Prepared in cooperation with the Washington Department of Fish and Wildlife

Assessing Fish Predation on Migrating Juvenile Steelhead 

and a Retrospective Comparison to Steelhead Survival 

Through the Priest Rapids Hydroelectric Project, 

Columbia River, Washington, 2009–11

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2012–1129

Assessing Fish Predation on Migrating Juvenile Steelhead 

and a Retrospective Comparison to Steelhead Survival 

Through the Priest Rapids Hydroelectric Project, 

Columbia River, Washington, 2009–11

By Jill M. Hardiman, Timothy D. Counihan, U.S. Geological Survey; Dave S. Burgess, Katrina E. Simmons, 

Washington Department of Fish and Wildlife; Glen S. Holmberg, U.S. Geological Survey; Josh Rogala, and Rochelle 

Polacek, Washington Department of Fish and Wildlife

Prepared in cooperation with the Washington Department of Fish and Wildlife

Open File Report 2012–1129

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012 

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Suggested citation:

Hardiman, J.M., Counihan, T.D., Burgess, D.S., Simmons, K.E., Holmberg, G., Rogala, J.A., and Polacek, R.R., 2012, 

Assessing fish predation on migrating juvenile steelhead and a retrospective comparison to steelhead survival through 

the Priest Rapids Hydroelectric Project, Columbia River, Washington, 2009–11: U.S. Geological Survey Open-File 

Report 2012–1129, 36 p. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

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

Contents

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

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

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

Study Area ................................................................................................................................................................. 3

Site Selection............................................................................................................................................................. 3

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

Electrofishing.......................................................................................................................................................... 4

Angling ................................................................................................................................................................... 5

Diet Analysis .............................................................................................................................................................. 5

Data Analyses............................................................................................................................................................ 7

Relative Abundance Indices................................................................................................................................... 7

Consumption Indices.............................................................................................................................................. 7

Predation Indices.................................................................................................................................................... 8

Retrospective Steelhead Survival and Predation Analysis ..................................................................................... 8

Results........................................................................................................................................................................... 9

River Environment ..................................................................................................................................................... 9

Sampling Effort .......................................................................................................................................................... 9

Catch Data................................................................................................................................................................10

Relative Abundance Indices......................................................................................................................................10

Diet Analyses............................................................................................................................................................11

Predation Indices ......................................................................................................................................................11

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

Acknowledgments ........................................................................................................................................................16

References Cited..........................................................................................................................................................17

Figures

Figure 1. Study area depicting Priest Rapids Hydroelectric Project from upstream of Wanapum Dam to Priest Rapids 

Dam, Columbia River, Washington.............................................................................................................................. 19

Figure 2. River conditions, discharge, and temperature in degrees Celsius (°C) at Priest Rapids Dam, Columbia 

River, Washington, from May to August, 2011 (dashed line), 2010 (dotted line), 2009 (gray line), and the 10-year 

average (solid line). ..................................................................................................................................................... 20

Figure 3. Summary of sample effort (number of electrofishing runs) for a predator indexing study by Washington 

Department of Fish and Wildlife, and the number of acoustic-tagged juvenile steelhead released for a concurrent 

survival study from May 8 to June 10, 2011, Priest Rapids Project, Columbia River, Washington.............................. 21

Figure 4. Distribution of sample effort (number of electrofishing runs) completed across the reaches, May 10–June 9, 

2011, Priest Rapids Project, Columbia River, Washington .......................................................................................... 22

Figure 5. Daily juvenile salmonid downstream passage counts at Rock Island Dam, Columbia River, Washington, in 

2009 (top), 2010 (middle), and 2011 (bottom), showing predator data-collection periods with vertical straight lines... 23

Figure 6. Total lengths of northern pikeminnow captured during electrofishing efforts, May 10–June 9, 2011, Priest 

Rapids Project, Columbia River, Washington. ............................................................................................................. 24
iv

Figure 7. Total lengths of smallmouth bass captured during electrofishing efforts, May 10–June 9, 2011, Priest 

Rapids Project, Columbia River, Washington. ............................................................................................................. 24

Figure 8. Total lengths of walleye captured during electrofishing efforts,May 10–June 9, 2011, Priest Rapids Project, 

Columbia River, Washington. ...................................................................................................................................... 25

Figure 9. Spatial representation of catch per unit effort (number of fish caught per 10 minutes of boat electrofishing; 

CPUE) for northern pikeminnow (total length greater than 170 mm) and smallmouth bass (total length greater than 

150 mm) during May 10 to June 9, 2011, at the Priest Rapids Hydroelectric Project, Columbia River, Washington. . 26

Figure 10. Diet composition by weight of northern pikeminnow in 2011 (May 10–June 9), by reach in the Priest 

Rapids Project, Columbia River, Washington. ............................................................................................................. 27

Figure 11. Proportion of fish in diet that are salmon and steelhead for northern pikeminnow by reach during sampling 

in 2011 (May 10–June 9) in the Priest Rapids Project, Columbia River, Washington. Reaches with no column indicate 

no diet sample ............................................................................................................................................................. 28

Figure 12. Diet composition by weight of smallmouth bass in 2011 (May 10–June 9), by reach in the Priest Rapids 

Project, Columbia River, Washington. ......................................................................................................................... 29

Figure 13. Proportion of fish in diet that are salmon and steelhead for smallmouth bass by reach in 2011 (May 10–

June 9) in the Priest Rapids Project, Columbia River, Washington ............................................................................. 30

Tables

Table 1. Percentage of species composition by number of fish captured in the 2011 sampling season, Priest Rapids 

Project, Columbia River, Washington. ......................................................................................................................... 31

Table 2. Summary of mean and standard error of catch per unit effort (catch per 10 minutes of electrofishing) for 

northern pikeminnow (total length greater than 170 mm) and smallmouth bass (total length greater than 150 mm), 

Priest Rapids Project,, Columbia River, Washington, May 2–June 7, 2009, May 4–June 2, 2010, and May 10–June 9, 

2011............................................................................................................................................................................. 32

Table 3. Summary of the number of northern pikeminnow diets analyzed in 2011 during electrofishing efforts by the 

Washington Department of Fish and Wildlife (WDFW) and by angling efforts by U.S. Department of Agriculture 

(USDA), Priest Rapids Project, Columbia River, Washington...................................................................................... 33

Table 4. Summary of the number of smallmouth bass (SMB), largemouth bass (LMB), and walleye (WAL) diets 

analyzed 2011, Priest Rapids Project, Columbia River, Washington........................................................................... 33

Table 5. Results of visual diet inspection of piscivores in the field versus laboratory analysis (number of prey fish 

estimated).................................................................................................................................................................... 33

Table 6. Summary of the mean consumption (Ci), abundance (Ai), and predation (Pi) indices, and standard errors ( ) 

for northern pikeminnow and smallmouth bass collected during randomized electrofishing efforts and site selected 

(“hotspot”) efforts for reaches, Priest Rapids and Wanapum Reservoirs, Columbia River, Washington, 2011............ 34

Table 7. Summary of mean predation indices (Pi) and standard errors for northern pikeminnow and smallmouth bass 

collected during May 2–June 7, 2009, May 4–June 2, 2010, and May 10–June 9, 2011, by randomized electrofishing 

efforts and site selection (“hotspot” = HS) efforts for reaches in Priest Rapids and Wanapum Reservoirs, Columbia 

River, Washington. ...................................................................................................................................................... 35
v 

Conversion Factors and Abbreviations and Acronyms

Inch/Pound to SI 

Multiply By To obtain

Length

inch (in) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

Flow rate

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

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

°F=(1.8×°C)+32 

SI to Inch/Pound

Multiply By To obtain

Length

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.62137 mile (mi)

Area

hectare (ha) 2.47105 acre

square kilometer (km2) 247.105 acre

Flow rate

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

kilogram (g) 2.20462 pound (lb)
vi

Abbreviations and Acronyms

Abbreviation or Acronym Meaning

BiOp Biological Opinion

BRZ Boat Restricted Zone

CPUE Catch per unit effort

CRRL Columbia River Research Laboratory

FERC Federal Energy Regulatory Commission

GPS Global Positioning System

GIS Geographic Information System

NMFS National Marine Fisheries Service

PRCC Priest Rapids Coordinating Committee

RPAs Reasonable and Prudent Alternatives

PRP Priest Rapids Project

SOP Standard Operating Procedures

USGS United States Geological Survey

UTM Universal Transverse Mercator

WDFW Washington Department of Fish and Wildlife

USDA U. S. Department of Agriculture

GRTS Generalized Random Tessellation Stratified

GPP Generator Powered Pulsator

PUD Grant County Public Utility District Number 2

spp species

RM River mile

SE Standard error
1 

Assessing Fish Predation on Migrating Juvenile 

Steelhead and a Retrospective Comparison to Steelhead

Survival Through the Priest Rapids Hydroelectric Project, 

Columbia River, Washington, 2009–11

By Jill M. Hardiman, Timothy D. Counihan, U.S. Geological Survey; Dave S. Burgess, Katrina E. Simmons, 

Washington Department of Fish and Wildlife; Glen S. Holmberg, U.S. Geological Survey; Josh Rogala, and 

Rochelle Polacek, Washington Department of Fish and Wildlife

Abstract

The U. S. Geological Survey (USGS) and the Washington Department of Fish and Wildlife 

(WDFW) have been working with the Public Utility District No. 2 of Grant County, Washington 

(Grant PUD), to increase their understanding of predator-prey interactions in the Priest Rapids 

Hydroelectric Project (PRP), Columbia River, Washington. For this study, the PRP is defined as the 

area approximately 6 kilometers upstream of Wanapum Dam to the Priest Rapids Dam tailrace, 397.1 

miles from the mouth of the Columbia River. Past year’s low survival numbers of juvenile steelhead 

(Oncorhynchus mykiss) through Wanapum and Priest Rapids Dams has prompted Grant PUD, on 

behalf of the Priest Rapids Coordinating Committee, to focus research efforts on steelhead migration 

and potential causal mechanisms for low survival. Steelhead passage survival in 2009 was estimated at 

0.944 through the Wanapum Development (dam and reservoir) and 0.881 through the Priest Rapids 

Development and for 2010, steelhead survival was 0.855 for Wanapum Development and 0.904 for 

Priest Rapids Development. The USGS and WDFW implemented field collection efforts in 2011 for 

northern pikeminnow (Ptychocheilus oregonensis), smallmouth bass (Micropterus dolomieu), and 

walleye (Sander vitreus, formerly Stizostedion vitreum) and their diets in the PRP. For predator 

indexing, we collected 948 northern pikeminnow, 237 smallmouth bass, 18 walleye, and two 

largemouth bass (Micropterus salmoides). The intent of this study was to provide standardized 

predation indices within individual reaches of the PRP to discern spatial variability in predation 

patterns. Furthermore, the results of the 2011 study were compared to results of a concurrent steelhead 

survival study. Our results do not indicate excessively high predation of Oncorhynchus spp. occurring 

by northern pikeminnow or smallmouth bass in any particular reach throughout the study area. 

Although we found Oncorhynchus spp. in the predator diets, the relative proportion was small.

Predation index values in 2011 were highest in the Priest Rapids mid-reservoir reach for northern 

pikeminnow and smallmouth bass. Predation indices generally were high in the tailrace areas for 

northern pikeminnow, and high in the forebay areas for smallmouth bass. Steelhead survival in 2011 

was consistently high throughout the study period and the PRP, although predation indices were 

relatively low, which suggests that fish predation did not significantly affect steelhead survival 

throughout the study area. Our efforts to correlate retrospective predation indices with survival 

estimates for 2009 and 2010 did provide some evidence for high predation occurring in some of the 

same reaches, which had low steelhead survival, such as the Priest Rapids tailrace in 2009. However, 
2 

for 2010, our results indicated that the loss of salmonids to predation were more contradictory to the 

survival results, where predation indices were higher for reaches in the Priest Rapids Development 

than in the Wanapum Development. Establishing correlations between steelhead survival and observed 

predation indices for previous research years, in 2009 and 2010 was confounded by the lack of 

coordination of these two studies during the initial study design, implementation period for such an 

analysis. Future efforts to correlate steelhead survival with fish predation would benefit from efforts to 

better coordinate the studies with consistent study reaches, and better timing of concurrent efforts. 

Introduction

Anadromous juvenile salmonids (Oncorhynchus spp.) migrating through the Columbia River 

experience a variety of hazards that affect their survival as they migrate from freshwater rearing 

habitats to the ocean. Predation by native and non-native fish predators in the hydroelectric 

impoundments in the Columbia River has been an important factor in contributing to the total mortality 

of seaward migrating salmonids. The Public Utility District No. 2 of Grant County, Washington (Grant 

PUD), on behalf of the Priest Rapids Coordinating Committee (PRCC), has requested that the U. S. 

Geological Survey (USGS), Columbia River Research Laboratory, and Washington Department of 

Fish and Wildlife (WDFW) assist them in their efforts to evaluate the effects of native and introduced 

predatory fish on migrating juvenile salmon. During 2011, Grant PUD and the PRCC requested that 

we evaluate the effects of predation on migrating juvenile steelhead (Oncorhynchus mykiss) to provide 

insight into recently observed low survival of steelhead passing through the Priest Rapids Reservoir 

(Timko and others, 2010, 2011). Predation in the Lower Columbia River has been well documented as 

a significant factor affecting the survival of downstream-migrating salmonids (Poe and others, 1991; 

Vigg and others, 1991; Ward and others, 1995; Petersen and Ward, 1999). However, little work has 

been conducted in the Columbia River, upstream of the confluence with the Snake River, regarding 

predator effects. The USGS, in collaboration with the WDFW, has been working with the Grant PUD 

to conduct research using past studies from the Lower Columbia River and using technical advances in 

biological sciences to increase our understanding of predator-prey interactions in the Priest Rapids 

Hydroelectric Project (PRP). For this study, the PRP is defined as the area approximately 6 km 

upstream of Wanapum Dam to the Priest Rapids Dam tailrace, Columbia River, Washington. 

In recent years, steelhead survival at Wanapum and Priest Rapids Dams has been below the 

Federal Energy Regulatory Commission performance standards for these dams. This became apparent, 

given the results of the “Behavior and Survival Analysis of Juvenile Steelhead and Sockeye Salmon 

through the Priest Rapids Hydroelectric Project” (hereafter referred to as “Steelhead Behavior and 

Survival Project”) that was conducted by Blue Leaf Environmental, Inc., in 2009 and 2010 (Timko and 

others, 2010, 2011). The survival of juvenile steelhead migrating through the PRP was lower than the 

stipulated survival standards: juvenile steelhead passage survival of 95 percent at each dam, and 93 

percent through a single development (one dam and reservoir; Federal Energy Regulatory 

Commission, 2008). Steelhead passage survival in 2009 was estimated at 0.9436 (standard error 

(SE)=0.0189) through the Wanapum Dam and reservoir and 0.8806 (SE=0.0206) for the Priest Rapids 

Dam and reservoir (Timko and others, 2010). Passage survival in 2010 was estimated at 0.8553 

(SE=0.0186) for steelhead through the Wanapum Dam and reservoir while survival was higher at 

Priest Rapids Dam at 0.9037 (SE=0.0171; Timko and others, 2011). The performance standards 

(passage survival rates) were established for Grant PUD under the “Reasonable and Prudent 

Alternatives” (RPAs) in the National Marine Fisheries Service (NMFS) 2004 Biological Opinion 

(BiOp) for the Priest Rapids Project (National Marine Fisheries Service, 2004) and were adapted into 

the “Terms and Conditions” of the 2008 NMFS BiOp (National Marine Fisheries Service, 2008). 
3 

Given that the estimated steelhead survival does not conform to the established survival 

standards, the PRCC has shifted the focus of the “Steelhead Behavior and Survival Project” to assess 

the behavior and survival of steelhead through the Priest Rapids Dam and reservoir. Similarly, the 

PRCC has directed the USGS and WDFW to collaborate with Blue Leaf Environmental, Inc., to 

redirect USGS and WDFW efforts to focus on assessing the effects of piscivorous fish on steelhead in 

the Priest Rapids Dam and reservoir. Thus, foregoing any research at the Wanapum Dam and reservoir 

in 2011 (the final year of the Predator Indexing Project), our objectives were to relate the relative 

predation, as well as relative abundances of predators to survival estimates, generated by Blue Leaf 

Environmental, Inc., for tagged steelhead in the same study periods, reaches, and years. 

Methods

Study Area

The 2011 study area consisted of the Priest Rapids Reservoir and approximately 6 km [3.7 river 

miles (RM)] upstream of Wanapum Dam in the Mid-Columbia River. Priest Rapids Reservoir is 

bounded by Wanapum Dam (RM 415) at the upstream extent, and Priest Rapids Dam (RM 397) at the 

downstream extent (fig. 1). The PRP consists of two run-of-the-river hydroelectric developments 

owned and operated by Grant PUD. The reservoir is approximately 18 mi long, with a shoreline of 56 

mi, and an approximate surface area of 7,580 acres (Pfeifer and others, 2001). River environment 

conditions for the study area were collected from the Columbia River DART website 

(http://www.cbr.washington.edu/dart/). 

Site Selection

In 2011, we focused our sampling effort in the Priest Rapids Reservoir and the Wanapum Dam 

forebay to evaluate predation effects of native and introduced predatory fish on juvenile salmonids as 

requested by the PRCC. The reservoir was divided into longitudinal sampling reaches consisting of: 

(1) Wanapum forebay, 

(2) Wanapum forebay near-Boat Restricted Zone (BRZ), 

(3) Wanapum forebay BRZ, 

(4) Wanapum tailrace BRZ, 

(5) Wanapum tailrace near-BRZ, 

(6) Wanapum tailrace, 

(7) Priest Rapids mid-reservoir, 

(8) Priest Rapids forebay, 

(9) Priest Rapids forebay near-BRZ, and 

(10) Priest Rapids forebay BRZ (fig. 1). 

Reaches were developed to encompass ecologically significant habitat areas, such as hydrologic 

influence of the dam operations and potential predator distribution or behavioral differences, as well as 

sampling design issues such as accessibility and comparability with previous studies. Past studies have 

shown that predation of juvenile salmonids varies longitudinally in impoundments of the Columbia 

River (Petersen, 1994) and that areas very near hydroelectric dams, such as the BRZ areas, are where 

relatively high predation of juvenile salmonids occurs (Ward and others, 1995). We used a Geographic 

Information System (GIS) to further delineate habitat that was available for sampling by incorporating 
4 

bathymetry data and satellite imagery. Shoreline areas falling within the depth range of about 10 ft or 

less were delineated within the GIS for sampling by electrofishing. Sample site selection was 

performed using a sample frame developed with a Generalized Random Tessellation Stratified (GRTS) 

design (Larsen and others, 2007) with a point grid resolution of 30×30 m within the delineated 

shoreline areas. Individual sites were selected using this framework, such that sites were random and 

spatially balanced in each reach.

A sampling schedule was developed so that all reaches would be sampled throughout the 

reservoir within a 2 day block. This schedule excluded the BRZ areas that required dam operations to 

be altered during sampling efforts and were scheduled separately. In the 2011 study year, 

environmental conditions did not allow dam operations to be altered to allow for sampling in the BRZ 

areas. Four sample sites per sample outing were distributed among the longitudinal reaches so that no 

overlap would occur for individual electrofishing runs within a reach. In addition to the regularly 

sampled points, a list of alternate points were generated. The alternate sample points were used in the 

event that the regularly sampled points could not be sampled because of environmental conditions (for 

example low water conditions). For the forebay and tailrace reaches, the four points were allocated so 

that three sites were within the larger forebay (Priest Rapids and Wanapum) or tailrace (Wanapum) 

reach,with one point allocated to the near-BRZ reach. A number of additional sites were selected to be 

sampled repeatedly throughout the study period based on results from the 2010 study period that 

indicated either high predator abundance or predation. These sample points are referred to as 

“hotspots” for analysis. Once sites were selected, coordinates were entered into a marine GPS for 

navigating to sampling locations. Sampling week days were randomly assigned to reaches by sample 

week throughout the entire study period. 

Fish Sampling 

Electrofishing

We used standardized operating procedures for electrofishing (available upon request) to 

collect predators. All sampling equipment was calibrated prior to departing. Electrofishing was 

conducted at night using two 5.5 m Smith Root 5.0 Generator Powered Pulsator (GPP) electrofishing 

boats following WDFW warm-water sampling protocol (Bonar and others, 2000). Boat electrofishing 

began no earlier than one-half hour after sunset (determined using the Mattawa site from 

http://www.usno.navy.mil/USNO/astronomical-applications/data-services/rs-one-year-us). We 

operated individual electrofishing boats in a downstream direction parallel to the shoreline at a rate of 

0.6–0.9 m/h, maintaining a distance from shore that allowed the inshore boom to fish entirely in the 

water, and avoiding areas that exceeded 10 ft in depth. Each site was electrofished for 600 s. Each crew 

consisted of one boat operator and two crew members that were outfitted with personal flotation 

devices, stationed at the front of the vessel, and equipped with 8-foot-long dipnets. To initiate fish 

galvanotaxis, we operated the GPP unit at approximately 1–2 amperes using a low power setting (50–

500 volts) with a frequency between 30 and 120 Hz DC. Depending on water conductivity, we 

adjusted our pulse frequency and percentage of range as necessary. In addition, to prevent unnecessary 

fish injury, we noted the behavior of fish within the electrical field and adjusted the power accordingly 

to promote the galvanotaxis. The following information was recorded for each sample site: water 

temperature, specific conductance, time of day, transect start and end GPS coordinates, initials of crew, 
5 

date, site designation, and power settings used to electrofish. During electrofishing, stunned fish were 

immediately placed into one of two onboard livewells equipped with a pump that continually added 

freshwater into the tank. After the completion of two sites, the boat was moored to shore, where staff 

collected the required biological information. In the event that transit time between sites was extended 

as a result of distance or environmental conditions, crews collected the pertinent data from the captured 

fish immediately after the completion of the first site.

Biological information was collected for the following target species: northern pikeminnow 

(Ptychocheilus oregonensis), smallmouth (Micropterus dolomieu) and largemouth bass (Micropterus 

salmoides), channel catfish (Ictalurus punctatus), and walleye (Sander vitreus, formerly Stizostedion 

vitreum), following standardized operating procedures (available upon request). Because of the 

potential for many of the captured fish to be consumed by anglers, under FDA guidelines it was not 

permissible to use the anesthetic commonly referred to as MS-222. Consequently, all fish captured 

were worked up in a non-anesthetized state. For all predatory fish (large- and smallmouth bass, 

walleye, and northern pikeminnow), diets and aging structures (scales, otoliths, and opercles) were 

collected, as well as total length, weight, sex, gonad maturity, and gonad weight where possible. The 

diets of walleye, smallmouth and largemouth bass, were collected using a lavage technique (non-lethal 

take) while northern pikeminnow and channel catfish stomachs were surgically removed (lethal take). 

All diets were preserved (either frozen whole, or contents soaked in 95-percent ethanol) and 

transported back to the laboratory to be analyzed for contents at a later date. In addition to the standard 

collections, we also performed a gross visual inspection of diets from collected predator stomachs at 

the request of PRCC members. The gross visual inspection was performed to assess the viability of 

rapid assessments in the field for salmonid predation. Non-predatory fish were worked up and released 

alive.

Because of a concurrent tagging and tracking project by Blue Leaf Environmental, Inc., in the 

Priest Rapids Development, all predatory fish were inspected for visual marks (dorsal hole punch or 

surgical scars) and scanned for PIT tags. Predatory fish that were tagged were worked up and released 

with minimal handling. Diet collection for these fish was accomplished by gastric lavage, and only 

scales were taken for aging. At the end of the work-up period, the boat operator and crew did qualitycontrol checks on a randomly selected group of diets and aging structures collected from each site to 

ensure proper documentation. 

Angling 

In addition to boat electrofishing, northern pikeminnow were collected by U.S. Department of 

Agriculture (USDA) staff, angling from the transformer deck of Wanapum Dam tailrace. Angling was 

done during the night and we typically collected fish in the morning. To reduce the likelihood of using 

fish that had been dead for a long period of time with advanced stages of stomach content digestion, 

we communicated with angling crews to schedule times to pick up their captured fish. Upon arrival at 

the transformer deck, captured northern pikeminnow were immediately placed in individual bags in the 

event that stomach contents were expelled so they could then be associated with the specific predator. 

Once fish were bagged they were placed on ice, and were processed following the identical field and 

laboratory procedures used for predators captured by electrofishing. 

Diet Analysis 

Diet analysis was conducted in a laboratory setting using two different methodologies (SOPs 

available upon request)—one methodology for northern pikeminnow and another methodology for 

bass and walleye. The methodology for processing northern pikeminnow stomachs involved pancreatin 
6 

digestion or maceration. Pancreatin digestion of northern pikeminnow gut contents works because a 

pikeminnow’s stomach digests at a high pH leaving the mineral content of bones intact. Bass, 

walleye, and other piscivorous fish use acidic digestion, which demineralizes prey-fish bones 

leaving flaccid wisps that are completely dissolved by pancreatin. Therefore, bass and walleye diets 

were preserved in ethanol and analyzed separately from northern pikeminnow. 

A major difference in the two methodologies is that prey fish are identified by diagnostic bones 

post pancreatin digestion for northern pikeminnow and therefore are not identifiable into more distinct 

categories (such as salmonid, non-salmonid) for pre-digestion prey weights. Northern pikeminnow 

diets were macerated with pancreatin and sodium sulfide nonahydrate between 40°C and 45°C. 

Pancreatin digests most tissue, but does not disintegrate or emulsify fat completely. A 1.5–2.0 molar 

solution of NaOH (lye) was therefore used to dissolve remaining fat. Samples were then rinsed 

through a 425-μm (#40) mesh sieve. The diagnostic bones we used to identify and enumerate fish 

(cleithra, dentaries, hyomandibular arches, pharyngeal arches, otoliths, and opercles) are paired 

structures on the left and right sides of fish. Therefore, bones were counted in pairs in order to avoid 

inflating the number of fish counted. For example, if we counted three left and two right 

salmon/steelhead cleithra of the same size, the total number of fish was recorded as three. For each 

individual northern pikeminnow diet that contained fish, the proportion of each prey fish count postmaceration was averaged to represent the mean percent composition of all diets analyzed. 

Diet contents were separated into five categories: fish, crayfish, mollusks, insects, and 

miscellaneous (unidentifiable material and vegetation/inorganics) and weighed. The most common 

item in northern pikeminnow stomachs is the miscellaneous/unidentifiable category, consisting 

primarily of a mucilaginous substance that presumably is digesta and sloughed intestinal intima. Each 

prey category was compiled and weighed for each northern pikeminnow pre-maceration; after 

weighing, all diet items were returned to the sample bag to be macerated. Prey items in smallmouth 

bass and walleye diets were identified to the lowest practical taxon and blotted wet weights were 

recorded. 

For each individual predator diet, the proportion of each prey item weight was averaged to 

represent the mean percent composition of all diets analyzed. Prey items were further identified within 

each prey category whenever possible. Prey fish categories included: Unknown fish species, Unknown 

salmonids, Unknown non-salmonids, Chinook (Oncorhynchus tshawytscha), Whitefish (Coregonus 

spp.), Salmon/Steelhead, Northern pikeminnow, Peamouth (Mylcheilus caurinus), Chiselmouth 

(Acrocheilus alutaceus), Redside shiner (Richardsonius balteatus), Dace spp., Cyprinid spp., Cottus

spp., Threespine stickleback (Gasterosteus aculeatus), Sucker spp., Walleye, Lampetra spp., Sand 

roller (Percopsis transmontana), and Lepomis spp. The unknown salmonid group consists of fish that 

could not be further identified and could include salmon, trout, char, or whitefish. The 

salmon/steelhead group includes fish in the genus Oncorhynchus. However, fish in that group cannot 

be identified beyond genus because their diagnostic bones are too similar. Chinook salmon were only 

identifiable because of the presence of coded wire tags or PIT tags. Zooplankton diet categories 

included: Daphnia spp., Bosminidae, Chydoridae, Copepoda, Ostracoda, and Sididae. Insect diet 

categories included: Insect parts, Diptera, Trichoptera, Lepidoptera, Ephemeroptera, Odonata, 

Orthoptera, Hemiptera, Hymenoptera, Coleoptera, Plecoptera, and unknown insects. Other diet items 

include: Amphipoda, Isopoda, Mollusca, Annelida, and Arachnida. For each individual predator diet 

that contained fish, the proportion of each prey fish was averaged to represent the mean percent 

composition of all diets analyzed.
7 

Data Analyses

Relative Abundance Indices

We characterized the relative abundance of predatory fish by estimating the catch-per-unit 

effort (CPUE; number captured per 10 min) and by density and abundance indices (Burley and Poe, 

1994). The CPUE was defined as the number of northern pikeminnow (total length greater than 170 

mm), smallmouth bass (total length greater than 150 mm), and walleye (total length greater than 180 

mm) captured during electrofishing efforts and then standardized to the number caught per 10 min. The 

abundance index (AI) was then estimated to be:

AI DI S i ii = × (1)

where:

AIi = Index of predator abundance in sampling area i, 

DIi = CPUE as the index of predator density in the sampling area i, and 

Si = Surface area (ha) for sampling area i, adjusted to include shoreline areas less than 10 ft in 

depth. 

Estimates of Si were derived using the GIS of the study area to estimate the area within each of the 

reaches sampled in 2011 that are less than 10 ft in depth. 

Consumption Indices

Previous studies have demonstrated the analytical techniques we used to develop consumption 

indices for northern pikeminnow and smallmouth bass (Ward and others, 1995; Fritts and Pearsons, 

2004). Ward and others (1995) based their consumption index on the concept of meal turnover-time 

(Windell, 1978; Rieman and others, 1991). We adopted the methods of Ward and others (1995) to 

estimate consumption of juvenile salmonids by northern pikeminnow collected within each reach. The 

consumption index (CINPM) used for northern pikeminnow was then: 

1.60 0.27 0.61 0.0209 ( ) CINPM T W n GW − = ⋅ ⋅ ⋅⋅ (2)

where:

T = water temperature (°C), 

W = predator weight (g), 

GW = mean total gut weight (g), and

n = mean number of salmonids per northern pikeminnow. 

We used the consumption index developed by Ward and Zimmerman (1999), who modified the 

relations developed by Rogers and Burley (1991) to describe smallmouth bass evacuation time as the 

consumption index for smallmouth bass (CISMB): 

( ( )) 23.015.0 29.0 .0 0407 − CI = nWe ⋅⋅⋅ GW T

SMB (3)
8 

where:

T = water temperature (°C), 

W = predator weight (g), 

GW = mean total gut weight (g), and

n = mean number of salmonids per smallmouth bass. 

Predation Indices 

We then combined the consumption indices with the abundance indices to calculate the 

predation index (Ward and others, 1995) as: 

PIi = AIi i ⋅CI (4)

where:

PIi = predation index for sample i,

AIi = abundance index for area i, and 

CIi = consumption index for sample i. 

Retrospective Steelhead Survival and Predation Analysis

An exploratory analysis was performed to compare survival estimates produced from acoustictagged steelhead to predation metrics generated by the USGS for concurrent time periods in 2009 and 

2010. This exploratory analysis was requested after the PRCC received the steelhead survival estimates 

for 2009 and 2010 (Timko and others, 2010, 2011), which prompted concern over the survival 

estimates being lower than the stipulated standards for the PRP. This analysis was not planned at the 

time the studies were conducted, thus the analysis was performed after the low survival estimates were 

already reported. The low steelhead survival estimates prompted the PRCC to further pursue potential 

correlations (such as predation) with the survival results. The original USGS study that was designed

during 2009 and 2010 was not formulated to coincide the sampling dates with the fish releases, or with 

the reaches used for survival estimation (Timko and others, 2010, 2011). 

The retrospective analyses were based on data from study periods used in both the USGS 

efforts and those by Timko and others in 2009 and 2010 (Timko and others, 2010, 2011) to estimate 

steelhead survival. In 2009, the acoustic-tagged steelhead releases occurred from May 2 to June 7. In

2010, acoustic-tagged steelhead releases occurred from May 4 to June 2. Relative abundance, 

consumption, and predation indices were calculated for northern pikeminnow and smallmouth bass for 

the study periods that coincided ( May 2–June 5, 2009; May 19–31, 2010) with the steelhead survival 

estimates generated by Timko and others (2010, 2011). Methodology for data collection for predator 

indexing in 2009 and 2010 for the USGS can be referenced in Counihan and others (2012), and for 

survival-estimation in the annual reports by Timko and others (2010, 2011). For these analyses, we

matched the reaches we used to the reaches used by Timko and others (2010, 2011), for survival 

estimates for 2009 and 2010, as best as possible. However, the survival study reaches and predatorindexing reaches are not directly comparable. Furthermore, the USGS combined the near-BRZ forebay 
9 

and tailrace with the larger forebay and tailrace reaches for study years 2010 and 2011 to better match 

the 2009 sample reaches as well as those used for survival estimation (Timko and others, 2010, 2011; 

Thompson and others, 2012). The mid-reservoir reaches between 2009 and the 2010 and 2011 study 

years were difficult to compare because the area available to sampling was expanded to cover the 

entire mid-reservoir area (all areas not defined as either forebay or tailrace) in the latter study years. In

2009, the Priest Rapids mid-reservoir reach consisted of a smaller area at approximately 3.7 mi (reach 

PM1) within the area defined as PM3 (fig. 1), which was used for the 2010 and 2011 study years. 

Results

River Environment

Discharge in the 2011 study period was considerably higher than the 10-year average (83 

percent higher from May 16 to July 18; Keeler, 2011), as well as those in the 2009 and 2010 study 

years (fig. 2). The elevated flows caused a high number of total dissolved-gas exceedance levels at 

Priest Rapids forebay, which were attributed to river flows in excess of Wanapum Dam’s hydraulic 

capacity requiring involuntary spill (Keeler, 2011). Water temperatures in 2011 were lower than the 

10-year average and the 2009 and 2010 study years (fig. 2). 

Sampling Effort

High river flows and wind conditions prevented the predator index sampling from occurring as 

planned. The initial sampling schedule included 22 sampling days from May 10 to May 31. We were 

able to complete 11 sampling days in this time, and an additional 5 sample days from June 1 to June 9 

(fig. 3). This encompassed the time period of tagged fish releases, but resulted in less frequent 

sampling than the daily fish releases. In order to accommodate for missed sampling trips, the study 

period was extended to June 9 to increase our predator collection efforts during the steelhead 

migration. In this time, 145 electrofishing runs within 8 of the 10 longitudinal reaches were completed. 

Sample efforts were originally planned to be allocated equally across reaches (forebay, mid-reservoir, 

and tailrace) with four efforts per outing (fig. 1). However, the sample efforts were not distributed 

across the reaches as planned throughout the study period because of adverse environmental conditions 

(fig. 4). The combined sample efforts for the four main study areas of Wanapum forebay, Wanapum 

tailrace, Priest Rapids mid-reservoir, and Priest Rapids forebay had similar weekly sample efforts, with 

the exception of the Wanapum tailrace (fig. 4). Wanapum tailrace was not sampled in the latter part of 

the study period. For the forebay areas, the near-BRZ reaches were sampled more frequently than 

prescribed in the study design in the latter half of the study period due to an emphasis on sampling in 

the prescribed “hotspot” sites in addition to the randomized sampling whenever WDFW was in this 

reach. 

The study period for predator collection coincided with the juvenile salmon outmigration 

period for steelhead and other juvenile salmonids. Smolt passage data are not collected at Priest Rapids 

Dam, but our study period encompassed the peak period of the steelhead outmigration at Rock Island 

Dam (upstream from Wanapum Dam, fig. 5). Furthermore, our data collection efforts for the 

retrospective analysis for 2009 and 2010 also captured the peak steelhead migration (fig. 5) in those 

years. 
10

Catch Data

We collected 5,313 fish comprising 23 species from the areas sampled in the 2011 predator 

indexing efforts. These results provide information about species composition during the study period, 

but are not a comprehensive list of fish species in the reservoir. For example, salmonids are avoided 

during electrofishing per our protocol, and non-target species are often counted and not netted. The 

most abundant fish captured were native catostomids (54.4 percent of total catch), primarily largescale 

sucker (37.7 percent of total catch), followed by northern pikeminnow (17.7 percent of total catch) 

(table 1). Non-native predators such as walleye and bass represented approximately 5 percent of the 

total catch. For predator indexing, we collected 948 northern pikeminnow from the study area. The 

total lengths of fish captured ranged from 38 to 532 mm; x = 180.5 mm (fig. 6). We also collected 237 

smallmouth bass, with total lengths ranging from 79 to 518 mm; x = 257.9 mm (fig. 7). Other 

predators collected include 18 walleye with total lengths ranging from 192 to 728 mm; x = 490.1 mm 

(fig. 8) and two largemouth bass with total lengths of 121 and 222 mm. 

In addition to the predator indexing efforts, northern pikeminnow also were collected from the 

USDA staff, from angling efforts at the transformer deck of Wanapum Dam tailrace. A portion of these 

fish were set aside for WDFW staff for diet analysis. From these efforts, we collected 135 northern 

pikeminnow, with total lengths ranging from 135 to 582 mm; x = 456.9 mm. 

Relative Abundance Indices

The highest CPUE of northern pikeminnow (total length greater than 170 mm) occurred in the 

Priest Rapids mid-reservoir reaches in 2011, and was consistently high in 2009 and 2010 (table 2). The 

exception was in 2010 when the Priest Rapids tailrace (almost double the next highest CPUE) reach 

yielded the highest CPUE of all sampled reaches and years; however this reach was not sampled in 

2011. For northern pikeminnow, the hotspot sampling sites resulted in similar CPUE results as the 

randomly selected sampling sites (table 2). For the retrospective analysis, an increasing catch trend was 

observed for northern pikeminnow from 2009 to 2011 in Priest Rapids forebay. Other CPUE results 

were relatively consistent across years for the Wanapum forebay and tailrace reaches for northern 

pikeminnow. 

For smallmouth bass (total length greater than 150 mm), the CPUE was highest in the Priest 

Rapids forebay and mid-reservoir reaches, in 2011 (table 2). The hotspot analysis for smallmouth bass 

yielded higher CPUE results for the Priest Rapids forebay (0.049, SE=0.022) than the random 

sampling data (0.015, SE=0.003), but results from the two sampling designs were similar in other 

reaches. Trends were more difficult to detect across years for the retrospective analysis. One notable 

result was the consistently low CPUE in the tailrace reaches for smallmouth bass (range 0–0.005). In

2011, there were consistently more northern pikeminnow captured in the mid-reservoir and forebay 

areas than smallmouth bass (fig. 9). This may be a result of the greater relative abundance of northern 

pikeminnow than smallmouth bass. 

Walleye were captured in just three reaches—Priest Rapids forebay, mid-reservoir, and 

Wanapum tailrace. This resulted in very low CPUE compared to the other piscivores (PF2, CPUE

=0.001; PM3, CPUE=0.003, and WT2, CPUE=0.002 ). Of the 18 captured walleye, the majority were

from Priest Rapids mid-reservoir (13 fish), with the remaining 5 captured walleye from the Priest 

Rapids forebay and the Wanapum tailrace.
11

Diet Analyses

Diet analyses was conducted on 561 northern pikeminnow collected via electrofishing and 

angling efforts (table 3). Fish were found in northern pikeminnow diets collected by electrofishing in 

the mid-reservoir and forebay of Priest Rapids Dam, and the forebay and tailrace of Wanapum Dam. 

Fish comprised 0.2–2.7 percent of the diets of northern pikeminnow from these reaches. The highest 

proportion of fish-prey items was in the Priest Rapids mid-reservoir (fig. 10). Of the 430 northern 

pikeminnow diets analyzed from electrofishing efforts, 13 (3 percent) of these fish contained salmon, 

consisting of a sum total of 17 identified salmonid prey items. Of the diets containing fish, the highest 

proportion containing salmonids occurred in the Priest Rapids forebay near-BRZ, followed by the 

Priest Rapids mid-reservoir, and the Wanapum tailrace reaches (fig. 11). 

For northern pikeminnow collected by USDA anglers from the Wanapum Dam tailrace, fish 

comprised 18.9 percent of the diet. Of the 131 diets analyzed, 64 (49 percent) of these fish contained 

salmon, consisting of a sum total of 75 identified salmonid prey items. Prey items were identified by 

CWT and PIT tags as well as visual inspection and from diagnostic bones during laboratory analysis.

For the smallmouth bass, fish comprised a much higher proportion of the diet than for northern 

pikeminnow (fig. 12 and table 4). Of the 164 non-empty diets, 18 of these fish (11 percent) contained 

salmon, consisting of 21 total salmonid prey items. Smallmouth bass caught in the Wanapum forebay 

(WF1 and WF2 combined) had the highest proportion of salmonid prey based on proportion of all fish 

prey counts, followed by Priest Rapids mid-reservoir, and the Priest Rapids forebay (PF1 and PF2) 

(fig. 13). For walleye, fish comprised more than 97 percent of their diets. Of the 12 non-empty walleye 

diets, 11 of these contained salmonid prey items.

We determined that visual inspection alone was an unreliable method for salmon predation 

assessment. The results from the laboratory analysis consistently produced higher numbers of salmon 

prey than the field inspections (table 5). Laboratory analysis for steelhead identification generally was 

only confirmed by the presence of a PIT tag, and field identification was not reliable. This was evident 

by a steelhead that was recorded in the field, however, upon further laboratory analysis, the fish 

identified in the field was not a steelhead. 

Predation Indices

For northern pikeminnow in our samples, predation of salmonids occurred in Priest Rapids 

forebay, mid-reservoir, and Wanapum tailrace reaches (table 6). The predation indices were 

consistently low across the reaches in 2011 for the randomized sampling, ranging from 0.068 to 1.191 

(table 6), with Wanapum tailrace the highest (Pi = 1.191), followed by Priest Rapids mid-reservoir (Pi

= 0.595). For the hotspot analysis, the predation indices ranged from 0 to 5.028, with predation only 

being documented in the the Priest Rapids near-BRZ forebay (Pi = 0.035) and the Priest Rapids midreservoir (Pi = 5.028). All other reaches were zero for the hotspot analysis. The predation index for the 

hotspots in Priest Rapids mid-reservoir was substantially higher than the randomly sampled sites 

within this reach. For the retrospective analysis, northern pikeminnow predation index values were 

high in Priest Rapids tailrace in 2009 and 2010 (table 7). Predation was not observed in the Priest 

Rapids mid-reservoir in the retrospective sampling window in 2009 and 2010, which contrasted with 

the relatively high index value for the 2011 analysis. No apparent trend could be distinguished across 

years for the Priest Rapids forebay and Wanapum tailrace and forebay reaches for northern 

pikeminnow. 
12

Smallmouth bass predation occurred in the Priest Rapids forebay, forebay near-BRZ reach, the 

mid-reservoir, and Wanapum forebay near-BRZ reach. The predation index for the randomized 

sampling ranged from 0.039 to 1.827; the hotspot predation indices being very similar, ranged from 

0.035 to 1.493 (table 6). The predation index for smallmouth bass was highest in Priest Rapids midreservoir, which was consistent with the northern pikeminnow results, followed by Wanapum forebay 

and the Priest Rapids forebay reaches. The hotspot analysis for smallmouth bass provided relatively 

consistent results to the randomized sites where predation occurred. For the retrospective analysis for 

smallmouth bass, the tailrace reaches were consistently zero across all years, and the Priest Rapids 

forebay predation indices were similar across years when predation occurred. 

Discussion

We documented the relative predation of juvenile salmonids from the forebay of Priest Rapids 

Dam to the forebay of Wanapum Dam during the peak steelhead migration from 2009 to 2011. The 

intent of this research was to relate the relative predation, as well as relative abundances, of predators 

to survival estimates generated by Blueleaf Environmental, Inc., for tagged steelhead within the same 

study periods, reaches, and years. Environmental conditions in 2011 precluded us from sampling the 

study area concurrently with releases of tagged steelhead and confounded our interpretation of the 

relation of predation and survival. Furthermore, survival estimates in 2011, unlike those in 2010, were 

consistently high throughout the study area, suggesting there were few if any spatial differences and/or 

relationships between predator abundance, predation, and survival (Timko and others, 2011; 

Thompson and others, 2012). Additionally, our results did not indicate excessively high predation of 

Oncorhynchus spp. occurring by northern pikeminnow or smallmouth bass in any particular reach 

throughout the study area. We cannot infer direct causal mechanisms for the low steelhead survival 

estimates observed in the previous years because our studies were not specifically designed to address 

this question. For instance, the reaches that survival was estimated for during 2009 and 2010 (Timko 

and others, 2010, 2011) did not directly comport to the reaches we measured predation over. However, 

the fact that our results indicate relatively low predation in the study periods and reaches, low 

steelhead survival likely may have been caused by more than predation by northern pikeminnow and 

smallmouth bass alone. However, our exploratory work may help managers better understand where 

predator abundance and predation is high throughout the project. 

During 2011, the predator index sampling efforts were confined to a relatively short time 

period (approximately 18 days) in which Thompson and others (2012) also were releasing tagged fish 

for survival estimation. The river environment during this time was such that on many occasions we

could not safely conduct sampling in some reaches, so discerning temporal differences within or across 

reaches was not plausible. However, the extension of our sampling period past the release period of 

Thompson and others (2012) allowed for better coverage of the entire steelhead migration. This period 

coincided directly with the steelhead migration and should still capture peak predation events and

provide insight to reaches where predator abundance and predation is high during the steelhead 

migration. 
13

We observed spatial trends in relative abundance, consumption, and predation indices over the 

study period. As has been true in past evaluations of fish predation in the study area (Burley and Poe, 

1994; Counihan and others, 2012), we observed the highest northern pikeminnow predation for the 

randomized sampling in one (WT2) of the two Wanapum Dam tailrace reaches we sampled during 

2011. However, dissimilar to what was observed during 2009 and 2010, we observed higher predation 

in the Priest Rapids mid-reservoir reach than in the Wanapum tailrace reach. The highest predation 

index value for northern pikeminnow was from the 2011 Priest Rapids mid-reservoir hotspot sampling 

efforts, which focused on sites near the mouth of Crab Creek. Furthermore, when we combined the two 

Wanapum tailrace sections (WT1 and WT2) and examined the trends from 2010 to 2011, the northern 

pikeminnow predation in the Wanapum tailrace was less than that observed for the Priest Rapids midreservoir reach, and much less than that observed for the Wanpum tailrace during 2010. The relatively 

high discharge observed in 2011 may have altered the hydraulic environment in the tailrace areas to 

make them less suitable for northern pikeminnow while making conditions more favorable for 

predation events to occur in the mid-reservoir reach. Mesa and Olson (1993) determined the swimming 

performance of northern pikeminnow and reported that water velocities from 3.28 to 4.27 ft/s may

reduce predation by northern pikeminnow around juvenile bypass outfalls at Columbia River dams. 

Thus, the high discharge during 2011 may have made areas in the Wanapum tailrace less favorable for 

northern pikeminnow, causing them to move downstream to the mid-reservoir reach. Futhermore, the 

result that predation was highest in the farthest downstream Wanapum tailrace reach (WT2) also 

indicates that pikeminnow may have been more prevalent in the downstream reaches. 

For the smallmouth bass, we also observed spatial trends across the reaches for the predation 

indices. The predation indices generally were higher in the Priest Rapids mid-reservoir and the forebay 

reaches, relative to the tailrace reaches, and to the northern pikeminow results in these reaches. For 

smallmouth bass in the Priest Rapids forebay, the consumption indices were high in the near-BRZ 

reach (PF1). However, the predation index was lower for the near-BRZ reach than for the larger 

forebay reach (PF2)—this was driven by a lower abundance index. The highest predation impact for 

smolts will always be a combination of high abundance and high consumption occurring from 

predators. One interpretation of a reach with a high consumption index is that the habitat conditions of 

that reach are conducive to predation events and therefore could be considered a reach of concern for 

management. The near-BRZ reach is a likely surrogate for the BRZ reach, which could not be sampled 

due to environmental conditions precluding altering dam operations to allow for safe sampling of these 

reaches. Thus, our results are likely to be biased low due to the lack of sampling opportunity in the 

BRZ reaches. We know these reaches are likely a concern for high predator abundances and predation 

based on the results of our evaluations (Counihan and others, 2012) and other predator studies (Ward 

and others, 1995; Petersen and Ward, 1999). Other studies in the Columbia River basin have indicated 

differential predator abundance and predation based on longitudinal differences in reservoir areas, such 

as the forebay, tailrace, and mid-reservoir reaches (Beamesderfer and Rieman, 1991; Burley and Poe, 

1994; Petersen, 1994). This is consistent with our results, and appears to be an interplay between 

capturing predation (consumption index) and abundance of predators. Furthermore, the tailrace 

environment also is an extremely difficult reach to sample with electrofishing gear, especially during 

the high discharges in 2011. Thus, there is the potential to underestimate predation in these areas. 
14

Our results showing that northern pikeminnow and smallmouth bass predation were mostly low 

throughout the study area coupled with the reported high steelhead survival estimates through the 

Priest Rapids Reservoir (Thompson and others, 2012) suggest that fish predation was not a significant 

factor affecting steelhead survival during 2011. The estimates of steelhead survival from Thompson 

and others (2012) for 2011 indicate that survival was uniformly high with only two areas indicating 

survival probabilities that were much less than 1. Namely, these reaches included the Wanapum Dam 

forebay (release to RM 416, 0.9661, SE=0.0057) and Wanapum Dam and tailrace area (RM 416–413, 

0.9816, SE=0.0046; Thompson and others, 2012). If fish predation were to have resulted in the 

mortality of steelhead in these areas, then we can infer from our data that smallmouth bass would have 

likely been the cause in the forebay area of Wanapum Dam and that northern pikeminnow would more 

likely to have been the cause in the tailrace, even though the geographic reaches in which our estimates 

encompass do not correspond directly to the survival reaches. For Wanapum Dam forebay, our reach 

WF2 is similar to the release to RM 416 of Thompson and others (2012), and our reaches WF1 and 

WT1 are similar to the RM 416–413 reach of Thompson and others (2012). The relatively low 

predation values we observed, and high survival observed by Thompson and others (2012) coupled 

with the presence of high discharges through the study area compared to 2009 and 2010 during the 

study period suggest that the high discharges may have promoted higher steelhead survival. Plumb and 

others (2006) documented relationships between flow and steelhead travel time and suggested that 

higher flows would result in higher steelhead survival. 

Northern pikeminnow collected by USDA anglers from the Wanapum Dam tailrace contained a 

higher percentage of prey fish and salmon in their diets than fish collected from other areas. However, 

we were unable to effectively electrofish the areas being fished by the USDA anglers, although few 

existing fish-sampling gears would be effective in the Wanapum Dam tailrace environment during the 

salmonind migration period due to discharge conditions and dam operations. The northern pikeminnow 

captured here contained a higher proportion of juvenile salmonids; this does not seem surprising as few 

other prey items are likely to be available at this location. Further, it seems reasonable to assume that 

the sole reason for the northern pikeminnow to occupy this habitat at this time of the year would be to 

consume juvenile salmon (Ward and others, 1995). Gadomski and Hall-Griswold (1992) documented 

that northern pikeminnow preferentially consume dead or dying prey to live prey in a laboratory 

environment. The tailrace area of a hydroelectric project is where smolts are more likely to be 

disoriented, moribund, or dead and are likely found in higher concentrations in the tailrace area than 

the mid-reservoir reaches. The overall effect of northern pikeminnow occupying this habitat ultimately 

depends on the number of fish. If there are few fish, the effect will not be that great despite the fact that 

they consume proportionally higher numbers of juvenile salmonids; if there were many fish, then the 

effect would be greater. Effort could be expended to enumerate the northern pikeminnow in this 

location, but the fact that there are anglers actively trying to remove them from the river implies that 

their presence and effect is undesirable. Given this, identifying new and unique ways to remove them 

may be the best use of available financial resources versus spending a considerable amount of time 

estimating the population size of northern pikeminnow at this location. However, given that northern 

pikeminnow continue to be abundant in the study area, northern pikeminnow removed from this 

location are likely to be readily replaced. 
15

The lack of a coordinated effort to simultaneously estimate survival and fish predation 

confounds our ability to retrospectively assess predation as a causal factor affecting the observed low 

steelhead survival through Priest Rapids Reservoir (Timko and others, 2010, 2011). During 2009 and 

2010, survival estimates presented by Timko and others (2010, 2011) were presented for dam and 

reservoir, and some smaller reaches (for both Priest Rapids and Wanapum dams). Although the reaches 

over which they estimated survival do not match the reaches that we used to estimate predation during 

these years, we did see evidence of high predation by northern pikeminnow in the Priest Rapids 

tailrace during 2009, which also was indicated by Timko and others (2010) as an area with lower 

survival. In 2009, Timko and others (2010) also estimated that survival was lower downstream of the 

Wanapum Dam and Priest Rapids Dam tailrace reaches. Our results for this same period indicated that 

the predation was highest by northern pikeminnow in the Priest Rapids tailrace and by smallmouth 

bass in the Priest Rapids mid-reservoir reach. The fact that we did not see evidence of more predation 

occurring during this period could be confounded by the fact that we were capturing, tagging, and 

releasing some of the northern pikeminnow for a mark recapture study (Counihan and others, 2012) 

during 2009. Thus, it is plausible that some of the large northern pikeminnow that were released 

contained salmonids in their stomachs. However, this procedure was not continued in the 2010 and 

2011 study years; during 2010, the highest predation index values were in the tailrace reaches of 

Wanapum and Priest Rapids Dams by northern pikeminnow, and in the Priest Rapids mid-reservoir by 

smallmouth bass. For northern pikeminnow, the predation index was more than four times higher in 

the Priest Rapids Dam tailrace versus the Wanapum Dam tailrace during 2010. However, the forebay 

reach of Wanapum Dam had a northern pikeminnow predation index of 0.251, while no predation was 

observed in the Priest Rapids forebay during this period. It is difficult to correlate the predation 

indices with the survival results for this area for the 2010 study period; the Priest Rapids Dam and 

reservoir survival (0.904, SE=0.017) was higher than Wanapum Dam and reservoir (0.855, SE= 0.019) 

survival estimates (Timko and others, 2011). If we assume predation is an indicator of low survival, 

our results indicate the potential for a high mortality of salmon in Priest Rapids tailrace. Predator 

movement between reaches also may confound correlations of survival to predation events if predators 

are consuming prey and then moving in between reaches. However, this is more likely to have 

occurred near the boundaries of reaches. 
16

If efforts to relate survival estimates to fish predation are continued, we recommend continued 

efforts to coordinate studies examining survival and predation. Estimating the survival of juvenile 

salmonids and the predation of juvenile salmonids over the same reaches and time periods should 

provide insight into the effects of fish predators. Identifying problem areas for survival in past years, 

and following up with a study examining predation effects in those reaches at a later date, like the 

following year, may be of little utility due to the interaction between predator distributions and juvenile 

salmonid migration paths that may change with varying environmental conditions (such as high or low 

discharge years). Clearly, efforts to assess predation in years with high flows, such as 2011, is difficult. 

Thus, efforts to relate predation and survival would benefit from being conducted over multiple years. 

Further, limiting the scope to assess one particular juvenile salmonid species seems problematic as 

well, as the interaction between predators and prey will vary seasonally from year to year. We also 

caution trying to relate the survival of juvenile salmonids and predation over very short reaches such as 

those originally proposed (1 mi) and those eventually used by Thompson and others (2012) during 

2011 (3 mi). The potential for bias in the estimates of survival from dead fish being transported 

through downstream arrays seems high and thus would bias associations with observed predation rates. 

Furthermore, there is an an increased potential for captured fish to move between small reaches and 

the associated predation values to be assigned to a different reach. 

For future predation analyses, we recommend coverage of the peak salmon migration periods 

and sampling efforts throughout the reservoir, as well as a focused effort in areas with high potential 

for either predator abundance and/or predation. Our results for 2011, specifically the hotspot analysis, 

as well as those from Thompson and others (2012), indicate that the area near the mouth of Crab Creek 

(in the Priest Rapids mid-reservoir reach) is an area of potential concern. Efforts could be expended to 

continue to explore incorporating hotspots in conjunction with the randomized monitoring sample 

frame within the reservoir relative to survival and predation, specifically Crab Creek. We also 

recommend emphasis on other areas, such as the near-BRZ, and BRZ for both forebay and tailrace 

reaches. Furthermore, research efforts could be expended to to look into alternate predator collection 

and removal methods, to better understand the differences between angled fish and electrofishing 

collection results in the tailrace environments.

Acknowledgments

 We thank Conrad Frost and Amy Puls of the USGS, and Curt Nelson, Fritz Wichterman, Klint 

Caillier, and seasonal and permanent staff of the WDFW for their assistance in the field and laboratory. 

We thank Lucinda Morrow and Lance Campbell of the WDFW aging laboratory. In addition, we thank 

Curt Dotson for contract administration and the members of the Priest Rapids Coordinating Committee 

for their comments and thoughts on this project. We also would like to acknowledge John Beeman and 

Matt Mesa, USGS, and Jeffrey Korth, WDFW, for their input and reviews of this report. 
17

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Fritts, A.L., and Pearsons, T.N., 2004, Smallmouth bass predation on hatchery and wild salmonids in 

the Yakima River, Washington: Transactions of the American Fisheries Society, v. 133, p. 880–895. 

Federal Energy Regulatory Commission, 2008, Priest Rapids Project license agreement: Federal 

Energy Reglatory Commission, accessed June 1, 2012, at 

http://www.gcpud.org/pudDocuments/naturalResourcesDocs/h1.pdf. 

Gadomski, D.M., and Hall-Griswold, J.A., 1992, Predation by Northern Squawfish on live and dead 

juvenile Chinook salmon: Transactions of the American Fisheries Society, v. 121, p. 680–685. 

Keeler, C., 2011, Summary of 2011 annual fish-spill season and total dissolved gas monitoring: 

Prepared for Public Utility District No. 2 of Grant County, Ephrata, Wash. 

Larsen, D.P., Olsen, A.R., Lanigan, S.H., Moyer, C., Jones, K.K., and Kincaid, T.M., 2007, Sound 

survey designs can facilitate integrating stream monitoring data across multiple programs: Journal of 

the American Water Resources Association, v. 43, p. 384–397. 

Mesa, M.G., and Olson, T.M., 1993, Prolonged swimming performance of Northern Squawfish: 

Transactions of the American Fisheries Society, v. 122, no. 6, p. 1,104–1,110. 

National Marine Fisheries Service, 2004, Biological opinion and Magnuson-Steven Fishery 

Conservation and Management Act—Interim Protection Plan for Operation of the Priest Rapids 

Hydroelectric Project, 115 p.. 

National Marine Fisheries Service, 2008, Biological Opinion and Magnuson-Steven Fishery 

Conservation and Management Act—New License for the Priest Rapids Hydroelectric Project, 74 p.. 

Petersen, J.H., 1994, Importance of spatial pattern in estimating predation on juvenile salmonids in the 

Columbia River: Transactions of the American Fisheries Society, v. 123, p. 924–930. 

Petersen, J.H., and Ward, D.L., 1999, Development and corroboration of a bioenergetics model for 

northern pikeminnow feeding on juvenile salmonids in the Columbia River: Transactions of the 

American Fisheries Society, v. 128, p. 784–801. 

Pfeifer, B., Hagen, J.E., Weitkamp, D., Bennett, D.H., Lukas, J., and Dresser, T., 2001, Evaluation of 

fish species present in the Priest Rapids Project area, Mid Columbia River, Washington, USA: Final 

Completion Report, Public Utility District No. 2 of Grant County, Ephrata, Wash. 
18

Plumb, J.M., Perry, R.W., Adams, N.S., and Rondorf, D.W., 2006, The effects of river impoundment 

and hatchery rearing on the migration behavior of juvenile steelhead in the Lower Snake River, 

Washington: North American Journal of Fisheries Management, v. 26, no. 2, p. 438–452. 

Poe, T.P., Hansel, H.C., Vigg, S., Palmer, D.E., and Prendergast, L.A., 1991, Feeding of predaceous 

fishes on out-migrating juvenile salmonids in John Day Reservoir, Columbia River: Transactions of 

the American Fisheries Society, v. 120, p. 405–420. 

Rieman, B.E., Beamesderfer, R.C., Vigg, S., and Poe, T.P., 1991, Estimated loss of juvenile salmonids 

to predation by northern squawfish, walleyes, and smallmouth bass in John Day Reservoir, Columbia 

River: Transactions of the American Fisheries Society, v. 120, p. 448–458. 

Rogers, J.B., and Burley, C.C., 1991, A sigmoid model to predict gastric evacuation rates of 

smallmouth bass (Micropterus dolomieui) fed juvenile salmon: Canadian Journal of Fisheries and 

Aquatic Sciences, v. 48, p. 933–937. 

Thompson, A.M., O’Connor, R.R., Timko, M.A., Sullivan, L.S., Rizor, S.E., Hannity, J.L., Wright, 

C.D., Fitzgerald, C.A., Meagher, M.L., Stephenson, J.D., Skalski, J.R., and Townsend, R.L., 2012, 

Evaluation of downstream juvenile steelhead survival and predator-prey interacations using JSATS 

through the Priest Rapids Reservoir in 2011: Final Report submitted to Grant County Public Utility 

District No. 2 of Grant County, Wash. 

Timko, M.A., Sullivan, L.S., Wright, C.D., Rizor, S.E., O’Connor, R.R., Fitzgerald, C.A., Meagher, 

M.L., Kikes, T.J., Stephanson, J.D., Skalski, J.R., and Townsend, R.L., 2010, Behavior and survival 

analysis of steelhead and sockeye through the Priest Rapids Hydroelectric project in 2009: Final 

Report submitted to Grant County Public Utility District No. 2 of Grant County, Wash. 

Timko, M.A., Sullivan, L.S., O’Connor, R.R., Wright, C.D., Rizor, S.E., Hannity, J.L., Fitzgerald, 

C.A., Meagher, M.L., Stephenson, J.D., Skalski, J.R., and Townsend, R.L., 2011, Behavior and 

survival analysis of juvenile steelhead and sockeye salmon through the Priest Rapids Hydroelectric 

project in 2010: Draft Report submitted to Grant County Public Utility District No. 2, Grant County, 

Wash. 

Vigg, S., Poe, T.P., Prendergast, L.A., and Hansel, H.C., 1991, Rates of consumption of juvenile 

salmonids and alternative prey fish by northern squawfish, walleyes, smallmouth bass, and channel 

catfish in John Day Reservoir, Columbia River: Transactions of the American Fisheries Society, v. 

120, p. 421–438. 

Ward, D. L., Petersen, J.H., and Loch, J.J., 1995, Index of predation on juvenile salmonids by northern 

squawfish in the Lower and Middle Columbia River and in the Lower Snake River: Transacations of 

the American Fisheries Society, v. 124, p. 321–324. 

Ward, D.L., and Zimmerman, M.P., 1999, Response of smallmouth bass to sustained removals of 

northern pikeminnow in the lower Columbia and Snake Rivers: Transactions of the American 

Fisheries Society, v. 128, no. 6, p. 1,020–1,035. 

Windell, J.T., 1978, Estimating food consumption rates of fish populations, in Bagenal, T., ed., 

Methods for assessment of fish production in fresh waters: London, Blackwell Scientific 

Publications, p. 227–254. 
19

 

Figure 1. Study area depicting Priest Rapids Hydroelectric Project from upstream of Wanapum Dam to Priest 

Rapids Dam, Columbia River, Washington. Study reaches are delineated and defined as: WF2, Wanapum 

forebay; WF1, Wanapum forebay near-BRZ (Boat Restricted Zone); WF0, Wanapum forebay BRZ; WT0, 

Wanapum tailrace BRZ; WT1, Wanapum tailrace near-BRZ; WT2, Wanapum tailrace; PM3, Priest Rapids midreservoir (study years 2010 and 2011); PM1, Priest Rapids mid-reservoir (study year 2009; delineated by gray 

line); PF2, Priest Rapids forebay; PF1, Priest Rapids forebay near-BRZ; PF0, Priest Rapids forebay BRZ.

WA

Wanapum Dam RM 416

Priest Rapids Dam RM 397

WF2

WF1

WF0

PF2

PM3

WT2

WT0

WT1

PF1

PF0

RM411

RM402

RM420

RM414

PM1
20 

Date

May Jun Jul Aug Sep 

Temperature °C

6

8

10

12

14

16

18

20

2009

2010

2011

10 yr avg.

Priest Rapids Dam

Discharge kcfs

0

100

200

300

400

Figure 2. River conditions, discharge, and temperature in degrees Celsius (°C) at Priest Rapids Dam, Columbia 

River, Washington, from May to August, 2011 (dashed line), 2010 (dotted line), 2009 (gray line), and the 10-year 

average (solid line). 
21

Figure 3. Summary of sample effort (number of electrofishing runs) for a predator indexing study by Washington 

Department of Fish and Wildlife, and the number of acoustic-tagged juvenile steelhead released for a concurrent 

survival study from May 8 to June 10, 2011, Priest Rapids Project, Columbia River, Washington. 

Date

Sample Effort

0

2

4

6

8

10

12

14

16

18

Steelhead Released

0

20

40

60

80

100

120

Sample Effort

Steelhead Released

5/9 5/16 5/23 5/30 6/6
22

Figure 4. Distribution of sample effort (number of electrofishing runs) completed across the reaches, May 10–

June 9, 2011, Priest Rapids Project, Columbia River, Washington. Reach locations: PF1, Priest Rapids forebay 

near-BRZ (Boat Restricted Zone); PF2, Priest Rapids forebay; PM3, Priest Rapids mid-reservoir; WT2, Wanapum 

tailrace; WT1, Wanapum tailrace near-BRZ; WF1, Wanapum forebay near-BRZ; WF2, Wanapum forebay.

Study Period

5/10 - 5/17 5/18 - 5/26 5/27 - 6/3 6/4 - 6/9

Sample Effort

0

5

10

15

20

25

30

PF1 

PF2 

PM3 

WT2 

WT1 

WF1 

WF2 
23 

0

2000

4000

6000

Date

Mar Apr May Jun Jul Aug Sep 

0

2000

4000

6000

Steelhead 

Other species

Number of smolts passing per day

0

10000

20000

30000

40000

Figure 5. Daily juvenile salmonid downstream passage counts at Rock Island Dam, Columbia River, Washington, 

in 2009 (top), 2010 (middle), and 2011 (bottom), showing predator data-collection periods with vertical straight 

lines. Daily steelhead (solid line) counts include all rearing types (wild, hatchery, and unknown), and other species 

include Chinook-0, Chinook-1, coho, and sockeye. Data are from the Fish Passage Center Columbia River Dart 

website (http://www.cbr.washington.edu/dart/). 
24

Figure 6. Total lengths of northern pikeminnow captured during electrofishing efforts, May 10–June 9, 2011, 

Priest Rapids Project, Columbia River, Washington. 

Figure 7. Total lengths of smallmouth bass captured during electrofishing efforts, May 10–June 9, 2011, Priest 

Rapids Project, Columbia River, Washington. 

0

20

40

60

80

100

120

0

30

6090

120150180210240270300330360390420450480510540570600

Number

Total Length (millimeters)

n = 948

0

2

4

6

8

10

12

14

16

0

20406080

100120140160180200220240260280300320340360380400420440460480500520540

Number

Total Length (millimeters)

n = 235
25

Figure 8. Total lengths of walleye captured during electrofishing efforts,May 10–June 9, 2011, Priest Rapids 

Project, Columbia River, Washington. 

0

1

2

3

4

70

100130160190220250280310340370400430460490520550580610640670700730760790

Number

Total Length (millimeters)

n = 18
26

Figure 9. Spatial representation of catch per unit effort (number of fish caught per 10 minutes of boat 

electrofishing; CPUE) for northern pikeminnow (total length greater than 170 mm) and smallmouth bass (total 

length greater than 150 mm) during May 10 to June 9, 2011, at the Priest Rapids Hydroelectric Project, Columbia 

River, Washington. The yellow symbols indicate sample sites were part of a “hotspot” sample frame, and orange 

sites were part of a randomized sample frame. 
27 

Reach

PF1 PF2 PM3 WT2 WT1 WF1 WF2

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

FISH 

CRAYFISH 

MOLLUSKS 

INSECT 

MISC 

Figure 10. Diet composition by weight of northern pikeminnow in 2011 (May 10–June 9), by reach in the Priest 

Rapids Project, Columbia River, Washington. Reaches with no column indicate no diet sample. Reach locations: 

PF1, Priest Rapids forebay near-BRZ (Boat Restricted Zone); PF2, Priest Rapids forebay; PM3, Priest Rapids 

mid-reservoir; WT2, Wanapum tailrace; WT1, Wanapum tailrace near-BRZ; WF1, Wanapum forebay near-BRZ; 

WF2, Wanapum forebay. 
28

Figure 11. Proportion of fish in diet that are salmon and steelhead for northern pikeminnow by reach during

sampling in 2011 (May 10–June 9) in the Priest Rapids Project, Columbia River, Washington. Reaches with no 

column indicate no diet sample. Reach locations: PF1, Priest Rapids forebay near-BRZ (Boat Restricted Zone); 

PF2, Priest Rapids forebay; PM3, Priest Rapids mid-reservoir; WT2, Wanapum tailrace; WT1, Wanapum tailrace

near-BRZ; WF1, Wanapum forebay near-BRZ; WF2, Wanapum forebay. 

Reach

PF1 PF2 PM3 WT2 WT1 WF1 WF2

Proportion

0.0

0.2

0.4

0.6

0.8

1.0 Salmon / 

Steelhead 
29 

Reach

PF1 PF2 PM3 WT2 WT1 WF1 WF2

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

FISH 

CRAYFISH 

MOLLUSKS 

INSECT 

MISC 

Figure 12. Diet composition by weight of smallmouth bass in 2011 (May 10–June 9), by reach in the Priest Rapids 

Project, Columbia River, Washington. Reaches with no column indicate no diet sample. Reach locations: PF1, 

Priest Rapids forebay near-BRZ (Boat Restricted Zone); PF2, Priest Rapids forebay; PM3, Priest Rapids midreservoir; WT2, Wanapum tailrace; WT1, Wanapum tailrace near-BRZ; WF1, Wanapum forebay near-BRZ; WF2, 

Wanapum forebay. 
30

Figure 13. Proportion of fish in diet that are salmon and steelhead for smallmouth bass by reach in 2011 (May 

10–June 9) in the Priest Rapids Project, Columbia River, Washington. Reaches with no column indicate no diet 

sample. Reach locations: PF1, Priest Rapids forebay near-BRZ(Boat Restricted Zone); PF2, Priest Rapids 

forebay; PM3, Priest Rapids mid-reservoir; WT2, Wanapum tailrace; WT1, Wanapum tailrace near-BRZ; WF1, 

Wanapum forebay near-BRZ; WF2, Wanapum forebay. 

Reach

PF1 PF2 PM3 WT2 WT1 WF1 WF2

Proportion

0.0

0.2

0.4

0.6

0.8

1.0 Unknown 

salmonids 

Salmon /

steelhead 

Chinook 
31

Table 1. Percentage of species composition by number of fish captured in the 2011 sampling season, Priest 

Rapids Project, Columbia River, Washington. 

[Reach locations: PF1, Priest Rapids forebay near-BRZ (Boat Restricted Zone); PF2, Priest Rapids forebay; PM3, Priest 

Rapids mid-reservoir; WT2, Wanapum tailrace; WT1, Wanapum tailrace near-BRZ; WF1, Wanapum forebay near-BRZ; 

WF2, Wanapum forebay; Overall, all locations combined.] 

Reach location

Fish PF1 PF2 PM3 WT2 WT1 WF1 WF2 Overall

Bluegill 0.21 0.10 0.00 0.00 0.00 0.00 0.40 0.07

Bridgelip sucker 0.00 0.00 0.00 0.00 0.00 1.91 0.40 0.28

Chiselmouth 1.04 1.06 1.88 0.00 0.00 34.90 2.62 5.65

Sculpin species 11.02 1.92 1.77 0.20 0.00 2.21 20.32 4.19

Carp 0.21 0.00 0.21 1.02 0.00 0.00 0.80 0.26

Largemouth bass 0.42 0.00 0.00 0.00 0.00 0.00 0.00 0.04

Longnose dace 0.00 0.00 0.00 0.00 0.00 0.00 1.01 0.09

Longnose sucker 0.62 1.82 2.60 0.82 3.85 1.62 12.47 2.96

Lamprey 0.00 0.00 0.00 0.00 0.00 0.29 0.20 0.06

Largescale sucker 29.11 33.88 53.18 36.53 41.03 24.15 12.47 37.71

Northern pikeminnow 32.64 39.54 13.85 2.86 0.43 7.22 9.86 17.74

Peamouth 2.91 4.32 2.08 0.41 0.00 1.33 8.05 2.81

Pumpkinseed 0.21 0.10 0.00 0.00 0.00 0.00 0.00 0.04

Redside shiner 6.44 8.35 2.45 1.63 0.00 23.12 24.95 8.50

Sandroller 0.00 0.00 0.00 0.00 0.00 0.29 0.00 0.04

Speckled dace 0.00 0.00 0.00 0.00 0.00 0.15 0.20 0.04

Sucker species 0.62 1.73 16.04 53.47 51.28 0.29 1.21 13.46

Smallmouth bass 13.31 6.05 3.91 0.20 0.00 2.21 3.82 4.44

Tench 0.00 0.00 0.05 0.00 0.00 0.00 0.00 0.02

Threespine stickleback 1.25 0.67 0.16 0.20 0.43 0.29 0.60 0.43

Walleye 0.00 0.29 0.68 0.41 0.00 0.00 0.00 0.34

Whitefish 0.00 0.00 1.09 2.24 2.99 0.00 0.00 0.73

Yellow perch 0.00 0.19 0.05 0.00 0.00 0.00 0.60 0.11
32

Table 2. Summary of mean and standard error of catch per unit effort (catch per 10 minutes of electrofishing) for

northern pikeminnow (total length greater than 170 mm) and smallmouth bass (total length greater than 150 mm), 

Priest Rapids Project,, Columbia River, Washington, May 2–June 7, 2009, May 4–June 2, 2010, and May 10–

June 9, 2011.

[Standard error is shown in parentheses ( ). All efforts were randomly selected throughout the reaches except for “hotspot” 

(HS) analysis in 2011, which were stationary sites repeatedly sampled throughout the study dates. Reaches were combined 

in 2010 and 2011 to match reaches for 2009, with the exception of the mid-reservoir areas. An “ns” indicates the reach was 

not sampled. Reach locations: PT, Priest Rapids tailrace; PF, Priest Rapids forebay; PM1, Priest Rapids mid-reservoir, 

2009; PM3, Priest Rapids mid-reservoir, 2010 and 2011; WT, Wanapum tailrace; WF, Wanapum forebay] 

Catch per 10 minutes

Northern pikeminnow Smallmouth bass

Reach 2009 2010 2011 2011 HS 2009 2010 2011 2011 HS

PT 0.013

(0.009)

0.082

(0.050)

ns ns 0.0 0.0 ns ns

PF 0.008

(0.003)

0.014

(0.007)

0.037

(0.005)

0.039

(0.007)

0.013

(0.011)

0.0 0.015

(0.003)

0.049

(0.022)

PM1 0.027

(0.008)

ns ns ns 0.014

(0.004)

ns ns ns

PM3 ns 0.017

(0.007)

0.044

(0.007)

0.042

(0.012)

ns 0.010

(0.010)

0.016

(0.006)

0.019

(0.008)

WT 0.004

(0.003)

0.005

(0.002)

0.007

(0.002)

0.005

(0.005)

0.0 0.0 0.0 0.005

(0.005)

WF 0.0 0.017

(0.008)

0.016

(0.003)

0.015

(0.006)

0.0 0.012

(0.006)

0.006

(0.002)

0.005

(0.002)
33

Table 3. Summary of northern pikeminnow diets analyzed in 2011 during electrofishing efforts by the Washington 

Department of Fish and Wildlife (WDFW) and by angling efforts by U.S. Department of Agriculture (USDA), Priest 

Rapids Project, Columbia River, Washington. 

[Including thenumber of stomachs recorded as empty before (pre-maceration) and number and percentage ()after 

maceration (post-maceration), as well as the number and percentage () of non-empty, the number and percentage containing 

fish bones, and the number and percentage with no fish bones for the post-maceration stomach counts] 

Pre-maceration Post-maceration

Collection Analyzed Empty Empty Non-empty Fish bones

No fish 

bones

WDFW

Electrofishing 430 0 229 (53%) 201 (47%) 20 (10%) 181 (90%)

USDA

Angling 131 0 50 (38%) 81 (62%) 76 (94%) 5 (6%)

Table 4. Summary of smallmouth bass (SMB), largemouth bass (LMB), and walleye (WAL) diets analyzed 2011, 

Priest Rapids Project, Columbia River, Washington. 

[Including the number and percentage ( ) of stomachs that were empty, non-empty, or contained fish and the number of fish 

prey items (Fish Count) within the stomachs] 

Species Analyzed Empty Non-empty Contained fish Fish count

LMB 1 1 (100%) - - -

SMB 202 38 (19%) 164 (81%) 131 (80%) 33 (20%)

WAL 18 6 (33%) 12 (67%) 11 (92%) 1 (8%)

Table 5. Results of visual diet inspection of piscivores in the field versus laboratory analysis (number of prey fish 

estimated). 

[Prey categories: SH, steelhead; SAL/SH, salmon or steelhead; UNK/Other, fish unidentified]

SH SAL/SH UNK/Other Total

Field 1 14 130 145

Laboratory 1 27 259 387
34

Table 6. Summary of the mean consumption (Ci), abundance (Ai), and predation (Pi) indices, and standard errors 

( ) for northern pikeminnow and smallmouth bass collected during randomized electrofishing efforts and site 

selected (“hotspot”) efforts for reaches, Priest Rapids and Wanapum Reservoirs, Columbia River, Washington, 

2011. 

[Ci, mean consumption index; Ai, mean abundance index; Pi, mean predation index. Reach locations: PF1, Priest Rapids 

forebay near-BRZ (Boat Restricted Zone); PF2, Priest Rapids forebay; PM3, Priest Rapids mid-reservoir; WT2, Wanapum 

tailrace; WT1, Wanapum tailrace near-BRZ; WF1, Wanapum forebay near-BRZ; WF2, Wanapum forebay] 

Northern pikeminnow Northern pikeminnow Hotspot

Reach Ci Ai Pi Ci Ai Pi

PF1 0.177

(0.116)

0.248 

(0.062)

0.068

(0.044)

0.123

(0.123)

0.301

(0.045)

0.035

(0.035)

PF2 0 2.077

(0.350)

0 0 2.519

(0.742)

0

PM3 0.033

(0.020)

12.71

(2.170)

0.595

(0.342)

0.244

(0.100)

12.354

(3.491)

5.028

(2.638)

WT2 0.153

(0.153)

1.109

(0.386)

1.191

(0.191)

0 0.624

(0.624)

0

WT1 0 0.047

(0.047)

0 0 0 0

WF1 0 0.246

(0.045)

0 0 0.131

(0.049)

0

WF2 0 0.918

(0.225)

0 0 0 0

Smallmouth bass Smallmouth bass Hotspot

Reach Ci Ai Pi Ci Ai Pi

PF1 0.205

(0.205)

0.124

(0.057)

0.039

(0.039)

0.073

(0.073)

0.619

(0.338)

0.035

(0.035)

PF2 0.167

(0.090)

0.797

(0.214)

0.157

(0.087)

0 1.260

(0.252)

0

PM3 0.334

(0.259)

4.627

(1.854)

1.827

(0.991)

0.179

(0.158)

5.572

(2.332)

1.493

(1.023)

WT2 0 0 0 0 0.624

(0.624)

0

WT1 0 0 0 0 0 0

WF1 0 0.071

(0.020)

0 0 0.044

(0.019)

0

WF2 0.463

(0.266)

0.475

(0.159)

0.947

(0.521)

0 0 0
35

Table 7. Summary of mean predation indices (Pi) and standard errors for northern pikeminnow and smallmouth 

bass collected during May 2–June 7, 2009, May 4–June 2, 2010, and May 10–June 9, 2011, by randomized 

electrofishing efforts and site selection (“hotspot” = HS) efforts for reaches in Priest Rapids and Wanapum 

Reservoirs, Columbia River, Washington. 

[Standard error is shown in parentheses ( ). A “ns” indicates the reach was not sampled. Reach locations: PT, Priest Rapids 

tailrace; PF, Priest Rapids forebay; PM1, Priest Rapids mid-reservoir, 2009; PM3, Priest Rapids mid-reservoir, 2010 and 

2011; WT, Wanapum tailrace; WF, Wanapum forebay] 

Northern pikeminnow Pi Smallmouth bass Pi

Reach 2009 2010 2011 2011 HS 2009 2010 2011 2011 HS

PT 1.949

(1.817)

5.492

(3.306)

ns ns 0 0 ns ns

PF 0 0 0.110

(0.077)

0.132

(0.132)

0.172

(0.172)

0 0.200

(0.094)

0.109

(0.109)

PM1 0 ns ns ns 0.240

(0.240)

ns ns ns

PM3 ns 0 0.595

(0.342)

5.028

(2.638)

ns 2.221

(0.000)

1.827

(0.991)

1.493

(1.023)

WT 0 1.228

(0.811)

0.177

(0.177)

0 0 0 0 0

WF 0 0.251

(0.251)

0 0 0 0 0.645

(0.375)

0
36

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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, WA 98115

http://wfrc.usgs.gov/
Hardiman and others Assessing Fish Predation on Migrating Juvenile Steelhead and a Retrospective Comparison to Steelhead Survival 

 Through the Priest Rapids Hydroelectric Project, Columbia River, Washington, 2009-11

Open-File Report 2012-1129