Prepared in cooperation with the Washington Department of Fish and Wildlife

Assessing Native and Introduced Fish Predation on 

Migrating Juvenile Salmon in Priest Rapids and 

Wanapum Reservoirs, Columbia River, Washington, 2009–11

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2012–1130

Assessing Native and Introduced Fish Predation on 

Migrating Juvenile Salmon in Priest Rapids and 

Wanapum Reservoirs, Columbia River, Washington, 

2009–11

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

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

Rochelle R. Polacek, Washington Department of Fish and Wildlife

Prepared in cooperation with the Washington Department of Fish and Wildlife

Open-File Report 2012–1130

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:

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

Assessing native and introduced fish predation on migrating juvenile salmon in Priest Rapids and Wanapum 

Reservoirs, Columbia River, Washington, 2009–11: U.S. Geological Survey Open-File Report 2012-1130, 68 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

Study Methods............................................................................................................................................................... 3

Study Area ................................................................................................................................................................. 4

Field Data Collection.................................................................................................................................................. 4

Site Selection ......................................................................................................................................................... 4

Sample Allocation................................................................................................................................................... 5

Boat Electrofishing ................................................................................................................................................. 5

Laboratory Analyses .................................................................................................................................................. 6

Aging Analysis........................................................................................................................................................ 6

Diet Analysis........................................................................................................................................................... 7

Data Analyses............................................................................................................................................................ 8

Relative Abundance Indices................................................................................................................................... 8

Consumption Indices.............................................................................................................................................. 9

Predation Indices.................................................................................................................................................. 10

Bioenergetics........................................................................................................................................................ 10

Results......................................................................................................................................................................... 11

River Conditions....................................................................................................................................................... 11

Catch Data ............................................................................................................................................................... 12

Northern Pikeminnow........................................................................................................................................... 12

Smallmouth bass.................................................................................................................................................. 13

Walleye................................................................................................................................................................. 14

Diet Analyses ........................................................................................................................................................... 14

Northern Pikeminnow........................................................................................................................................... 14

Smallmouth Bass ................................................................................................................................................. 15

Walleye................................................................................................................................................................. 16

Predation Indices ..................................................................................................................................................... 16

Northern Pikeminnow........................................................................................................................................... 16

Smallmouth Bass ................................................................................................................................................. 16

Bioenergetics ........................................................................................................................................................... 17

Discussion ................................................................................................................................................................... 17

Acknowledgments........................................................................................................................................................ 24

References Cited......................................................................................................................................................... 24

Figures

Figure 1. Study area sampled in the Columbia River, Washington, 2009 ............................................................... 29

Figure 2. Study area sampled in the Columbia River, Washington, 2010 ............................................................... 30

Figure 3. River discharge (1,000 cubic feet second) and water temperature as measured in the tailrace of 

Priest Rapids, Wanapum, and Rock Island Dams, Columbia River, Washington, from May to October.................. 31

Figure 4. Length frequency histograms for northern pikeminnow during Burley and Poe sampling overall in

2009 (May 27–June 12 and August 3–20), in spring 2009 (May 27–June 12), and in summer 2009 

(August 3–20), Priest Rapids Project, Columbia River, Washington........................................................................ 32
iv

Figure 5. Length frequency histograms for northern pikeminnow during predator index sampling overall in 2009 

(May 1–August 27), in spring 2009 (May 7–June 11), and in summer 2009 (June 23–August 5), Priest Rapids 

Project, Columbia River, Washington. n, total number of fish.................................................................................. 33

Figure 6. Length frequency histograms for northern pikeminnow during predator index sampling overall in 2010 

(May 19–September 3), in spring 2010 (May 19–June 8), and in summer 2010 (June 28–August 11), Priest Rapids 

Project, Columbia River, Washington ...................................................................................................................... 34

Figure 7. Mean length and one standard error for northern pikeminnow during Burley and Poe sampling in spring 

2009 (May 27–June 12) and summer 2009 (August 3–20), by reaches, Priest Rapids Project, Columbia River, 

Washington.............................................................................................................................................................. 35

Figure 8. Mean length and one standard error for northern pikeminnow during predator index sampling overall in 

2009 (May 1–August 27), in spring 2009 (May 7–June 11), and in summer 2009 (June 23–August 5), by reaches, 

Priest Rapids Project, Columbia River, Washington ................................................................................................ 36

Figure 9. Mean length and one standard error for northern pikeminnow during predator index sampling overall in 

2010 (May 19–September 3), in spring 2010 (May 19–June 8), and in summer 2010 (June 28–August 11), by 

reaches, Priest Rapids Project, Columbia River, Washington ................................................................................. 37

Figure 10. Age frequency (number) of northern pikeminnow captured, Priest Rapids Project, Columbia River, 

Washington, 2009–11.............................................................................................................................................. 38

Figure 11. Mean length at age and one standard error for northern pikeminnow captured, Priest Rapids Project, 

Columbia River, Washington, 2009–11 ................................................................................................................... 38

Figure 12. Length frequency histograms for smallmouth bass during Burley and Poe sampling overall in 2009 (May 

27–June 12 and August 3–20), in spring 2009 (May 27–June 12), and in summer 2009 (August 3–20), Priest 

Rapids Project, Columbia River, Washington .......................................................................................................... 39

Figure 13. Length frequency histograms for smallmouth bass during predator index sampling overall in 2009 

(May 1–August 27), in spring 2009 (May 7–June 11) and in summer 2009 (June 23–August 5), Priest Rapids 

Project, Columbia River, Washington ...................................................................................................................... 40

Figure 14. Length frequency histograms for smallmouth bass during predator index sampling overall in 2010 

(May 19–September 3), in spring 2010 (May 19–June 8), and in summer 2010 (June 28–August 11), Priest 

Rapids Project, Columbia River, Washington .......................................................................................................... 41

Figure 15. Mean length and one standard error for smallmouth bass during Burley and Poe sampling in spring 

2009 (May 27–June 12) and summer 2009 (August 3–20), by reaches, Priest Rapids Project, Columbia River,

Washington.............................................................................................................................................................. 42

Figure 16. Mean length and one standard error for smallmouth bass during predator index sampling overall in 

2009 (May 1–August 27), in spring 2009 (May 7–June 11), and in summer 2009 (June 23–August 5), by reaches, 

Priest Rapids Project, Columbia River, Washington ................................................................................................ 43

Figure 17. Mean length and one standard error for smallmouth bass during predator index sampling overall in 

2010 (May 19–September 3), in spring 2010 (May 19–June 8), and in summer 2010 (June 28–August 11), by 

reaches, Priest Rapids Project, Columbia River, Washington ................................................................................. 44

Figure 18. Age frequency (number) of smallmouth bass captured, Priest Rapids Project, Columbia River, 

Washington, 2009–11.............................................................................................................................................. 45

Figure 19. Mean length at age and one standard error for smallmouth bass captured Priest Rapids Project, 

Columbia River, Washington, 2009–11 ................................................................................................................... 45

Figure 20. Length frequency histograms for walleye during Burley and Poe sampling overall in 2009 (

May 27–June 12 and August 3–20), in spring 2009 (May 27–June 12), and in summer 2009 (August 3–20),

Priest Rapids Project, Columbia River, Washington ............................................................................................... 46

Figure 21.Length frequency histogram for walleye during predator index sampling overall in 2009 (

May 1–August 27), and in summer 2009 (August 3–20), Priest Rapids Project, Columbia River, Washington ....... 47
v

Figure 22. Length frequency histograms for walleye during predator index sampling overall in 2010 (May 19–

September 3), in spring 2010 (May 19–June 8), and in summer 2010 (June 28–August 11), Priest Rapids 

Project, Columbia River, Washington ...................................................................................................................... 48

Figure 23. Age frequency (number) of walleye captured, Priest Rapids Project, Columbia River, Washington, 

2009–11................................................................................................................................................................... 49

Figure 24. Mean length at age and one standard error for walleye captured, Priest Rapids Project, Columbia 

River, Washington, 2009–11. Fish 11 years old and older are combined................................................................ 49

Figure 25. Proportion of diet for northern pikeminnow during Burley and Poe sampling in spring 2009 

(May 27–June 12) and summer 2009 (August 3–20), by reaches, Priest Rapids Project, Columbia River, 

Washington.............................................................................................................................................................. 50

Figure 26. Proportion of diet for northern pikeminnow during Predator index sampling in spring 2009 

(May 7–June 11) and summer 2009 (June 23 - August 5), by reaches, Priest Rapids Project, Columbia River, 

Washington.............................................................................................................................................................. 51

Figure 27. Proportion of diet of northern pikeminnow collected during predator index sampling overall in 2010 

(May 19–September 3), in spring 2010 (May 2–June 9), and in summer 2010 (June 27–August 11), by reaches, 

Priest Rapids Project, Columbia River, Washington. ............................................................................................... 52

Figure 28. Proportion of fish that are salmon in the diets of northern pikeminnow during predator index

sampling in 2010 overall (May 19–September 3), in spring 2010 (May 2–June 9), and summer 2010 

(June 27 - August 11), by reach, Priest Rapids Project, Columbia River, Washington ............................................ 53

Figure 29. Proportion of diet for smallmouth bass during Burley and Poe sampling in spring 2009 

(May 27–June 12) and in summer 2009 (August 3–20), by reaches, Priest Rapids Project, Columbia River, 

Washington.............................................................................................................................................................. 54

Figure 30. Proportion of diet for smallmouth bass during predator index sampling in spring 2009 

(May 7–June 11) and summer 2009 (June 23 - August 5), by reaches, Priest Rapids Project, Columbia River, 

Washington.............................................................................................................................................................. 55

Figure 31. Diet composition of smallmouth bass captured during predator index sampling overall in 2010 

(May 19–September 3), in spring 2010 (May 2–June 9), and in summer 2010 (June 27–August 11), by reaches, 

Priest Rapids Project, Columbia River, Washington ................................................................................................ 56

Figure 32. Proportion of fish in diet of walleye that are salmon during predator index sampling overall in 2010 

(May 19–September 3), in spring 2010 (May 19– June 8), and in summer 2010 (June 28–August 11) by reaches, 

Priest Rapids Project, Columbia River, Washington ................................................................................................ 57

Figure 33. Estimated weights of prey items consumed by northern pikeminnow during the 2009 and 2010 

study periods using bioenergetics modeling ............................................................................................................ 58

Figure 34. Proportion of prey items consumed by northern pikeminnow from bioenergetics modeling during the 

2009 and 2010 study periods................................................................................................................................... 59

Figure 35. Estimated weights of prey items consumed by smallmouth bass during the 2009 and 2010 study 

periods using bioenergetics modeling...................................................................................................................... 60

Figure 36. Proportion of prey items consumed by smallmouth bass from bioenergetics modeling during the 

2009 and 2010 study periods................................................................................................................................... 61
vi

Tables

Table 1.Estimated area of each reach that is less than 3 meters in depth, Priest Rapids Project, Columbia 

River, Washington, 2009–10.................................................................................................................................... 62

Table 2.Percentage of each age class of northern pikeminnow and smallmouth bass captured in 2009 and 

2010 in less than 3 m depth in the Priest Rapids Project, Columbia River, Washington.......................................... 63

Table 3.Catch of northern pikeminnow in 10 minute period during Burley and Poe sampling in spring 

(May 27–June 12) and summer (August 3–20), and during predator index sampling in spring (May 7–June 11) 

and summer (June 23 - August 5), by reaches, Priest Rapids Project, Columbia River, Washington, 2009............ 64

Table 4.Catch of northern pikeminnow (greater than 170 millimeters in length) and smallmouth bass 

(greater than 150 millimeters in length) per 10 minute period captured during electrofishing runs in spring 

(May 2–June 9) and summer 2010 (June 27–August 11), by reaches, Priest Rapids Project, Columbia River, 

Washington.............................................................................................................................................................. 65

Table 5.Predation indices estimated during the Burley and Poe sampling (Burley and Poe, 2009) in spring 

(May 27–June 12) and summer 2009 (August 3–20), and during Burley and Poe’s 1993 study (Burley and Poe, 

1994). ...................................................................................................................................................................... 66

Table 6.Predation indices and standard errors (in parentheses) estimated for northern pikeminnow and 

smallmouth bass captured during predator index sampling in spring (May 7–June 11) and summer 2009 

(June 23–August 5), by reaches, Priest Rapids Project, Columbia River, Washington............................................ 67

Table 7.Predation indices and standard errors (in parentheses) estimated for northern pikeminnow and

smallmouth bass captured during predator index sampling in spring (May 2–June 9), and summer 2010 

(June 27–August 11), by reaches, Priest Rapids Project, Columbia River, Washington.......................................... 68
vii

Conversion Factors and Abbreviations and Acronyms

Conversion Factors

Inch/Pound to SI 

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 0.4047 hectare (ha)

acre 0.004047 square kilometer (km2)

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

Mass

ounce, avoirdupois (oz) 28.35 gram (g) 

I 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) 0.62137 mile (mi)

Area

hectare (ha) 0.003861 square mile (mi2)

hectare (ha) 2.47105 acre

Flow rate

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)
viii

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

°F=(1.8×°C)+32 

Abbreviations and Acronyms

Abbreviation or Acronym Definition

BiOp Biological Opinion

BRZ Boat Restricted Zone

CPUE Catch per unit effort

EPA U.S. Environmental Protection Agency

EMAP Environmental Monitoring and Assessment Program

FERC Federal Energy Regulatory Commission

GIS Geographic information system

GPP Generator Powered Pulsator

GPS Global Positioning System

Grant PUD Public Utility District No. 2 of Grant County, Washington

n number

PRCC Priest Rapids Coordinating Committee

PRP Priest Rapids Project

RM River mile

SE Standard error

SOP Standard Operating Procedures

spp species

USGS U.S. Geological Survey

WDFW Washington Department of Fish and Wildlife
1

Assessing Native and Introduced Fish Predation on 

Migrating Juvenile Salmon in Priest Rapids and Wanapum 

Reservoirs, Columbia River, Washington, 2009–11

By Timothy D. Counihan1, Jill M. Hardiman1, Dave S. Burgess2, Katrina E. Simmons2, Glen S. Holmberg1, 

Josh A. Rogala2, and Rochelle R. Polacek2

Abstract 

Hydroelectric development on the mainstem Columbia River has created a series of 

impoundments that promote the production of native and non-native piscivores. Reducing the effects of 

fish predation on migrating juvenile salmonids has been a major component of mitigating the effects of 

hydroelectric development in the Columbia River basin. Extensive research examining juvenile salmon 

predation has been conducted in the lower Columbia River. Fewer studies of predation have been done 

in the Columbia River upstream of its confluence with the Snake River; the most comprehensive 

predation study being from the early 1990s. The Public Utility District No. 2 of Grant County, 

Washington initiated a northern pikeminnow removal program in 1995 in an attempt to reduce predation 

on juvenile salmonids. However, there has been no assessment of the relative predation within the Priest 

Rapids Project since the removal program began. Further, there is concern about the effects of 

piscivores other than northern pikeminnow (Ptychocheilus oregonensis), such as channel catfish

(Ictalurus punctatus), smallmouth bass (Micropterus dolomieu), and walleye (Sander vitreus, formerly 

Stizostedion vitreum). The Public Utility District No. 2 of Grant County, Washington and the Priest 

Rapids Coordinating Committee requested that the U.S. Geological Survey, in collaboration with the 

Washington Department of Fish and Wildlife, assist them in evaluating the effects of native and 

introduced predatory fish on migrating juvenile salmon. From 2009 to 2010, we conducted sampling in 

the 103 kilometers (64 river miles) of the Columbia River from the tailrace of Rock Island Dam 

downstream to the tailrace of Priest Rapids Dam. To assess predation, we used electrofishing to collect 

northern pikeminnow, smallmouth bass, and walleye to analyze their diets during 2009 and 2010. In 

2009, we used methods to allow comparisons to a previous study conducted in 1993. During 2009, we 

also used an alternate sampling strategy using habitat data and geographic information system software 

to select sites and allocate samples. In 2010, we used the data collected during 2009 to further refine our 

sampling design, with the intent of using the data collected during 2010 to formulate a design strategy 

for implementation during 2011. Based on the results of 2011, we would then propose a strategy for 

future studies. However, during 2011, our efforts were redirected to specifically address factors that 

may be affecting steelhead trout survival in the Priest Rapids Reservoir, Columbia River. 

 

1 U.S. Geological Survey

2 Washington Department of Fish and Wildlife
2

We used the catch and diet data collected in 2009 and 2010 to estimate relative abundance, 

consumption, and predation indices for northern pikeminnow and smallmouth bass. Despite extensive

sampling in the study area in 2009 and 2010, very few channel catfish and walleye were captured. The 

mean total lengths of northern pikeminnow were much lower than those observed in 1993; suggesting 

that efforts to remove northern pikeminnow in the study area may be shifting the population towards 

smaller fish. The northern pikeminnow predation index values were lower in 2009 than in the 1993 

study. The reduced predation levels observed may be due to the prevalence of smaller pikeminnow in 

our catches than in catches reported in 1993. Predation by smallmouth bass was lower in 2009 than in 

2010, and generally was greater than predation for northern pikeminnow. Predation for northern 

pikeminnow was concentrated in the tailrace areas of Priest Rapids, Wanapum, and Rock Island Dams; 

predation for smallmouth bass was concentrated in the forebay and mid-reservoir sections of the study 

area. Our results indicate areas where control measures for smallmouth bass could be concentrated to 

reduce predation in the Priest Rapids Project.

Introduction

Hydroelectric development in the Columbia River basin has transformed the Columbia River 

from a high-gradient riverine system to a series of impoundments created by hydroelectric dams. 

Anadromous juvenile salmonids migrating through the Columbia River experience a variety of hazards 

that affect their survival as they migrate from freshwater rearing habitats to the ocean. Direct effects 

associated with dam passage (for example, instantaneous mortality, injury, and loss of equilibrium) and 

indirect effects (such as predation, disease, and physiological stress) contribute to the total mortality of 

seaward-migrating salmonids. Many studies (Raymond, 1979; Stier and Kynard, 1986; Iwamato and 

others, 1994; Muir and others, 1995; Bickford and Skalski, 2000; Timko and others, 2007a, 2007b) have 

been conducted to estimate dam, reach, and route-specific (that is through spillways, bypass areas, and 

turbines) survival of juvenile salmon to help identify the potential sources of mortality. Based on these 

studies and the endangered or threatened status of anadromous salmonid stocks in the Columbia River 

basin, management actions are being implemented to improve survival of juvenile salmonid migrants. In 

some instances, management strategies are in response to stipulated criteria as part of Federal Energy 

Regulatory Commission (FERC) hydroelectric project relicensing agreements. For instance, as part of 

the FERC license issued to the Public Utility District No. 2 of Grant County, Washington (Grant PUD) 

for the operation of the Priest Rapids Project on April 17, 2008 (Federal Energy Regulatory 

Commission, 2008), performance standards (passage survival rates) were established for Grant PUD in 

the National Marine Fisheries Service 2004 Biological Opinion (National Marine Fisheries Service, 

2004), as adapted in the “Terms and Conditions” of the 2008 Biological Opinion (National Marine 

Fisheries Service, 2008). The 2006 Priest Rapids Project Salmon and Steelhead Settlement Agreement 

(U.S. Fish and Wildlife Service and others, 2006) requires that the same survival standards be met for 

salmonid species not listed under the Endangered Species Act. 

Grant PUD is working to improve juvenile salmonid survival through their hydroelectric 

developments and the river environment affected by the construction and operation of these structures, 

collectively referred to as the Priest Rapids Project (PRP). Management actions to improve survival 

include altering dam operations, modifying the physical structure of hydroelectric projects, and reducing 

predator effects. For instance, surface flow alternatives to promote egress through the near-dam 

environment have resulted in improved passage at Priest Rapids and Wanapum Dams, where surface 

bypass systems are in operation; a prototype top-spill bypass was installed at Priest Rapids Dam in 2006 

(Harmon and Parks, 1980; Ransom and Steig, 1995; Coutant and Whitney, 2000; Johnson and others, 

2005; Robichaud and others, 2005; Timko and others, 2007a, 2007b). In 2008, modifications to the 
3

operation of the prototype top-spill included additional bottom and sluiceway spill at adjacent gates, 

which increased passage effectiveness and warranted further testing (Sullivan and others, 2001). These 

alterations have resulted in some improvements in fish collection efficiency and survival. 

Predation in the Columbia River is a significant factor affecting survival of downstream 

migrating salmonids (Beamesderfer and Rieman, 1991; Burley and Poe, 1994; Ward and others, 1995; 

Petersen and Ward, 1999; Petersen, 2002). Beamesderfer and others (1996) estimated that about 16.4 

million out-migrating juvenile anadromous salmonids were consumed annually by northern 

pikeminnow (Ptychocheilus oregonensis) in the Columbia and Snake Rivers prior to the Northern 

Pikeminnow Management Program for the lower Columbia and Snake Rivers and Northern 

Pikeminnow Removal Programs implemented collectively by Grant PUD, Public Utility District No. 1 

of Chelan County, Washington, and Public Utility District No. 1 of Douglas County, Washington. When 

compared to the estimated 200 million juvenile anadromous salmonids produced in the combined 

Columbia–Snake River systems, northern pikeminnow are believed to consume approximately 8 percent

of all downstream migrants, although 6.5 percent are believed to be consumed downstream of The 

Dalles Dam (Beamesderfer and others, 1996). 

Extensive research on juvenile salmon predation has been conducted in the Columbia River 

downstream of its confluence with the Snake River. Fewer studies of predation on juvenile salmonids 

have been done in the Columbia River upstream of its confluence with the Snake River, the most 

comprehensive study was from the early 1990s (Burley and Poe, 1994). The Grant PUD initiated a 

northern pikeminnow removal program in 1995 in an attempt to reduce predation on juvenile salmonids 

(Garner and Keeler, 2008, 2009). However, no assessment has been made of the relative predation 

within the PRP since the removal program began. Furthermore, there is concern about the effects of fish 

predators other than northern pikeminnow, such as channel catfish (Ictalurus punctatus), smallmouth 

bass (Micropterus dolomieu), and walleye (Sander vitreus, formerly Stizostedion vitreum). The Grant 

PUD and the Priest Rapids Coordinating Committee (PRCC) requested that the U.S. Geological Survey 

(USGS), in collaboration with the 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. From 2009 to 2010, we developed and conducted research to increase our understanding of 

predator-prey interactions within the PRP.

Our objectives in this study were to assess the current status of predation on juvenile salmonids 

migrating through the Priest Rapids and Wanapum Reservoirs on the Columbia River, Washington. 

Specifically, we were to repeat the methods of a previous study (Burley and Poe, 1994) to assess the 

current status of predation on juvenile salmonids from the tailrace of Rock Island Dam to the tailrace of 

Priest Rapids Dam on the Columbia River. In addition, we were to implement alternate study design and 

sampling protocols that could be used for future studies of juvenile salmonid predation within the PRP. 

Study Methods

We conducted field collections of fish predators and their diets in 2009–11. In 2009, we 

implemented design and sampling strategies to allow comparisons to a Mid-Columbia Predation Index 

Study from 1993 (Burley and Poe, 1994; hereafter referred to as Burley and Poe). We replicated this 

study, with the exception that we sampled from the tailrace of Rock Island Dam to the tailrace of Priest 

Rapids Dam and modified some data collection and laboratory analysis protocols to conform to current 

standards and regulatory requirements. We also explored alternate sampling strategies that incorporated 

habitat data and geographic information system (GIS) software to select sites and allocate samples. In 

2010, we continued our sampling using modified methods in our study design and sample frame design 

that incorporated the results from 2009. In 2011, the PRCC redirected the original study objectives to 
4

specifically assess predation effects of juvenile steelhead migrating through the Priest Rapids 

Development only. The results of the 2011 work are described in a separate report (Hardiman and 

others, 2012). We used fish collected in 2011, however, to describe certain characteristics of the 

predator populations (such as fish ages) and to present the results in this report. 

Study Area

The PRP study area included approximately 64 river miles, from the Rock Island Dam (RM 453) 

tailrace to about 8 mi downstream of the Priest Rapids Dam (RM 397) in the Columbia River (fig. 1). 

The PRP consists of two run-of-the-river hydroelectric developments owned and operated by Grant 

PUD. The Priest Rapids Reservoir is about 18 mi in length, with a shoreline of 56 mi and an 

approximate surface area of 7,580 acres (Pfeifer and others, 2001). The Wanapum Reservoir is 38 mi in 

length, with 91 mi of shoreline and a surface area of 14,590 acres (Pfeifer and others, 2001).

Environmental conditions during the study periods were obtained for Priest Rapids, Wanapum, and 

Rock Island Dams from the University of Washington’s Columbia River Data Access in Real Time

(DART) Web site (http://www.cbr.washington.edu/dart/). 

Field Data Collection 

Site Selection

The study area was divided into strata based on the longitudinal position of reaches in each 

reservoir. The construction of hydroelectric projects on the Columbia River has formed a series of 

impoundments that have characteristics typical of lakes and streams. The consequences of impoundment 

are relatively predictable; the reservoirs are more like streams immediately downstream of the upstream 

dam and more like lakes near the downstream dam. As such, reservoirs typically can be divided into 

three zones (riverine, transitional, and lacustrine), corresponding to riverine conditions (tailrace area); 

transition to lake conditions (mid-reservoir); and lake-like conditions near the downstream dam (forebay 

area). Past predation studies have shown that predation of juvenile salmonids varies longitudinally in 

impoundments of the Columbia River (Petersen, 1994) and that areas near hydroelectric dams, that are 

typically restricted to boat use (Boat Restricted Zones; BRZ), are areas where predation of juvenile 

salmonids is relatively high (Ward and others, 1995). The work of Petersen (1994) demonstrated that 

failure to account for this spatial variability resulted in bias in predation estimates. Therefore, we 

structured our sampling strategy, in part, based on the development of longitudinal strata in the study 

area.

Sampling consisted of Burley and Poe’s efforts in 2009 and additional predator indexing efforts 

using a modified sampling design in 2009 and 2010 that incorporated the use of a GIS containing 

habitat features. In 2009, the longitudinal strata (that is, forebay, mid-reservoir, and tailrace) replicated 

the Burley and Poe (1994) study, and were approximately 3.7 mi in length with the exception of the 

BRZ areas (fig. 1). For the Burley and Poe efforts each of the longitudinal strata were divided into 

transects that were approximately 1,640 ft in length and were randomly selected for shoreline (depths of 

less than 10 ft) electrofishing efforts. For the predator index sampling, a GIS was used to generate a 

systematic grid of points spaced every 50 ft with a depth criterion of less than 10 ft within each of the 

longitudinal strata. Points were then randomly selected for electrofishing sites each week. Because of

the smaller sizes of the BRZ areas, the entire available shoreline was sampled whenever access was 

provided to these areas. 
5

Modifications to the sample design were incorporated into the 2010 sample framework, based on 

assessment of the 2009 sampling efforts. One constraint was the limited access to sampling in the BRZ 

areas because of high flows and coordination needed to cease some dam operations to safely access 

these areas. Therefore, we added additional strata immediately upstream of the forebay and downstream 

of the tailrace BRZs in 2010 (fig. 2). By adding these reaches, we were able to sample the near-BRZ 

areas weekly without affecting dam operations. Another sample area modification was to expand the 

mid-reservoir reaches in both Wanapum and Priest Rapids Reservoirs from the approximate 3.7-mi

reach used in 2009 to the entire river area that was not included in one of the other sampling reaches 

(fig. 2). This eliminated the possibility of missing potentially important areas not sampled in 2009. 

Sample Allocation

The sample allocation for the Burley and Poe efforts was designed to replicate sampling periods 

to capture the spring and summer periods as achieved in the 1993 study (Burley and Poe, 1994). Efforts 

were allocated over a 10-day sampling period for the spring and summer, where each strata would be 

covered twice, consisting of six randomly selected transects, with the exception of the BRZs (only two

transects). To determine when to initiate sampling, because there was no rationale for mimicking the 

actual dates sampled in 1993, we used water temperature as a criterion to begin sampling during the 

spring and summer periods. Sampling for the spring was initiated when water temperatures reached 

approximately 12°C, and for the summer, when water temperatures were approximately 19°C. Burley 

and Poe’s spring sampling occurred from May 27 to June 12, 2009, and summer sampling occurred 

from August 3 to 20, 2009. 

For the 2009 and 2010 predator indexing efforts, we sampled continuously throughout the 

juvenile salmonid migration period, and then retrospectively determined the spring and summer periods 

based on Smolt Passage Indices presented on the Columbia River DART web site for Rock Island Dam. 

The sample design was such that the entire study area would be covered each week; and week days were 

randomly assigned to reaches by sample week throughout the study. Efforts allocated to the BRZ 

sampling were less than those allocated for the other strata because of the coordination and alteration of 

dam operations required to access BRZ areas. To determine the spring migration period, we summed the 

smolt index values for yearling Chinook, coho, and sockeye salmon, and steelhead trout, and then 

assumed the middle 90 percent of the run as the sampling period. In 2010, logistics prevented us from 

sampling until May 19, which was later than these criteria would dictate. We used the smolt passage 

index at Rock Island Dam for sub-yearling Chinook salmon to define the summer period so that the 

beginning of the summer period was the first day that the index values for Rock Island Dam exceeded 

and subsequently did not go below values for either yearling Chinook, coho, and sockeye salmon, and 

steelhead trout. In 2009, sampling occurred from May 1 to August 27, with the spring migration period 

defined as May 4–June 11, and the summer migration period defined as June 22–August 7. For 2010, 

sampling occurred from May 19 to September 3, with the spring migration period defined as May 19–

June 9, and the summer period defined as June 27–August 11. 

Boat Electrofishing

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

predators in 2009 and 2010. Electrofishing efforts were conducted along the shoreline at preselected 

sites using two 18 ft-long (5.5 m-long), Smith Root® 5.0 Generator Powered Pulsator (GPP) 

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

individual electrofishing boats were operated parallel to the shoreline at a rate of 0.6–0.9 m/h, 

maintained a distance from shore that allowed the inshore boom to fish entirely in the water, and 
6

avoided areas that exceeded 10 ft in depth. To facilitate fish galvanotaxis, we operated the GPP unit at 

approximately 1–2 amperes (amps) using a low power setting (50–500 volts) with a frequency between 

30–120 Hz DC. To prevent unnecessary fish injury, we noted the behavior of fish within the electrical 

field and adjusted the power accordingly. 

Time, personnel, and direction of travel associated with sampling also were standardized. The 

goal of each electrofishing boat was to electrofish each site for 600 s. The number of crew on an 

individual boat also was regulated to maintain a constant effort between times and boats. Each crew 

consisted of one boat operator and two dip netters stationed at the front of the vessel, and each crew 

member was outfitted with a personal flotation device. Electrofishing was always conducted 

downstream. 

For the Burley and Poe efforts, electrofishing began 90 min before sunrise (determined using the 

Mattawa site from http://www.usno.navy.mil/USNO/astronomical-applications/data-services/rs-oneyear-us) and continued until we attained a target catch of 15 northern pikeminnow from each section 

sampled. For the predator indexing efforts, electrofishing began no earlier than 30 min after sunset 

(determined using the Mattawa site from http://www.usno.navy.mil/USNO/astronomicalapplications/data-services/rs-one-year-us) and continued until all sites were completed, weather 

permitting. 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, date, site designation, 

and power settings used to electrofish. During electrofishing, stunned fishes were placed immediately in 

one of the two onboard livewells equipped with a pump that continually added freshwater into the tank. 

After the completion of two 600-s electrofishing runs, the boat operator moored the electrofishing boat 

on shore where WDFW or USGS staff collected the required biological information from the captured 

fish. 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 fishes immediately after the completion 

of the first site.

Following standardized operating procedures (available upon request), biological information 

was collected for the following target species: northern pikeminnow, smallmouth and largemouth bass, 

channel catfish, and walleye. Because of the potential for the captured fishes to be consumed by anglers, 

we did not use the anesthetic commonly referred to as MS-222 per U.S. Food and Drug Administration 

guidelines. Therefore, all fish captured as non-lethal take were worked up in a non-anesthetized state. 

The collection of data from identified fish included the length, weight, and aging structures, such as 

scales for non-lethal-take fish and opercles for lethal-take fish. Hard structures for aging were collected 

from northern pikeminnow, smallmouth bass, and walleye for the duration of the fieldwork. The diets of 

walleye, smallmouth and largemouth bass (Micropterus salmoides), and walleye were collected using a 

lavage technique (non-lethal take), while northern pikeminnow and channel catfish (lethal take) 

stomachs were surgically removed. 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. 

Laboratory Analyses

Aging Analysis

Scales and opercles collected in the field were transported to the Large Lakes Research Team 

Laboratory in Ellensburg, Washington, to be prepared for aging analysis according to standardized 

operating procedures (available upon request). Personnel at the WDFW aging laboratory read scales 

using a standard office microfiche that had the ability to alter magnifications levels. Initially, a 

magnification that permitted a view of the entire scale was used to examine circuli. The areas where 
7

circuli were concentrated indicated an annulus or year mark (Jearld, 1983). Each annulus from the focus 

or center of the scales was identified and counted to provide an estimate of a length at age for an 

individual fish, and data were sent to the Large Lakes Research Team Laboratory. Cleaned opercular 

bones were placed proximal side up in a petri dish containing 95-percent ethanol and viewed under a 

dissecting microscope between 60 and 120 magnification. Samples were viewed under reflected light 

and contrasted against a solid black background. Annuli were counted on the proximal surface in a 

plane from the center to the anterior opercle edge similar to Le Cren (1947). Annuli were distinguished 

as the band of transparent growth occurring during the slow growing season (assumed winter months) 

and soon after the opaque fast growth (assumed spring and summer months). Fish were assumed to have 

a birth date of January 1; therefore, annuli forming at the opercle edge in fall months were not counted 

unless there was opaque growth beyond the annuli, although for spring collections, annuli at the edge 

were counted (occurring after the universal birth date of January 1). Up to three readings on older, more 

difficult structures were made per sample until a consistent reading could be determined. The age 

estimation was recorded and the opercle was placed back in the sample envelope and sealed.

Mean length at age and the standard error (SE) were calculated for each age class for the three 

predators. Length-at-age data for northern pikeminnow, smallmouth bass, and walleye were combined 

for the 2009, 2010, and 2011 sampling seasons in order to increase our sample size and to reduce the 

amount of variation associated with aging fish. Aging data should yield a mean length-at-age trend that 

increases as a group of fish ages. This trend was not the case for all our predatory fish 10 years or older. 

Therefore, length-at-age frequencies for fish determined to be 10 years or older omitted data for fish that 

had a mean length of less than that of fish estimated to be 1 year younger. Decreasing confidence in age 

estimates for older fish when scales are used have been noted in other studies (Donabauer 2010, 

Erickson, 1983; Isermann and others, 2003; Hanchin, 2011). 

Diet Analysis

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

available upon request), one for northern pikeminnow and another for bass and walleye. The 

methodology for processing northern pikeminnow stomachs involved pancreatin digestion or 

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

pikeminnow’s stomach digests at a high pH, leaving the mineral content of bones untouched. 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 apart from northern pikeminnow diets. 

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- to 2.0-molar solution of 

NaOH (lye) was, therefore, used to dissolve the remaining fat. Next, samples were rinsed through a 425-

μm (#40) mesh sieve. The diagnostic bones we used to identify and to enumerate fishes (cleithra, 

dentaries, hyomandibular arches, pharyngeal arches, otoliths, and opercles) are paired structures on the 

left and right sides of the fish. Therefore, bones were counted in pairs so as not to inflate the number of 

fish counted. For example, if we counted three left and two right salmon or 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 post-maceration was averaged to represent the 

mean percent composition of all diets analyzed. 
8

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 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 in each 

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

salmonids, Unknown non-salmonids, Chinook, Whitefish spp., Salmon/Steelhead, Northern 

Pikeminnow, Peamouth, Chiselmouth, Redside Shiner, Dace spp., Cyprinid spp., Cottus spp., 

Threespine Stickleback, Sucker spp., Walleye, Lampetra spp., Sandroller, 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. Fish in 

that group cannot be identified beyond genus because their diagnostic bones are too similar. Chinook 

salmon were only identified as such 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.

Data Analyses

Analysis of the data was organized into study year, data collection methodology (that is, Burley 

and Poe or predator indexing), and sampling period (such as overall, spring, and summer), as defined in 

the section, “Sample Allocation.” Metrics for relative abundance, consumption, and predation were

calculated for these periods using the methodology described below for northern pikeminnow and 

smallmouth bass. Because so few other predators (such as walleye, largemouth bass, and channel 

catfish) were captured during our efforts, and those that were captured were from a limited geographic 

area, we determined that developing consumption or predation indices for these species was of limited 

utility.

Relative Abundance Indices

To estimate the relative abundance indices of northern pikeminnow (> 250 mm) for the Burley 

and Poe efforts, the density index (DIBandP) was estimated as the proportion of nonzero catches 

(Counihan and others, 1999). To compare this index to the original values presented in Burley and Poe 

(1994), we calculated the proportion of nonzero catches from the density index they used: 

/1 proportion of witheffort catchzero (1)
9

For all other efforts, we estimated the relative abundance of predatory fish by estimating the CPUE

(number captured per 10 min of electrofishing) of northern pikeminnow (> 170 mm), smallmouth bass 

(> 150 mm), and walleye (> 180 mm) as the DICPUE (Ward and others, 1995).

The abundance index (AI) for each species was then estimated to be:

iii AI DI ×= S (2) 

where:

AIi = Index of predator abundance in sampling area i, 

DIi = 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 3 m in depth. 

To compare our results with those of Burley and Poe, we recalculated the abundance indices they 

presented based on current estimates of Si. Estimates of Si were derived using the GIS of the study area 

to estimate the area within each of the strata sampled in 2009–10 that are less than 3 m in depth (table 

1). 

Consumption Indices

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

indices for northern pikeminnow (CINPM) and smallmouth bass CISBM (Ward and others, 1995; Ward and 

Zimmerman, 1999). 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, using the following 

consumption index: 

.0 0209 )( 27.060.1 − 61.0 CI NPM nWT ⋅⋅⋅⋅= GW (3) 

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 as:

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

SMB (4) 

where:

T = water temperature (°C), 

W = predator weight (g), 

GW = mean total gut weight (g), and

n = mean number of salmonids per smallmouth bass. 
10

Predation Indices 

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

predation index (Ward and others, 1995) as: 

AIPI CIiii ⋅= (5)

where:

PIi = predation index for sample i,

AIi = abundance index for area i, and 

CIi = consumption index for sample i. 

For the comparisons to Burley and Poe, the PIi was estimated according to the procedures in 

their report (Burley and Poe, 1994). The predation index values for the predator index sampling in 2009 

and 2010 were estimated for each electrofishing effort and then averaged by strata. Reservoir-wide 

estimates were summed across strata as done by Burley and Poe (1994) and as a mean for a stratified 

random sample as done by Cochran (1977) for the predator index sampling in 2009 and 2010. 

Bioenergetics

The advent of bioenergetics modeling has enabled researchers to estimate the impacts of 

predators on biota within a system (Hanson and others, 1997). Using data from standard food habit 

studies that examine instantaneous diets, bioenergetics modeling allows a researcher to estimate 

energetic requirements of individual or predator cohorts (Brandt and Hartman, 1993). We used the Fish 

Bioenergetics 3.0 model (Hanson and others, 1997) to estimate prey consumption for northern 

pikeminnow and smallmouth bass of different ages during spring and summer periods. The 

bioenergetics model uses the following input parameters: water temperature, predator diet, prey energy 

density, predator size (weight), predator abundance, and predator age distribution, and works on the 

generalized formula: 

Energy consumed = Respiration + Waste + Growth.

This can be further divided into a more specific mass balance equation (Hanson and others, 1997): 

Consumption = (respiration + active metabolism + specific dynamic action) + (egestion + excretion) + 

(somatic growth + gonad production). 

The Fish Bioenergetics software (Hanson and others, 1997) contains many parameter sets for 

different fishes, but lacks the parameters necessary to model the bioenergetics of northern pikeminnow. 

Petersen and Ward (1999) compiled the physiological parameters necessary to model the energetic 

requirement of northern pikeminnow for various situations. Using the available parameters, we 

constructed model simulations in the Fish Bioenergetics software for northern pikeminnow in the PRP. 

The model output is based on total weight of prey items consumed by each predator cohort. For our 

modeling simulations, a cohort is a group of fish of the same age class and species, and the modeled 

population output is the sum of all individual cohort model runs for each species. 
11

Temporal, biological, and environmental parameters are required to fully populate the 

bioenergetics model. To estimate the energetic requirement of the fish species evaluated during our 

study period, water temperature data were obtained from the Grant County Public Utility District

Natural Resource link 

(http://www.gcpud.org/naturalResources/fishWaterWildlife/waterqualityMonitoring.html). The diet 

composition of individual predators throughout the study period was obtained from our field collections. 

The proportion of a diet for an individual was calculated by dividing the sum of each individual prey 

item by the total weight of the diet contents for that individual. Diet data from field collections were 

compiled by day, species, and age class, and were averaged for each model day. Diet contents were then 

assigned constant energy densities using various literature sources (Cummins and Wuycheck, 1971; 

Stewart and others, 1983). For model simulations, we estimated the Bioenergetics software p-value 

(proportion of maximum consumption) based on hypothetical consumption rates that would likely have 

been experienced in the field. We used a p-value of 0.5 for both smallmouth bass and northern 

pikeminnow in the PRP and conducted model runs assuming fish consumed 50 percent of their 

maximum consumption rate. Values commonly range between 0.2 and 0.6 estimated from observed 

growth of fishes in the field for bioenergetics modeling (Dieterman and others, 2004; Mateo, 2007; 

McCarthy and others, 2009).

We used our estimated ages of the fish collected to partition the proportion of the modeled 

population into age classes (table 2) or individual cohorts for the bioenergetics modeling. The age data 

was further used to determine mean length at age, and the mean weight of each age class for both 

smallmouth bass and northern pikeminnow. Because we did not have an accurate population estimate 

for the species of interest, we used our field data to estimate a hypothetical population for modeling 

purposes. The total numbers of predators captured were used for the population estimate. Modeling was 

further partitioned into spring and summer periods for the 2009 and 2010 study years, relative to our 

study periods based on juvenile salmonid migration times. 

Results

River Conditions

River discharge and water temperatures in 2009 were lower than the 10-year average from midJune to early-July, and remained lower than the 10-year average for the remainder of the field season

(fig. 3). Conversely, in 2010, river discharge generally was higher from mid-June to early-July, and 

consistently higher than the 10-year average. Water temperatures in 2009–10 were similar to the 10-year 

average.
12

Catch Data

Northern Pikeminnow

Northern pikeminnow were captured during 2009 Burley and Poe sampling efforts, and also 

during 2009 and 2010 predator index sampling efforts. During the Burley and Poe sampling, we 

captured and measured 1,225 northern pikeminnow ranging from 43 to 580 mm in total length in Priest 

Rapids and Wanapum Reservoirs (fig. 4). Similar overall numbers of fish were captured between the 

spring (n=601) and summer (n=624) periods. The fish captured during the spring period, ranging from 

43 to 531 mm in total length, were slightly smaller than fish captured during the summer efforts, 

ranging from 50 to 580 mm in total length. The CPUE of northern pikeminnow greater than 250 mm in 

total length during the 2009 Burley and Poe sampling was highest in the Rock Island tailrace and

generally was higher in Wanapum reservoir than in Priest Rapids reservoir for both spring and summer 

periods (table 3). 

The predator index sampling during 2009 covered a longer time period (overall, May 1–August 

27) than during Burley and Poe, but fewer fish were captured (n=1,025). The northern pikeminnow 

captured ranged from 40 to 567 mm in total length (fig. 5); smaller than those captured during Burley 

and Poe efforts. During the spring, we captured 392 northern pikeminnow ranging from 45 to 520 mm

in total length. Fewer fish were captured during the summer (n=361), but the overall total lengths were 

larger, as seen during the Burley and Poe sampling (ranging from 61 to 539 mm in total length) (fig. 5). 

The CPUE of northern pikeminnow greater than 170 mm during 2009 predator index sampling was 

highest in Rock Island tailrace during the spring, and generally was higher in the Wanapum reservoir 

than in the Priest Rapids reservoir during the spring and summer periods (table 3).

Sampling started and ended later in 2010 than in 2009. However, we captured and measured the 

greatest number of northern pikeminnow in 2010 compared to all other sampling efforts (n=2,581). The

northern pikeminnow captured ranged from 33 to 581 mm in total length (fig. 6). During the spring 

period, we captured 544 northern pikeminnow ranging from 42 to 510 mm in total length; in the 

summer period, we captured almost double that number with 990 northern pikeminnow ranging from 42 

to 581 mm in total length (fig. 6). The CPUE of northern pikeminnow greater than 170 mm in total 

length during the 2010 spring period was highest in the Priest Rapids tailrace near-BRZ reach, followed 

by the Wanapum mid-reservoir (table 4). For the summer period, the CPUE was highest in the 

Wanapum mid-reservoir followed by the Priest Rapids tailrace near-BRZ reach.

The mean lengths of northern pikeminnow captured during sampling efforts varied by strata in 

the PRP (figs. 7–9). Generally, we found that larger northern pikeminnow were more prevalent near the 

dams than in the mid-reservoir reaches. During the Burley and Poe sampling, the largest mean northern 

pikeminnow lengths were from fish in the forebay of Wanapum Dam and the tailrace of Priest Rapids 

Dam (fig. 7). This trend was evident in both the spring and summer periods. For the 2009 and 2010 

predator indexing efforts, the largest mean northern pikeminnow lengths were from fish in the tailrace 

and forebay of Wanapum Dam and in the tailrace of Priest Rapids Dam (figs. 8 and 9). 
13

Aging analysis was completed for all northern pikeminnow (fig. 10) captured in 2009, 2010, and 

2011 in the PRP. The analysis indicated that ages of the captured fish ranged from 1 to 24 years (median 

age = 3 years). The mean length at age was estimated for all age classes (fig. 11). For all northern 

pikeminnow captured in the Priest Rapids reservoir, the relationship between length and weight is 

described by the equation: 

log10 (weight) = 2.9974(log10 length) – 5.1203; r2 = 0.9625; 

For Wanapum Reservoir the relation is described by: 

log10 (weight) = 3.0422(log10 length) – 5.2224; r2

 = 0.9813. 

Smallmouth bass

We captured smallmouth bass in the PRP during the Burley and Poe efforts in 2009 and during 

the predator indexing efforts in 2009 and 2010. We generally captured fewer smallmouth bass than 

northern pikeminnow for all sampling efforts. We captured and measured 272 smallmouth bass during 

the Burley and Poe sampling in the PRP ranging from 35 to 518 mm in total length (fig. 12). In the 

spring, we captured 168 smallmouth bass ranging from 51 to 517 mm in total length and in the summer, 

we captured 104 smallmouth bass ranging from 35 to 518 mm in total length. The CPUE of smallmouth 

bass greater 150 mm in length in the spring and summer Burley and Poe sampling was highest in the 

forebay of Priest Rapids Dam and in the mid-reservoir section of Priest Rapids Reservoir (table 3). 

Fewer smallmouth bass were captured in the 2009 predator indexing sampling than during the 

Burley and Poe sampling. During predator index sampling, we captured 232 smallmouth bass in Priest 

Rapids and Wanapum Reservoirs that ranged from 21 to 479 mm in total length (fig. 13). In the spring, 

we captured 48 bass that ranged from 105 to 467 mm in total length. The capture number more than 

doubled for the summer period (n=112), with smallmouth bass that ranged from 112 to 450 mm in total 

length (fig. 13). The CPUE for predator indexing was highest in the forebay BRZs of Priest Rapids and 

Wanapum Dams in the spring and summer periods (table 3). 

The 2010 sampling resulted in the highest number of smallmouth bass being captured out of all 

the sampling efforts; this followed the same trend as the northern pikeminnow capture results. We 

captured and measured 687 smallmouth bass ranging from 46 to 515 mm in total length in Priest Rapids 

and Wanapum Reservoirs (fig. 14). In the spring sampling, we captured 149 bass ranging from 71 to 

469 mm in total length; in the summer sampling, we captured 294 bass ranging from 73 to 515 mm in 

total length (fig. 14). The CPUE of smallmouth bass greater than 150 mm in length in the spring and 

summer periods was highest in the forebay areas of Priest Rapids and Wanapum Dams (table 4).

We also observed spatial trends in the mean length of smallmouth bass captured across the strata 

sampled in 2009 and 2010 (figs. 15–17). During Burley and Poe spring 2009 sampling, the largest mean 

smallmouth bass lengths were from the tailrace and forebay of Wanapum Dam and the tailrace of Priest 

Rapids Dam (fig. 15). A similar trend was evident in the Burley and Poe summer sampling with the 

exception that only one bass was captured in the Priest Rapids tailrace. For the 2009 and 2010 predator 

index sampling, the largest mean smallmouth bass lengths were from the Priest Rapids mid-reservoir

and tailrace reaches, and the Wanapum forebay and mid-reservoir reaches (figs. 16 and 17).

Our aging analyses of smallmouth bass (fig. 18) captured in 2009, 2010, and 2011, in the Priest 

Rapids Project indicate that the ages ranged from 1 to 14 years (median age = 3 years). The mean length 

at age was estimated for smallmouth bass and is presented in figure 19. For all smallmouth bass 

captured in Priest Rapids Reservoir, the relation between length and weight is described by the equation: 

log10 (weight) = 3.1151(log10 length) – 5.1566; r2

 = 0.9864; 

for Wanapum Reservoir, the relation is described by: 

log10 (weight) = 3.1417(log10 length) – 5.2164; r2 = 0.9829. 
14

Walleye

Very few walleye were captured across all sampling efforts and study years. During the Burley 

and Poe sampling, we captured 13 walleye in Priest Rapids and Wanapum Reservoirs, ranging from 100

to 775 mm in total length (fig. 20). In the spring, we captured seven walleye that ranged from 425 to 

775 mm in total length; in the summer, we captured six walleye that ranged from 100 to 481 mm in total 

length. During the 2009 predator index sampling, we captured 18 walleye in Priest Rapids and 

Wanapum Reservoirs that ranged from 165 to 685 mm in total length (fig. 21). Of these only 3 walleye

were captured in the spring period, while 15 walleye were captured in the summer, ranging from 165 to 

685 mm in total length (fig. 21). We captured more than three times as many walleye in 2010 (n=59),

ranging from 184 to 786 mm in total length (fig. 22). In the spring 2010 predator index sampling, we 

captured 15 walleye ranging from 200 to 771 mm in total length, and in the summer, we captured 21 

walleye ranging from 194 to 693 mm in total length (fig. 22). The CPUE for walleye was low in both 

the spring and summer for all sampling periods (< 0.005) in 2009 and 2010, with the highest values 

from the Priest Rapids BRZ (CPUE = 0.01) in both the spring and summer. 

Our aging analyses of walleye (fig. 23) captured in 2009, 2010, and 2011 in the Priest Rapids 

Project indicate that walleye ages range from 1 to 16 years (median age = 3 years; n=34). The mean 

length at age relation for walleye is described in figure 24. We did not develop a relationship between 

length and weight or examine the spatial variability in mean lengths because so few walleye were 

captured.

Diet Analyses

Northern Pikeminnow

When we evaluated the diets of northern pikeminnow, we found the highest proportion of the 

diet was consistently the miscellaneous prey category. That is, the highest proportion by weight could 

not be identified into any of the other prey categories during the pre-maceration process. Of the diets 

collected as part of the Burley and Poe spring sampling, the miscellaneous prey category constituted on 

average 59 percent, with insects as the next dominant item at 31 percent, followed by fish (6 percent), 

mollusks, and crayfish (fig. 25). The diets from the Priest Rapids tailrace reach had the highest 

percentage of fish (18 percent) in the spring sampling, followed by the Priest Rapids forebay (10 

percent), and Rock Island tailrace (6 percent). For the summer sampling, the diet proportions were 

similar to the spring with 61 percent as miscellaneous, 30 percent insects, and 6 percent mollusks, 

followed by fish (1.5 percent) and crayfish (fig. 25). The proportion of fish (6.5 percent) was highest in 

the tailrace of Rock Island Dam in the summer (fig. 25). Of the diets with fish prey items captured 

during Burley and Poe sampling, northern pikeminnow containing salmon occurred in four strata in the 

spring (Priest Rapids tailrace, Wanapum tailrace, Wanapum forebay, and Wanapum mid-reservoir), and

no salmon were observed in the diets in the summer.
15

In the 2009 predator index sampling, we observed similar trends with fish constituting a 

relatively minor component of the diets of northern pikeminnow captured, but being more prevalent 

near the dams (fig. 26). Sampling fish comprised on average 2 percent of the diet in the spring and 3 

percent in the summer. Percentages of fish prey items were highest in the Wanapum forebay BRZ (11 

percent) in the spring, and in the tailraces of Priest Rapids (13 percent) and Wanapum (13 percent) dams

(fig. 26) in the summer. Of the northern pikeminnow with fish in their diets, salmon were present in low 

proportions in the Priest Rapids tailrace only in spring (0.44) and summer (0.25). 

Although fish were a relatively minor component of the northern pikeminnow diet in the 2010 

predator index sampling, they were again most prevalent in the diets of northern pikeminnow captured 

in the reaches nearest to Priest Rapids, Wanapum, and Rock Island Dams (fig. 27). In the spring 

sampling, the average proportion of the diet consisting of fish (16 percent) was higher than in all of the 

2009 sampling efforts. However, the most dominant prey items were still in the miscellaneous category 

(61 percent), followed by insects (19 percent), and then fish, mollusks, and crayfish. In the summer, the 

average proportion of diet consisting of fish was much lower (1.1 percent), with most reaches sampled 

having no northern pikeminnow captured with fish in their diets (fig. 27). The proportion of the fish 

prey that was salmon in northern pikeminnow was variable among strata, and was highest in the tailrace 

of Wanapum Dam (fig. 28). The occurrence of salmon within the fish prey items generally was higher 

in the tailrace areas than in the forebay and mid-reservoir areas, and was higher in the spring than in the 

summer (fig. 28).

Smallmouth Bass

The diets of smallmouth bass generally had a much higher proportion of fish prey items than 

northern pikeminnow diets. For smallmouth bass collected during the spring Burley and Poe sampling, 

fish constituted the highest percentage (84 percent) on average of the diet. The same result was seen for 

the summer sampling, with fish constituting an average of 67 percent of the smallmouth bass diet. This 

trend was consistent across most of the reaches sampled in the spring and summer periods, with the 

exception of the Priest Rapids forebay (fig. 29). Of the smallmouth bass diets with fish, salmon were 

documented only in the Priest Rapids mid-reservoir reach in the spring. 

During the 2009 predator index sampling, fish generally were generally the most prevalent diet 

item in smallmouth bass. On average, fish were 55 percent of the spring smallmouth bass diet, and 76 

percent of their summer diet. Fish were the most prevalent smallmouth bass diet item in all reaches

where diets were collected, with the exception of the Priest Rapids tailrace BRZ (fig. 30). Juvenile 

salmonids were found in the diets of bass collected in the forebay of Priest Rapids Dam and in the Priest 

Rapids mid-reservoir reach in the spring. In the summer, salmon were found in the diets of bass 

collected in four reaches: Priest Rapids tailrace, Priest Rapids forebay, Priest Rapids mid-reservoir, and 

Wanapum mid-reservoir. In all cases, the proportion of fish in the diets that were salmon never 

exceeded 0.25. 

As in the 2009 sampling, fish generally were the dominant prey item for smallmouth bass 

captured in 2010 (fig. 31). On average, fish were 83 percent of the diet in the spring and 57 percent of 

the diet in the summer. This trend was consistent across all reaches with the exception of crayfish that 

were the dominant prey item in the summer in the forebay BRZs for both Priest Rapids and Wanapum 

Dams. fig. 31). Salmonids were observed in the diets of smallmouth bass captured in eight reaches

concentrated in the forebays and mid-reservoir reaches of Priest Rapids and Wanapum Dams in 2010. 
16

Walleye

The diets of walleye, collected during the Burley and Poe sampling and the 2009 and 2010 

predator index sampling, consisted primarily of fish. The proportion of fish in the diets was mostly near 

1, with the exception of a fish collected in the Rock Island tailrace reach in 2009 that had no fish in its 

stomach. Otherwise, the proportion of walleye diets that were fish was never less than 0.89. The 

proportion of fish in the diets of walleye that were salmon was concentrated in the tailrace of Priest 

Rapids Dam, and ranged from 0.5 to -1. In 2010, the distribution of walleye collected that had salmon 

was higher (fig. 32). The proportion of salmon in the diets of walleye captured in 2010 was highest in 

the tailrace of Priest Rapids Dam in the spring and in the tailrace of Wanapum Dam in the summer (fig. 

32). 

Predation Indices

Northern Pikeminnow

Northern pikeminnow predation indices estimated for the Burley and Poe sampling were very 

low, and were much lower than those estimated in 1993 (Burley and Poe, 1994). Predation index values 

for 2009 ranged from 0 to 31 in the spring, and no predation was evident in samples from the summer 

(table 5). The predation index estimates we calculated based on the data from Burley and Poe (1994) 

ranged from 0 to 71 in the spring and 0 to 120 in the summer. For the 2009 predator index sampling, the 

estimated predation indices that used CPUE as the density index also indicated very low predation in the 

study area in the spring and summer with only the Priest Rapids tailrace being greater than zero (table 

6). The northern pikeminnow predation indices for 2010 were higher and more widely distributed 

throughout the study area than in 2009, ranging from 0 to 1.918 (table 7). Northern pikeminnow 

predation in 2010 was highest in the Wanapum mid-reservoir (1.918, SE=1.211) and Wanapum tailrace 

(1.018, SE=1.018) reaches in the spring, and were less than 0.196 in the summer with evidence of 

predation occurring only in the Rock Island tailrace and the Priest Rapids mid-reservoir reaches. 

Smallmouth Bass

The predation indices for smallmouth bass during the 2009 predator index sampling indicated 

that predation of salmonids in the study area was low in all areas in the spring and summer (table 6). In

the spring, predation indices were less than 0.240 for all reaches, and predation of salmonids was

documented in the Priest Rapids mid-reservoir and the Priest Rapids forebay reaches (PF1 and PF0) 

only. For the summer, predation of salmonids was documented in the Priest Rapids tailrace near-BRZ, 

the Priest Rapids mid-reservoir, the Priest Rapids forebay BRZ, and the Wanapum mid-reservoir, with 

the highest index value from the Priest Rapids tailrace (1.073, SE=1.073). In 2010, our results suggest 

that predation of juvenile salmonids by smallmouth bass was more widespread than in 2009 (table 7). In

the spring of 2010, predation was highest in the Priest Rapids mid-reservoir reach (5.940, SE 2.731), 

followed by the Wanapum forebay BRZ (0.90, SE=0.90) and the Priest Rapids forebay near-BRZ reach

(0.114, SE=0.114). In the summer, predation was again highest in the Priest Rapids mid-reservoir 

(1.760, SE=1.152), the Priest Rapids forebay reach (1.055, SE=0.776), and the Wanapum forebay BRZ 

and near-BRZ reaches. In the spring and summer, predation was higher in the forebay and mid-reservoir 

reaches than in the tailraces.
17

Bioenergetics

We observed seasonal differences in total and fish consumption by northern pikeminnow and 

smallmouth bass in 2009 and 2010. The output from the bioenergetics model results indicated that the 

northern pikeminnow modeled population (n=928) consumed 6,447 g of fish in the spring 2009 (fig. 

33), which was approximately 15 percent of their diet by weight (fig. 34). In the summer, the weight of 

fish consumed (5,002 g) was 3 percent less than in the spring sampling period. In 2010, the modeled 

population (n=1,118) consumed 11,865 g of fish in the spring (9 percent of their diet) (fig. 34) and 

20,995 g in the summer (fig. 33). Even though the proportion of the modeled population diet that was 

composed of fish was only 3 percent, the total grams of fish consumed by northern pikeminnow in the 

summer, was much higher (fig. 34). 

A higher proportion of the diets of smallmouth bass were composed of fish than northern 

pikeminnow. We estimated smallmouth bass (n=165) consumed 1,124 g of fish in spring 2009 and 

4,192 g in summer 2009 (fig. 35). This comprised approximately 60 and 40 percent of their total diet 

(fig. 36). We further estimated that 168 g (9 percent of diet) of salmonids were consumed in the spring 

and 801 g (8 percent of diet) in the summer. In 2010, the modeled population of smallmouth bass 

(n=372) consumed 1,582 grams (55 percent of diet) of fish in the spring and 12,448 g (48 percent of 

diet) in the summer. The salmonid consumption was estimated to be 354 g (12 percent of diet) in the 

spring and 2,667 g (11 percent of diet) in the summer.

Discussion

The predation indices estimated from the Burley and Poe sampling in 2009 were much lower 

than those we calculated from the 1993 data of Burley and Poe (1994). This may be a result of efforts to 

reduce the abundance of northern pikeminnow in the Priest Rapids project by physically capturing and 

removing them (Garner and Keeler, 2008). The reduced predation may be, in part, a result of changes in 

the northern pikeminnow population characteristics brought about by the northern pikeminnow removal 

program. The mean total lengths we observed in the study reaches were much lower than those reported 

in Burley and Poe (1994). For instance, Burley and Poe (1994) reported a mean fork length of 436 mm 

for northern pikeminnow captured in the Wanapum Dam tailrace; the mean total length of northern 

pikeminnow we captured in this reach was less than 150 mm in both the spring and summer periods. 

Grant PUD also has noted a decrease in the average size of northern pikeminnow captured in 2011 

compared to previous years but note that the reduction may be due to gear bias (Curt Dotson, Grant 

County Public Utility District, written communication 2011). However, our results summarizing data 

from northern pikeminnow captured using a different gear, electrofishing, corroborate Grant PUD’s 

observations. 

Reductions in the size of northern pikeminnow may be resulting in a decrease in predation 

because consumption of juvenile salmonids increases with the size of northern pikeminnow (Vigg and 

others, 1991). Rieman and Beamesderfer (1991) suggest that continuous exploitation of northern 

pikeminnow greater than 250 mm in fork length would result in a 50 percent or greater reduction in 

predation. When evaluating the effects of the pikeminnow removal program, Zimmerman and Ward 

(1999) documented post-removal program predation index values that were 44–91 percent lower than 

mean values prior to the implementation of the removal program throughout the lower Columbia River 

basin. Zimmerman and Ward (1999) note that the observed declines in relative predation were 

consistent with changes in the size and age structure of northern pikeminnow populations associated 

with the Northern Pikeminnow Management Program; that is, there was a shift towards smaller, 

younger individuals (Knutsen and Ward, 1999). Although the overall mean size of northern pikeminnow 
18

captured as part of our electrofishing efforts has decreased compared to 1993, we observed similar 

trends in mean length in the longitudinal reaches. Specifically, we observed a trend of larger fish in 

reaches nearest the dams, as did Burley and Poe (1994), suggesting that larger fish within the population 

still are found near dams. Our analysis of the diets of northern pikeminnow also suggests that the fish

captured near Wanapum and Priest Rapids Dams were more likely to have fish as a component of their 

diets, and that salmon were found in their diets. 

Very low northern pikeminnow predation indices were observed in 2009 for both the Burley and 

Poe and the predator index sampling, despite differences in the diel timing of these efforts. The 

electrofishing efforts conducted during the predator indexing began no earlier than 30 min after sunset, 

while the Burley and Poe electrofishing began 90 min before sunrise and continued sampling until a 

target catch of 15 northern pikeminnow were captured from each section sampled. That a similar result 

was attained for the different approaches suggests that the low levels of predation observed during the 

2009 Burley and Poe efforts were not a function of the timing of the sampling. Although conducting 

electrofishing at night during the 2009 and 2010 predator index sampling versus early-morning resulted 

in a higher CPUE in most reaches and seasons for both northern pikeminnow and smallmouth bass, the 

increased collections did not result in higher predation index values. Furthermore, the timing of the 

summer 2009 Burley and Poe efforts were conducted past the peak migration period for sub-yearling 

Chinook salmon. Another factor that may have affected our results was that we were tagging and 

releasing northern pikeminnow with the intent of recapturing them; which occurred infrequently enough 

that we discontinued the efforts in 2010. Because there were so few northern pikeminnow captured in

2009 that were in the larger size categories, releasing the few we did capture likely resulted in us 

releasing predators that were the most likely to contain salmonids in their diets. The release of predators 

as part of our tagging effort could have contributed to the lack of documented predation in the summer 

2009. However, there was little evidence of predation from our sampling efforts in the summer 2010 in 

many of the reaches. 

We observed a shift in the diet composition of northern pikeminnow collected in the PRP from 

1993 to the present (2011). Burley and Poe (1994) reported that the average proportion of northern 

pikeminnow diet that was fish was 0.66 in the spring and 0.35 in the summer; the largest proportion we 

observed was less than 0.2. The shift towards insects and food items other than fish may be a reflection 

of the reduction in the average size of northern pikeminnow captured; the shift is not likely the result of 

a reduced prey base because of the constant supply of hatchery juvenile salmonids migrating through 

the PRP. The ecological implications of the shift to a greater portion of northern pikeminnow being 

smaller are unclear and beyond the scope of this report. However, if our observations are indicative of 

the diets of most of the northern pikeminnow population in the study area, than it seems reasonable to 

assume that there would be consequences of a shift away from piscivory. 

The results of our study suggest that there are areas within the PRP that can be targeted to 

mitigate the predation of smallmouth bass on juvenile salmonids. However, our results do not suggest 

that juvenile salmonids were a major constituent in the diet of smallmouth bass. Despite the higher 

proportion of fish observed in the diets of smallmouth bass, the predation indices during the 2009 

predator index sampling supplied little evidence to suggest that juvenile salmonid predation by bass was 

very prevalent in the study area. The fact that juvenile salmon do not constitute a significant portion of 

the diet of smallmouth bass also has been observed in other studies. For instance, Naughton and others 

(2004) observed that juvenile salmonids constituted a maximum of 11 percent of the diets of 

smallmouth bass captured in the forebay of the Lower Granite Dam, Snake River, and only 5 percent in 

other areas of the Snake and Clearwater Rivers. Similar to our results for northern pikeminnow, 

predation indices were higher and more widespread throughout the study area for smallmouth bass in 
19

spring 2010 than in 2009. However, dissimilar to what we observed for northern pikeminnow, predation 

was high and more widespread in summer 2010 suggesting that smallmouth bass predation of subyearling Chinook salmon may be higher than predation of northern pikeminnow. In 2010, predation by 

smallmouth bass was highest in the Priest Rapids mid-reservoir reach and concentrated in the forebay 

areas of Priest Rapids and Wanapum Dams. Similarly, Naughton and others (2004) observed that the 

highest monthly consumption rates of juvenile salmonids by smallmouth bass were in the forebay areas 

in April 1996 and in the forebay BRZ in July 1997 at Lower Granite Dam, Snake River. Our findings 

also agree with those of Vigg and others (1991), who found that in the John Day Reservoir, Columbia 

River, consumption of juvenile salmonids by smallmouth bass was highest in the forebay. Ward and 

Zimmerman (1999), who found smallmouth bass consumption of juvenile salmonids usually was 

highest in the summer in the forebay of John Day Reservoir, and also was evident downstream of 

Bonneville Dam (rkm 190–197). Ward and Zimmerman (1999) also observed that consumption of 

juvenile salmonids by smallmouth bass was highly variable across reservoir reaches (for example 

forebay, mid-reservoir, and tailrace areas) and seasons and generally was low. 

The results of our diet analyses for smallmouth bass suggest that fish were a more significant 

constituent in the diets of smallmouth bass compared to northern pikeminnow. Our diet analysis results 

are consistent with those previously reported for the study area. Burley and Poe (1994) reported that the 

diets of smallmouth bass collected in their survey of the mid-Columbia River consisted of 87 percent 

fish, 12 percent crustaceans, and 1 percent other items. The diet composition of smallmouth bass 

collected in the study area, however, seems to differ from the observed diets of smallmouth bass 

collected in other reaches of the Columbia and Snake Rivers, where smallmouth bass are relatively more 

abundant. Specifically, smallmouth bass in other reaches of the Columbia and Snake Rivers have been 

shown to contain a higher percentage of crustaceans. For instance, Naughton and others (2004) reported 

that crustaceans comprised the highest percentage of the diet (by weight) of smallmouth bass 175–249 

mm in total length at all locations they sampled in the Snake River in 1996 and 1997, except for the 

Clearwater River arm, where non-salmonid fishes were the primary prey item. For smallmouth bass 

250–389 mm in total length, Naughton and others (2004) observed that crustaceans were the primary 

diet item in 1997. Burley and Poe (1994) reported that crustaceans constituted 42 percent of the dietary 

totals for smallmouth bass collected in the John Day Reservoir. Zimmerman (1999) also found that in

spring and summer, the proportional weight of crayfish was highest in the impounded reaches of the 

Columbia (50 percent) and Snake (52 percent) Rivers. From the results reported from other areas of the 

Columbia and Snake Rivers, crustaceans appear to be an important diet item of smallmouth bass. If the 

diets of smallmouth bass we collected are reflective of the availability of crustaceans as prey items in 

the study area, perhaps the lack of crustaceans available as prey may be limiting smallmouth bass 

numbers in the study area. Low densities of crayfish, for instance, could be due to a lack of suitable 

habitat in the study area or predation by another fish species, such as the northern pikeminnow. 

Low numbers of walleye have been reported by other researchers sampling fishes in the Priest 

Rapids Project. Despite sampling the study area for approximately 4 months during each of 2009 and 

2010 with electrofishing gear, we captured very few walleye. Burley and Poe (1994) captured only 16 

walleye in the study area during their sampling efforts in 1993. Electrofishing may be inefficient at 

capturing walleye. Schoenebeck and Hansen (2005) suggest that the relationship between electrofishing 

catch rates and population size may depend on habitat and may vary seasonally. However, Rogers and 

others (2003) found that the electrofishing catch rate of adult walleye was positively related to adult 

walleye density, and that the electrofishing catch rate of the total walleye population was positively 

related to total walleye density. In a study that examined the fish population structure in the Priest 

Rapids Project, only 35 walleye were captured despite their efforts to capture fish with various gear in 
20

addition to electrofishing gear (Pfeifer and others, 2001). Our catches suggest little walleye recruitment 

is occurring in the study area. Pfeifer and others (2001) collected some smaller individuals in the 

backwaters of the Wanapum reservoir and hypothesized that walleye may have spawned successfully in 

the reservoir or recruited from upstream sources, principally Lake Roosevelt; our catch data lead us to 

concur with this assessment.

Our bioenergetics modeling provided additional insight into the interactions of predators and 

juvenile salmonids in the study area. The bioenergetics modeling output indicates that fish (in general) 

and salmon (in particular) consumed by weight were greater in the summer than in the spring sampling 

periods, with the exception of the 2009 northern pikeminnow data; despite the results that the proportion 

of fish in the diets of northern pikeminnow and smallmouth bass were slightly less in the summer. We 

used daily diets and temperatures based on field-data collections, but held the proportion of maximum 

consumption constant across the study period as inputs into the bioenergetics model. Given that water 

temperatures increased in the summer migration period, we expect a concomitant increase in the 

energetic requirements of northern pikeminnow and smallmouth bass. Increased energetic demands can 

either be manifested as a loss in weight or an increase in either total consumption or increase in 

consumption of higher energy density prey items, such as fish to compensate for the higher energy 

demands. Our results show that northern pikeminnow and smallmouth bass total consumption and fish 

consumption by weight were higher in the summer periods, suggesting that predation effects from 

northern pikeminnow and smallmouth bass may be higher for juvenile salmon (namely, sub-yearling 

Chinook salmon) migrating in the summer sampling period. Our efforts to characterize the diets of 

northern pikeminnow and smallmouth bass in the study area and couple that information to a 

bioenergetics framework also could provide a way to assess the effects of existing removal programs, 

such as the Grant PUD northern pikeminnow removal program or the potential effects of new removal 

programs, such as those for smallmouth bass. For instance, one could model the predicted reductions in 

salmon eaten that would occur if a certain number of smallmouth bass were removed. Converting the 

reductions in the salmon eaten by weight to numbers of salmon would require assumptions regarding 

the relative proportions of various salmon species in the diet and the size distributions of the species. 

However, such a modeling exercise would provide context to the relative benefits expected in light of 

the cost of implementing such a program. 

In 2009, our fish collection efforts resulted in the capture of few northern pikeminnow that 

were large fish (> 250 mm) with salmon in their stomachs. Thus, a result of using this data for 

bioenergetics modeling is that very few salmonids were consumed relative to other prey items in the 

modeling scenarios. However, this result is consistent with diet results from the northern pikeminnow 

removal program studies, where the proportion of northern pikeminnow diets that were smolts ranged 

from 0.8 to 1.8 percent for study years 2008 and 2009 (Garner and Keeler, 2008, 2009). In 2010, fish 

and salmon consumption was higher than in 2009, and likely was a result of the collection of more large 

predators with salmon in their stomachs in 2010. The estimated weight of fish consumed by northern 

pikeminnow may be underestimated as a result of the differences in their morphology and physiology 

and the processing of their stomach contents. Much of the material in a northern pikeminnow’s stomach 

often consisted of miscellaneous material, some of which may have been salmonid prey items, but was 

not discernible during the pre-maceration as fish. Thus, even though salmonids and other fish can be 

detected in the diets post-maceration, a weight was not assigned to this prey item to be incorporated into 

the diet by weight analysis. Thus, the fish prey weights are underrepresented in the diet proportions for 

northern pikeminnow. We chose not to estimate a weight associated with bones found post-maceration, 

as we did not have a method to consistently assign a weight to these fish such that it would be 

represented properly with the other items found in the stomach at that time. Smallmouth bass have a true 
21

stomach and partitioned digestive tract, making the collection and identification of prey items

considerably easier. Furthermore, the focus of this study was on predation indices, which use the counts 

of salmon found in stomachs to estimate predation and not a proportion of weights of salmon in the diet. 

In theory, diagnostic bones can be measured and used to estimate the size of a fish prey item at the time 

of consumption; however, this would then overestimate the weight of fish in the diet relative to the other 

items in which pre-consumption weights could not be estimated. 

Our inability to access the BRZ areas of Priest Rapids and Wanapum Dams during the 2009 

Burley and Poe sampling confounds comparisons to the results of the study conducted in 1993. Burley 

and Poe (1994) observed relatively high consumption index values in the BRZs. Ward and others (1995) 

and others have observed that predation is disproportionately large near dams, with 33 percent of the 

overall predation occurring in the BRZs. To attain access to the BRZs, it was necessary to coordinate 

with the dam operators so that discharge through the spillway could be discontinued to allow our 

electrofishing crews safe access to these areas. Despite our efforts at coordination, we encountered 

issues that precluded us from completing scheduled sampling events. In addition to coordination 

(miscommunication) and logistical issues (river flows that precluded the cessation of spill), during the 

2009 and 2010 sampling seasons, we also encountered environmental conditions (such as high winds 

and river flows) that kept us from sampling the BRZ areas. Although we were unable to access the 

BRZs as part of the 2009 Burley and Poe sampling, we did have limited success as part of our other 

sampling efforts in 2009 and 2010. 

We recommend that future predation studies in the Priest Rapids Project include a design 

scheme to allocate sample efforts to areas immediately adjacent to the BRZ areas, such as was done in 

2010. The addition of a reach as close to the dam as possible but not in the BRZ allowed us to allocate 

efforts to areas thought to have higher consumption rates. We recommend retaining these reaches in 

future studies of predation in the PRP. The logistical constraint and safety issues we encountered trying 

to sample the BRZ areas biased estimates of these areas and, therefore, our assessment of predation in 

the study area. Studies examining predation in other reaches of the Columbia River have shown these 

areas can have high densities of predators (Ward and others, 1995). Evaluations of diets of northern 

pikeminnow collected off the transformer deck of the Wanapum Dam suggest these fish were more 

likely to contain salmonids than fish captured in other areas (Hardiman and others, 2012). That the 

predation indices for smallmouth bass were as high as they were for the forebay areas, especially for 

Wanapum Dam in the summer 2010 sampling, suggest relatively high levels of predation given the 

small area contained within that reach. Therefore, although the data we present suggest that predation is 

lower now (2010) than in 1993, our results suggest areas where control efforts for smallmouth bass 

could be focused if managers chose this as an action to mitigate the predation losses caused by 

smallmouth bass. Although our catch data for walleye suggest that this species is not abundant in the 

study area, our CPUE and diet analyses suggest areas where efforts to reduce the numbers of this 

species could be focused; the tailraces of Priest Rapids and Wanapum Dams are the areas where salmon 

were documented as being consumed by walleye.

Future predation indexing should either be conducted throughout the migration season, or the 

migration run timing variability because of environmental conditions or hatchery practices, and whether 

monitoring for predation of multiple species or of one particular species is desired need to be 

incorporated into the sampling design. Our efforts to repeat the timing of the 1993 study were 

confounded by the lack of reported criteria used to time the 1993 fieldwork. We surmised from the 

Burley and Poe (1994) report that the logistics of conducting such an effort over a large geographical 

area, much larger than the PRP, dictated to some extent when sampling was conducted in a particular 

river reach. In the absence of specific criteria, we chose to use water temperature as a criterion to begin 
22

sampling. Water temperature was selected as a criterion because of the bioenergetic implications of 

predator activity associated with changing temperatures (Cech and others, 1994) and our desire to 

sample under conditions similar to those of the original work. This decision resulted in our Burley and 

Poe summer sampling period occurring towards the end of the summer migration. Conversely, the 2009 

and 2010 predator index sampling was structured so that sampling efforts mostly encompassed both the 

spring and summer migration periods. Sampling continuously throughout the juvenile salmonid 

migration in 2009 and 2010 allowed us to use juvenile salmonid passage information to place the 

predation sampling in the context of the migration of multiple juvenile salmonid species. How to 

strategize the timing of efforts to characterize predation in a particular area potentially is problematic for 

future efforts to examine predation in the PRP, especially for the purposes of determining trends over 

time.

Progress towards the development of a comprehensive long-term monitoring strategy was 

confounded by the redirect of the original objectives and tasks for the third and final year of this study 

to address predation of steelhead in 2011 (Hardiman and others, 2012). However, the PRCC has 

expressed continued interest in establishing a predation monitoring program in the Priest Rapids Project. 

Toward this end, we provide recommendations on how to proceed with the development of such a 

program. Monitoring programs that address a diverse set of objectives and information should occur 

nationwide and provide information on: regulation compliance, the status of aquatic resource conditions, 

effectiveness of management and regulatory programs, and policy planning and decision-making 

processes. Considerable expenditures have been made on such programs, often with mixed results and 

information provided. The U.S. Environmental Protection Agency (EPA) initiated the Environmental 

Monitoring and Assessment Program (EMAP) to advance the science of natural resource monitoring at 

regional and national scales. A significant task in the development of EMAP has been the statistical 

design and analysis methodologies to support meeting the goal of "with known confidence" in the 

design of monitoring studies. This effort has drawn heavily on existing survey design literature and 

applications in other areas. 

The EPA reviewed past and current aquatic monitoring programs and identified some common 

characteristics of the design and analyses for such programs that fail—that is, do not meet the 

expectations for producing information regarding the status and trends of aquatic resources—and 

generally categorized them into four broad classes. 

The objectives for monitoring are not clearly, precisely stated and understood. 

Monitoring measurement protocols, survey design, and statistical analysis become scientifically outof-date.

Monitoring results are not directly tied to management decision-making. 

Results are not timely nor communicated to key audiences in terms they can understand. 
23

Organizations such as EPA that conduct national and regional monitoring, and regional groups 

such as the Pacific Northwest Aquatic Monitoring Partnership (http://www.pnamp.org/) have identified 

key aquatic resource survey design components necessary for the formulation of a long-term monitoring 

effort. For instance, the EPA suggests that the following are necessary components for a monitoring 

program: 

• Objectives stated precisely and quantitatively. 

• Target population explicitly, precisely defined. 

• Sample frame constructed that represents the target population. 

• Decision on which survey design will best provide information to meet objectives. 

• Selection of sampling sites using survey design. 

• Implementation of consistent measurement protocols at sampled sites. 

• Statistical analysis that matches survey design. 

We suggest that the completion and inclusion of these components are necessary to enact a longterm monitoring activity for predation in the Priest Rapids Project. We strongly recommend that the 

PRCC pursue the completion of these key elements as they make progress towards the development of 

long-term monitoring programs. 

We further recommend that the PRCC convene an expert panel that can work through the 

completion of these components. Participation in regional monitoring groups, such as the Pacific 

Northwest Aquatic Monitoring Partnership, can help to facilitate the process. Specifically, with respect

to predation monitoring, decisions need to be made as to what metrics are necessary to assess status and 

trends in predation. We present a variety of metrics that are dependent on collecting predators and 

examining their diets. The metrics we present have been used before, but are labor-intensive and, 

therefore, costly (Petersen and Ward, 1999). Furthermore, the results of these types of studies are not 

always easily interpreted by key audiences; a characteristic listed above as being problematic for the 

success of a monitoring program. Metrics other than those used in this study may be as good or better at 

communicating the status and trends of the underlying driver behind assessing predation: the mortality 

of juvenile salmonids from fish predators in the Priest Rapids Project. For instance, survival goals have 

been established for the study area; perhaps survival metrics could serve as a metric to assess mitigation 

efforts to reduce predators in the study area. Increases in survival should be an indication of reduced 

mortality from fish predators. Alternately, various other methods have been used to justify and evaluate 

efforts to mitigate the effects of predators, including monitoring the movements of tagged predators and 

prey, measuring growth and fecundity of predators, and modeling how juvenile salmonid mortality

varies with predator density, river flow, and other variables (Petersen and Ward, 1999). The formulation 

of an expert panel can help the PRCC work through the development and justification of metrics used to 

assess predation of juvenile salmon in the study area. However, valuable information can be derived 

from assessing the diets of fish predators that can help to directly assess predation of juvenile salmonids 

and the effects of fish predators on other components of the ecosystem in addition to juvenile salmon. 

With respect to the development of a sample frame, the USGS has formulated a sample frame 

for the PRP using a Generalized Random Tessellation Stratified algorithm (Larsen and others, 2007). 

This sample frame encompasses both the river channel and upland areas. The sample frame was 

developed as part of efforts to facilitate the development of a long-term monitoring program for aquatic 

invasive species and as part of efforts to initiate an integrated status and trends monitoring program for 

aquatic resources in the Columbia River basin (U.S. Geological Survey; unpub. Data, 2011). The 

sample frame is available upon request and is slated to be made available through a tool being 

developed by the Pacific Northwest Aquatic Monitoring Partnership

(http://www.pnamp.org/project/3263).
24

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 want 

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

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

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

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29

Figure 1. Study area sampled in the Columbia River, Washington, 2009. Reach locations: PT1, Priest Rapids 

Tailrace; PT0, Priest Rapids Tailrace BRZ; PF0, Priest Rapids Forebay BRZ; PF1, Priest Rapids Forebay; PM1, 

Priest Rapids Mid-Reservoir; WT1, Wanapum Tailrace; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay 

BRZ; WF1, Wanapum Forebay; WM1, Wanapum Mid-Reservoir; RT1, Rock Island Tailrace. RM, river mile.

RT1

WF1

WT1

WM1

PF1

PM1

PT1

RM448

RM443

RM437

RM421

Rock Island Dam

RM453

Wanapum Dam

RM416

RM416

Priest Rapids Dam

RM397

RM411

RM407

RM402

RM389

RM396

WA

PT0 PF0

WT0

WF0
30

Figure 2. Study area sampled in the Columbia River, Washington, 2010. Reach locations: PT1, Priest Rapids 

Tailrace; PT0, Priest Rapids Tailrace BRZ; PF0, Priest Rapids Forebay BRZ; PF1, Priest Rapids Forebay; PM1, 

Priest Rapids Mid-Reservoir; WT1, Wanapum Tailrace; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay 

BRZ; WF1, Wanapum Forebay; WM1, Wanapum Mid-Reservoir; RT1, Rock Island Tailrace. RM, river mile.

WA

Wanapum Dam 

RM 416

Priest Rapids Dam RM 397

PT2

WF1

WF0

PF2

PM3

WT2

WT0 WT1

PF0 PT1

RM411

RM402

RM416

RM414

PF1 PT0

RT1

WF2

WM1

RM448

RM437

RM421

Rock Island Dam RM453

RM426
31

Figure 3. River discharge (1,000 cubic feet second) and water temperature as measured in the tailrace of Priest 

Rapids, Wanapum, and Rock Island Dams, Columbia River, Washington, from May to October. Discharge data 

from tailrace outflow. Temperature data from the Water Quality Meter station, downloaded from the University of 

Washington Columbia River Data access in real time Web site. 

Date

May Jun Jul Aug Sep Oct 

Temperature °C

6

8

10

12

14

16

18

20

Temperature °C

6

8

10

12

14

16

18

20

Temperature °C

6

8

10

12

14

16

18

20

2009

2010

10 year average

Rock Island

Discharge kcfs

50

100

150

200

250

300

Wanapum

Discharge kcfs

50

100

150

200

250

300

Priest Rapids

Discharge kcfs

50

100

150

200

250

300
32

Figure 4. Length frequency histograms for northern pikeminnow during Burley and Poe sampling overall in 2009 

(May 27–June 12 and August 3–20), in spring 2009 (May 27–June 12), and in summer 2009 (August 3–20), Priest 

Rapids Project, Columbia River, Washington. n, total number of fish.

0

50

100

150

0

306090

120150180210

240270300330360390420450480510540570600

Number

Total Length (millimeters)

Overall

n = 1,225

0

50

100

150

0

306090

120150180210240270300330360390420450480510540570600

Number

Total Length (millimeters)

Spring

n = 601

0

50

100

150

0

306090

120150180210240270300330360390420450480510540570600

Number

Total Length (millimeters)

Summer

n = 624
33

Figure 5. Length frequency histograms for northern pikeminnow during predator index sampling overall in 2009 

(May 1–August 27), in spring 2009 (May 7–June 11), and in summer 2009 (June 23–August 5), Priest Rapids 

Project, Columbia River, Washington. n, total number of fish.

0

20

40

60

80

100

120

0

306090

120150180210

240270300330360390420450480510540570600

Number

Total Length (millimeters)

Overall

n = 1,025

0

20

40

60

80

100

120

0

306090

120150180210240270300330360390420450480510540570600

Number

Total Length (millimeters)

Spring

n = 392

0

20

40

60

80

100

120

0

306090

120150180210240270300330360390420450480510540570600

Number

Total Length (millimeters)

Summer

n = 361
34

Figure 6. Length frequency histograms for northern pikeminnow during predator index sampling overall in 2010 

(May 19–September 3), in spring 2010 (May 19–June 8), and in summer 2010 (June 28–August 11), Priest Rapids 

Project, Columbia River, Washington. n, total number of fish.

0

50

100

150

200

0

306090

120150180210

240270300330360390420450480510540570600

Number

Total Length (millimeters)

Overall

n = 2,581

0

50

100

150

200

0

306090

120150180210240270300330360390420450480510540570600

Number

Total Length (millimeters)

Spring

n = 544

0

50

100

150

200

0

306090

120150180210240270300330360390420450480510540570600

Number

Total Length (millimeters)

Summer

n = 990
35

Figure 7. Mean length and one standard error for northern pikeminnow during Burley and Poe sampling in spring 

2009 (May 27–June 12) and summer 2009 (August 3–20), by reaches, Priest Rapids Project, Columbia River, 

Washington. Reach locations: PT1, Priest Rapids Tailrace; PF1, Priest Rapids Forebay; PM1, Priest Rapids MidReservoir; WT1, Wanapum Tailrace; WF1, Wanapum Forebay; WM1, Wanapum Mid-Reservoir; RT1, Rock Island 

Tailrace.

0

50

100

150

200

250

300

350

PT1 PF1 PM1 WT1 WF1 WM1 RT1

Mean Total Length (millimeters)

Reach

Spring

0

50

100

150

200

250

300

350

PT1 PF1 PM1 WT1 WF1 WM1 RT1

Mean Total Length (millimeters)

Reach

Summer
36

Figure 8. Mean length and one standard error for northern pikeminnow during predator index sampling overall in 

2009 (May 1–August 27), in spring 2009 (May 7–June 11), and in summer 2009 (June 23–August 5), by reaches, 

Priest Rapids Project, Columbia River, Washington. Reach locations: PT1, Priest Rapids Tailrace; PT0, Priest 

Rapids Tailrace BRZ; PF0, Priest Rapids Forebay BRZ; PF1, Priest Rapids Forebay; PM1, Priest Rapids MidReservoir; WT1, Wanapum Tailrace; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay BRZ; WF1, 

Wanapum Forebay; WM1, Wanapum Mid-Reservoir; RT1, Rock Island Tailrace.

0

50

100

150

200

250

300

350

PT1 PT0 PF0 PF1 PM1 WT1 WT0 WF0 WF1 WM1 RT1

Mean Total Length 

(millimeters)

Reach

Overall

0

50

100

150

200

250

300

350

PT1 PT0 PF0 PF1 PM1 WT1 WT0 WF0 WF1 WM1 RT1

Mean Total Length 

(millimeters)

Reach

Spring

0

50

100

150

200

250

300

350

PT1 PT0 PF0 PF1 PM1 WT1 WT0 WF0 WF1 WM1 RT1

Mean Total Length 

(millimeters)

Reach

Summer
37

Figure 9. Mean length and one standard error for northern pikeminnow during predator index sampling overall in 

2010 (May 19–September 3), in spring 2010 (May 19–June 8), and in summer 2010 (June 28–August 11), by 

reaches, Priest Rapids Project, Columbia River, Washington. Reach locations: PT2, Priest Rapids Tailrace; PT1, 

Priest Rapids Tailrace near-BRZ; PT0, Priest Rapids Tailrace BRZ; PF0, Priest Rapids Forebay BRZ; PF1, Priest 

Rapids Forebay near-BRZ; PF2, Priest Rapids Forebay; PM3, Priest Rapids Mid-Reservoir; WT2, Wanapum 

Tailrace; WT1, Wanapum Tailrace near-BRZ; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay BRZ; WF1, 

Wanapum Forebay near-BRZ; WF2, Wanapum Forebay; WM3, Wanapum Mid-Reservoir; RT2, Rock Island 

Tailrace; RT1, Rock Island Tailrace near-BRZ.

0

100

200

300

400

PT2PT1PT0

PF0PF1PF2

PM3

WT2WT1WT0

WF0WF1WF2

WM3

RT2RT1

Mean Total Length 

(millimeters)

Reach

Overall

0

100

200

300

400

PT2PT1PT0

PF0PF1PF2

PM3

WT2WT1WT0

WF0

WF1WF2

WM3

RT2RT1

Mean Total Length 

(millimeters)

Reach

Spring

0

100

200

300

400

PT2PT1PT0

PF0PF1PF2

PM3

WT2WT1WT0

WF0WF1WF2

WM3

RT2RT1

Mean Total Length 

(millimeters)

Reach

Summer
38

Figure 10. Age frequency (number) of northern pikeminnow captured, Priest Rapids Project, Columbia River, 

Washington, 2009–11. n, total number of fish.

Figure 11. Mean length at age and one standard error for northern pikeminnow captured, Priest Rapids Project, 

Columbia River, Washington, 2009–11. Fish 9 years old and older are combined. n, total number of fish.

30

380

642

351

180

114

54 35 46 35 25 31 8 5 7 1 3 11 5 1 1 1

0

100

200

300

400

500

600

700

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 22 23 24

Number

Age (years)

n = 1,966

0

50

100

150

200

250

300

350

400

450

500

1 2 3 4 5 6 7 8 9+

Mean Total Length (millimeters)

Age (years)

n = 1,966
39

Figure 12. Length frequency histograms for smallmouth bass during Burley and Poe sampling overall in 2009 (May 

27–June 12 and August 3–20), in spring 2009 (May 27–June 12), and in summer 2009 (August 3–20), Priest 

Rapids Project, Columbia River, Washington. n, total number of fish.

0

5

10

15

20

25

0

306090

120150180210240270300330360390420450480510

540

Number

Total Length (millimeters)

Overall

n = 272

0

5

10

15

20

25

0

306090

120150180210240270300330360390420450480510540

Number

Total Length (millimeters)

Spring

n = 168

0

5

10

15

20

25

0

306090

120150180210240270300330360390420450480510540

Number

Total Length (millimeters)

Summer

n = 104
40

0 

5 

10 

15 

20 

0 

30 60 90 

120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 

Number 

Total Length (millimeters)

Overall

n = 232 

0 

5 

10 

15 

20 

0 

30 60 90 

120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 

Number 

Total Length (millimeters)

Spring 

n = 48

0 

5 

10 

15 

20 

0 

30 60 90 

120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 

Number 

Total Length (millimeters)

Summer

n = 112

Figure 13. Length frequency histograms for smallmouth bass during predator index sampling overall in 2009 (May 

1–August 27), in spring 2009 (May 7–June 11) and in summer 2009 (June 23–August 5), Priest Rapids Project, 

Columbia River, Washington. n, total number of fish.
41

Figure 14. Length frequency histograms for smallmouth bass during predator index sampling overall in 2010 (May 

19–September 3), in spring 2010 (May 19–June 8), and in summer 2010 (June 28–August 11), Priest Rapids 

Project, Columbia River, Washington. n, total number of fish.

0

10

20

30

40

50

60

0

306090

120150180210240270300330360390420450480510540

Number

Total Length (millimeters)

Overall

n = 687

0

10

20

30

40

50

60

0

306090

120150180210240270300330360390420450480510540

Number

Total Length (millimeters)

Spring

n = 149

0

10

20

30

40

50

60

0

306090

120150180210240270300330360390420450480510540

Number

Total Length (millimeters)

Summer

n = 294
42

Figure 15. Mean length and one standard error for smallmouth bass during Burley and Poe sampling in spring 

2009 (May 27–June 12) and summer 2009 (August 3–20), by reaches, Priest Rapids Project, Columbia River, 

Washington. Reach locations: PT1, Priest Rapids Tailrace; PF1, Priest Rapids Forebay; PM1, Priest Rapids MidReservoir; WT1, Wanapum Tailrace; WF1, Wanapum Forebay; WM1, Wanapum Mid-Reservoir; RT1, Rock Island 

Tailrace.

0

50

100

150

200

250

300

350

400

PT1 PF1 PM1 WT1 WF1 WM1 RT1

Mean Total Length (millimeters)

Reach

Spring

0

50

100

150

200

250

300

350

400

PT1 PF1 PM1 WT1 WF1 WM1 RT1

Mean Total Length (millimeters)

Reach

Summer
43

Figure 16. Mean length and one standard error for smallmouth bass during predator index sampling overall in 2009 

(May 1–August 27), in spring 2009 (May 7–June 11), and in summer 2009 (June 23–August 5), by reaches, Priest 

Rapids Project, Columbia River, Washington. Reach locations: PT1, Priest Rapids Tailrace; PT0, Priest Rapids 

Tailrace BRZ; PF0, Priest Rapids Forebay BRZ; PF1, Priest Rapids Forebay; PM1, Priest Rapids Mid-Reservoir; 

WT1, Wanapum Tailrace; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay BRZ; WF1, Wanapum Forebay; 

WM1, Wanapum Mid-Reservoir; RT1, Rock Island Tailrace.

0

100

200

300

400

PT1 PT0 PF0 PF1 PM1 WT1 WT0 WF0 WF1 WM1 RT1

Mean Total Length 

(millimeters)

Reach

Overall

0

100

200

300

400

PT1 PT0 PF0 PF1 PM1 WT1 WT0 WF0 WF1 WM1 RT1

Mean Total Length 

(millimeters)

Reach

Spring

0

100

200

300

400

PT1 PT0 PF0 PF1 PM1 WT1 WT0 WF0 WF1 WM1 RT1

Mean Total Length 

(millimeters)

Reach

Summer
44

Figure 17. Mean length and one standard error for smallmouth bass during predator index sampling overall in 2010

(May 19–September 3), in spring 2010 (May 19–June 8), and in summer 2010 (June 28–August 11), by reaches, 

Priest Rapids Project, Columbia River, Washington. Reach locations: PT2, Priest Rapids Tailrace; PT1, Priest 

Rapids Tailrace near-BRZ; PT0, Priest Rapids Tailrace BRZ; PF0, Priest Rapids Forebay BRZ; PF1, Priest Rapids 

Forebay near-BRZ; PF2, Priest Rapids Forebay; PM3, Priest Rapids Mid-Reservoir; WT2, Wanapum Tailrace; 

WT1, Wanapum Tailrace near-BRZ; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay BRZ; WF1, 

Wanapum Forebay near-BRZ; WF2, Wanapum Forebay; WM3, Wanapum Mid-Reservoir; RT2, Rock Island 

Tailrace; RT1, Rock Island Tailrace near-BRZ.

0

100

200

300

400

500

PT2PT1PT0

PF0PF1PF2

PM3

WT2WT1WT0

WF0WF1WF2

WM3

RT2RT1

Mean Total Length 

(millimeters)

Reach

Overall

0

100

200

300

400

500

PT2PT1PT0

PF0PF1PF2

PM3

WT2WT1WT0

WF0WF1

WF2

WM3

RT2RT1

Mean Total Length 

(millimeters)

Reach

Spring

0

100

200

300

400

500

PT2PT1PT0

PF0PF1PF2

PM3

WT2WT1WT0

WF0WF1WF2

WM3

RT2RT1

Mean Total Length 

(millimeters)

Reach

Summer
45

Figure 18. Age frequency (number) of smallmouth bass captured, Priest Rapids Project, Columbia River, 

Washington, 2009–11. n, total number of fish.

Figure 19. Mean length at age and one standard error for smallmouth bass captured Priest Rapids Project, 

Columbia River, Washington, 2009–11. Fish 10 years old and older are combined. n, total number of fish.

9

182 181

210

117

52

28 27

9 3 10 4 5 1

0

50

100

150

200

250

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Number

Age (years)

n = 838

0

100

200

300

400

500

600

0 1 2 3 4 5 6 7 8 9 10+

Mean Total Length (millimeters)

Age (years)

n = 838
46

Figure 20. Length frequency histograms for walleye during Burley and Poe sampling overall in 2009 (May 27–June 

12 and August 3–20), in spring 2009 (May 27–June 12), and in summer 2009 (August 3–20), Priest Rapids Project, 

Columbia River, Washington. n, total number of fish.

0

1

2

70

100130160190220250280310340370400430460

490520550

580610640670700730760790

Number

Total Length (millimeters)

Overall

n = 13

0

1

2

70

100130160190220250280310340370400430460490520550580610640670700730760790

Number

Total Length (millimeters)

Spring

n = 7

0

1

2

70

100130160

190220250280

310340370400430460490

520550580610640

670700730760790

Number

Total Length (millimeters)

Summer

n = 6
47

Figure 21. Length frequency histogram for walleye during predator index sampling overall in 2009 (May 1–August 

27), and in summer 2009 (August 3–20), Priest Rapids Project, Columbia River, Washington. There were not 

enough fish captured in 2009 during night electrofishing sampling to generate length frequency histograms for 

spring or by reservoir. n, total number of fish.

0

1

2

3

70

100130160190220250280

310340370400

430460490520550580610640670700730760790

Number

Total Length (millimeters)

Overall

n = 18

0

1

2

3

70

100130160190220250280310340370400430460490520550580610640670700730760790

Number

Total Length (millimeters)

Summer

n = 15
48

Figure 22. Length frequency histograms for walleye during predator index sampling overall in 2010 (May 19–

September 3), in spring 2010 (May 19–June 8), and in summer 2010 (June 28–August 11), Priest Rapids Project, 

Columbia River, Washington. n, total number of fish.

0

2

4

6

70

100130160190220250280310340370400430460490520550580610640670700730760790

Number

Total Length (millimeters)

Overall

n = 59

0

2

4

6

70

100130160190220250280310340370400430460490520550580610640670700730760790

Number

Total Length (millimeters)

Spring

n = 15

0

2

4

6

70

100130160190220250280310340370400430460490520550580610640670700730760790

Number

Total Length (millimeters)

Summer

n = 21
49

Figure 23. Age frequency (number) of walleye captured, Priest Rapids Project, Columbia River, Washington, 

2009–11. n, total number of fish.

Figure 24. Mean length at age and one standard error for walleye captured, Priest Rapids Project, Columbia River, 

Washington, 2009–11. Fish 11 years old and older are combined. n, total number of fish.

2

34

11

17

12

4 4 3 4 4

2 2 1 2 2 1

0

5

10

15

20

25

30

35

40

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Number

Age (years)

n = 105

0

100

200

300

400

500

600

700

800

900

0 1 2 3 4 5 6 7 8 9 11+

Mean Total Length (millimeters)

Age (years)

n = 105
50

Spring

PT1 PF1 PM1 WT1 WF1 WM1 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0 FISH 

CRAYFISH 

MOLLUSKS 

INSECT 

MISC 

Summer

Reach

PT1 PF1 PM1 WT1 WF1 WM1 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

Figure 25. Proportion of diet for northern pikeminnow during Burley and Poe sampling in spring 2009 (May 27–

June 12) and summer 2009 (August 3–20), by reaches, Priest Rapids Project, Columbia River, Washington. 

Reaches with no column indicate no diet sample. Reach locations: PT1, Priest Rapids Tailrace; PF1, Priest Rapids 

Forebay; PM1, Priest Rapids Mid-Reservoir; WT1, Wanapum Tailrace; WF1, Wanapum Forebay; WM1, Wanapum 

Mid-Reservoir; RT1, Rock Island Tailrace. 

 
Spring

PT1 PT0 PF0 PF1 PM1 WT1 WT0 WF0 WF1 WM1 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0 FISH 

CRAYFISH 

MOLLUSKS 

INSECT 

MISC 

Summer

Reach

PT1 PT0 PF0 PF1 PM1 WT1 WT0 WF0 WF1 WM1 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

51

Figure 26. Proportion of diet for northern pikeminnow during Predator index sampling in spring 2009 (May 7–June 

11) and summer 2009 (June 23 - August 5), by reaches, Priest Rapids Project, Columbia River, Washington. 

Reaches with no column indicate no diet sample. Reach locations: PT1, Priest Rapids Tailrace; PT0, Priest Rapids 

Tailrace BRZ; PF0, Priest Rapids Forebay BRZ; PF1, Priest Rapids Forebay; PM1, Priest Rapids Mid-Reservoir; 

WT1, Wanapum Tailrace; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay BRZ; WF1, Wanapum Forebay; 

WM1, Wanapum Mid-Reservoir; RT1, Rock Island Tailrace. 
52

Spring

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

FISH 

CRAYFISH 

MOLLUSKS 

INSECT 

MISC 

Summer

Reach

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

Overall

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2WM3 RT2 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

Figure 27. Proportion of diet of northern pikeminnow collected during predator index sampling overall in 2010 (May 

19–September 3), in spring 2010 (May 2–June 9), and in summer 2010 (June 27–August 11), by reaches, Priest 

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

PT2, Priest Rapids Tailrace; PT1, Priest Rapids Tailrace near-BRZ; PT0, Priest Rapids Tailrace BRZ; PF0, Priest 

Rapids Forebay BRZ; PF1, Priest Rapids Forebay near-BRZ; PF2, Priest Rapids Forebay; PM3, Priest Rapids MidReservoir; WT2, Wanapum Tailrace; WT1, Wanapum Tailrace near-BRZ; WT0, Wanapum Tailrace BRZ; WF0, 

Wanapum Forebay BRZ; WF1, Wanapum Forebay near-BRZ; WF2, Wanapum Forebay; WM3, Wanapum MidReservoir; RT2, Rock Island Tailrace; RT1, Rock Island Tailrace near-BRZ. 
53

Overall

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion 

0.0

0.2

0.4

0.6

0.8

1.0 Salmon / 

steelhead

Unknown 

salmonids

Chinook 

salmon

Spring

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion 

0.0

0.2

0.4

0.6

0.8

1.0

Summer

Reach

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion 

0.0

0.2

0.4

0.6

0.8

1.0

Figure 28. Proportion of fish that are salmon in the diets of northern pikeminnow during predator index sampling in 

2010 overall (May 19–September 3), in spring 2010 (May 2–June 9), and summer 2010 (June 27 - August 11), by 

reach, Priest Rapids Project, Columbia River, Washington. Reach locations: PT2, Priest Rapids Tailrace; PT1, 

Priest Rapids Tailrace near-BRZ; PT0, Priest Rapids Tailrace BRZ; PF0, Priest Rapids Forebay BRZ; PF1, Priest 

Rapids Forebay near-BRZ; PF2, Priest Rapids Forebay; PM3, Priest Rapids Mid-Reservoir; WT2, Wanapum 

Tailrace; WT1, Wanapum Tailrace near-BRZ; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay BRZ; WF1, 

Wanapum Forebay near-BRZ; WF2, Wanapum Forebay; WM3, Wanapum Mid-Reservoir; RT2, Rock Island 

Tailrace; RT1, Rock Island Tailrace near-BRZ.
Spring

PT1 PF1 PM1 WT1 WF1 WM1 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0 FISH 

CRAYFISH 

MOLLUSKS 

INSECT 

MISC 

Summer

Reach

PT1 PF1 PM1 WT1 WF1 WM1 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

54

Figure 29. Proportion of diet for smallmouth bass during Burley and Poe sampling in spring 2009 (May 27–June 

12) and in summer 2009 (August 3–20), by reaches, Priest Rapids Project, Columbia River, Washington. Reaches 

with no column indicate no diet sample. Reach locations: PT1, Priest Rapids Tailrace; PF1, Priest Rapids Forebay; 

PM1, Priest Rapids Mid-Reservoir; WT1, Wanapum Tailrace; WF1, Wanapum Forebay; WM1, Wanapum MidReservoir; RT1, Rock Island Tailrace. 

 
55

Spring

PT1 PT0 PF0 PF1 PM1 WT1 WT0 WF0 WF1 WM1 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0 FISH 

CRAYFISH 

MOLLUSKS 

INSECT 

MISC 

Summer

Reach

PT1 PT0 PF0 PF1 PM1 WT1 WT0 WF0 WF1 WM1 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

Figure 30. Proportion of diet for smallmouth bass during predator index sampling in spring 2009 (May 7–June 11) 

and summer 2009 (June 23 - August 5), by reaches, Priest Rapids Project, Columbia River, Washington. Reaches 

with no column indicate no diet sample. Reach locations: PT1, Priest Rapids Tailrace; PT0, Priest Rapids Tailrace 

BRZ; PF0, Priest Rapids Forebay BRZ; PF1, Priest Rapids Forebay; PM1, Priest Rapids Mid-Reservoir; WT1, 

Wanapum Tailrace; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay BRZ; WF1, Wanapum Forebay; 

WM1, Wanapum Mid-Reservoir; RT1, Rock Island Tailrace. 

 
Spring

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

FISH 

CRAYFISH 

MOLLUSKS 

INSECT 

MISC 

Summer

Reach

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

Overall

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

56

Figure 31. Diet composition of smallmouth bass captured during predator index sampling overall in 2010 (May 19–

September 3), in spring 2010 (May 2–June 9), and in summer 2010 (June 27–August 11), by reaches, Priest 

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

PT2, Priest Rapids Tailrace; PT1, Priest Rapids Tailrace near-BRZ; PT0, Priest Rapids Tailrace BRZ; PF0, Priest 

Rapids Forebay BRZ; PF1, Priest Rapids Forebay near-BRZ; PF2, Priest Rapids Forebay; PM3, Priest Rapids MidReservoir; WT2, Wanapum Tailrace; WT1, Wanapum Tailrace near-BRZ; WT0, Wanapum Tailrace BRZ; WF0, 

Wanapum Forebay BRZ; WF1, Wanapum Forebay near-BRZ; WF2, Wanapum Forebay; WM3, Wanapum MidReservoir; RT2, Rock Island Tailrace; RT1, Rock Island Tailrace near-BRZ. 

 
57

Overall

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0 Unknown

Salmonids

Chinook 

Salmon / 

Steelhead 

Spring

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

Summer

Reach

PT2 PT1 PT0 PF0 PF1 PF2 PM3 WT2 WT1 WT0 WF0 WF1 WF2 WM3 RT2 RT1

Proportion

0.0

0.2

0.4

0.6

0.8

1.0

Figure 32. Proportion of fish in diet of walleye that are salmon during predator index sampling overall in 2010 (May 

19–September 3), in spring 2010 (May 19– June 8), and in summer 2010 (June 28–August 11) by reaches, Priest 

Rapids Project, Columbia River, Washington. Reach locations: PT2, Priest Rapids Tailrace; PT1, Priest Rapids 

Tailrace near-BRZ; PT0, Priest Rapids Tailrace BRZ; PF0, Priest Rapids Forebay BRZ; PF1, Priest Rapids 

Forebay near-BRZ; PF2, Priest Rapids Forebay; PM3, Priest Rapids Mid-Reservoir; WT2, Wanapum Tailrace; 

WT1, Wanapum Tailrace near-BRZ; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay BRZ; WF1, 

Wanapum Forebay near-BRZ; WF2, Wanapum Forebay; WM3, Wanapum Mid-Reservoir; RT2, Rock Island 

Tailrace; RT1, Rock Island Tailrace near-BRZ. 
58

 

Figure 33. Estimated weights of prey items consumed by northern pikeminnow during the 2009 and 2010 study 

periods using bioenergetics modeling. Bioenergetics modeling was based on the diets of 928 and 1,118 individuals 

during the 2009 and 2010 study periods, respectively.

0

100,000

200,000

300,000

400,000

500,000

600,000

Crayfish Fish Insects Misc Mollusks

Weight (g)

Prey Items

2009 Spring

2009 Summer

2010 Spring

2010 Summer
59

0 

10 

20 

30 

40 

50 

60 

70 

80 

90 

100 

2009 Spring 2009 Summer 2010 Spring 2010 Summer

Proportion 

Time

Mollusks 

Misc

Insects

Fish

Crayfish

Figure 34. Proportion of prey items consumed by northern pikeminnow from bioenergetics modeling during the 

2009 and 2010 study periods. Bioenergetics modeling was based on the diets of 928 and 1,118 individuals during 

the 2009 and 2010 study periods, respectively.
60

Figure 35. Estimated weights of prey items consumed by smallmouth bass during the 2009 and 2010 study periods 

using bioenergetics modeling. Bioenergetics modeling was based on the diets of 165 and 372 individuals during the 

2009 and 2010 study periods, respectively.

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

Weight (g)

Prey Items

2009 Spring

2009 Summer

2010 Spring

2010 Summer
61

Figure 36. Proportion of prey items consumed by smallmouth bass from bioenergetics modeling during the 2009 

and 2010 study periods. Bioenergetics modeling was based on the diets of 165 and 372 individuals during the 

2009 and 2010 study periods, respectively.

0

10

20

30

40

50

60

70

80

90

100

2009 Spring 2009 Summer 2010 Spring 2010 Summer

Proportion

Time

Salmonid

Other fish

Mollusk

Insects/zoop

Crayfish

Cottid
62

Table 1. Estimated area of each reach that is less than 3 meters in depth, Priest Rapids Project, Columbia River, 

Washington, 2009–10.

[BRZ, Boat Restricted Zone in the forebay and tailrace of each dam; HA, hectares; <, less than; M, meters]

YEAR RESERVOIR REACHES AREA (HA) < 3 M DEPTH

2009 Hanford Priest Rapids Tailrace 120.75

2009 Hanford Priest Rapids Tailrace BRZ 11.11

2009 Priest Rapids Priest Rapids Forebay BRZ 0.576

2009 Priest Rapids Priest Rapids Forebay 101.86

2009 Priest Rapids Priest Rapids Mid-Reservoir 89.18

2009 Priest Rapids Wanapum Tailrace 174.51

2009 Priest Rapids Wanapum Tailrace BRZ 2.33

2009 Wanapum Wanapum Forebay BRZ 4.44

2009 Wanapum Wanapum Forebay 134.90

2009 Wanapum Wanapum Mid-Reservoir 74.88

2009 Wanapum Rock Island Tailrace 30.46

2010 Hanford Priest Rapids Tailrace 116.73

2010 Hanford Priest Rapids Tailrace near-BRZ 2.12

2010 Hanford Priest Rapids Tailrace BRZ 2.82

2010 Priest Rapids Priest Rapids Forebay BRZ 1.37

2010 Priest Rapids Priest Rapids Forebay near-BRZ 5.30

2010 Priest Rapids Priest Rapids Forebay 88.89

2010 Priest Rapids Priest Rapids Mid-Reservoir 176.21

2010 Priest Rapids Wanapum Tailrace 155.1

2010 Priest Rapids Wanapum Tailrace near-BRZ 19.71

2010 Priest Rapids Wanapum Tailrace BRZ 3.16

2010 Wanapum Wanapum Forebay BRZ 3.36

2010 Wanapum Wanapum Forebay near-BRZ 4.11

2010 Wanapum Wanapum Forebay 133.37

2010 Wanapum Wanapum Mid-Reservoir 295.37

2010 Wanapum Rock Island Tailrace 28.13

2010 Wanapum Rock Island Tailrace near-BRZ 2.39
63

Table 2. Percentage of each age class of northern pikeminnow and smallmouth bass captured in 2009 and 2010 

in less than 3 m depth in the Priest Rapids Project, Columbia River, Washington. 

[Composition is based on the total number of each species sampled during the field season] 

 2009 2010

Age Northern Pikeminnow Smallmouth Bass

Northern 

Pikeminnow Smallmouth Bass

0 54.0

1 14.2 12.5

2 7.5 30.9 13.6 36.6

3 7.0 26.1 24.2 24.2

4 5.4 21.2 19.7 16.9

5 4.2 14.5 12.8 12.1

6 2.6 7.3 7.0 4.8

7 1.6 3.9 1.9

8 1.4 2.7 3.5

9 2.0 3.6
64

Table 3. Catch of northern pikeminnow in 10 minute period during Burley and Poe sampling in spring (May 27–

June 12) and summer (August 3–20), and during predator index sampling in spring (May 7–June 11) and summer 

(June 23 - August 5), by reaches, Priest Rapids Project, Columbia River, Washington, 2009. 

[Catch per unit effort for Burley and Poe (2009) represents catches of northern pikeminnow greater than 250 millimeters and 

smallmouth bass greater than 150 millimeters, and for Predator Index 2009 represents catches of northern pikeminnow 

greater than 170 mm and smallmouth bass greater than 150 millimeters. Reach locations: PT1, Priest Rapids Tailrace; PT0, 

Priest Rapids Tailrace BRZ (Boat Restricted Zone); PF0, Priest Rapids Forebay BRZ; PF1, Priest Rapids Forebay; PM3, 

Priest Rapids Mid-Reservoir; WT1, Wanapum Tailrace; WT0, Wanapum Tailrace BRZ; WF0, Wanapum Forebay BRZ; 

WF1, Wanapum Forebay; WM3, Wanapum Mid-Reservoir; RT1, Rock Island Tailrace] 

Catch per 10 minutes

Northern pikeminnow Smallmouth bass

Burley and Poe 

2009

Predator Index 

2009

Burley and Poe 2009 Predator Index 

2009

Reaches Spring Summer Spring Summer Spring Summer Spring Summer

PT1 0.004 0.002 0.012 0.011 0.001 0 0 0.002

PT0 a a 0b 0.020 a a 0.010 0.012

PF0 a a 0.007 0.010 a a 0.033 0.098

PF1 0.004 0.018 0.009 0.005 0.015 0.012 0.012 0.012

PM3 0.010 0.023 0.026 0.009 0.019 0.003 0.020 0.007

WT1 0.002 0.002 0.003 0.005 0.002 0.002 0 0

WT0 a a 0b c a a 0 c

WF0 a a 0.075 0.040 a a 0.030 0.035

WF1 0.021 0.013 0 0.016 0.007 0.017 0 0.008

WM3 0.022 0.028 0.039 0.036 0.003 0.0004 0.001 0.006

RT1 0.026 0.038 0.072 0.037 0 0 0 0 a- Boat restricted zones were not sampled as part of the 2009 Burley and Poe sampling due to logistical restraints. b- Only one electrofishing effort was conducted. c- Not sampled.
65

Table 4. Catch of northern pikeminnow (greater than 170 millimeters in length) and smallmouth bass (greater 

than 150 millimeters in length) per 10 minute period captured during electrofishing runs in spring (May 2–June 9) 

and summer 2010 (June 27–August 11), by reaches, Priest Rapids Project, Columbia River, Washington. 

[Reach locations: PT2, Priest Rapids Tailrace; PT1, Priest Rapids Tailrace near-BRZ; PT0, Priest Rapids Tailrace BRZ; PF0, 

Priest Rapids Forebay BRZ; PF1, Priest Rapids Forebay near-BRZ; PF2, Priest Rapids Forebay; PM3, Priest Rapids MidReservoir; WT2, Wanapum Tailrace; WT1, Wanapum Tailrace near-BRZ; WT0, Wanapum Tailrace BRZ; WF0, Wanapum 

Forebay BRZ; WF1, Wanapum Forebay near-BRZ; WF2, Wanapum Forebay; WM3, Wanapum Mid-Reservoir; RT2, Rock 

Island Tailrace; RT1, Rock Island Tailrace near-BRZ.] 

Catch per 10 minutes

Northern pikeminnow Smallmouth bass

Reaches Spring 2010 Summer 2010 Spring 2010 Summer 2010

PT2 0 0.011 0 0

PT1 0.127 0.064 0 0.010

PT0 0.080 0.008 0.015 0.016

PF0 0.075 0.035 0.155 0.100

PF1 0.018 0.021 0.036 0.087

PF2 0.032 0.033 0.010 0.028

PM3 0.030 0.034 0.020 0.015

WT2 0.030 0.005 0 0.002

WT1 0.046 0 0 0

WT0 0.074 0.025 0 0.020

WF0 0.060 0.010 0.110 0.060

WF1 0.027 0.017 0.013 0.033

WF2 0.006 0.019 0.010 0.007

WM3 0.091 0.098 0.001 0.005

RT2 0.043 0.055 0 0

RT1 0.036 0.052 0 0
66

Table 5. Predation indices estimated during the Burley and Poe sampling (Burley and Poe, 2009) in spring (May 

27–June 12) and summer 2009 (August 3–20), and during Burley and Poe’s 1993 study (Burley and Poe, 1994). 

[Values presented for Burley and Poe’s original study were estimated using a different density index (proportion of positive 

efforts) that was calculated from the values presented in their report and using different estimates of the areal extent of the 

proportion of the relevant reaches less than 3 meters in depth] 

Predation Index

Northern pikeminnow

Burley and Poe 2009 Burley and Poe 1993

Reaches Spring Summer Spring Summer

PT1 31.0 0 71.0 109

PF1 0 0 2.38 0

PM3 0 0 0 0

WT1 7.57 0 56.5 120

WF1 2.13 0 17.1 0

WM3 9.42 0 0 0

RT1 0 0 11.6 1.29
67

Table 6. Predation indices and standard errors (in parentheses) estimated for northern pikeminnow and 

smallmouth bass captured during predator index sampling in spring (May 7–June 11) and summer 2009 (June 23–

August 5), by reaches, Priest Rapids Project, Columbia River, Washington. 

[Reach locations: PT1, Priest Rapids Tailrace; PT0, Priest Rapids Tailrace BRZ (Boat Restricted Zone); PF0, Priest Rapids 

Forebay BRZ; PF1, Priest Rapids Forebay; PM1, Priest Rapids Mid-Reservoir; WT1, Wanapum Tailrace; WT0, Wanapum 

Tailrace BRZ; WF0, Wanapum Forebay BRZ; WF1, Wanapum Forebay; WM1,Wanapum Mid-Reservoir; RT1,Rock Island 

Tailrace] 

 

Predation Index 

Northern pikeminnow Smallmouth bass

Reaches Spring 2009 Summer 2009 Spring 2009 Summer 2009

PT1 1.754 (1.637) 0.565(0.565) 0 1.073 (1.073)

PT0 0 0 0 0

PF0 0 0 0.002 (0.002) 0.012 (0.007)

PF1 0 0 0.0.129 (0.129) 0

PM1 0 0 0.240 (0.240) 0.225 (0.225)

WT1 0 0 0 0

WT0 0 0 0 a

WF0 0 0 0 0

WF1 0 0 0 0

WM1 0 0 0 0.306 (0.306)

RT1 0 0 0 0 a - Not sampled
68

Table 7. Predation indices and standard errors (in parentheses) estimated for northern pikeminnow and 

smallmouth bass captured during predator index sampling in spring (May 2–June 9), and summer 2010 (June 27–

August 11), by reaches, Priest Rapids Project, Columbia River, Washington. 

[Reach locations: PT2, Priest Rapids Tailrace; PT1, Priest Rapids Tailrace near-BRZ; PT0, Priest Rapids Tailrace BRZ; PF0, 

Priest Rapids Forebay BRZ; PF1, Priest Rapids Forebay near-BRZ; PF2, Priest Rapids Forebay; PM3, Priest Rapids MidReservoir; WT2, Wanapum Tailrace; WT1, Wanapum Tailrace near-BRZ; WT0, Wanapum Tailrace BRZ; WF0, Wanapum 

Forebay BRZ; WF1, Wanapum Forebay near-BRZ; WF2, Wanapum Forebay; WM3, Wanapum Mid-Reservoir; RT2, Rock 

Island Tailrace; RT1, Rock Island Tailrace near-BRZ.] 

Predation Index

Northern pikeminnow Smallmouth bass

Reaches Spring 2010 Summer 2010 Spring 2010 Summer 2010

PT2 0 0 0 0

PT1 0.117 (0.051) 0 0 0

PT0 0 0 0 0.0

PF0 0 0 0.015 (0.015) 0.012 (0.012)

PF1 0 0 0.114 (0.114) 0.038 (0.025)

PF2 0 0 0 1.055 (0.776)

PM3 0.421(0.421) 0.196 (0.196) 5.940 (2.731) 1.760 (1.152)

WT2 1.018 (1.018) 0 0 0

WT1 0.157 (0.157) 0 0 0

WT0 0.015 (0.015) 0 0 0

WF0 0 0 0.090 (0.090) 0.109 (0.044)

WF1 0.011 (0.011) 0 0 0.113 (0.113)

WF2 0 0 0 0

WM3 1.918 (1.211) 0 0 0

RT2 0.274 (0.184) 0 0 0

RT1 0.011 (0.011) 0.016 (0.016) 0 0
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, WA 98115

http://wfrc.usgs.gov/
Counihan and others—Assessing Native and Introduced Fish Predation on Migrating Juvenile Salmon, Priest Rapids and Wanapum Reservoirs, Columbia River, Wash., 2009–11—Open-File Repot 2012–1130