U.S. Fish and Wildlife Service

Occupancy and Distribution of Larval Pacific 

Lamprey and Lampetra spp. in Wadeable 

Streams of the Pacific Northwest

FY 2015 Annual Report

Jeffrey C. Jolley, Gregory S. Silver, Julianne E. Harris, Erin C. Butts, and 

Carrie Cook-Tabor

U.S. Fish and Wildlife Service

Columbia River Fish and Wildlife Conservation Office

Vancouver, WA 98683
On the cover: USFWS personnel evaluating Balm Grove Dam on Gales Creek of the Tualatin 

River Basin. Photo taken in June 2015 by Jeff Jolley.

The correct citation for this report is:

Jolley, J.C., G.S. Silver, J.E. Harris, E.C. Butts, and C. Cook-Tabor. 2016. Occupancy and 

Distribution of Larval Pacific Lamprey and Lampetra spp. in Wadeable Streams of the Pacific 

Northwest. U.S. Fish and Wildlife Service, Columbia River Fish and Wildlife Conservation 

Office, Vancouver, WA. 35 pp.
Occupancy and Distribution of Larval Pacific Lamprey

and Lampetra spp. in Wadeable Streams of the Pacific 

Northwest

Study funded by

U.S. Fish and Wildlife Service

and authored by

Jeffrey C. Jolley

Gregory S. Silver

Julianne E. Harris

Erin C. Butts

Carrie Cook-Tabor

U.S. Fish and Wildlife Service

Columbia River Fish and Wildlife Conservation Office

1211 SE Cardinal Court, Suite 100

Vancouver, WA 98683

And

U.S. Fish and Wildlife Service

Western Washington Fish and Wildlife Conservation Office

510 Desmond Drive Southeast, #102

Lacey, WA 98503

Final

August 30, 2016
Disclaimers

The findings and conclusions in this report are those of the authors and do not necessarily 

represent the views of the U.S. Fish and Wildlife Service.

The mention of trade names or commercial products in this report does not constitute 

endorsement or recommendation for use by the federal government.
5

Occupancy and Distribution of Larval Pacific Lamprey and Lampetra spp. in 

Wadeable Streams of the Pacific Northwest

Jeffrey C. Jolley, Gregory S. Silver, Julianne E. Harris, Erin C. Butts, and Carrie Cook-Tabor

U.S. Fish and Wildlife Service

Columbia River Fish and Wildlife Conservation Office

1211 SE Cardinal Court, Suite 100

Vancouver, WA 98683

and

U.S. Fish and Wildlife Service

Western Washington Fish and Wildlife Conservation Office

510 Desmond Drive Southeast, #102

Lacey, WA 98503

Abstract – Pacific Lamprey Entosphenus tridentatus are declining throughout their range. 

In 2015, we used an occupancy-based sampling approach to answer several questions about

larval lamprey occurrence in wadeable streams to assess status and aid conservation. 

Specifically, our project goals were to: 1) determine detection probability in a Washington 

Coastal system (Chehalis River) known to be occupied by Pacific Lamprey through intensive 

occupancy sampling; 2) rapidly determine occupancy in a 5th level Hydrologic Unit Code (HUC

5; Middle Nisqually River) where Pacific Lamprey occupancy was not known; 3) determine 

occupancy at small spatial scales in two streams on the Tualatin River National Wildlife Refuge

prior to stream restoration; 4) evaluate Pacific Lamprey occupancy above and below a potential 

passage barrier (Balm Grove Dam) in Gales Creek within the Tualatin River basin; and 5) 

evaluate potential recolonization of Pacific Lamprey in the White Salmon River basin following 

a large dam removal (Condit Dam). Using our approach, larval lampreys were highly detectable

in areas of the coastal Washington Chehalis River basin known to be occupied. Only Lampetra

spp. occupied the Nisqually River Middle HUC 5. Small streams on the Tualatin River National 

Wildlife Refuge were occupied by Lampetra spp. only. Balm Grove Dam on Gales Creek is a 

likely barrier to upstream passage of Pacific Lamprey, as larvae occupied below the dam but 

were absent above it. Pacific Lampreys were found to occupy areas above the former site of 

Condit Dam on the White Salmon River indicating recent colonization. The occupancy-based 

approach proved applicable for addressing a variety of questions over divergent spatial scales, 

and was efficient since larvae are highly detectable. High detectability of larvae leads to a 

reduced sample effort required to attain a desired level of confidence that lamprey are absent 

when they are not observed.
6

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7

Table of Contents

List of Tables .................................................................................................................................. 8

List of Figures................................................................................................................................. 8

Introduction..................................................................................................................................... 9

Methods......................................................................................................................................... 10

Results and Discussion ................................................................................................................. 18

Conclusions................................................................................................................................... 26

Acknowledgements....................................................................................................................... 27

Literature Cited ............................................................................................................................. 28
8

List of Tables

Table 1. Number of reaches sampled for larval lamprey occupancy, number of reaches where 

Pacific Lamprey, Lampetra spp., and unidentified larvae (Unid) were collected and associated 

detection probability (dunit) in 2015. ............................................................................................. 11

Table 2. Mean total length (TL), standard error (SE), and number (N) of Pacific Lamprey and 

Lampetra spp. collected in 2015................................................................................................... 19

List of Figures

Figure 1. Larval Pacific Lamprey (PCL) and Lampetra spp. (LAM) occupancy in stream reaches 

within the Chehalis River Basin, WA in 2015.............................................................................. 10

Figure 2. Larval Lampetra spp. (LAM) and unidentified (UNID) occupancy in stream reaches 

within the Nisqually River Basin, WA in 2015. ........................................................................... 12

Figure 3. Larval Lampetra spp. (LAM) and unidentified (UNID) occupancy in stream reaches 

within the Tualatin River Basin, OR in 2015. All reaches were within Tualatin River National 

Wildlife Refuge (green shaded area). ........................................................................................... 13

Figure 4. Larval Pacific Lamprey (PCL), Lampetra spp. (LAM) and unidentified (UNID) 

occupancy in stream reaches upstream and downstream of Balm Grove Dam within Gales Creek 

of the Tualatin River Basin, OR in 2015. ..................................................................................... 14

Figure 5. Larval Pacific Lamprey (PCL), Lampetra spp. (LAM), and unidentified (UNID) 

occupancy in stream reaches within the White Salmon River Basin, WA in 2015...................... 16

Figure 6. Length frequency (total length in mm) of Pacific Lamprey and Lampetra spp. larvae 

collected in the Chehalis River Basin, WA in 2015. .................................................................... 20

Figure 7. Length frequency (total length in mm) of and Lampetra spp. larvae collected in the 

Middle Nisqually River Basin, WA in 2015................................................................................. 21

Figure 8. Length frequency (total length in mm) of Lampetra spp. larvae collected in the 

Tualatin River National Wildlife Refuge within the Tualatin River Basin, OR in 2015.............. 22

Figure 9. Length frequency (total length in mm) of Pacific Lamprey and Lampetra spp. larvae 

collected above and below Balm Grove Dam on Gales Creek of the Tualatin River Basin, OR in 

2015............................................................................................................................................... 24

Figure 10. Length frequency (total length in mm) of Pacific Lamprey and Lampetra spp. larvae 

collected above and below the former Condit Dam site on the White Salmon River, WA and 

selected tributaries in 2015. .......................................................................................................... 25
9

Introduction

Pacific Lamprey Entosphenus tridentatus have experienced a decline in distribution and 

abundance across their range (Close et al. 2002). They are culturally important to Native 

American tribes and are also ecologically important (Docker et al. 2015). In 2003, the Pacific 

Lamprey was petitioned for listing under the Endangered Species Act, and Oregon and Idaho list 

Pacific Lamprey as a species of concern. However, managers lack critical information about life 

history, basic biology and ecology, and status (e.g., occupancy, distribution, and abundance) of 

Pacific Lamprey necessary to develop effective conservation strategies (Luzier et al. 2011; Mesa 

and Copeland 2009). Furthermore, developing range-wide occupancy and distribution data for 

informing threats analyses remains a necessity (Wang and Schaller 2015). 

Relatively recently, occupancy sampling approaches have received considerable study 

and use to examine distribution and assess status of species in a landscape (MacKenzie and 

Royle 2005; MacKenzie et al. 2006). Occupancy can be a more efficient method to assess status 

as compared to abundance, but may also be more relevant to a highly fecund, panmictic species 

like Pacific Lamprey, since distribution may be a more important consideration for status than 

abundance in any particular system. The benefits of using standard approaches to data collection 

are many and include ease of comparison, examination of time-series, and evaluation of 

management regimes (Bonar et al. 2009). Standardizing sampling methods can be particularly 

important for occupancy sampling, since an occupancy approach can be used to answer a variety 

of questions (e.g., access/passage barrier, distribution, recolonization) about a species at multiple 

spatial scales (MacKenzie and Royle 2005; Baumgardt et al. 2014; Chandler et al. 2015). 

Pacific Lampreys have a complex life history that includes larval (i.e., ammocoete), 

migratory juvenile (i.e., macrophthalmia), and adult marine phases (Scott and Crossman 1973). 

Larvae lived burrowed in fine sediments of small streams (Claire et al. 2007; Stone and Barndt 

2005; Torgersen and Close 2004) and large rivers (Jolley et al. 2012). At this life stage they 

filter feed on detritus and organic material (Sutton and Bowen 1994) until metamorphosing

(typically July to December ) into the migratory juvenile life stage (McGree et al. 2008). Within 

the Columbia River Basin, the migration season is protracted (Mesa et al. 2015; Moser et al. 

2015), with most major downstream migrations to the Pacific Ocean occurring during spring,

while others begin migration in the fall (Beamish and Levings 1991; Moser et al. 2015). The 

sympatric Western Brook Lamprey Lampetra richardsoni does not have a major migratory or 

marine life stage although adults may locally migrate upstream before spawning (Renaud 1997). 

In addition, the co-occurring River Lamprey L. ayresii, paired-species to the Western Brook 

Lamprey (Docker 2009), has a migratory parasitic life-history similar to the Pacific Lamprey

(Jolley et al. 2016). Often, the larval stage is sampled to identify occupancy and assess status of 

lamprey species (Moser et al. 2007; Cowx et al. 2009).

We applied an occupancy-based study design to investigate and document larval lamprey 

occupancy in several stream basins in Oregon and Washington. Use of an efficient, statisticallyrigorous, probabilistic study design allowed us to efficiently address several questions relating to 

occupancy in a standardized fashion to help range-wide conservation. The approach is flexible 

and can be adapted to a specific project objective (Jolley et al. 2012; Harris and Jolley in review).

We applied the design to the following project goals in 2015: 1) determine detection probability 

in a Washington Coastal system (Chehalis River) through intensive occupancy sampling; 2) 

rapid determination of lamprey presence in a 5th level Hydrologic Unit Code (HUC 5; Middle 

Nisqually River); 3) determine occupancy at small spatial scales in two streams on the Tualatin 
10

River National Wildlife Refuge prior to stream restoration; 4) evaluate Pacific Lamprey 

occupancy above and below a potential passage barrier (Balm Grove Dam) in Gales Creek 

within the Tualatin River basin; and 5) evaluate potential recolonization of Pacific Lamprey in 

the White Salmon River basin following a large dam removal (Condit Dam). Specific objectives 

within each goal were to: 1) evaluate whether selected areas are occupied by larvae; 2) determine 

species occupying selected areas; 3) determine detection probability of larvae in selected areas;

and 4) describe size-structure of larvae in selected areas.

Methods

Study area descriptions

Intensive sampling for determination of detection probability specific to a Washington coastal 

basin

The Chehalis River Basin was selected as representative of a Washington Coastal basin.

The Chehalis River is a 5th order river; the basin originates from several areas including the 

Cascade Mountain Range of Central Washington and the Olympic Mountain Range of Northwest 

Washington. The basin covers 6,889 km2 and enters Grays Harbor of the Pacific Ocean (Figure 

1). The study units were the subbasins of the Black (3rd order; all stream orders hereafter are 

expressed at the 1:100,000 scale), Skookumchuck (4th order), and Newaukum (4th order) rivers 

Figure 1. Larval Pacific Lamprey (PCL) and Lampetra spp. (LAM) occupancy in stream 

reaches within the Chehalis River Basin, WA in 2015.
11

(Table 1). Although targeted lamprey surveys have not previously occurred, the Newaukum and 

Skookumchuck rivers were known to be occupied with Pacific Lamprey (Pacific Lamprey Data 

Clearinghouse, USGS ScienceBase 2016) while recent (i.e., 10 years) occupancy within the 

Black River was unknown. Occupancy data for drainages of the Washington Coast were so 

sparse that a threats analysis was not possible (Luzier et al. 2011). We have not previously 

conducted directed occupancy sampling in a coastal drainage of Washington and results can 

inform specific detection probabilities in these habitats as well as serve as a representative 

sentinel basin for tracking long-term trends in occupancy and distribution. In addition, 

discussions are underway regarding a proposed dam on the Chehalis River upstream from our 

subbasins of interest (Figure 1).

Table 1. Number of reaches sampled for larval lamprey occupancy, number of reaches where 

Pacific Lamprey, Lampetra spp., and unidentified larvae (Unid) were collected and associated 

detection probability (dunit) in 2015.

Rapid determination of lamprey presence within a basin

The area of interest within the Nisqually River Basin was the Middle Nisqually River 

HUC 5 (Table 1). The Nisqually River is a 5th order river; the basin originates in the Cascade 

Mountain Range of Central Washington from the southern slope of Mount Rainier and the basin 

covers 1,339 km2

, entering the southern end of Puget Sound of the Salish Sea (Figure 2). The 

middle HUC5 was sampled on the Nisqually River mainstem, Ohop Creek (3rd order), the 

Mashell River (4th order), and an unnamed tributary to the Nisqually River (2nd order). The 

Basin Sample unit

Stream 

order

Reaches 

sampled

Reaches 

occupied by 

Pacific lamprey d unit

Reaches 

occupied by 

Lampetra spp. d unit Unid

Chehalis Black River 3 10 7 0.7 8 0.8 3

Skookumchuck River 4 10 9 0.9 9 0.9 7

Newaukum River 4 10 10 1.0 6 0.6 3

Nisqually Middle Nisqually River 

HUC 5 5 4 0 0.0 4 1.0 2

Tualatin Chicken Creek 3 7 0 0.0 5 0.7 1

Rock Creek South - lower 1 7 0 0.0 2 0.3 0

Rock Creek South - upper 1 4 0 0.0 0 0.0 0

Gales Creek - upstream of 

Balm Grove Dam 3 6 0 0.0 6 1.0 1

Gales Creek - downstream 

of Balm Grove Dam 3 6 6 1.0 6 1.0 1

White 

Salmon Trout Lake Creek 3 6 0 0.0 3 0.5 5

Mill Creek 1 10 0 0.0 2 0.2 0

Buck Creek 2 6 0 0.0 0 0.0 0

Rattlesnake Creek 3 3 0 0.0 0 0.0 0

White Salmon River 

mainstem 5 12 9 0.8 12 1.0 8
12

Nisqually River Basin, in general, is known to be occupied with Pacific Lamprey and Lampetra

spp. but collections have only been made in the Nisqually Lower HUC 5 (Hayes et al. 2013). 

Occupancy data for drainages of Puget Sound were so sparse that a threats analysis was not 

possible (Luzier et al. 2011). Occupancy in the middle HUC5 is unknown and the upper HUC5 

was historically blocked to fish passage in the LaGrande Gorge and currently by LaGrande Dam

(Figure 2).

Determination of presence at a small spatial scale to provide information at a National Wildlife 

Refuge prior to stream restoration

The Tualatin River is a 5th order river; the basin originates in the Coast Range Mountains

of northern Oregon and covers 1,844 km2, entering the Willamette River at Rkm 46 (Columbia 

River Basin; Figure 3). Portions of the Tualatin River Basin are on the Tualatin River National 

Wildlife Refuge (TRNWR). Chicken Creek (3rd order) and Rock Creek South (1st order) within 

TRNWR served as pre-restoration monitoring of stream restoration projects (Figure 3). Planned 

stream restorations include removing the diversion on Chicken Creek allowing the creek to flow 

Figure 2. Larval Lampetra spp. (LAM) and unidentified (UNID) occupancy in stream 

reaches within the Nisqually River Basin, WA in 2015.
13

Figure 3. Larval Lampetra spp. (LAM) and unidentified (UNID) occupancy in stream 

reaches within the Tualatin River Basin, OR in 2015. All reaches were within Tualatin 

River National Wildlife Refuge (green shaded area).
14

to the historic channel. A more natural, meandering stream channel will be constructed for Rock 

Creek; the creek is presently ditched and channelized (Figure 3).

Evaluate a potential passage barrier to Pacific Lamprey

Gales Creek (3rd order), within the Tualatin River Basin, contained a potential Pacific 

Lamprey passage barrier (Balm Grove Dam; Rkm 20; Figure 4). The dam is 0.9 m tall and 2.4 m 

tall when flash boards are installed with a 90o corner over the top and is constructed with a 

concrete bottom, measuring approximately 4.6-6.1 m, with sheet flow and an additional small 

concrete sill at the top of the dam. If fish passage was restored, at least 40 km of stream is 

accessible to anadromous fish upstream of the dam (T. Stahl and T. Alsbury, 2005 memorandum 

to Oregon Fish and Wildlife Commission, Oregon Department of Fish and Wildlife, on Scoggins 

Dam Fish Passage Waiver – Net Benefit Determination, available: 

http://www.dfw.state.or.us/agency/commission/minutes/05/Feb/E_2_analysis.pdf).

Figure 4. Larval Pacific Lamprey (PCL), Lampetra spp. (LAM) and unidentified (UNID) 

occupancy in stream reaches upstream and downstream of Balm Grove Dam within Gales 

Creek of the Tualatin River Basin, OR in 2015.
15

Evaluate potential recolonization of Pacific Lamprey following a large dam removal

The White Salmon River is a 5th order river that enters the Columbia River at Rkm 269. 

The 1,000 km2 basin originates from the southern slope of Mount Adams in the Cascade 

Mountain Range of southern Washington (Figure 5). Condit Dam was located on the White 

Salmon River 5.3 km upstream from its confluence with the Columbia River, and was a 

complete barrier to upstream migrating fish for nearly 100 years. In 2011, the dam was breached 

and completely removed in 2012. Prior to the dam removal, larval lamprey surveys were 

conducted in the White Salmon River Basin above and below the dam (Silver et al. 2010). Predam removal surveys indicated the following watersheds would potentially be suitable for 

Pacific Lamprey recolonization following dam removal: the mainstem White Salmon River

above the former dam site, Rattlesnake Creek (3rd order), Buck Creek (2nd order), and Mill Creek 

(1st order). Trout Lake Creek (3rd order) was found to be occupied with Lampetra spp. larvae but 

may not be accessible to anadromous Pacific lamprey. In 2015, these watersheds were sampled 

for larval lamprey. 

Occupancy assessment

We used an adaptive probabilistic framework to identify the number and the specific 

location of sampling sites needed to assess occupancy and to estimate detection probability 

within a unit. When presence is established (see below) in a unit, occupancy is known with a 

probability of 1; however, when presence is not established, the unit could be truly unoccupied 

or it could be occupied but not enough individuals were detected. The posterior probability of 

unit occupancy if presence is not established, P(F|Co), can be estimated by a model developed by 

Peterson and Dunham (2003):

(1) P(F|Co) = 

P(Co|F) ∙P(F)

P(Co|F)∙P(F)+ P(Co|~F)∙P(~F)

,

P(F) is the prior probability of larval lamprey presence and P(~F) is the prior probability of 

absence (i.e., 1- P(F)). We set P(F) = P(~F) = 0.5 (uninformed) since larval lamprey presence in 

each unit was considered unknown. P(Co|F) is the probability of not detecting presence when it 

occurs (C0 = no detection; Peterson and Dunham 2003) and is a function of detection probability 

and the number of sites sampled, n. Detection probability, or the probability of detecting 

presence from a random site within an occupied unit, dunit, is based on sampling method and 

gear. For practical purposes we assumed dunit was the proportion of sampled sites in which 

presence of the specific larval lamprey species was observed. P(Co|F) is then estimated as (1-

dunit)

n

. Thus, identifying n requires a unit-and gear-specific detection probability (assumed or 

estimated) and a predetermined acceptably low level for P(F|Co), the posterior probability of unit

occupancy if larval lamprey presence is not established. 

Sample effort varied depending on the objective for each project goal. To select the 

number of sample sites, we drew on information from previous occupancy sampling in other 

basins, with detection rates commonly higher than 0.75 (Poirier et al. 2010; Silver et al. 

2010). We assumed a relatively conservative detection rate of 0.40 (given an area was 

occupied). We chose sampling efforts to meet the goals of different objectives to achieve a 

chosen level of certainty of lamprey absence when they are not detected (Table 1).

In the Chehalis Basin, we set our sampling effort at 10 reaches (reaches are defined 

below) per subbasin which would yield greater than 99% certainty of lamprey absence when they 

were not detected (given an assumed d=0.4). The more intensive effort also allows empirical 
16

Figure 5. Larval Pacific Lamprey (PCL), Lampetra spp. (LAM), and unidentified (UNID) 

occupancy in stream reaches within the White Salmon River Basin, WA in 2015.
17

determination of detection probability for a representative coastal system, for which one has not 

been explicitly determined in these habitats. 

We targeted four reaches in the middle Nisqually HUC 5 which would yield over 80% 

certainty that lampreys were absent if we did not detect them (given an assumed d=0.4). We 

wanted to sample the minimum number of reaches in this area to rapidly determine if larval 

lamprey were present in the HUC5.

Seven reaches in Chicken Creek and Rock Creek South within the Tualatin River 

National Wildlife Refuge were sampled and only four reaches were sampled in Upper Rock 

Creek South due to unfavorable conditions. This effort would yield 97% and 89% certainty of 

lamprey absence when they were not detected (given an assumed d=0.4).

Gales Creek within the Tualatin River Basin was divided into two study areas 

downstream and upstream of Balm Grove Dam. We sampled six reaches below (12 Rkm) and 

above (17 Rkm) Balm Grove Dam which would yield a 96% certainty of lamprey absence when 

they were not detected (given an assumed d=0.4).

In the White Salmon River basin, we targeted up to 10 reaches, (but most often fewer 

were sampled due to passage barriers) in each of Mill, Buck, Rattlesnake, and Trout Lake creeks, 

and 12 reaches in the White Salmon River mainstem, 6 upstream and 6 downstream of the 

former Condit Dam site (96% certainty).

We used a Generalized Random Tessellation Stratified (GRTS) approach, to produce 

sampling reaches in a random, spatially-balanced order (Stevens and Olsen 2004). This 

approach was used to generate an unbiased sample design that would allow the quantification of 

detection probabilities.

Field sampling

After the stream reach was located, a 50-m reach upstream of the point was measured and 

flagged, and water temperature and conductivity were recorded. Sampling for larval lampreys 

was conducted using an AbP-2 backpack electrofisher (ETS Electrofishing Systems, Madison, 

WI). The electrofisher delivered three pulses per second DC at 125 V and 25% duty cycle, with 

a 3:1 burst pulse train (i.e., three pulses on, one pulse off). Electrofishing effort within each 

reach was targeted at preferred larval lamprey rearing habitat where depositional silt and sand 

substrates are dominant (henceforth Type I habitat, Slade et al. 2003), while areas with hard bedrock 

and boulder substrates were sampled less intensively. Habitats that were too deep to wade or had 

high water velocity were not sampled.

Collected lampreys were anesthetized in a solution of tricaine methanesulfonate (MS222), and those fish >60 mm total length (TL) were identified as Pacific Lamprey or Lampetra

spp. according to caudal pigmentation (Goodman et al. 2009; Docker et al. 2016), and classified 

according to developmental stage (i.e., ammocoete, macrophthalmia, or adult). Most lampreys 

<60 mm TL were difficult to identify and were deemed “unidentified”. A sample of caudal fin

tissue was removed from a subset of larvae and stored in ethanol for future determination or 

verification of species using genetic methods (Spice et al. 2011; Docker et al. 2016). Lampreys 

were measured (TL in mm), placed in a recovery bucket of fresh river water, and released after 

resuming active swimming behavior. Occupancy of a reach was defined as collection of two 

larvae of the same species that were greater than 20 mm difference in TL or five individuals of 

the same species that were within 20 mm in TL. If these criteria are not met, we conservatively 

erred with the determination that the area is not occupied. Length-frequency histograms were 

constructed for each species to describe size structure.
18

Results and Discussion

Intensive sampling for determination of detection probability specific to a Washington coastal 

sentinel basin

Pacific Lamprey and Lampetra spp. were widespread and occupied all subbasins sampled 

within the Chehalis River Basin. Detection rates were high for both species ranging from 0.6 to 

1.0 (Table 1). There was a wide size range of larvae (Figure 6); a total of 163 Pacific Lamprey

larvae (49-123 mm TL), 139 Lampetra spp. larvae (48-138 mm TL), 5 Pacific Lamprey

transforming (92-116 mm TL), 2 Lampetra spp. transforming (122-123 mm TL), and 3 Western 

Brook Lamprey adults (118-132; definitive species identification is possible at adult stage) were 

captured (Table 2).

High detection rates of larvae in the Chehalis River Basin was similar to other studies 

(Poirier et al. 2010; Silver et al. 2010; Dunham et al. 2013; Reid and Goodman 2015) and further 

corroborate that the sampling approach is effective at quickly determining occupancy in an area. 

Furthermore, high detection rates yield minimal reach effort requirements to attain a high level 

of certainty of larval lamprey absence when they are not detected. The Chehalis River mainstem 

is undammed and free-flowing and could contain high abundance of larval Pacific Lamprey. It is 

possible that the high detection rates encountered are related to high abundance. Overall, our 

study suggests a robust population of Pacific Lamprey is currently present in the Chehalis River 

Basin.

Construction of a new dam to provide flood control for the cities of Chehalis and 

Centralia is being proposed. This could have a negative effect on lamprey populations in the 

Chehalis Riverbasin, as dams on the mainstem Columbia and Snake rivers have been implicated 

as a major contributor to the decline of Pacific Lamprey (Moser et al. 2002; Mesa et al. 2010; 

Jackson and Moser 2012; Keefer et al. 2012). In some cases dam construction and operation has 

led to the extirpation of Pacific Lamprey in upstream areas (Beamish and Northcote 1989). The 

proposed dam site is upstream of the basins sampled in this study, thus the extent of impacts to 

lamprey in areas affected by the dam is currently unknown although Pacific Lamprey are known 

to occur in this area (M. Winkowski, Washington Department of Fish and Wildlife, personal 

communication). Nevertheless, given the abundance and widespread occurrence of Pacific 

Lamprey in the Chehalis River basin, it is imperative that any dam constructed on the river in the 

future be equipped with fish ladders engineered to facilitate upstream passage of adult Pacific 

Lamprey.

Rapid determination of lamprey presence within a basin

The Nisqually River middle HUC5 was occupied with Lampetra spp. but not with Pacific 

Lamprey larvae. Lampetra spp. occupied all four sites sampled in the basin (Ohop Creek, 

Mashell River, and mainstem Nisqually River). Assuming the lowest detection rate found for 

Pacific Lamprey in the nearby Chehalis River Basin of 0.7, we are over 99% confident that 

Pacific Lamprey larvae are absent within the Nisqually River middle HUC5 given the lack of 

detection in four sampled reaches. A wide range of sizes of larvae were collected (Figure 7); a

total of 37 larvae (49-162 mm TL) and 1 transforming (122 mm TL) Lampetra spp. were 

captured (Table 2). Hayes et al. (2012) collected juvenile Pacific Lamprey in the lower 

Nisqually River with salmonid juvenile screw traps indicating some use of the Nisqually River 

by Pacific Lamprey. It is unclear why we did not detect any Pacific Lamprey larvae as no 

barriers to adult migration were known in these areas.
19

Table 2. Mean total length (TL), standard error (SE), and number (N) of Pacific Lamprey 

and Lampetra spp. collected in 2015.

Determination of presence at a small spatial scale to provide information at a National Wildlife 

Refuge prior to stream restoration

Pacific Lamprey larvae did not occupy Chicken Creek or Rock Creek South of the 

Tualatin River National Wildlife Refuge. Lampetra spp. occupied Chicken Creek and lower 

Rock Creek South but not in the upper Rock Creek South. A wide range of sizes were collected 

(Figure 8); a total of 38 Lampetra spp. larvae (43-188 mm TL) and 13 adult Western Brook 

Lamprey (119-157 mm TL) were captured (Table 2).

Although equivocal, these relatively smaller order tributaries are more likely to be 

occupied by Western Brook Lamprey (i.e., Lampetra spp.) while Pacific Lamprey are more often 

found in larger order streams (Stone 2006). Pacific Lamprey larvae are known to occur in the 

Tualatin River Basin (Schultz et al. 2016) and restoration of natural meanderings and function of 

refuge stream reaches might lead to recolonization of Pacific Lamprey. Pacific Lamprey can 

detect pheromones released by larvae (Yun et al. 2011) but it is unknown if the pheromone 

signal from Lampetra spp. larvae may provide this cue. It is unknown if Pacific Lamprey larvae 

occupy the remainder of the Chicken Creek watershed above the refuge or if adult migrating 

lampreys are able to successfully navigate the diversion structure. Most stream habitat 

restoration projects in the Pacific Northwest have a salmonid focus; it is unknown how these 

actions may benefit lamprey species. Our study provides baseline data for lamprey occupancy in 

these areas but future work should invest in documenting impacts of habitat changes to lamprey 

Basin Sample unit Pacific lamprey Lampetra spp.

Mean TL SE N Mean TL SE N

Chehalis Black River

75 2 40 90 3 58

Skookumchuck River 78 2 57 74 3 49

Newaukum River 81 2 66 89 4 32

Nisqually Middle Nisqually HUC5 - - -

82 4 37

Tualatin Chicken Creek - - - 101 7 18

Rock Creek South lower - - - 143 5 20

Gales Creek below Balm Grove Dam 91 3 33 90 2 73

Gales Creek above Balm Grove Dam - - - 85 3 65

White 

Salmon Trout Lake Creek - - - 69 3 66

Rattlesnake Creek - - - 65 - 1

Mill Creek - - - 93 4 10

Mainstem White Salmon above Condit 52 3 13 61 3 107

Mainstem White Salmon below Condit 58 4 43 79 3 122
20

Figure 6. Length frequency (total length in mm) of Pacific Lamprey and Lampetra spp. 

larvae collected in the Chehalis River Basin, WA in 2015.

Black River

0

5

10

15

20

Pacific lamprey

Lampetra spp. 

Newaukum River

Number

0

5

10

15

20

Skookumchuck River

Size group (mm)

40-49 50-5960-6970-79 80-89 90-99

100-109 110-119120-129

130-139 140-149150-159 160-169170-179180-189

0

5

10

15

20
21

Figure 7. Length frequency (total length in mm) of and Lampetra spp. larvae collected in 

the Middle Nisqually River Basin, WA in 2015.

Nisqually River

Size group (mm)

40-4950-5960-6970-7980-8990-99

100-109110-119120-129130-139140-149150-159160-169170-179180-189

Number

0

2

4

6

8

10

12

Lampetra spp.
22

Figure 8. Length frequency (total length in mm) of Lampetra spp. larvae collected in the 

Tualatin River National Wildlife Refuge within the Tualatin River Basin, OR in 2015.

Chicken Creek

Number

0

1

2

3

4

5

6

Lampetra spp.

Lower Rock Creek

Size group (mm)

40-49

50-5960-6970-79

80-89

90-99

100-109110-119120-129130-139140-149150-159160-169170-179180-189

0

1

2

3

4

5

6
23

species. An increased effort is necessary to monitor lamprey populations associated with stream 

restoration activities to provide information on beneficial conservations actions.

Evaluate a potential passage barrier to Pacific Lamprey

Pacific Lamprey and resident Lampetra spp. larvae occupied all six reaches (dunit =1.0) 

below Balm Grove Dam on Gales Creek of the Tualatin River while only Lampetra spp. 

occupied reaches above the dam (dunit =1.0). A wide range of sizes were collected (Figure 9); a

total of 33 Pacific Lamprey larvae (69-134 mm TL) and 138 Lampetra spp. larvae (42-147 mm 

TL) were captured (Table 2). Moderately abundant Pacific Lamprey spawning habitat was 

observed downstream of Balm Grove Dam, in the most proximate reaches. Potential Pacific 

Lamprey redds were observed, likely from earlier in the season. The lower sampling reaches, 

located within the floodplain and agricultural areas, were more degraded, slack, and composed of

over 90% Type 1 habitat (Slade et al. 2003). Moderately abundant Pacific Lamprey spawning 

habitat was observed above the dam but there were no signs of recent spawning use. The 

availability of adult Pacific Lamprey spawning habitat, the abundant larval rearing habitat 

(highlighted by the abundant Lampetra spp. larvae), and lack of detection of Pacific Lamprey 

larvae above the dam strongly suggests that Balm Grove Dam is a passage barrier to migrating 

adults. The height and physical configuration of the dam is likely unpassable by migrating adults 

(Moser et al. 2002). Balm Grove Dam has also been suggested as a partial or complete passage 

barrier to migrating salmonids (Bio-Surveys 2015). We recommend the removal of this defunct 

dam to improve passage conditions for migrating adult Pacific Lamprey and salmonids, and to 

reconnect the stream corridor. There is 40-56 km of stream upstream of the dam that would 

likely be accessible to anadromous fish (T. Stahl and T. Alsbury, 2005 memorandum to Oregon 

Fish and Wildlife Commission, Oregon Department of Fish and Wildlife, on Scoggins Dam Fish 

Passage Waiver – Net Benefit Determination, available: 

http://www.dfw.state.or.us/agency/commission/minutes/05/Feb/E_2_analysis.pdf).

Evaluate potential recolonization of Pacific Lamprey following a large dam removal

In the White Salmon River Basin, five reaches in Trout Lake Creek were occupied with 

Lampetra spp. (dunit = 0.83). Lampetra spp. occupied two reaches of Mill Creek, and one larva

was detected in 1 of 6 reaches in the Rattlesnake Creek basin, which did not meet our definition 

of occupancy. No reaches in Buck Creek were occupied by lampreys. In the White Salmon 

River mainstem above the former dam site, Lampetra spp. occupied six reaches and Pacific 

Lamprey occupied four reaches. Larval Pacific Lamprey and Lampetra spp. were detected 

downstream of Condit Dam after the removal in 6/6 reaches (dunit = 1.00; Table 1) in wadeable 

areas of the mainstem of the White Salmon River. A wide range of sizes were collected 

throughout the basin (Figure 9); 43 Pacific Lamprey larvae (10–107 mm TL), and 122 Lampetra

spp. larvae (6–143 mm TL), and 7 Lampetra spp. transforming (120–170 mm TL) were captured.

Pacific Lamprey larvae occurred in several areas within the White Salmon River Basin that were 

unavailable to them prior to the removal of Condit Dam. The presence of anadromous Pacific 

Lamprey upstream of the former dam site suggests natural recolonization and habitat utilization 

of new areas by Pacific Lamprey can occur relatively rapidly after barrier removal (i.e., about 

three years; Silver et al. 2010). Pacific Lamprey larvae were detected in four reaches above the 

dam site, as well as in newly created habitats formed by sediment deposition at the river mouth

following dam breach and removal (Silver et al. 2010). This is the first rigorously documented 
24

Gales Creek below Balm Grove Dam

Number

0

5

10

15

20 Pacific lamprey

Lampetra spp. 

Size group (mm)

40-4950-5960-6970-7980-8990-99

100-109110-119120-129130-139140-149150-159160-169170-179180-189

0

5

10

15

20 Gales Creek above Balm Grove Dam

Figure 9. Length frequency (total length in mm) of Pacific Lamprey

and Lampetra spp. larvae collected above and below Balm Grove Dam 

on Gales Creek of the Tualatin River Basin, OR in 2015.
25

Mill Creek

Number

0

5

10

15

20

Pacific lamprey

Lampetra spp. 

Trout Lake Creek

40-4950-5960-6970-79 80-8990-99

100-109110-119120-129130-139140-149150-159160-169170-179180-189

0

5

10

15

20

White Salmon River above Condit Dam site

White Salmon River below Condit Dam site

Size group (mm)

40-4950-5960-6970-7980-8990-99

100-109110-119120-129130-139140-149 150-159160-169170-179

180-189

Figure 10. Length frequency (total length in mm) of Pacific Lamprey and Lampetra spp. 

larvae collected above and below the former Condit Dam site on the White Salmon River, 

WA and selected tributaries in 2015.

natural recolonization by Pacific Lamprey following a dam removal. The dam was breached in 

October of 2011, and the structure was fully removed by September of 2012. Pacific Lamprey

naturally recolonized the North Fork Toutle River which was devastated after the eruption of 

Mount St. Helens in 1980 (Lin et al. 2008) and colonized Babine Lake in British Columbia after 

the removal of rockslide that was a suspected barrier (Farlinger and Beamish 1984). Pacific 

Lampreys have also recolonized Indian Creek of the Elwha River after the removal of Elwha 

Dam in 2012 (R. Paradis, Lower Elwha Klallam Tribe, personal communication). Sea Lampreys 

have been shown to recolonize portions of stream basins rapidly following dam removals (Hogg 

et al. 2013; Lasne et al. 2014) when they were already present downstream of the dam. 

Colonization of new or completely vacant stream basins may be slower (Pess et al. 2014; Starr 

2008) which may be due to the lack of a larval pheromone signal. Pheromones from streamrearing larvae have been shown to orient adult migrating Sea Lamprey (Bjerselius et al. 2000; 

Johnson et al. 2012) and experimentally for Pacific Lamprey (Yun et al. 2011).

The small size of the Pacific Lamprey larvae upstream of the Condit Dam site (26-67 mm 

TL) further corroborates that these individuals were likely produced from recent spawning 

events. Although adult Pacific Lamprey spawning migrations are protracted (Clemens et al. 
26

2009) they typically enter river basins in spring (Beamish 1980) and continue spawning activities 

into the summer (Stone 2006); there were three full migration seasons for adults to enter the 

basin (2013-2015). Although estimating age from length is prone to large error rates (Meeuwig 

and Bayer 2005) as size at age is highly variable (Hess et al. 2015) these larvae were likely age

0, age 1 and age 2. Meewig and Bayer (2005) reported mean length of 46 mm and 51 mm for 

age 1 and age 2 Pacific Lamprey larvae, respectively. Quintella et al. (2003) reported Sea 

Lamprey larvae were 50 mm at 12 months and Potts et al. (2015) reported age 1 Sea Lamprey

were 30-60 mm TL.

The extent of the White Salmon River basin that could be available and recolonized by 

Pacific Lamprey remains unclear. Because spawning substrates of Pacific Lamprey and 

anadromous Rainbow Trout Oncorhynchus mykiss (i.e., steelhead trout) overlap (Bjornn and 

Reiser 1991; Scott and Crossman 1973), anadromous steelhead trout intrinsic spawning habitat 

potential (Burnett et al. 2007; NMFS 2013) has been used to predict habitats where Pacific 

Lamprey may occur (Luzier et al. 2011). Although Rattlesnake Creek had a high intrinsic 

potential for steelhead trout (NMFS 2013) no lamprey larvae were detected in 2007 (Silver et al. 

2010) and one Lampetra spp. larva was detected in 2015. Rattlesnake Creek is a potential area 

in the system that Pacific Lamprey could recolonize since it is has habitat that is suitable for 

larval rearing. It is also unclear what constitutes a natural passage barrier for adult Pacific 

Lampreys as they can climb some structures (Close et al. 2002; Kemp et al. 2009) but detailed 

evaluations of their full capabilities have not been conducted. There are a series of waterfalls on 

the mainstem White Salmon River including Husum Falls (Rkm 12.6), BZ Falls (Rkm 20), and 

Big Brother Falls (Rkm 27). Allen (2012) suggested that a series of falls beginning at Rkm 25 is 

likely a barrier to all anadromous fish. Other anadromous fish have rapidly recolonized the basin 

up to Husum Fall including anadromous steelhead trout, Chinook Salmon O. tshawytscha (Allen 

et al. 2016), as well as an anecdotal report of an adult Bull Trout Salvelinus confluentus (Engle et 

al. 2013). A Chinook Salmon redd was also reported in 2012 upstream of Husum Falls. The 

importance of stream connectivity to the resident Western Brook Lamprey is unclear and it is 

curious that a robust population is present in Trout Lake Creek, located upstream of all the

aforementioned passage barriers. If Pacific Lamprey were to navigate those barriers, there 

appears to be abundant rearing habitat in the streams of Trout Lake Valley.

Conclusions

The occupancy-based sampling techniques outlined in this study can be adapted to assess 

potential passage barriers or within-basin distribution across multiple spatial scales. Adopting a 

probabilistic sampling scheme that utilizes concepts of occupancy and detection probability has 

shown to be an efficient and valuable method for rapidly establishing occupancy for a variety of 

applications (Jolley et al. 2012; Harris and Jolley in review). The study design is adaptable and 

efficient because larval lampreys are highly detectable when they occupy an area (Dunham et al. 

2013; Reid and Goodman 2015). Data generated through occupancy sampling opens up new 

avenues such as abundance estimation and modeling the effects of habitat covariates (Bailey et 

al. 2004; Chelgren and Dunham 2015; Harris and Jolley in review). Adherence to a randombased approach to site selection assures inferences can be applied to entire study units of interest 

and are not limited to specific habitats. Increasing the amount of intensive-sampling applied to a 

broad range of situations will allow application across the range. Follow-up monitoring of

restoration areas (habitat, barriers) will allow an increased understanding of the effectiveness of 
27

these conservation actions. Establishing sentinel basins across the range could allow long-term 

evaluation of an increasing or decreasing regional or range-wide population trend and help 

inform management in prioritizing and reducing population stressors.

Acknowledgements

We thank A. Berry, K. Brakensiek, S. Hansen, M. Kavanagh, L. Low, M. Lowe, R. Miller, T. 

Sheffels, and B. Silver for field assistance. D. Hines, C. Wang, and T. Whitesel provided GIS, 

analytical, and project guidance. M. Mizell, M. White (Confederated Tribes of the Chehalis 

Reservation), and K. Hibdon (Thurston County Parks) provided site access in the Black River.
28

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35

U.S. Fish and Wildlife Service

Columbia River Fish and Wildlife Conservation Office

1211 SE Cardinal Court, Suite 100

Vancouver, WA 98683

 

August 2016