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Deschutes Whirling Disease Risk Assessment Zielinski ad Bartholomew 2009 Executive Summary

Risk of Introduction and Establishment of Salmonid Whirling Disease

in the Deschutes River Basin, Oregon, USA

Christopher M Zielinski and Jerri L Bartholomew

Center for Fish Disease Research, Department of Microbiology, Oregon State University,

Nash Hall 220, Corvallis, Oregon 97331, USA.

Final Report to the Pelton Round Butte Fish Committee

October 8, 2009

EXECUTIVE SUMMARY

Introduction Located in the high desert region of Central Oregon, the Deschutes River is 278 river

kilometers (Rkm) long and is a major tributary of the Columbia River. It is fed by seven

main tributaries; five of these enter the river below the Pelton Round Butte Hydroelectric

Project (PRB), the other two enter in waters above the PRB. The position of the PRB

effectively divides the system into two sections that are commonly referred to as the

upper and lower Deschutes River basins. The PRB consists of series of three dams; the

lowermost Reregulating Dam (Rkm 161) serves to even downstream lower Deschutes

River flows while Pelton (Rkm 166) and Round Butte (Rkm 188) dams are high head

hydroelectric peaking dams. The dam complex currently serves as a barrier to upstream-

migrating native fish populations of summer steelhead trout (Oncorhynchus mykiss), and

spring Chinook (O. tshawytscha) and sockeye salmon (O. nerka). The dam complex also

isolates bull trout (Salvelinus confluentus) populations upstream from those in tributaries

to the Deschutes River downstream of the complex. Both bull trout and summer

steelhead are now listed “Threatened” under the Federal Endangered Species Act (ESA)

(Nehlsen 1995; USFWS 2002). During construction of the three-dam complex, both

upstream and downstream fish-passage facilities were constructed. However, the

Final Report to the Pelton Round Butte Fish Committee October 8, 2009

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construction of Round Butte Dam in 1964 resulted in confusing surface currents in Lake

Billy Chinook, the reservoir upstream, and the original fish-passage system was

discontinued due to the inability to effectively capture downstream-migrants. The last

anadromous fish migrated through the PRB in 1968.

With the recent federal relicensing of the hydro project (Federal Energy Regulatory

Commission [FERC] #2030), a plan for the reestablishment of anadromous salmonid

stocks into waters above the PRB and safe passage with new facilities has been

developed (PGE and Tribes 2004). The reintroduced fish runs would consist of steelhead

trout, and spring-run Chinook and sockeye salmon, all of which were historically present

above the PRB (Nehlsen 1995). The plan also involves physically connecting bull trout

populations that exist below the PRB with those upstream to achieve connectivity goals

in the draft bull trout recovery plan (USFWS 2002) for genetic conservation (Engelking

2002) and population stability.

Part of the process for evaluating the passage plan was the development of a Fish Health

Management Program (PGE and Tribes 2006). The program is, in part, designed to

minimize and communicate the magnitude of risk associated with passing serious fish

pathogens upstream of the PRB until evaluation of the fish passage effort confirms that

the reestablishment of anadromous fish species can be successful (FERC PRB License

Article 419). With the goal of minimizing the risk of passing serious fish pathogens

along with evaluating the magnitude of risk, several potential pathogens of concern were

identified: infectious hematopoietic necrosis virus (IHN), Renibacterium salmoninarum

(bacterial kidney disease) and the myxozoan parasites Ceratomyxa shasta

(ceratomyxosis) and Myxobolus cerebralis (Engelking 1999). At the forefront of concern

is M. cerebralis, the pathogen responsible for whirling disease in salmonids (Wolf and

Markiw 1984). Salmonid whirling disease is of concern because it has demonstrated the

ability to cause high mortality rates in juvenile salmonids in some North American

populations. For example, resident rainbow trout (O. mykiss) populations in the

intermountain west have suffered devastating losses in some basins where M. cerebralis

has become established (Walker and Nehring 1995; Vincent 1996; Baldwin et al. 1998).


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In addition, while the other pathogens of concerns are enzootic in the Deschutes River

basin, M. cerebralis is an introduced pathogen native to waters in Europe (Bartholomew

et al. 2002).

Determining the potential impacts of this parasite/disease on important resident salmonid

populations in the upper Deschutes Subbasin was given high importance among the

numerous pre-passage evaluation studies that were undertaken. Resident fish species

upstream of PRB that are reported to be susceptible to M. cerebralis infection; include:

redband (rainbow) trout (O. mykiss), mountain whitefish (Prosopium williamsoni) and

kokanee salmon (O. nerka) (Hedrick et al. 1999; Thompson et al. 1999; Baldwin et al.

2000; Sollid et al. 2002; Bartholomew et al. 2003). In addition, establishment of M.

cerebralis in the upper Deschutes River Basin could reduce the survival and natural

production of the reintroduced anadromous fish species (Engelking 2002).

The life cycle of M. cerebralis involves two spore stages (myxospore and

triactinomyxon) and two obligate hosts: a salmonid and the aquatic oligochaete worm

Tubifex tubifex (Wolf and Markiw 1984). The myxospore stage develops within the

salmonid host and the triactinomyxon stage develops within T. tubifex. At present, there

are five known mitochondrial lineages of T. tubifex present in North America (I, III, IV,

V, VI) (Beauchamp et al. 2001; Arsan et al. 2007). Overall, populations comprised of

lineage I and III are known to be susceptible to M. cerebralis while lineages IV, V, and

VI are resistant (Beauchamp et al. 2002, 2005; DuBey et al. 2005; Arsan et al. 2007).

The signs of M. cerebralis infection in the fish include black tail, whirling behavior, gross

structural abnormalities, and death (Halliday 1976; Rose et al. 2000; Hedrick et al. 2001;

MacConnell et al. 2002).

Introduction of myxospores of the parasite into the lower Deschutes River Basin is

known to have occurred by stray (origin outside of Deschutes River Basin) hatchery

summer steelhead and to a lesser extent spring Chinook salmon (Leek 1987; Engelking

2002). Stray wild summer steelhead and spring Chinook salmon are also suspected to

transport myxospores of M cerebralis into the lower Deschutes River (Engelking 2002).


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The closest enzootic area to the Deschutes River is in NE Oregon where M. cerebralis is

established in the Imnaha and Grand Ronde river basins (Lorz et al.1989; Sandell et al.

2002; Sollid et al. 2004). The infected hatchery steelhead are thought to have strayed

during their adult migration after being infected as juveniles in one of these Snake River

Basin streams.

Summary of Research Goals and Results

Goal #1: Validate the assumption that, despite frequent introduction, M. cerebralis was

not established in the lower Deschutes River Basin.

Results: We were unable to validate the assumption M. cerebralis is not established in

the lower Deschutes River Basin. With the use of sentinel fish as biological screens we

where able to determine the parasite had become established in more than one location

within the lower Deschutes River Basin. The establishment level is very low and none of

the sentinel fish held for myxospore development showed any clinical symptoms of

whirling disease. No location ever tested positive for more than one consecutive year.

The establishment appears to be limited to small locales within Trout, Buck Hollow, and

Mud Springs creeks and near Oak Springs Hatchery (Rkm 76) on the main stem

Deschutes River.

Goal #2: Determine the distribution, relative abundance, lineage composition, and

susceptibility of selected Deschutes River Basin T. tubifex populations to infection by M.

cerebralis; and the ability of this parasite to proliferate within its alternate host and thus

amplify the potential for disease.

Results:

Distribution

Populations of T. tubifex were found in waters above and below the PRB and had an

intermittent or “patchy” distribution. Of the 63 oligochaete collection locations, only 20

had the confirmed presence of T. tubifex. Final Report to the Pelton Round Butte Fish Committee October 8, 2009

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Relative Abundance

The relative abundance of T. tubifex in oligochaete samples from 12 locations analyzed

varied from 0 to 95%. The sites with 0% relative abundance were Bake Oven and Buck

Hollow creeks, small east side tributaries to the lower Deschutes River where steelhead

spawn. Samples from these streams were void of oligochaetes. However, some must

have been present in these streams as M. cerebralis was detected at a very low level in

sentinel fish exposed to Buck Hollow Creek. Samples containing the highest percentages

of T. tubifex came from Trout Creek, another larger east side tributary to the lower

Deschutes River, and Wizard Falls Trout Hatchery located on the Metolius River, a

tributary to the Deschutes upstream of PRB.

Genetic Lineage Composition

Genetic analysis from subsamples consisting of 60 worms each per site (n=10) revealed

T. tubifex populations consisted of two lineages, III and VI. Four populations were pure

lineage III, one was pure lineage VI, and the remaining populations were a mixture,

dominated by lineage III. As noted above lineage III worms are susceptible to M.

cerebralis infection in contrast to lineage VI worms which are resistant.

Susceptibility to and Proliferation of M. cerebralis

Overall, susceptibility to and proliferation of M. cerebralis varied significantly between

the worm populations with populations containing all lineage III worms having the

highest infection rates and triactinomyxon production numbers. Triactinomyxons were

detected in all groups exposed to M cerebralis myxospores except those from a site on

the lower Deschutes River at Oaks Springs Hatchery (Rkm 76). The Trout Creek group

had the highest overall production by nearly one order of magnitude over the Wizard Fall

Hatchery reference group in both susceptibility challenges, and nearly two orders of

magnitude over the lowest producer, the Crooked River group. Only lineage III worms

were found susceptible to the parasite and capable of producing triactinomyxons.


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Goal #3: Evaluate the probability of introduction and establishment of M. cerebralis into

the upper Deschutes River Basin.

Results:

Probability of Introduction

• Dispersal via human movement of fish into upper Deschutes River Basin

Negligible if pathogen free eggs, fry, and smolts are moved

Low if only marked naturally-reared adult salmon and steelhead

from above PRB are moved back upstream.

Moderate if only wild stocks are moved.

Low if only Deschutes River Basin hatchery stock are moved

High if stray hatchery fish are moved. Passage of stray hatchery

fish is not part of the current passage plan but was examined as

part of this risk assessment due to the potential for

misidentification.

Low-moderate for dispersal via private pond stocking

Negligible to low if ponds don’t have outflow into upper

Deschutes River Basin.

• Dispersal via recreation: low

• Dispersal via natural dispersal into and within Deschutes River Basin (birds): low

• Dispersal via bait used in the commercial crayfish fishery: moderate, but could be

lowered to negligible.

Assignment of an overall risk for introduction into the upper Deschutes River Basin will

almost completely be dependent on what sources are used for reintroduction purposes;

whether they are eggs, fry, Deschutes River Basin hatchery, stray hatchery or wild stocks.

Risks associated with other pathways of introduction into the upper Deschutes River

Basin could be significantly reduced with education of the public regarding certified

sources of fish for stocking of private ponds, types of crayfish bait that should not be


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used, cleaning of angler equipment and the proper dumping of bilge or engine coolant

water.

Probability of Establishment If introduced, conditions are appropriate for the parasite to complete its life cycle in both

the upper and lower basin. However, not all areas within the Deschutes River Basin have

an equal probability for establishing M. cerebralis and most contain abiotic and biotic

factors that will help impede establishment and/or the effects of the parasite. These

factors include:

• The majority of areas sampled did not have a confirmed presence of T. tubifex.

Discontinuous T. tubifex occurrence has the potential of limiting spatial and

temporal overlap of both hosts and spore stages.

• In most samples that contained T. tubifex, the relative abundances were extremely

low even though these locations appeared to contain optimum habitat.

• Optimal habitats for T. tubifex appear to be relatively rare in the Deschutes River

Basin. Optimum habitat appears to be confined to small sections of the lower end

of most tributaries and the upper main stem Deschutes River.

• The presence of resistant T. tubifex lineage VI may help mitigate the effects of M.

cerebralis.

• The majority of young salmonids in the Deschutes River Basin emerge and rear

during their most susceptible life stages when water temperatures are not

conducive for parasite proliferation in either host with the exception of redband

trout.

• High summer water temperatures in some tributaries are not conductive to

parasite survival, especially when reaching 20˚C or higher. Other tributaries run

year around with exceptionally cool water (below 10˚C), which is not conductive

to parasite proliferation.

These factors may come into play if introduction of the parasite into the upper Deschutes

River Basin does occur by synergistically limiting the establishment of M. cerebralis

along with its impact on resident and reintroduced salmonids populations.

Deschutes Basin Whirling Disease Risk Assessment Zielinski ad Bartholomew 2009

Risk of the Introduction and Establishment of Salmonid Whirling Disease

in the Deschutes River Basin, Oregon, USA

Christopher M Zielinski and Jerri L Bartholomew

Center for Fish Disease Research, Department of Microbiology, Oregon State University,

Nash Hall 220, Corvallis, Oregon 97331, USA.

Final Report to the Pelton Round Butte Fish Committee

October 8, 2009


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INTRODUCTION

Background

With the recent federal relicensing of the Pelton Round Butte Hydroelectric Project

(Federal Energy Regulatory Commission [FERC] Project #2030), a plan for the

reestablishment of anadromous salmonid stocks into waters above the hydro project and

safe passage with new facilities has been developed (PGE and Tribes 2004). The Pelton

Round Butte (PRB) complex, starting at river kilometer (Rkm) 161 on the Deschutes

River in Central Oregon, presently blocks upstream migration of anadromous salmonids

into upper river basin waters. Part of the process for evaluating the passage plan was the

development of a Fish Health Management Program (PGE and Tribes 2006). This

program is, in part, designed to minimize and communicate the magnitude of risk

associated with passing serious fish pathogens upstream of the PRB, at least until

evaluation of the fish passage effort confirms that the reestablishment of anadromous fish

species can be successful (FERC License Article 419). With the goal of minimizing the

risk of passing serious fish pathogens along with evaluating the magnitude of risk, several

potential pathogens of concern were identified in the Fish Health Management Program.

These included infectious hematopoietic necrosis virus causing IHN virus disease,

Renibacterium salmoninarum causing bacterial kidney disease, and the myxozoan

parasites Ceratomyxa shasta causing ceratomyxosis and Myxobolus cerebralis

(Engelking 1999). At the forefront of concern is M. cerebralis, the parasite responsible

for whirling disease in salmonids (Wolf and Markiw 1984), because it has caused high

rates of juvenile mortality in some salmonid populations in North America. For example,

rainbow trout populations (Oncorhynchus mykiss) in the intermountain west have

suffered devastating losses in some basins where M. cerebralis has become established

(Walker and Nehring 1995; Vincent 1996; Baldwin et al. 1998). In addition, while the

other pathogens of concerns are enzootic in the Deschutes River basin, M. cerebralis is

an introduced pathogen native to waters in Europe (Bartholomew et al. 2002).


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Annual, or at least periodic, introductions of M. cerebralis myxospores into the

Deschutes River Basin have occurred at least since 1984 as a result of infected stray adult

salmonids. Hatchery summer steelhead (O. mykiss) and to a lesser extent spring Chinook

salmon (O. tshawytscha), originally rearing in enzootic areas of the Columbia River

Basin, on their return adult migration, stray into the cooler waters of the Deschutes River

(Leek 1987; Engelking 2002). For unexplained reasons, many of these stray hatchery

summer steelhead spawn and then die in the Deschutes River Basin, depositing whirling

disease spores. The closest enzootic areas to the Deschutes River Basin are in NE

Oregon and NW Idaho, where M. cerebralis is established in certain watersheds of the

Snake River Basin, including the Imnaha and Grand Ronde rivers (Lorz et al.1989;

Sandell et al. 2002; Sollid et al. 2004). The infected stray adult salmonids found in the

Deschutes River are thought to have reared as juveniles in these Snake River tributaries.

Life Cycle and Epizootiology of Myxobolus cerebralis

The life cycle of M. cerebralis involves two spore stages (myxospore and actinospore)

and two obligate hosts: a salmonid and the aquatic oligochaete worm Tubifex tubifex

(Wolf and Markiw 1984). The myxospore stage develops within the salmonid host and

the triactinomyxon actinospore stage develops within T. tubifex (El-Matbouli and

Hoffmann 1998; El-Matbouli et al. 1999). The triactinomyxon is released into the water

column by infected T. tubifex, where it must come into contact with a susceptible

salmonid species (El-Matbouli and Hoffmann 1998). Upon contact, sporoplasm cells

penetrate the epidermis and enter the peripheral and central nervous system, where they

multiply and travel to the cartilage (El-Matbouli et al. 1995). Within the cartilage, the

parasite consumes the tissue and causes lesions in the skull, gills, and vertebral column

(Hedrick et al. 1998, MacConnell and Vincent 2002). The myxospore stage takes about 3

months to develop within the salmonid host at 15°C (Markiw 1992). Upon death of the

salmonid host and subsequent decomposition, myxospores are released into the

environment (El-Matbouli et al. 1992). Upon ingestion by T. tubifex, the myxospore

penetrates between the epithelial cells of the intestine and multiplies. After 3 months at

15°C, triactinomyxons are released into the water (Markiw 1986, El-Matbouli and

Hoffmann 1998, Stevens et al. 2001).


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The signs of M. cerebralis infection in the salmonid host include black tail, whirling

behavior and gross structural abnormalities (Halliday 1976; Rose et al. 2000; Hedrick et

al. 2001; MacConnell et al. 2002) and these may result in death. Whirling behavior is

common in susceptible small salmonids when they are heavily infected with the parasite.

Abundant cartilage in the skeleton of young salmonids renders them extremely

susceptible to the effects of the parasite (MacConnell et al. 2002). Rose et al. (2000)

demonstrated that whirling behavior of infected salmonids is a neuropathological

consequence of lower brain stem and spinal cord constriction. Black tail results when the

parasite infects cartilage in the posterior spinal column, causing pressure on the caudal

nerves, which control pigmentation in the caudal area (Halliday 1976). Permanent

skeletal deformities of the head and spine occur from parasite-induced damage to

cartilage.

Study Area

The Deschutes River Basin encompasses a diverse array of habitats and is the second

largest watershed in the state of Oregon (Figure 1). The basin covers about 10,700

square miles in central Oregon, making it one of the major subbasins of the Columbia

River system. It is cradled on its western side by the Cascade Mountains, on its south by

lava plateaus, by the Ochoco Mountains on its east and to the North by the Columbia

River. The basin measures 278 km long in the north-south direction and up to 211 km at

its greatest width. Much of the main stem Deschutes River and major tributaries are

protected by scenic waterway designations and in-stream water rights. There are also

existing surface water rights for out-of-stream uses, such as for irrigation and municipal

uses. Above PRB, there is a hydraulic connection between ground water and surface

water flows (State of Oregon Water Resources Department [OWRD] 2006). Because of

this connection, ground water withdrawals affect surface water flows. Due to the

underlying geology, virtually all ground water not consumptively used in the upper

Deschutes River Basin discharges to surface water near Pelton Dam. The entire summer

flow of the Deschutes River at Madras is ground water discharge (Gannett et al. 2003;

OWRD 2006). Supported by glaciers, snow-melt, and ground-water discharge, the


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http://www.wrd.state.or.us/OWRD/index.shtml


tributaries originating on the western side of the basin, including the Warm Springs and

Metolius rivers and Shitike Creek, drain the higher Cascade Mountains and generally

have cooler water temperatures throughout the year. Tributaries originating on the

eastern side of the basin including Buck Hollow, Bake Oven, and Trout creeks are

warmer and supported mainly by run-off flows (O’Connor et al. 2003; Olsen et al.

1992;).

N

10 0 10 20 30 Kilometers

Pelton Dam and Lake Simtustus

Round Butte Dam and Lake Billy Chinook

Crooked Rive r

Ochoco Dam and Reservoir

Bowman Dam and Prineville Reservoir

Warm Springs River

Shitike Creek

Metolius River

Des

chu t

es R

iver

Des

chut

es R

iver

Wickiup Dam and Reservoir

Crane Prairie Damand Reservoir

Suttle LakeOpal Springs Dam

Big Falls

Oregon

Columbia River

Crescent Lake

Steelhead Falls

Figure 1. Location of the Deschutes River and major tributaries in Central Oregon.

Study Approach

With potential to cause significant mortality and population level losses, the risk

associated with M. cerebralis in the Deschutes River Basin needed to be determined as

part of the reintroduction and fish passage effort. In order to evaluate potential risks, we


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conducted a series of studies from 1998 through 2007 guided by a risk-assessment model

developed by Bartholomew et al. (2002). The model, tailored specifically for assessing

the risk associated with M. cerebralis invading new salmonid habitats, is designed to

identify key life cycle and environmental variables that will influence pathways of

introduction, the potential for establishment, and the consequences of establishment to

salmonid populations. Following this model we studied the carrier rate of stray

salmonids entering the Deschutes Basin, the susceptibility of Deschutes Basin salmonids

to infection by M. cerebralis (Sollid et al. (2002), and the distribution and susceptibility

of T. tubifex within the Deschutes Basin (Zielinski et al. in preparation[a]). The

preliminary assumption within the model is that the parasite has not yet become

established in the watershed. The three steps involved in the risk assessment model are:

(1) the release assessment; (2) the exposure assessment; and (3) the consequence

assessment. Using this model we evaluated the probability of introduction and

establishment of M. cerebralis within the lower Deschutes River Basin as well as into

waters above the PRB with the coming fish passage program. To apply the model, we

identified three questions that needed to be answered to advance the anadromous fish

reintroduction plan. These are:

1. Can the life cycle of M. cerebralis be completed in the Deschutes River Basin?

2. If it can be completed, how virulent is M. cerebralis to salmonid stocks

upstream of the PRB?

3. If M. cerebralis becomes established in the upper Deschutes River Basin, is it

likely to cause population losses in important salmonid populations and

would these losses threaten sustainability?

Surveys and Studies of the Alternate Host, Tubifex tubifex

To determine the distribution of T. tubifex in the Deschutes River Basin, oligochaete

worms were collected in sediment samples from 63 locations below and above the PRB

from 1999-2007. The sample dates varied for each collection location and most sites

were only sampled once during summer months. Samples sites were selected based on

their close proximity to anadromous fish spawning sites (below PRB) or potential future

spawning locations (above PRB). The potential spawning locations above the PRB were


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determined based on historic spawning distribution data for anadromous fish and the

ability for the habitat to support successful recruitment of adult fish and rearing of fry.

At each site, sediment was collected using a 500 μm kicknet and strained through a 500

μm sieve using stream water then placed in 4-liter bags on ice and transported to the John

L. Fryer Salmon Disease Laboratory (SDL) at Oregon State University, Corvallis,

Oregon. In the laboratory, the samples were sieved (500 μm) once more and the

remaining sediment plus sample oligochaete populations were transferred to labeled

containers containing clean sand, aerated well water, and held at 13˚C in an incubator.

A subsample from each site was examined for the presence of T. tubifex. Each sediment

sample was placed in a shallow 21 x 44 cm tray with chilled ice water. Sexually mature

worms with a pink coloration and the ability to coil were isolated for further inspection

under a dissection microscope to observe if hair chaete were present (Kathman and

Brinkhurst 1998). If hair chaete were present, the worms were mounted on a glass

microscope slide and observed under a compound microscope for the presence of

pectinate chaete and differentiating sexual organs (Brinkhurst 1991) to confirm they were

T. tubifex and not another oligochaete species.

Once various Deschutes T. tubifex populations were concentrated, they were held at the

SDL were genetic lineage was determined and a series of studies on their susceptibility to

M. cerebralis and their relative efficiency at amplifying the parasite were conducted.

Details and results of these T. tubifex presence and density surveys, and the various

studies conducted in the lab are detailed in Zielinski et al. (in-preparation[a]). Results of

these studies and their application to the risk assessment are discussed in the “exposure

assessment” section of this report.

Validation of Preliminary Assumption

The first step in the risk assessment was to validate the assumption that despite frequent

introduction of myxospores from stray salmonids, M. cerebralis was not established in

the lower Deschutes River Basin. Earlier sampling of resident salmonids in the lower


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Deschutes River had not found the characteristic spores in a survey of 1745 resident

salmonids (Engelking 2002). Surveys for the establishment of M. cerebralis in the lower

Deschutes River Subbasin in this study used a different method described in detail in

Zielinski et al. (in preparation [b]). In this study, susceptible rainbow trout and steelhead

fry (both are referred to as sentinel fish) were exposed at various sites every year from

1998 to 2007 (except in 2004) then were tested for infection by M. cerebralis. Sentinel

fish were held at most sites twice annually: once in mid-late spring and once in early-mid

fall. Site locations varied between years and included sites at the mouths of the five

lower Deschutes River tributaries and in the lower main stem river. Tributary sites were

selected based on three criteria: (1) downstream proximity to anadromous fish spawning

sites, (2) confirmed presence of T. tubifex populations (most sites) and (3) ease of access.

During the course of the study, M. cerebralis was detected in sentinel fish exposed to

lower Deschutes Subbasin waters at six different locations in three out of 9 study years

(2001, 2002 and 2007). In 2001 and 2002, M. cerebralis was detected by polymerase

chain reaction (PCR), a very sensitive genetic test, in sentinel fish from Mud Springs,

Buck Hollow and Trout creeks and one main-stem site near Oak Springs Hatchery (Rkm

76). In 2007, sentinel fish exposed in at least two out of eight sites in Trout Creek tested

positive for M. cerebralis by quantitative PCR, and diagnostic M. cerebralis myxospores

were visually observed in pepsin-trypsin digest (PTD) preparations of the tissue. As a

result of this study, we conclude that our preliminary assumption, that M. cerebralis has

not become established in the lower Deschutes River Basin, is not valid.

1. RELEASE ASSESSMENT

The release assessment explores potential pathways of pathogen introduction above the

PRB. With establishment of M. cerebralis in the lower river basin, we examined ways

the parasite could be introduced into upper Deschutes system as well as moved within the

lower Deschutes River Basin. Identifying parasite life stage(s) with the highest potential

for disseminating over significant distances was the first step in determining probable

routes of dispersal. Of the two obligate hosts, the fish host is more mobile; thus,

myxospores are more likely to be distributed over a broader area than triactinomyxons


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(Arsan et al. 2007). The release assessment focused on potential routes of

introduction/dissemination of the myxospore stage. These routes include human

movement of fish, natural dispersal, and recreational activities. Human movement of fish

examines the transfer of anadromous salmonids above the PRB for reintroduction

purposes and the stocking of private ponds within the Deschutes River Basin. Natural

dispersal is focused on movement of the parasite by stray anadromous salmonids and by

pisciverous bird movements. Recreational activity considers movement of the parasite by

anglers, boaters, and for personal and commercial crayfish harvesting.

To categorize the risk associated with each potential introduction route we assigned each

a qualitative probability of occurrence as described by Bartholomew et al. (2005):

• high – the event would be expected to occur

• moderate – less than an even chance of the event occurring

• low - the event is unlikely to occur

• negligible – the chance of the event occurring is so small that it can be ignored.

Human Movement of Fish

Transfer of Salmonids

The risk of disseminating the parasite above the PRB as a result of transfer of salmonids

varies with age, species and origin of the fish selected for transfer. This introduction

route can be broken into the four categories. The first consists of the movement of

salmonid eggs, fry and smolts to waters above the PRB. This movement is predicted to

not be significant in regard to the passage of M. cerebralis into waters above the PRB

since the parasite is not transmitted vertically from parent to offspring and any fry and/or

smolts being passed will be raised in M. cerebralis-free water. The other two categories

involve transfer of adult salmonids. Adult salmon and steelhead entering the Deschutes

River presently are of three origins. Two hatcheries in the Deschutes River Basin produce

spring Chinook salmon and/or summer steelhead that are released below the dams; these

are referred to as hatchery salmonids. Salmonids from other river basins with markings


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indicating they were raised to smolt stage at hatcheries outside of the Deschutes River

Basin are termed out-of-basin or stray hatchery fish. Unmarked salmonids (fish without

clipped fins) are presumed of natural or wild origin (Deschutes River Basin or unknown

source).

In the future (starting in approximately 2011) marked, naturally-reared salmonids

released as fry upstream of PRB, and passed downstream through the new fish facilities

starting in 2009, will return to the Deschutes River Basin as marked adults. If reservoir

passage survival criteria for smolts are met according to the Fish Passage Plan (PGE and

Tribes 2004), these fish will be passed upstream over PRB to spawn naturally. These fish

are referred to as naturally-reared, marked adult salmonids.

Although passage of stray fish is not considered in the current fish passage plan, these

fish were considered in the risk assessment because they provide information to assess

parasite establishment in the lower basin and because they present a risk in the event of

human error. The likelihood of introducing M. cerebralis as a result of passage of wild

and stray hatchery steelhead trout and salmon was assessed using data from two studies

that determined the prevalence of infection among adult salmonids captured at traps. In

the first of these, Engelking (2002) examined 642 fish and found that 18% of stray

hatchery steelhead and 8% of stray hatchery Chinook salmon were infected. Infection

prevalence was high (25%) among stray hatchery sockeye salmon; however, this is most

likely due to the low sample size (n=4). Currently, no natural production of sockeye

salmon exists within the Deschutes River Basin, although a population of land-locked

sockeye salmon (kokanee) does exist within the upper Deschutes River Basin. It is this

population that will be used to attempt to reestablish a wild anadromous sockeye salmon

run in the Deschutes River Basin (PGE and Tribes 2004).

The Engelking study also demonstrated 8% of unmarked (wild) steelhead, and 5% of

unmarked Chinook salmon had M. cerebralis spores (Engelking, 2002). Based on the

rate of straying of these species into the lower Deschutes River (ODFW Mid-Columbia

Fish District Report, 2005), an assumption was made that the infected unmarked fish


10


originated in other watersheds. This assumption was further tested when Engelking

(2002) examined 1745 resident yearling salmonids collected from the lower Deschutes

River Basin and did not find a single infected fish.

The infection prevalence among adult stray hatchery summer steelhead collected at fish

traps was again assessed in 2007. Two groups of fish were tested; 120 fish were

collected from the Pelton Trap located immediately below the Reregulating Dam on the

lower main stem of the Deschutes River and 25 fish were taken from a trap located in the

lower section of Trout Creek. The infection prevalence was 27% and 16% for steelhead

from the Pelton and Trout Creek traps, respectively.

As described previously, sentinel fish studies demonstrated the establishment of M.

cerebralis within Trout Creek. However, resident fish samples from Trout Creek and

other streams with known establishment have been negative for M. cerebralis. The

infection prevalence among the sentinel fish was very low and no location was positive

for more than a consecutive year. Thus, the lack of detection in resident salmonids may

be a result of very low parasite release, or the resident fish sampled may have originated

from areas in Trout Creek above where M. cerebralis occurs.

In summary, a low to moderately high percentage (8 to 27% depending on species) of

stray anadromous hatchery salmonids and a low percentage (5 to 8%) of suspected stray

anadromous wild salmonids entering the Deschutes River were found to be infected and

carried myxospores of M. cerebralis. Infection has been detected at very low levels in

susceptible sentinel fish, but not in resident or Deschutes stock hatchery salmonids

sampled from the Deschutes River (Engelking 2002).

Assessment of Risk

Transfer of eggs, fry, and smolts poses a negligible risk for introduction of the parasite.

However, movement of adult salmonids for reintroduction purposes poses varying risks

for transporting and disseminating M. cerebralis above the PRB. If stray hatchery

summer steelhead were used for reintroduction purposes they would present the highest


11


risk for introduction because they have both the highest stray rate and highest infection

prevalence. Between 1983 and 2005, 17 to 53% of adult hatchery steelhead collected

annually below the PRB during peak migration were strays (Mid-Columbia Fish District

Annual Report, 2005) and infection prevalence as high as 27% has been documented. In

addition, susceptible species like steelhead are capable of harboring hundreds of

thousands to over a million myxospores per fish (Markiw 1992; Sollid et al. 2002).

Deschutes River origin hatchery fish pose a very low risk for disseminating the parasite

as they are raised to smolt stage in hatcheries free of the parasite. At this age, they have

acquired an age-dependent resistance to the parasite (Sollid et al. 2003; Ryce 2003; Ryce

et al. 2004). During their journey through the lower Deschutes River Basin this

resistance would decrease the likelihood and severity of infection in these fish. This is

particularly true as long as the establishment of M. cerebralis in the lower river system

stays at its current low level and with limited geographical range. Likewise, salmonids

(steelhead, Chinook and sockeye salmon) reared upstream of PRB that emigrate

downstream through the new facilities and are marked for future identification pose a

very low risk of becoming infected and disseminated the parasite upstream of the PRB

when passed back upstream as adults.

The primary risk in moving fish of wild origin at this time is the inability to differentiate

between wild fish of Deschutes River Basin origin and infected stray wild fish (out of

basin origin). It is important to note that the infection prevalence in fish presumed to be

stray wild salmonids is low (Engelking 2002) and the likelihood of wild salmonids of

Deschutes River Basin origin becoming infected is very low due to the limited and low

level establishment of M. cerebralis in the lower river system. This, however, could

change in the future if establishment of the parasite becomes widespread within the lower

Deschutes River Basin. It is also important to note not all hatchery fish are marked prior

to release; marking rates rarely exceed 97% (T. Amandi, senior fish health specialist

ODFW, personal communication). If wild adults were passed, it is possible that some

infected unmarked hatchery fish would be passed due to the inability to differentiate them


12


from wild salmonids. The risk posed from this is still considered low due to the limited

number of unmarked hatchery fish that are released yearly.

In summary: The risk of introduction is high if stray adult hatchery salmonids,

particularly steelhead, are transferred above the PRB. The risk is moderate if only wild

salmonids are transferred above the PRB. The risk is low if only Deschutes River Basin

hatchery salmonids are transferred. The risk will also be low if only fish produced

upstream of the PRB are passed back upstream. The risk is negligible if only eggs, fry,

and smolts raised in M. cerebralis free water are transferred. It is important to note that

cumulative risk over time for each of the pathways would be compounded with every

new transfer of salmonids except for the transfer of eggs, fry, and smolts. The risk as a

result of transfer of any in basin stock will increase if the parasite becomes more

widespread in the lower Deschutes River Basin.

Stocking of Private Ponds

The second route of introduction via human movement of fish is the stocking of private

ponds within the Deschutes River Basin with rainbow trout (O. mykiss). From 1999-

2001, 19 requests were made to stock seven private ponds within the Deschutes River

Basin with fish from a private rearing facility located in the Willamette River basin (N.

Hurtado, ODFW fish pathologist, personal communication). Myxobolus cerebralis was

detected in October 2001 in juvenile rainbow trout reared at this private facility

(Bartholomew et al. 2007). A total of 16 ponds throughout the state, including one in the

Deschutes River Basin, that had received potentially infected fish were monitored for

establishment of the parasite using sentinel rainbow trout fry. Infection was not detected

in sentinel fish held in any of these ponds which indicated that the parasite life cycle did

not become established as a result of the stocking. Additionally, the six private ponds in

the Deschutes River Basin stocked with potentially infected fish that were not monitored

are land locked, and would not likely have resulted in introduction of the parasite into the

Deschutes River Basin.


13


Assessment of Risk

The risk of introducing M. cerebralis by stocking of private ponds does exist because

potentially infected fish have been transferred from a private facility infected with M.

cerebralis to ponds within the Deschutes River Basin. ODFW does govern and regulate

such transfers of fish and it is against their policy for the transfer of known infected fish

to areas where the parasite is not known to exist. Although private facilities are required

to be tested annually for the parasite, the public’s lack of awareness of the need to acquire

a permit to buy, transport and place fish in their private ponds adds an additional layer of

complexity associated with this risk (Arsan et al. 2007). Although the stocking of

potentially-infected fish into private ponds described above does not appear to have

resulted in parasite establishment, this type of monitoring is limited and the experience of

other states suggests that private stocking of ponds can be a primary route of introduction

(reviewed by Bartholomew and Reno 2002). Also, it is possible private pond owners can

and do stock their ponds from rearing facilities located in other states. For these reasons,

the risk associated with introducing the parasite into the Deschutes River basin is low-

moderate. With public education this risk level could be lowered.

Recreational Movement

Anglers

Transfer of M. cerebralis on fishing equipment has been demonstrated in controlled

laboratory studies. In one study, felt-soled waders transported myxospores and

triactinomyxons (P. Reno, Oregon State University, personal communication). In that

same study, myxospores were vulnerable to desiccation and had decreased viability after

8 hours. Hedrick et al. (2008) also documented myxospores were vulnerable to

desiccation. In that study, no T. tubifex produced triactinomyxons when exposed to

myxospores allowed to dry at 22°C for 18.5 hours. In another study in which wader

materials were tested for their ability to trap myxospores, rubber had the fewest number

of adhering myxospores and felt the highest, with lightweight nylon and neoprene

intermediate (Gates et al. 2007). The researchers found that the felt used on the bottom

of wader boots is so effective at trapping myxospores that they were unable to release any


14


even when rinsed off with water pressure equal to the output of a residential garden hose

(simulating the cleaning of angler equipment at home). Another study demonstrated that,

on average, anglers transport 22 g of sediment on their boots and waders between access

sites (Gates et al. 2006). They also found that forty percent of anglers reported that they

occasionally, rarely, or never cleaned their fishing equipment between uses. Because of

their fragility, it is unlikely that triactinomyxon stages could be transported long distances

on fishing equipment; however, infected T. tubifex adhered to a felt sole may provide a

suitable environment for triactinomyxons to remain viable (Arsan et al. 2007).

Boating

Within the Deschutes River Basin, boating takes place in both the upper and lower river

system. The highest concentration of boats occurs on Lake Billy Chinook, the reservoir

behind Round Butte Dam, and on the lower main-stem section of the Deschutes River

(Oregon State Marine Board 2005). In fact, Lake Billy Chinook is the second most

popular boating destination in the state with more then 50,000 registered boat use days

recorded in 2004. This is in part due to active fisheries of kokanee and bull trout but also

for general recreational purposes such as water skiing. Boat use on the lower Deschutes

River consists of a mixture of rafting and fishing boats with more then 70,000 rafting trip

permits issued in 2006 (Bureau of Land Management Lower Deschutes River Boater Pass

schedule) and 20,000 registered boat use days recorded in 2004 (Oregon State Marine

Board 2005). For the entire river, most recreational boating occurs during the months of

May-October (Oregon State Marine Board 2005) when warmer water temperatures may

help facilitate the production and release of the triactinomyxon stage.

Boat bilge and engine cooling water represent a possible route by which recreational

activities could introduce M. cerebralis (Arsan et al 2007). The triactinomyxon stage

could be transported this way because it is neutrally buoyant and once in the water

column may be retained in the bilge or engine cooling water. The triactinomyxon stage

can survive many days to weeks depending on water temperature (El-Matbouli et al.

1999, Markiw 1992). These studies also determined triactinomyxons were particularly

sensitive to water temperatures above 19°C, with a significant decrease in viability


15


occurring even when exposed at this temperature for 2 days or less. With most boating

activity occurring during the hotter summer months, the triactinomyxon stage would need

to endure water temperatures greater than 19°C in bilge or engine cooling water.

However, if the parasite is able to survive for at least two days this would be more than

adequate for movement of boats from the lower river to the upper or from the enzootic

area of NE Oregon. Boat movement from the lower to the upper river basin does occur,

with boats previously being used in the lower Deschutes River Basin being used for a

total of 4850 boat use days during 2004 in the upper Deschutes River Basin (Oregon

State Marine Board 2005). Movement of boats from the enzootic areas of NE Oregon to

the Deschutes River Basin also occurs (Oregon State Marine Board 2005). In 2004, boats

moved from NE Oregon to the Deschutes River Basin were registered for approximately

1950 boat use days in the Deschutes River Basin.

Assessment of Risk

Many events would have to align for introduction to occur by recreational activities. The

first is the attachment of myxospores or infected T. tubifex to fishing equipment or the

retaining of triactinomyxons in bilge or engine cooling water. The probability of this

occurring is low since the availability of both spore stages and infected T. tubifex is very

limited in the natural environment and especially in the Deschutes River Basin.

Prevalence of M. cerebralis infection in worms found in natural bodies of water is

typically less then 10%, even in streams where whirling disease has caused significant

mortality in rainbow trout (DuBey and Caldwell 2004; Beauchamp et al. 2002; Kerans

and Zale 2002; Rognlie and Knapp 1998; Kruger et al. 2006; Zendt and Bergersen 2000).

Similarly, myxospore availability is low or patchy even in areas where whirling disease

has caused severe declines in rainbow trout abundances (Kerans and Zale 2002). This is

also the case for the triactinomyxon stage. Two water filtration studies conducted in

Colorado by Thompson and Nehring (2000) and Nehring (2006) revealed very low

average concentrations (0.006 triactinomyxon/L) of triactinomyxons in waters where

infection rates among wild rainbow trout ranged from 45 to 100 percent.


16


For a boat to transport one triactinomyxon in bilge water collected from a natural source

containing an average of 0.006 triactinomyxon/L, it would have to collect 166.666 L of

bilge and engine cooling water. This volume of water is unlikely to be transported by a

single boat since the majority of boats registered in the state of Oregon are less than 26 ft

in length and almost 50 percent are less than 16 ft (Oregon State Marine Board 2005). As

long as the current rate of boat movement between the Deschutes River Basin and NE

Oregon remains low, then the long term cumulative transfer risk of triactinomyxons will

also stay low. The risk of transferring triactinomyxons from the lower to the upper

Deschutes River Basin is currently low due to the limited establishment of M. cerebralis.

In addition to being picked up and transferred, the parasite would have to survive

transport. The second and final events that would need to align with the first event are

the survival of the spores while being transported and movement of angler equipment and

boats to a new location. Both can occur, but the very limited availability of spores and

their vulnerability to desiccation make the risk of introducing M. cerebralis by

recreational activities low. It is important to mention areas with the heaviest use would

be of the greatest risk for introduction, i.e. Lake Billy Chinook and the lower main stem

river.

Natural Dispersal

Stray Anadromous Fish

As discussed previously in the human movement of fish section, stray adult salmonids

introduce M. cerebralis into the lower river system (Leek 1987, Engelking et al. 2002,

chapter 3). See human movement of fish section for assessment of risk.

Birds

Arsan et al. (2007) was the first to describe the chain of events that need to occur for

birds to introduce M. cerebralis into a new location. The events are as follows:

piscivorous bird eats infected fish, bird retains infected fish in gut for duration of flight to

new location, myxospores survive passage through gut of bird, and finally the bird


17


releases viable spores over new water body. Myxospores have been shown to remain

viable after passing through the alimentary canal of mallard ducks (Anas platyrhynchos)

(Taylor and Lott 1978; El-Matbouli et al. 1992) and black-crowned night herons

(Nycticorax nycticorax) (Taylor and Lott 1978). Myxospores may not even have to pass

through the digestive system for dispersal since many bird species regurgitate pellets

containing inedible pieces of tissue such as bone, cartilage, and scales. Questions

regarding whether or not most species would egest waste over or near water are

unanswered for most species. One species, the Caspian tern (Sterna caspia), has been

documented to do neither and prefers to egest and regurgitate pellets within 30 cm of its

nest or roost (Bergman 1953). On the other hand, bald eagles (Haliaeetus leucocephalus)

(Mersmann 1992) and California gulls (Larus californicus) (Vermeer 1970), are often

seen egesting over water but regurgitate pellets over land. For the majority of

piscivorous birds, little is known about pellet-casting and defecation habits.

The Deschutes River Basin contains over 25 species of piscivorous birds, many of which

are also found in the upper Columbia River Basin) and in NE Oregon (Unites States

Forest Service 2007). The largest piscivorous eating birds in the Deschutes River Basin

are egrets, eagles, osprey, mergansers, cormorants and herons, all of which are

accomplished fish catchers (United States Geological Service [USGS] 2007). Most of

the piscivorous birds stay within close proximity of the three major rivers, the Crooked,

Metolius and Deschutes (USGS 2007).

The closest area to the Deschutes River Basin with the highest density of piscivorous

birds is the lower Columbia River estuary where colonies of Caspian terns and double-

crested cormorants (Phalacrocorax auritus) occur with bird numbers in the thousands

(Collis et al., 2002). There are also smaller colonies of Caspian terns close to the

enzootic area of NE Oregon, and these birds feed on juvenile steelhead as their primary

source of food (Collis et al., 2001).

Some bird species use the Deschutes River Basin as a temporary stopping ground on

their migration to and from northern breeding grounds (USGS 2007). What type of


18


migratory path a bird species takes will influence whether it crosses into the Deschutes

River Basin from an enzootic area. Migration paths are not always narrow in nature and

often, depending on the species and food availability, can be classified as broad or

converging (Bellrose and Graber 1963). The converging and broad routes take birds over

large ranges of territory not only in the typical longitudinal direction but also diagonally

crossing latitudinal patterns. These types of routes move birds into and from the

enzootic region of NE Oregon. Also within migratory routes, patterns of migration exist

(Martell 2001). Two patterns of migration that would play a role in introducing M.

cerebralis into the Deschutes River Basin are loop and dog-leg patterns. Each is

characterized by large swings of movement in latitudinal directions. For example,

California Gulls often fly northward along the Pacific Coast until reaching the Columbia

River, they then travel hundreds of miles eastward bringing them into the Deschutes

River Basin and NE Oregon then migrate back in the opposite direction before heading

north (Diem and Condon 1967).

For introduction into the Deschutes River Basin or dissemination within, a bird would

have to eat an infected fish in one of three locations: the enzootic area of NE Oregon and

Snake River Basin, the upper Columbia River, or within the lower Deschutes River

Basin. Eating an infected fish in one of the first two locations and then introducing the

parasite into the Deschutes River Basin would require extensive travel while retaining the

meal. Migration patterns of some piscivorous birds would bring them from NE Oregon

or the upper Columbia River into the Deschutes River Basin; whether or not they make it

to the Deschutes while retaining a meal is dependent on their migration speeds and food

retention times, which are not documented for most bird species. Migratory speeds are

known for three species of piscivorous birds found within this region. The bald eagle can

travel an average of 210 km/day (Kerlinger, 1995), the osprey (Pandion haliaetus)

between 108-431 km/day (Alerstam, 2003; Hake et al., 2001), and the caspian tern has an

average speed of 40–50 km/d (Jozefik, M. 1969). Food retention times have been

documented for three species of birds found in the region. The double-crested cormorant,

takes 1-2 days (Brugger 1993), the caspian tern takes at least 33 hours to pass 90% of

eaten food (Chavez, K. J. H. 1997), and the bald eagle takes approximately 62 hours to


19


pass rainbow trout marked with trace minerals. Food retention times are not available

for osprey but would most likely be similar to bald eagles due to similar gut morphology

(Stone et al. 1978).

The enzootic area of NE Oregon is approximately 200 km from the Deschutes River

Basin. Both bald eagles and osprey could fly from NE Oregon following the Columbia

River to the Deschutes River Basin in little over a day, thus a meal of infected fish could

be retained. Migration times would be even less for birds catching infected outmigrating

smolts passing the Deschutes River Basin on their migration down the Columbia River.

Bald eagles in the upper Columbia River have been documented to follow and feed on

runs of outmigrating steelhead smolts (Servheen, C. and W. English. 1979). Some of

these runs would most likely include infected steelhead smolts from the Snake River

Basin.

Assessment of Risk

With fast flight speeds and long food retention times it is possible M. cerebralis

myxospores could be introduced into the Deschutes River Basin by piscivorous birds.

However, the likelihood of this happening is dependent on several events. These events

not only have to occur in sequence but also are confounded by biotic and abiotic

influences such as the limited distribution of M. cerebralis within the enzootic areas and

its geographical distance from the Deschutes River Basin. Therefore, introduction of M.

cerebralis via bird migration into the Deschutes River Basin is unlikely and has a low

probability of occurring. It is also unlikely at the current level in which M. cerebralis is

established in the lower Deschutes River Basin that resident and migratory birds would

disseminate the parasite within the system, including into waters above the PRB.

However, the risk for dissemination by piscivorous birds into the upper Deschutes River

Basin would increase substantially if the establishment of M. cerebralis were to become

more widespread in the lower river system.


20


Commercial Dispersal

Crayfish Fishery

The only commercial fishery in the Deschutes River Basin is the harvest of signal

crayfish (Pacifastacus leniusculus) from Lake Billy Chinook. The harvest reached its

peak in 1987 with 69,967 kg taken from the lake (Lewis 1997). Presently, harvest

continues but at a much smaller scale. The risk that this fishery presents for introduction

of M. cerebralis is the type of bait used. The source of bait for this fishery is unknown

but if it included carcasses or heads from salmonids originating from the enzootic areas,

these could harbor myxospore stages of the parasite. This represents a data gap that

needs further research in order to fully comprehend the risks associated crayfish

harvesting practices.

Assessment of Risk

Based on two criteria, a moderate risk is associated with this route of introduction. First,

not knowing the source of salmonid parts used as bait elevates the risk level. Second,

movement of infected fish is the most common route for disseminating M. cerebralis

(Hoffman, 1970, 1990; Hedrick et al., 1998; Bartholomew et al. 2002). Because

commercial harvesting licenses must be requested annually, management agencies could

educate licensees to use bait that is from sources that do not contain M. cerebralis

infected fish. Harvest of crayfish in the Crooked and Deschutes river arms of Lake Billy

Chinook is managed by ODFW and harvest in the Metolius River arm is managed by the

Confederated Tribes of the Warm Springs Reservation of Oregon (CTWSRO). Both

could work together to educate the commercial crayfish harvesters and further decrease

the risk of introduction by this route.

Summary of the Release Assessment

The probability of introduction of M. cerebralis into the Deschutes River Basin is

summarized as:

• Dispersal via human movement of fish into upper Deschutes River Basin

Negligible if pathogen free eggs or fry are moved


21


Low if marked naturally-reared adult salmon and steelhead from

above PRB are moved back upstream.

High if stray hatchery adult steelhead are moved. Passage of

stray hatchery fish is not part of the current passage plan, but was

examined as part of this risk assessment due to the potential for

misidentification.

Moderate if only wild stocks are moved.

Low if only Deschutes River Basin hatchery stocks are moved.

Low-moderate for dispersal via private pond stocking.

Negligible to low if ponds don’t have outflow into Deschutes

River Basin.

• Dispersal via recreation: low

• Dispersal via natural fish movement within lower Deschutes River Basin (stray

fish): high

• Dispersal via natural dispersal into and within Deschutes River Basin (birds): low

• Dispersal via bait in the commercial crayfish fishery: moderate, but could be

lowered to negligible.

Overall, the risk of M. cerebralis introduction into the lower Deschutes River Basin is

high due to significant numbers of infected stray hatchery steelhead and Chinook salmon

entering the river. Assignment of an overall risk for introduction into the upper

Deschutes River Basin will almost completely be dependent on what sources are used for

reintroduction purposes; whether they are eggs, fry, Deschutes River Basin hatchery,

stray hatchery or wild stocks. The long term goal for the reintroduction plan is to

eventually move all stocks of adult anadromous salmonids entering the lower Deschutes

River Basin. If this occurs, the likelihood of introducing M. cerebralis into waters above

the PRB is high, especially over the long term where cumulative passage will increase the

likeliness of introduction. The one unknown regarding introduction into the upper

Deschutes River Basin is the possible use of crayfish bait that contains salmonid parts

from infected individuals.


22


2. EXPOSURE ASSESSMENT

The exposure assessment explores the risk of parasite establishment (Bartholomew et al.

2005). The likelihood for the establishment, proliferation, and spread of M. cerebralis is

dependent upon several environmental and biological factors (Kerans and Zale 2002).

These include susceptibility of hosts, spatial-temporal overlap of hosts, and favorable

environment (e.g. water temperatures, substrate type) (Figure 2); each of which affects

the outcome of the exposure assessment (Arsan et al. 2007).

Susceptible T. tubifex?

Establishment does NOT occur

Susceptible salmonids?

Event occurs


Event does NOT occur

Establishment occurs, at what

level Event occurs

Event occursEvent does NOT

occur

Introduction occurs


Spatial, temporal overlap of hosts?

Event does NOT occur

Figure 2. Diagram of the steps needed for establishment of Myxobolus cerebralis. If any one step does not occur then establishment is halted. Diagram borrowed and modified with permission from Arsan et al. (2007).


23


Susceptibility of the Alternate Hosts

Oligochaete Host Susceptibility

Tubifex tubifex is composed of several distinct genetic lineages with differences in

susceptibility and resistance to infection by M. cerebralis (Beauchamp et al. 2002, 2005;

Kerans et al. 2004; Dubey and Caldwell 2004; DuBey et al. 2005; Hallett et al. 2005;

Arsan et al. 2007). At present, there are five known mitochondrial lineages present in

North America (I, III, IV, V, VI) (Beauchamp et al. 2001; Arsan et al. 2007). Overall,

populations comprised of lineage I and III have shown to be susceptible to M. cerebralis

while lineages IV, V, and VI are resistant (Beauchamp et al. 2002, 2005; DuBey et al.

2005; Arsan et al. 2007).

A survey to determine what lineages were present in waters above and below the PRB

and a laboratory study that evaluated the susceptibility of selected populations is

described in Zielinski et al. (in preparation [a]). In summary, groups of 60 worms each

were taken from ten populations of T. tubifex collected from the Deschutes River Basin.

Populations from sites below the PRB were collected from areas with close downstream

proximity to anadromous fish spawning locations. Populations from sites above the PRB

were from locations where anadromous fish could potentially spawn. Lineage was

determined for each worm by performing a genetic assay (Sturmbauer et al. 1999;

Beauchamp et al. 2002). Lineage III and VI worms were found both above and below the

PRB. Out of ten populations, four were pure lineage III and one was pure lineage VI; the

remainder were a mixture, dominated by lineage III (Table 1).


24


Table 1. Genetic lineage composition of 60 worm subsamples from 10 Deschutes River basin sites. The dominant lineage is listed first.

Subsample

Lineages/Percentage

Warm Springs River III (60%) VI (40%) Shitike Creek III (80%) VI (20%) Oak Springs Hatchery VI (100%) Trout Creek III (100%) Lake Billy Chinook (Metolius River arm) III (100%) Lake Billy Chinook (Crooked River arm) III (60%) VI (40%) McKay Creek III (60%) VI (40%) Crooked River III (60%) VI (40%) Whychus Creek III (100%) Wizard Falls Hatchery #1 III (100%)

Tubifex worms from nine populations were further tested in controlled laboratory

challenges to determine the susceptibility of the population to infection by M. cerebralis

and their ability to produce the triactinomyxon actinospore stage infective to fish.

Consistent with other studies, populations containing all lineage III worms were more

susceptible and produced greater numbers of triactinomyxons when compared to samples

containing at least one resistant lineage (Baxa et al. 2008; Arsan et al. 2007; Hallett et al.

2008). Triactinomyxons were detected in all exposed groups except those from the

Deschutes River near Oaks Springs Hatchery, which were all lineage VI. Compared with

populations that were pure lineage III, overall population susceptibility and

triactinomyxon production decreased as the proportion of lineage VI worms increased

(Warm Springs and Crooked rivers and the Shitike and McKay creek populations).

Among populations of lineage III T. tubifex, there was variability in triactinomyxons

production, with the population from Trout Creek producing 10 times more

triactinomyxons than other lineage III populations (Figure 3). Interestingly, Trout Creek

is the only location in which establishment of the M. cerebralis life cycle has been

demonstrated by microscopic observation of characteristic myxospores.


25


TriaTriactinomyxon Number ctinomyxon Release Per Susceptible Worm

0 50

100 150 200 250 300 350 400 450

Wizard Falls

Hatchery #1

McKay Creek

Crooked River

TroutCreek

WizardFalls

Hatchery#2

ShitikeCreek

WarmSprings

River

OakSprings

Hatchery

LB Metolius

River LB

CrookedRiver

Sample

Figure 3. Average total triactinomyxon release per susceptible worm per population. LB = Lake Billy Chinook. Error bars represent one standard deviation.

In conclusion, only lineage III worms were found susceptible to the parasite and capable

of producing triactinomyxons. Populations containing a mixture of lineages produced the

fewest numbers of triactinomyxons and had the lowest infection rates.

Salmonid Host Susceptibility

Relative susceptibilities of selected Deschutes River salmonid species to experimentally

induced infection by M. cerebralis were determined by Sollid et al. (2002). They

demonstrated that indigenous salmonids from both above and below the PRB are

susceptible to infection. Of these, redband (rainbow) trout were most susceptible, with

steelhead following close behind in overall susceptibility. Kokanee were susceptible to

infection but did not show any outward clinical signs, and Chinook salmon were the least

susceptible indigenous anadromous salmonid species. However, susceptibility for most

species is age-dependent and it was shown subsequently that Chinook salmon are

susceptible until approximately three weeks post-hatch, after which they develop

resistance to clinical disease (Sollid et al. 2003). In contrast, rainbow trout remain highly

susceptible until approximately seven weeks post-hatch, after which time they become

resistant to development of clinical disease, although still susceptible to infection (Ryce

et al. 2004; Sollid et al. 2003). Bull trout are reported to be highly resistant to infection

by M. cerebralis (Hedrick et al. 1999), but they do become infected, and infection


26


prevalence increases with cumulative exposure (Bartholomew et al. 2003). Thus,

although bull trout are at low risk for the effects of disease, they do present a risk for

further disseminating the parasite. Mountain whitefish (Prosopium williamsoni) have

been documented to be susceptible to infection in both laboratory experiments and field

surveys (Gelwick et al. 2000; MacConnell et al. 2002; Schisler et al. 2008).

In conclusion, susceptible salmonid hosts exist in all waters of the Deschutes River Basin

but the degree of susceptibility is dependent on the species, age and exposure dosage.

The three Deschutes River Basin salmonid species of most concern for the effects of M.

cerebralis infection are redband trout, steelhead, and mountain whitefish, all of which are

highly susceptible during early life stages to even low concentrations of triactinomyxons.

Rainbow trout and steelhead remain highly susceptible up to 7 weeks of age unlike

Chinook salmon which develop resistance at 3 weeks post hatch. Bull trout, kokanee and

Chinook salmon are the least susceptible to the effects of M. cerebralis.

Spatial, Temporal Overlap of Hosts

For the parasite to establish after introduction of myxospores, obligatory spatial overlap

of myxospores and T. tubifex, with subsequent spatial overlap of salmonids and

triactinomyxons would be required (Arsan et al. 2007). This part of the exposure

assessment is focused on the distribution and relative abundances of T. tubifex and the

distribution of susceptible early life stages of salmonid species present in waters both

above and below the PRB.

Distribution of Tubifex tubifex

To determine the distribution of T. tubifex in the Deschutes River Basin, sediment

samples from 63 locations below and above the PRB were collected and examined for T

tubifex from 1999-2007 (Figures 4 and 5). Samples sites were selected based on their

close proximity to anadromous fish spawning sites (below PRB) or potential future

spawning locations (above PRB). Results showed that of the 63 collection locations, 20

had the confirmed presence of T. tubifex. The sample locations varied in substrate

composition from sand/cobble to fine organic matter/clay. Overall, most sites with the


27


confirmed presence of T. tubifex were characterized by slow moving water or areas with

protection from fast flowing currents and a sediment make-up of fine organic matter/clay

where spawning habitat for anadromous fish is limited. These types of areas are where

myxospores would most likely settle as they are dispersed passively by water currents

(Kerans and Zale 2002).

N

2130

45

15

28

14

18

Figure 4. Map of lower Deschutes River subbasin with locations of grey triangles and squares representing oligochaete sampling locations: triangles represent sites with a confirmed presence of T. tubifex whereas squares represent sites with no confirmed T. tubifex. Dams are depicted by grey rectangles. Areas within creeks and rivers containing both, T. tubifex habitat and yearly water temperatures above 10°C for at least three consecutive months are outlined with grey. Numbers indicate river kilometers.


28


N

Figure 4.2. Map of upper Deschutes River basin with locations of grey triangles and squares representing oligochaete sampling locations: triangles represent sites with a confirmed presence of T. tubifex whereas squares represent sites with no confirmed T. tubifex. Dams are depicted by grey rectangles. Areas within creeks and rivers containing both, T. tubifex habitat and yearly water temperatures above 10°C for at least three consecutive months are colored grey. Numbers indicate river kilometers.

Figure 5. Map of upper Deschutes River subbasin with locations of grey triangles and squares representing oligochaete sampling locations: triangles represent sites with a confirmed presence of T. tubifex whereas squares represent sites with no confirmed T. tubifex. Dams are depicted by grey rectangles. Areas within creeks and rivers containing both, T. tubifex habitat and yearly water temperatures above 10°C for at least three consecutive months are colored grey. Numbers indicate river kilometers.

7.1

12

15

28 9.4

1 0

1 8

1 4

24

1.8

51

10

39.5 3

3

5

166 38

123


29


Relative Abundances of Tubifex tubifex

The relative abundance of T. tubifex in samples from 12 aquatic worm samples collected

from sites above and below the PRB varied from 0 to 95% (Figure 6). The sites with 0%

relative abundance included Bake Oven and Buck Hollow creeks where no oligochaetes

were found in sediment samples. At most sites, T. tubifex made up less than 2% of the

overall oligochaete population (Figure 6) and these generally consisted of mixed

populations of lineages III and VI. Samples containing the highest percentages of T.

tubifex were from Trout Creek, tributary to the lower Deschutes River, and Wizard Falls

Hatchery on the Metolius River upstream of PRB (95% and 85% respectively). The high

percent sites were all composed of lineage III worms (see Susceptibility of Oligochaete

host above).

Percent Tubifex tubifex From Each Population Sample

1 1 0 0 1.6

95

2 2 2.5

20

1.6

85

0102030405060708090

100

WarmSpringsRiver

ShitikeCreek

BuckHollowCreek

BackOvenCreek

OakSprings

Hatchery

TroutCreek

LBMetolius

River

LBCrooked

River

McKayCreek

CrookedRiver

WychusCreek

WizardFalls

Hatchery

Sample

Perc

ent R

elat

ive

Abu

ndan

ce

Figure 6. Percent relative abundance of Tubifex tubifex in each oligochaete sample. LB = Lake Billy Chinook. Sites are listed in order of lower and upper river section with Warm Springs - Trout Creek representing lower river sites and LB Metolius - Wizard Falls Hatchery representing upper river sites.

Present Distribution of Salmonid Host Early Life Stages

Early life stages of redband trout are present predominantly in areas dominated by

groundwater hydrology such as the Metolius River and its tributaries, the lower and

middle section of the Crooked River, and the Deschutes River up to Big Falls (Cramer


30


and Beamesderfer 2006). Redband trout fry emerge during the months of May through

September and fry typically rear close to their natal habitat.

Summer steelhead spawn in the lower main stem river and its tributaries, with a majority

being produced in the eastern tributaries (Lichatowich et al. 1998). Summer steelhead fry

emerge from March through May. The summer steelhead fry of eastern tributaries

migrate from their natal streams to the main stem Deschutes River or to cooler

headwaters soon after emerging due to low, warm water but fry from cooler west-side

tributaries rear close to spawning locations.

Spring Chinook salmon spawn and rear in the lower part of Shitike Creek (Fritsch 1995)

and the upper half of the Warm Springs River and its tributaries (Lindsay et al. 1989).

The majority of spring Chinook salmon fry emerge from January through April and a

portion migrates downstream in the fall and the remainder the next spring as yearling

smolts. Fall Chinook salmon spawn and rear throughout the lower main stem river below

the PRB with most fry emerging between February and April and moving to the ocean

their first year of life (Lichatowich et al. 1998).

Kokanee salmon fry emerge from January to early May in the upper Deschutes River up

to Steelhead Falls (Rkm 203), the lower 1.2 Rkm of the Crooked River, and the Metolius

River and its tributaries (Thiesfeld et al. 1999). Most fry emigrate downstream to Lake

Billy Chinook within days of emerging from the gravel (D. Ratliff, senior PGE biologist,

personal communication).

Bull trout spawn and fry initially rear in the Metolius River and its tributaries, the Warm

Springs River, and Shitike Creek. Subadults rear in Lake Billy Chinook and the lower

main stem of the Deschutes River (Lichatowich et al. 1998). Fry emerge from December

through February. Juveniles (age 0 - 3) disperse from their rearing sites all months of the

year with an apparent peak migration out of Metolius tributaries during May and June

(Ratliff et al. 1996).


31


Little is known about the life history of Deschutes mountain whitefish but young are

found throughout the Deschutes River Basin. In the Deschutes River Basin mountain

whitefish may form three distinct life-history patterns that have been documented in other

river systems, these include: a lacustrine pattern where the life cycle is completed entirely

within a lake, a riverine pattern where the life cycle is completed entirely within flowing

water, and an adfluvial pattern where the life cycle involves migrations between lakes

and rivers (Thompson 1974). Mountain whitefish spawn in the late fall or early winter

when water temperatures are between 0 to 10°C. The fry emerge from late February to

April and the newly-emerged fry may drift some distance downstream before moving

into shallow, low velocity areas along the river margins (McPhail and Troffe 1998).

In conclusion: the majority of young salmonids in the Deschutes River Basin emerge

January to May, coinciding with the period when water temperatures are not conducive

for parasite proliferation (see Favorable Environment section below). One exception is

redband trout fry, which can emerge later when water temperatures are conductive for

parasite proliferation.

Future Distribution of Anadromous Fish in the Upper Deschutes River Basin

Two species of anadromous fish are being considered for reintroduction purposes in

waters above the PRB: summer steelhead and spring Chinook salmon. Both species were

historically present in the upper Deschutes River Basin (Nehlsen 1995). Steelhead will

be introduced into Whychus Creek, and into the Crooked River Basin in Ochoco and

McKay creeks, and the mainstem Crooked River. Spring Chinook salmon will be

introduced into Whychus Creek, in the upper Deschutes system, the mainstem Metolius

River and its tributary Lake Creek, and into the mainstem Crooked River.

Favorable Environment

Abiotic environmental conditions, in particular water temperature, sediment type, and

flow rate influence parasite success either directly or through their influence on host


32


ecology (Bartholomew et al. 2005). The focus of this section will be on two categories:

habitat for T. tubifex and favorable water temperatures for parasitic proliferation and

infection.

Habitat for Tubifex tubifex

Habitat for T. tubifex is defined as areas with fine sediment, low flow, and organic matter

(Brinkhurst and Gelder 1991). These areas are found sporadically distributed throughout

the Deschutes River Basin with the majority found in the lower sections of most

tributaries. Some however, are found in the upper reaches of the Deschutes River above

its confluence with Lake Billy Chinook (Figures 4 and 5). It is important to note, not all

habitat in the Deschutes River Basin with fine sediment, low flow, and organic matter

contains populations of T. tubifex, as documented in our worm survey study (Zielinski et

al. in preparation [a]). However, T. tubifex was more often found in this type of habitat

than in areas with fast flowing water, course sediment and little or no organic matter. In

most samples that contained T. tubifex the relative abundances were extremely low even

though these locations appeared to contain optimum habitat (Figure 6).

Water Temperatures

Parasite development in both hosts is influenced by water temperature (Halliday 1973;

El-Matbouli et al. 1999; Hedrick and El-Matbouli 2002; Blazer et al. 2003; Kerans et al.

2005). In T. tubifex, water temperatures below 10°C significantly delay the development

of triactinomyxons and those above 20°C disrupt the parasite’s development (El-

Matbouli et al. 1999). The optimum water temperature for development and release of

triactinomyxons has been demonstrated to be 10 to 15°C (El-Matbouli et al. 1999).

Highest prevalence of infection in fish occurs when average daily water temperatures are

between 11-14°C (Baldwin et al. 2000; Granath and Gilbert 2002). Severities of

infection in sentinel rainbow trout exposed in the Madison River, Montana, were highest

at water temperatures between 12.8 and 15.8˚C (MacConnell and Vincent 2002) and

water temperature was correlated positively to the severity of infection in sentinel trout

exposed in Willow Creek, Montana (Baldwin et al. 2000), and three drainages in Idaho

(Hiner and Moffitt 2002).


33


Water temperature also affects prevalence of the parasite in T. tubifex. A laboratory

study completed by Kerans and Zale (2002) in which populations of T. tubifex were

infected at the same rate then held at different water temperatures demonstrated the

following infection percentages: 22.2% at 17˚C, 14.6% at 13˚C, and 11.4% at 9˚C with

no infected worms at 20˚C. In the Metolius River and its tributaries where water

temperatures are well below 10˚C for 11 months of the year (USGS 2006) parasite

development rates in T. tubifex and total numbers of triactinomyxons released would be

decreased by the sub-optimal water temperatures. The areas at highest risk for having

extended periods of 3 months (minimum amount of time for parasitic development in

either host) or more of optimum water temperatures between 10-15˚C are the Warm

Springs, Crooked and Deschutes Rivers and Trout, Buck Hollow, Bake Oven, and Shitike

Creeks (USGS 2006). Some of these areas also have seasonally high water

temperatures above 20˚C for extended periods; these include the lower sections of Warm

Springs River, Trout, Buck Hollow and Bake Oven creeks. In conclusion, almost all

streams in the Deschutes River Basin, except for the Metolius River and it tributaries,

have extended period of water temperatures favorable for parasite proliferation and

infection (Figures 4 and 5).

3. CONSEQUENCE ASSESSMENT

If introduced, conditions are appropriate for the parasite to complete its life cycle in both

the upper and lower basin. However, not all areas within the Deschutes River Basin have

an equal probability for establishing M. cerebralis and most contain abiotic and biotic

factors that will help impede establishment and/or the effects of the parasite.

• The majority of areas sampled did not have a confirmed presence of T. tubifex.

Discontinuous T. tubifex occurrence has the potential of limiting spatial and

temporal overlap of both hosts and spore stages.

• In most samples that contained T. tubifex, the relative abundances were extremely

low even though these locations appeared to contain optimum habitat.


34


• Optimal habitats for T. tubifex appear to be relatively rare in the Deschutes River

Basin. Optimum habitat appears to be confined to small sections of the lower end

of most tributaries and the upper main stem Deschutes River.

• The presence of resistant T. tubifex lineage VI may help mitigate the effects of M.

cerebralis.

• The majority of young salmonids in the Deschutes River Basin emerge and rear

during their most susceptible life stages when water temperatures are not

conducive for parasite proliferation in either host with the exception of redband

trout.

• High summer water temperatures in some tributaries are not conductive to

parasite survival, especially when reaching 20˚C or higher. Other tributaries run

year around with exceptionally cool water (below 10˚C), which is not conductive

to parasite proliferation.

All of the above factors will come into play if introduction does occur in the upper

Deschutes River Basin. They will severely limit the establishment of M. cerebralis along

with its impact on resident and reintroduced salmonids. The lower river system is a

good model for what would occur in the upper system as both share the factors listed

above. Even with 20 plus years of introduction via stray anadromous fish, M. cerebralis

has had severely limited establishment along with no evidence of impact on resident

salmonids in the lower Deschutes River Basin. Infection in resident salmonids has never

been detected even in locations where sentinel fish have become infected. The same

could be true for the upper Deschutes River Basin, if introduction of M. cerebralis occurs

there.

CONCLUSIONS AND RISK MANAGEMENT

The current low overall relative abundances and sporadic distribution of T. tubifex within

the Deschutes River Basin is a vital characteristic of the system that would limit

establishment of and the disease caused by M. cerebralis. Adding to the natural

resistance of the system is the presence of lineage VI worms, which are completely


35


resistant to M. cerebralis and may further mitigate the effects of M. cerebralis by

deactivating the myxospore stage. Beauchamp et al. (2006) demonstrated the ability of

resistant T. tubifex to actively impair the infectivity of M. cerebralis myxospores by some

mechanism sufficient to result in a 70% overall reduction in triactinomyxon production

compared to parasite-exposed pure cultures of susceptible oligochaetes. This may

explain why the numbers of triactinomyxons produced decreased substantially with

increased percentages of lineage VI worms in our worm susceptibility study.

The highest risk for introducing M. cerebralis into the upper Deschutes River Basin is by

indiscriminate movement of adult salmonids for reintroduction purposes. Lower risks are

associated with moving Deschutes River hatchery fish, eyed eggs, and fry raised in

pathogen free water and the movement of marked returning adults. Risks associated with

other pathways of introduction into the upper Deschutes River Basin could be

significantly reduced with education of the public regarding certified sources of fish for

stocking of private ponds, types of crayfish bait that should not be used, cleaning of

angler equipment and the proper dumping of bilge or engine coolant water.

Any increase in the level of establishment in the lower Deschutes River Basin would

raise the risk of not only birds as a vector but also anglers, boats and the passage of adult

salmonids over the PRB. For this reason, future monitoring should involve sampling of

resident salmonids and the use of sentinel fish. Detection of the parasite in these fish

should not only be done with traditional methods such as PTD and diagnosis by

microscopic observation of the characteristic myxospores, but also with highly sensitive

genetic techniques such as PCR. The reason for this is that traditional methods can miss

low-level or early infections and thus underestimate establishment as was observed

during out sentinel fish studies.

As outlined in the introduction, three questions were of particular interest in regards to M.

cerebralis, the reintroduction of salmonids, and the Fish Health Management Program.

The questions and answers are as follows:


36


Question 1: Can the life cycle of M. cerebralis be completed in the Deschutes River

Basin?

Answer: Yes, It can be completed in both the lower and upper Deschutes River

subbasins.

Question 2: If it can be completed, how virulent is whirling disease to stocks upstream

of the PRB?

Answer: All resident salmonids show varying degrees of susceptibility with redband

trout being the most susceptible. All have varying levels of age-dependent resistance,

with redband trout and steelhead having the slowest development of resistance.

However, they can all become infected at any age if the parasite exposure is great

enough. If establishment occurred, salmonids would be exposed to low levels of the

parasite as predicted in the establishment assessment. A low-level establishment would

not likely result in population effects on resident salmonids species.

Question 3: If M. cerebralis becomes established in the upper Deschutes River

Subbasin, is it likely to cause population losses in important salmonid populations?

Would these losses threaten survival and sustainability?

Answer: Establishment of M. cerebralis upstream of the PRB is not likely to cause

population losses in important salmonid populations including kokanee, redband and bull

trout and mountain whitefish. Establishment would most likely be similar to that of the

lower river system with no substantial impact on resident salmonids. Under present

conditions it is unlikely that significant losses would occur. Whirling disease under

present conditions would not threaten survival and sustainability of important salmonid

populations.

In the future, any changes to physical conditions within the Deschutes River Basin, may

alter the probability of parasite establishment and proliferation. High densities of T.

tubifex are often associated with dominance of fine sediments, nutrient enrichment, and

human disturbance (Brinkhurst and Kennedy 1965; Aston 1973; Kaster 1980; Thorpe and

Covich 1991; Sauter and Gude 1996; Lazim and Learner 1987; Robbins et al. 1989;


37


Modin 1998; Zendt and Bergersen 2000). Areas subject to these conditions will need to

be monitored for increases in worm densities and possible establishment of M. cerebralis.

ACKNOWLEDGMENTS

We would also like to thank Don Stevens, who worked many long hours collecting field

data for this project. We would like to thank the countless Oregon Department of Fish

and Wildlife personnel that helped answer our questions and collect fish samples. Thank

you to the biologists at Portland General Electric and The Confederated Tribes of Warm

Springs, Oregon. Finally, a special thank you to Teena Abbott for her willingness and

determination to voluntarily work hard long days collecting field data, her effort was

greatly appreciated. Portland General Electric and the Confederated Tribes of the Warm

Springs Reservation of Oregon, Co-licensees of the Pelton Round Butte Hydro Project,

funded the monitoring and research involved in this Risk Assessment. We would also

like to thank Antonio Amandi, Richard Holt, Craig Banner, Rick Stocking and Don

Ratliff for their contributions to study design and their constructive criticism of this

report.


38


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