because only some taxa of chironomids can tolerate shortterm drying of the substrate (Fillion, 1967; Pinder, 1986). Because of the limited scope of our 

invertebrate sampling, we did not attempt to quantify annual losses of chironomids due to reservoir 

drawdown. Benson and Hudson (1975) found that reducing fall drawdown by 4–5 m yielded a threefold increase in benthic macroinvertebrate production during May. For newly impounded reservoirs, 

colonization of the benthos by chironomids begins the summer following impoundment when their 

growth, reproduction, and dispersal are highest (Paterson and Fernando, 1969; Voshell and 

Simmons, 1984). Similarly, our results suggest that recolonization of the benthos by chironomids 

and our 

catch rates decreased gradually from 2006 to 2007, suggesting that summer water-level 

manipulations, and perhaps other factors, were affecting chironomid populations. Our results also 

indicate that chironomid densities were severely reduced after the complete dewatering of 2007. In 

the fall, chironomids may have been subjected to increased predation due to high densities of 

predators and altered swimming behavior of the insects (Yamagishi and Fukuhara, 1971; Mousavi 

and others, 2002; Takagi and others, 2005). 
15

occurs the summer after Beulah Reservoir is dewatered and that the amount of biomass available for 

bull trout is low during late fall and winter following a dewatering event. More study is needed to 

understand the relation between chironomid density in the benthos, their densities in open water, and 

the influence of summer drawdowns. 

Despite the reduction of nearshore habitat available for fish in the fall, our catches of fish in 

fyke nets were similar between spring and fall 2006, indicating that fish densities did not change 

appreciably during a year of moderate drawdown. In 2007, however, which included a complete 

dewatering of the reservoir, catch rates of most species significantly decreased from spring to fall.

Our results suggest that moderate reductions in water volume (1) do not elicit widespread emigration 

of fish from the reservoir, (2) concentrates fish in offshore areas, and (3) leads to stable or even 

increased catch rates of many species of fish. For example, the catch rates by gillnet of northern 

pikeminnow, suckers, and white crappie all increased in fall 2006. In contrast, complete dewatering 

events may lead to large-scale emigrations of fish upstream and downstream. Although we did not 

sample the river upstream of the reservoir, the fish kill that occurred downstream of Agency Valley 

Dam late in summer 2007 indicates that many fish left the reservoir. Although sampling was more 

difficult in the run-of-river conditions left after dewatering, the low densities of fish probably were 

real and not an artifact of sampling because poor catch rates persisted into spring 2008. This 

indicates that fish density remained low and that limited recovery of the fish community occurred 

over the winter. Because we only evaluated two levels of drawdown in Beulah Reservoir and the 

response of different species to these manipulations varies, it is difficult to ascribe clear cause-effect 

relations between drawdown, water volume, and fish abundance. Our results indicate that higher 

reservoir levels and increased shoreline cover from spring through fall improves the overwinter 

survival of many fishes or that immigration of fish into the reservoir from tributaries increases when 

river flows were higher. Miranda and others (1984) and Willis (1986) suggested that higher reservoir 

levels and increased shoreline cover improve survival for subyearling largemouth bass (Micropterus 

salmoides). 

Besides affecting the overall catch rates of some species, water-level management in Beulah 

Reservoir had obvious effects on the sizes of fish in the reservoir. Specifically, the dewatering event 

of 2007 severely reduced the numbers of all sizes of dace, northern pikeminnow <200 mm, redside 

shiners from 20 to 65 mm, and suckers of all sizes. The severe drawdown completely eliminated 

white crappie. Our results indicate that the effects of a single dewatering in Beulah Reservoir occur 

quickly and can be dramatic. The drastic changes in water levels probably lowered the reproductive 

success of fishes by dewatering eggs and larvae, reducing the amount of spawning substrate, and 

exposing juveniles to increased predation (Lantz and others, 1967; Heman and others, 1969; Estes, 

1972; Willis, 1986). Some fish, however, seemed a bit more resilient to the dewatering event of 

2007. For example, our catch rates of juvenile dace increased in spring 2008, indicating that these 

fish may have persisted in tributaries while the reservoir was dewatered and they moved into the 

reservoir after it started to refill. Indeed, Gonzalez and others (1998) documented downstream 

migrations of dace in the NFMR upstream of Beulah Reservoir during fall. The presence of largersized native fishes, like suckers, also suggests that these fish show some resilience to reservoir 

dewatering. 
16

Although our results clearly indicate that a single dewatering event can negatively affect fish 

populations in Beulah Reservoir, some evidence suggests that the effects may be short lived. For 

example, our catch rates of fish in 2006 and spring 2007 and the sizes of fish represented in those 

catches were indicative of robust populations with diverse age structures. In particular, most 

individuals collected were smaller and consisted of a high proportion of subyearling fish. Our work 

in 2006 was conducted about 1.5–2.5 years after consecutive dewatering events in Beulah Reservoir 

from 2002 to 2004. Petersen and Kofoot (2002) and Petersen and others (2003) sampled Beulah 

Reservoir during summers 2001 and 2002—prior to the 3 consecutive years of dewatering—and also 

reported that diverse age structures of fish were present. Thus, if severe drawdown events were 

having lasting effects on prey fish populations in Beulah Reservoir, we would have expected some 

evidence of this when we started sampling in 2006. However, the reasons why our catches in 2006 

were high and contained many small fish could be due to higher reproductive success from adults 

that persisted in the reservoir, increased recruitment in a relatively predator-free post-dewatering 

environment, and emigration of fishes from areas upstream of the reservoir. In 2006, there also was 

increased shoreline cover and spawning areas available for fish at the higher pool levels, which is 

important for fish production (Stroud, 1948; Johnson, 1963; Scott and Crossman, 1973; Nelson and 

Walburg, 1977; Martin and others, 1981). Paller (1997) documented similar increases in smaller 

individuals and decreases in larger individuals of various warm water species after a 4-year 

drawdown of Par Pond, South Carolina. In the end, documenting the long-term effects of a single 

drawdown or multiple, consecutive drawdowns on fish populations was beyond the scope of our 

study. We found that about 8 months after a complete dewatering event, many aspects of the fish 

populations in Beulah Reservoir change substantially, including relative abundance, absolute 

abundance, and size structure. We also noted that high catch rates of fish with diverse size structures 

are possible in Beulah Reservoir about 1.5 years after three consecutive dewatering events.

However, we have little context to interpret this information. Based on a review of gillnet catch data 

from 1955 to 1970, Petersen and others (2003) also concluded that reservoir dewatering does not 

cause long-term changes to the fish community in Beulah Reservoir. Sampling of the fish 

populations in Beulah Reservoir in the absence of moderate or severe drawdowns would provide 

information on the natural seasonal variability of selected metrics and a baseline for understanding 

the effects of drawdown. 

Our estimates of population size for most species were limited by our ability to recapture 

fish. Therefore, we only estimated abundance for the species that were most common and 

consistently recaptured, including redside shiners, northern pikeminnow, and RBT. The moderate 

drawdown of 2006 may have affected populations of redside shiners and northern pikeminnow, both 

of which had higher abundance in the spring than in the fall. However, there are many possible 

explanations for this change in population size, including an influx of subyearling fishes from 

tributaries during late fall and early winter, changes in vulnerability of fish to the gear, losses due to 

predation, outmigration of larger fishes, and direct losses due to the drawdown. Although abundance 

estimates were lower during the fall, fish biomass may continue to increase from the fall through 

spring if over-winter growth occurs. In contrast, the dewatering of the reservoir in 2007 reduced the 

populations of northern pikeminnow and redside shiners by more than 90 percent. Population 

estimates for RBT, however, were higher after dewatering than before dewatering. Gonzalez and 

others (1998) documented downstream migrations of wild RBT in the NFMR during fall through 

spring and perhaps some RBT migrated into Beulah Reservoir during fall, avoided entrainment, and 

dealt with the stressful conditions associated with the dewatering. As such, the effects of dewatering 

on RBT populations may not be evident until several years later, when reductions in adult population 
17

size may result in limited recruitment. The situation with RBT in Beulah Reservoir is even more 

confusing considering the planting of hatchery fish that occurs most springs (table 6). Because RBT 

could potentially compete with bull trout for resources in Beulah Reservoir, the ecology and biology 

of RBT warrant further study. 

The dewatering of Beulah Reservoir and the resultant changes in prey populations may have 

had some effects on bull trout. Of all the bull trout we captured in Beulah Reservoir, only fish from 

spring 2008 had condition factors lower than 1.0. The mean condition factor of bull trout in Beulah 

Reservoir during spring 2008 also was 17 percent lower than that of similar sized fish from Lake 

Billy Chinook, Oregon (Beauchamp and Van Tassell, 2001). Given the decreases in abundance of 

fish following the dewatering event of 2007, it seems plausible that prey availability for bull trout in 

the winter 2007 and spring 2008 was sparse, perhaps resulting in lower fish conditions. Furthermore, 

the extent of reservoir drawdown is correlated with the estimated spawning population size of bull 

trout in the NFMR (fig. 29; Schwabe and others, 2006). However, sampling bull trout was not a 

focus of our study, sample sizes were low, and more information is needed to confirm any notions 

about prey availability, condition of bull trout, and the effects of water-level management on their 

health and well being. 

Our bioenergetics modeling simulations suggested that current water-level management 

could limit bull trout production in Beulah Reservoir and the NFMR. Our analysis revealed that 

under a moderate drawdown, there probably would be enough prey fish in the fall to support a large 

population of bull trout under some growth and diet scenarios. After a complete dewatering of the 

reservoir, however, predation by even a modest population of bull trout could exceed or remove a 

high proportion of the prey biomass. The key variables determining the extent of predator impact in 

Beulah Reservoir were the biomass of prey available in the fall, the number of bull trout inhabiting 

the reservoir, diet composition of bull trout, and their growth rate. Of these, we had empirical data 

on prey biomass, and the diet and growth of bull trout to base our simulations on. We did not know 

how many bull trout actually used the reservoir for overwintering and modeled two hypothetical 

populations based on recovery criteria. Obviously, this was a key variable in our bioenergetics 

analysis and is an area of study for future research. Because of the possibility that bull trout can 

deplete their forage base, dewatering of Beulah Reservoir may adversely affect the health and wellbeing of bull trout. 

There are several ways to improve our bioenergetics analysis. First, as we just alluded to, 

more information is needed on the number and percentage of bull trout of all sizes in the NFMR that 

actually use Beulah Reservoir during certain times. This probably is the biggest gap in our 

knowledge of the potential predatory impact of bull trout in Beulah Reservoir and is fundamental for 

evaluating the adequacy of the forage base. Such information also would help define the life history 

of bull trout in the NFMR and could be useful for establishing recovery criteria. For example, if 

substantial numbers of resident bull trout exist in the NFMR, then a recovery criterion based on 

adult population size (for example, 1,000–1,500 fish; U.S. Fish and Wildlife Service, 2002) could be 

distributed amongst interconnected populations. We also need more information on the activity, diet, 

feeding rates, and growth of bull trout from fall through spring to parameterize the model with 

relevant data from the field. For example, we used diet information and growth data from only a 

small number of individuals and were not able to partition diet data into size cohorts or describe 

temporal variation with much detail. Evaluating the predatory impact of other species, particularly 

northern pikeminnow and redband trout, would provide a more thorough understanding of predatorprey interactions in Beulah Reservoir and potential effects of reservoir management on bull trout. 

Clearly, bull trout predation does not occur in a vacuum and a more thorough, food-web based 
18

analysis would be more informative and useful. Finally, evaluating recruitment of prey fish during 

the simulation period would provide a better depiction of prey dynamics within the reservoir. 

Addressing these, and perhaps other, issues about bull trout and their prey in the NFMR basin would 

not only improve model results, but it also would allow for more informed decision making 

regarding reservoir operations and species recovery. 

Summary 

The overall goal of our research was to evaluate the effects of seasonal drawdowns on 

measures of insect and fish production in Beulah Reservoir. We wanted to use this information to 

determine a pool level, or volume, in Beulah Reservoir to maintain after drawdown to ensure 

adequate forage for bull trout that may overwinter in the reservoir. In 2 consecutive years, we 

evaluated the effects of a moderate (about a 68-percent decrease in pool volume) drawdown and a 

complete dewatering of Beulah Reservoir and concluded that adequate forage for bull trout would 

exist after the moderate but not the severe drawdown. The complete dewatering, and to a lesser 

extent the moderate drawdown, resulted in decreases in the abundance of pelagic and benthic 

invertebrates, the relative abundance and biomass of several species of fish, the absolute abundance 

of northern pikeminnow and redside shiners, and changes in the size and species composition of our 

catches. These changes were evident at least 8 months after the dewatering event and we do not 

know how long it will take fish and insect populations to recover. Based on bioenergetics modeling 

simulations, we hypothesize that the forage fish populations remaining after a moderate drawdown 

would be sufficient for a large population of bull trout under several growth and diet scenarios. After 

a complete dewatering, however, there would be inadequate time for forage fish populations to 

recover and the biomass of prey would be insufficient for even a modest overwintering population of 

bull trout. 

Because we sampled Beulah Reservoir before and after only two drawdown events, we can

not determine the minimum pool volume needed to sustain a suitable forage fish population for bull 

trout. Continued monitoring of fish populations in Beulah Reservoir over a wider range of 

drawdown scenarios, particularly moderate to high levels, would increase our understanding of the 

effects of reservoir operations and may allow for minimum pool recommendations. In addition, 

because of their importance to the diet of bull trout, more detailed information on the responses of 

aquatic insect populations to drawdown in Beulah Reservoir would be prudent. Finally, and perhaps 

most importantly, detailed study of the movements, distribution, habitat use, and general life history 

of bull trout in the NFMR and Beulah Reservoir are needed to develop recommendations for water 

management of the reservoir. 

Acknowledgments

We thank Katy Hanna, Alexis Koenings, and Joseph Mullins for their invaluable assistance 

in the field and during data processing. Tim Walters and Ray Perkins from Oregon Department of 

Fish and Wildlife, and Lawrence Schwabe and Jason Fenton of the Department of Fish and Wildlife 

of the Burns Paiute Tribe, provided field gear and technical advice. This manuscript benefited from 

the careful peer review of Tim Walters, Sally Sauter, and biologists from the Bureau of Reclamation. 
19

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23

Figure 1. Map showing general location and tributaries of Beulah Reservoir, Oregon. 
24

2004

0

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Reservoir volume (%) 40

Outflow from Beulah Reservoir

Inflow into Beulah Reservoir

2005

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Data not available

Figure 2. Discharge of water entering and exiting Beulah Reservoir

and reservoir volume, 2004–08. Our study occurred during years 

marked with an asterisk.
25

Figure 3. Photographs of a fish kill that occurred downstream of Agency Valley Dam on the North Fork of the 

Malheur River during the fall of 2007 after the end of irrigation season and the release of water. By November, 

the fish kill extended about 500 m downstream Photograph taken by Brien P Rose, USGS, November 2007). 
26

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan

Temperature (°C)

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Figure 4. Mean daily temperatures at the outflow of Beulah Reservoir, 2006–08. 
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0 5 10 15 20

4/11

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Temperature (°C)

0 5 10 15 20

2006 2007 2008

Figure 5. Water-temperature profiles in three regions of Beulah Reservoir, 2006–08. The length of each profile 

corresponds to depths of the reservoir for each period. The middle and north sites were not sampled on April 

17, 2006, and data were not collected at some sites due to insufficient water depths or because the sites were 

dry.
28

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Dissolved Oxygen (mg/L)

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4/11

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5/09

Dissolved Oxygen (mg/L)

4 6 8 10 12 14

2006 2007 2008

Figure 6. Dissolved-oxygen profiles in three regions of Beulah Reservoir, 2006–08. The length of each profile 

corresponds to depths of the reservoir for each period. The middle and north sites were not sampled on April 

17, 2006, and data were not collected at some sites due to insufficient water depths or because the sites were 

dry.
29

2006

0

1

2

3

2007

Secchi Disk Depth (m) 

0

1

2

3

2008

Mar May Jul Sep Nov 

0

1

2

3

Figure 7. Mean (± SD) Secchi disk depths measured in Beulah Reservoir, 2006–08. Means were derived from 

sampling at three sites (when water depths permitted).
30

Spring 2006

Spring 2007

Spring 2008

Number per m

3

0.0

0.5

1.0

1.5 NS NS

Spring 2006

Fall 2006

Spring 2007

Fall 2007

Spring 2008

Number per m

3

0.0

0.5

1.0

1.5

2.0

2.5

3.0

NS ** *** NS

Figure 8. Mean (± 95-percent confidence intervals) and median (white circles) catch per m3 of Hydracarina 

(upper panel) and chironomids (lower panel) collected with vertical net tows in Beulah Reservoir during spring, 

2006–08. Asterisks denote significant differences in catch rates between consecutive sampling periods (that is, 

* = P < 0.05, ** = P < 0.01, *** = P < 0.001). NS = not significant.
31

Frequently 

Occasionally

Usually Not 

Number per m

2

0

500

1000

1500

2000

Frequently 

Occasionally

Usually Not 

Mass per Individual (g)

0.00

0.01

0.02

0.03

0.04

0.05

0.06

b

b

b b

a

a

Figure 9. Mean (± 95-percent confidence intervals) and median (white circles) catch per m2 (upper panel) and 

mass (lower panel) of benthic dipterans collected from locations in Beulah Reservoir that were frequently, 

occasionally, or usually not dewatered during typical summer drawdown, 2006–07. Means with letters in 

common did not differ significantly (P > 0.05). 
32

Spring 2006

Spring 2007

Spring 2008

Chironomids per m

2

0

400

800

1200

1600

a

a

b

 

Figure 10. Mean (± 95-percent confidence intervals) and median (white circles) catch per m2 of benthic 

dipterans collected during the spring in Beulah Reservoir, 2006–07. Areas of reservoir substrate that were 

frequently, occasionally, or usually not dewatered were pooled for analysis. Means with letters in common did 

not differ significantly (P > 0.05).
33

Figure 11. Locations of sampling with fyke nets in Beulah Reservoir, 2006–08. 
34

Figure 12. Locations of sampling with gillnets in Beulah Reservoir, 2006–08.
35

Spring 06

Fall 06

Spring 07

Fall 07

Spring 08

Fish per Hour

0.0

0.1

0.2

0.3

0.4

***

NS

NS

NS

 

Figure 13. Mean (and 95-percent confidence interval) and median (white circles) catch per hour of fyke nets for 

species of dace in Beulah Reservoir, 2006–08. Asterisks denote significant differences in catch rates between 

consecutive sampling periods (that is, * = P < 0.05, ** = P < 0.01, *** = P < 0.001). NS = not significant. 
36

Fork Length (mm)

20 30 40 50 60 70 80

Fish per 24 Hours

0.5

1.0

1.5

2.0

Spring 2006

Fall 2006

Spring 2007

Fall 2007

Spring 2008

Figure 14. Catch rates of fyke nets for longnose and speckled dace of different sizes in Beulah Reservoir, 

2006–08. Data were plotted at 5-mm intervals. 
37

0

1

2

3

4

5

6 NS NS NS NS

Fish per Hour 0

2

4

6

8

10 *** NS *** NS

Fyke Nets

Gillnets

all

Spring 06

Fall 06

Spring 07

Fall 07

Spring 08

Fish per Hour

0

1

2

3

4

5

6 NS NS NS NS

<156mm

Spring 06

Fall 06

Spring 07

Fall 07

Spring 08

0

1

2

3

4

5

6 *** *** ** NS

 all

<156mm

Figure 15. Mean (and 95-percent confidence interval) and median (white circles) catch per hour of fyke nets 

and gillnets for northern pikeminnow in Beulah Reservoir, 2006–08. Asterisks denote significant differences in 

catch rates between consecutive sampling periods (that is, * = P < 0.05, ** = P < 0.01, *** = P<0.001). NS = not 

significant. 
38

100 120 140 160 180 200

5

10

15

20

25

30

Gillnet

200 250 300 350 400 450 500

1

2

3

4

5

Fyke Net

0 20 40 60 80 100

Fish per 24 Hours

4

8

12

16

20

Fork Length (mm)

Fork Length (mm)

100 200 300 400 500

Fish per 24 Hours

0.1

0.2

0.3

0.4

0.5

0.6

Spring 2006

Fall 2006

Spring 2007

Fall 2007

Spring 2008

Figure 16. Catch rates of fyke nets (upper panels) and gillnets (lower panels) for northern pikeminnow of 

different sizes in Beulah Reservoir, 2006–08. Data were plotted at 5-mm intervals for fish <201 mm and at 20-

mm intervals for fish >200 mm. Data for larger fish were multiplied by 0.25 to facilitate visual comparisons 

across sizes.
39

Fyke Nets

Gillnets

Spring 06

Fall 06

Spring 07

Fall 07

Spring 08

Fish per Hour

0

2

4

6

8 NS NS *** NS

Spring 06

Fall 06

Spring 07

Fall 07

Spring 08

Fish per Hour

0.0

0.2

0.4

0.6

0.8 NS NS NS NS

Figure 17. Mean (and 95-percent confidence interval) and median (white circles) catch per hour of fyke nets 

and gillnets for redside shiners in Beulah Reservoir, 2006–08. Asterisks denote significant differences in catch 

rates between consecutive sampling periods (that is, * = P < 0.05, ** = P < 0.01, *** = P < 0.001). NS = not 

significant.
40

Fyke Net

Fork Length (mm)

0 20 40 60 80 100 120

Fish per 24 Hours

0

5

10

15

20

25

Spring 2006

Fall 2006

Spring 2007

Fall 2007

Spring 2008

Gillnet

Fork Length (mm)

0 20 40 60 80 100 120

Fish per 24 Hours

0.0

0.5

1.0

1.5

2.0

2.5

 

Figure 18. Catch rates of fyke nets (upper panel) and gillnets (lower panel) for redside shiners of different sizes 

in Beulah Reservoir, 2006–08. Data were plotted at 5-mm intervals.
41

0.0

0.2

0.4

0.6

0.8

1.0 NS NS *** NS

Fyke Nets

Gillnets

Fish per Hour

0

1

2

3

4

5 *** *** *** ***

All

Spring 06

Fall 06

Spring 07

Fall 07

Spring 08

Fish per Hour

0.0

0.2

0.4

0.6

0.8

1.0 * NS *** NS

 <156mm

 All

Spring 06

Fall 06

Spring 07

Fall 07

Spring 08

0.00

0.05

0.10

0.15

0.20

0.25 NS NS NS NS

<156mm

Figure 19. Mean (and 95-percent confidence interval) and median (white circles) catch per hour of fyke nets 

and gillnets for species of suckers in Beulah Reservoir, 2006–08. Asterisks denote significant differences in 

catch rates between consecutive sampling periods (that is, * = P < 0.05, ** = P < 0.01, 

*** = P < 0.001). NS = not significant.
42

Gillnet

100 200 300 400 500

Fish per 24 Hours

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Fyke Net

0 20 40 60 80 100

Fish per 24 Hours

0.0

0.5

1.0

1.5

2.0

100 200 300 400 500

0.0

0.1

0.2

0.3

0.4

0.5

Spring 2006

Fall 2006

Spring 2007

Fall 2007

Spring 2008

Fork Length (mm)

Fork Length (mm)

Figure 20. Catch rates of fyke nets (upper panels) and gillnets (lower panel) for bridgelip and largescale 

suckers of different sizes in Beulah Reservoir, 2006–08. Data were plotted at 5-mm intervals for fish 

<201 mm and at 20-mm intervals for fish >200 mm. Data plotted for larger fish were multiplied by 0.25 to 

facilitate comparisons across sizes.
43

Spring 06

Fall 06

Spring 07

Fall 07

Spring 08

Fish per Hour

0.00

0.05

0.10

0.15

0.20

0.25

0.30 *** NS *** NS

Spring 06

Fall 06

Spring 07

Fall 07

Spring 08

Fish per Hour

0.0

0.5

1.0

1.5

2.0

2.5

3.0 *** *** ** NS

Fyke Nets

Gillnets

Figure 21. Mean (and 95-percent confidence interval) and median (white circles) catch per hour of fyke nets 

and gillnets for white crappies in Beulah Reservoir, 2006–08. Asterisks denote significant differences in catch 

rates between consecutive sampling periods (that is, * = P < 0.05, ** = P < 0.01, *** = P < 0.001). NS = not 

significant.
44

Fyke Net

Fork Length (mm)

50 100 150 200 250 300

Fish per 24 Hours

0.0

0.1

0.2

0.3

0.4

0.5

Spirng 2006

Fall 2006

Spring 2007

Fall 2007

Spring 2008

Gillnet

Fork Length (mm)

50 100 150 200 250 300

Fish per 24 Hours

0

2

4

6

8

10

12

14

Figure 22. Catch rates of fyke nets (upper panel) and gillnets (lower panel) for white crappies of different sizes 

in Beulah Reservoir, 2006–08. Data were plotted at 5-mm intervals.
45

Redside Shiner

Number of Fish

0

50000

100000

150000

200000

250000

300000

< 68 mm

>67 MM

Total

Redband Trout

Spring 2006 Fall 2006 Spring 2007 Fall 2007 Spring 2008

0

2000

4000

6000

8000

10000

12000

14000

16000

Northern Pikeminnow

0

20000

40000

60000

80000

100000

120000

Figure 23. Population abundance estimates (± 95-percent confidence intervals) for northern pikeminnow 

<156 mm, and redside shiners and redband trout of different sizes in Beulah Reservoir, 2006–08. No data were 

available for redband trout in the spring of 2006. 
46

Spring 2007

N = 5

0

1

2

3

4

Fall 2007

N = 5

Number of Fish

0

1

2

3

4

Spring 2008

N = 17

Fork Length (mm)

200 250 300 350 400 450 500

0

1

2

3

4

Figure 24. Length-frequency distributions of bull trout collected from Beulah Reservoir, 2007–08. 
47

Fork Length (mm)

200 250 300 350 400 450 500

Fulton Condition Factor

0.8

0.9

1.0

1.1

1.2

1.3

Spring 2008

Spring 2007

Fall 2007

Figure 25. Fulton’s condition factor of bull trout collected from Beulah Reservoir, 2007–08. 
48

High Growth Rate (2.30 g/d)

0

5

10

15

20

25

30

2

4

6

8

10

12

14

16

18

Medium Growth Rate (0.64 g/d)

Daily Consumption (g)

0

5

10

15

20

25

30

Reservoir Temperature (°C)

2

4

6

8

10

12

14

16

18

Low Growth Rate (0.01 g/d)

Simulation Day

0 50 100 150 200

0

5

10

15

20

25

30

2

4

6

8

10

12

14

16

18

Zooplankton

Fish

Macroinvertebrates

Reservoir Temperature (°C)

Figure 26. Per capita daily consumption estimates of bull trout for three prey types in Beulah Reservoir under 

three different growth rates and assuming a diet comprised of 20 percent zooplankton, 60 percent fish, 20

percent invertebrates. Bioenergetics model simulations ran for 210 days from fall through spring. Growth rates 

of bull trout were representative of fish in Lake Billy Chinook, Oregon (upper panel), Beulah Reservoir and 

North Fork of the Malheur River (middle panel), and a maintenance growth rate of about 

1 percent during the simulation period (lower panel). The solid line denotes the mean daily water temperature in 

Beulah Reservoir during the simulation period.
49

Fish and Invertabrate Diet

0

20

40

60

80

100

Medium growth rate (0.64 g/day)

High growth rate (2.30 g/day)

Low growth rate(0.01 g/day)

All Fish Diet

Simulation Day

0 50 100 150 200

Prey Fish Population Remaining (%)

0

20

40

60

80

100

Figure 27. Predicted biomass of fish <156 mm remaining in Beulah Reservoir after consumption by 

hypothetical populations of 188 (black lines) or 1,000 (grey lines) bull trout with three different growth rates 

during a 210-day period from fall through spring, 2006–07. Prey fish biomass remaining was calculated by 

subtracting the cumulative consumption estimates of each population from the initial prey fish biomass in the 

fall. The simulations in the top graph assumed a diet comprised of 60 percent fish, 20 percent invertebrates, 

and 20 percent zooplankton, and the bottom graph assumed a diet comprised of only fish. 
50

Simulation Day

0 50 100 150 200

0

20

40

60

80

100

Fish and Invertebrate Diet

Prey Fish Biomass Remaining (%)

0

20

40

60

80

100

Medium growth rate (0.64 g/d) 

High growth rate (2.30 g/d) 

Low growth rate (0.01 g/d) 

All Fish Diet

Figure 28. Predicted biomass of fish <156 mm remaining in Beulah Reservoir after consumption by a 

hypothetical population of 188 (black lines) or 1,000 (grey lines) bull trout with three different growth rates 

during a 210-day period from fall through spring, 2007–08. Prey fish biomass remaining was calculated by 

subtracting the cumulative consumption estimates of each population from the initial prey fish biomass in the 

fall. The simulations in the top graph assumed a diet comprised of 60 percent fish, 20 percent invertebrates, 

and 20 percent zooplankton, and the bottom graph assumed a diet comprised of only fish. 
51

Minimum Reservoir Pool (acre-feet)

Water Year

1992 1994 1996 1998 2000 2002 2004 2006 2008

Redd Count

0

20

40

60

80

100

120

140

160

1991 1993 1995 1997 1999 2001 2003 2005 2007

0

5000

10000

15000

20000

25000

30000

Redd count

Minimum pool level

Redd Count Year

Figure 29. Number of bull trout redds counted in the North Fork Malheur River and the minimum pool level of 

Beulah Reservoir. The x- axes are plotted offset by 1 year. 
52

Table 1. Day of simulation, reference month, temperature range in Beulah Reservoir during that month, and 

temperatures used in bioenergetics modeling of bull trout feeding and growth. 

Simulation 

day Reference month

Temperature range

°C 

Temperature

modeled

1 November 8.5-5.7 8.5

31 December 4.5-4.5 4.5

61 January 5.0-2.0 5.0

91 February 5.0-4.5 5.0

121 March 8.0-6.0 8.0

151 April 13.0-12.5 13.0

181 May 15.5-8.5 15.5

Table 2. Percent occurrence ( %O), percent of total number (%N), and percent of total mass (%M) of prey 

items consumed by bull trout in Beulah Reservoir during the spring (N=5) and fall (N=5) of 2007.

Spring 2007 Fall 2007

Prey type %O %N %M %O %N %M

Northern pikeminnow 20 <1 59 0 0 0 

Redband trout 20 <1 25 60 30 88

Redside shiner 20 <1 12 60 70 12

Zooplankton 40 95 1 0 0 0

Diptera 20 3 4 0 0 0
53

Table 3. Mean length (and SD), weight, sample size, and energy density of fish from Beulah Reservoir that we 

used in bioenergetics modeling of bull trout feeding and growth. 

[Fish were collected during the spring and fall of 2006. The energy densities of invertebrates were from the literature and 

assumed constant between seasons. RBT = redband trout, RSS = redside shiner, NPM = northern pikeminnow] 

Season and

Species

Mean

length

(mm)

Mean

weight

(g) N

Mean caloric

density 

(kJ/g)

Spring

RBT 108 (9.3) 14.7 (3.8) 9 5.08 (0.46) 

RSS 78 (6.1) 6.8 (1.4) 10 5.63 (0.45) 

NPM 99 (18.4) 12.9 (10.4) 10 5.36 (0.61) 

All Fish 94 (18) 11.3 (7.0) 29 5.37 (0.55) 

- - - 

Fall

RBT 88 (9.2) 6.7 (2.2) 9 5.56 (0.24) 

RSS 70 (12.7) 4.6 (2.3) 11 5.07 (0.30) 

NPM 63 (5.5) 2.7 (0.6) 10 5.47 (0.37) 

All Fish 73 (13.9) 4.6 (2.4) 30 5.35 (0.37) 

Daphnia - - - 3.56 

Invertebrates - - - 3.35 

Table 4. Fyke net and gillnet sampling effort and overall catch rates of fish in Beulah Reservoir, 2006–08. 

Fyke net Gillnet

Sampling 

period

N

of 

sets

Hours 

sampled

Mean 

soak time 

(h)

Overall

catch

(fish/hour)

N

of 

sets

Hours 

sampled

Mean 

soak time

(h)

Overall

catch

(fish/hour)

Spring 2006 71 1,650 23 6.0 142 61.3 0.4 1.8

Fall 2006 39 1,143 29 5.0 47 16.6 0.4 10.6

Spring 2007 76 1,813 24 6.5 73 27.2 0.4 4.7

Fall 2007 65 1,514 23 1.5 83 38.2 0.5 4.5

Spring 2008 100 2,378 24 1.2 80 36.5 0.5 1.8
54

Table 5. Bioenergetics model estimates of consumption (kg) by bull trout with a hypothetical population size of 

188 or 1,000 fish using three growth rates and two diet scenarios. 

[Simulations ran for 210 days during late fall through spring] 

Fish and invertebrate

diet

____________________________

_

All fish

Diet

________________

____

Growth rate and 

population size Fish

MacroinvertebratesZooplankton Fish

0.01 g/day

188 49.1 11.3 11.3 60.5

1,000 261.0 60.0 60.0 321.8

0.64 g/day

188 105.1 25.4 25.4 130.5

1,000 559.2 135.2 135.2 694.4

2.30 g/day

188 255.3 65.9 65.9 319.8

1,000 1,357.9 350.4 350.4 1701.0

Table 6. Number of rainbow trout that were stocked in Beulah Reservoir, 2004–08. 

[T = triploid fish] 

 

Year Date stocked Number stocked Fish Stock

2004 5/12/2004 35,000 Oak Springs

2005 5/12/2005 30,000 Oak Springs

2006 5/16/2006 79,974 Oak Springs (T=47%)

2007 5/15/2007 80,000 Oak Springs (T)

2008 5/15/2008 29,970 Oak Springs (T)
Publishing support provided by the U.S. Geological Survey 

Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the 

Director, Western Fisheries Research Center

U.S. Geological Survey, 6505 NE 65th Street

Seattle, Washington 98115

http://wfrc.usgs.gov/
Rose and Mesa— Minimum Pool and Bull Trout Prey Base Investigations at Beulah Reservoir—Final Report for 2008—Open-File Report 2009-1068