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Assessing the Feasibility of Native Fish Reintroductions: 

A Framework and Example Applied to Bull Trout in the 

Clackamas River, Oregon

Jason Dunham, Kirsten Gallo

Open-File Report 2008-1007

U.S. Department of the Interior

U.S. Geological Survey
ii Report Title

U.S. Department of the Interior

Dirk Kempthorne, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Dunham, J., Gallo, K., 2008, Assessing the Feasibility of Native Fish Reintroductions: A Framework and Example 

Applied to Bull Trout in the Clackamas River, Oregon: Reston, Virginia, U.G. Geological Survey, Open-File Report 2008-

1007, p. 23.

In fulfillment of requirements for Technical Assistance Agreement No: 13420-06-H628
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Contents

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

Keywords................................................................................................................................................1

Introduction.....................................................................................................................................................1

Methods...........................................................................................................................................................2

Framework for assessing the feasibility of a reintroduction.........................................................2

Assessment of the recipient habitat for a possible reintroduction..............................................2

Was the recipient habitat likely historically occupied by a self-sustaining 

population of bull trout? .......................................................................................2

Is it unlikely that bull trout are present now in the recipient habitat? .......................2

Is the recipient habitat currently suitable for supporting spawning and early 

rearing of at least one or more self-sustaining local populations of bull 

trout? .......................................................................................................................3

Have past, present, and potential future threats in the recipient habitat been 

mitigated sufficiently to justify a reintroduction? ............................................4

Is natural recolonization of the recipient habitat unlikely in the short term?...........4

Assessment of donor populations to be used for a possible reintroduction..............................4

System for evaluating the evidence ..................................................................................................5

Applying the Framework to the Clackamas River Basin .........................................................................6

Study system .........................................................................................................................................6

Assessment of the recipient habitat..................................................................................................8

Evidence supporting the potential feasibility of the Clackamas River for a bull 

trout reintroduction ..............................................................................................8

Was the recipient habitat likely historically occupied by a self-sustaining 

population of bull trout? .......................................................................................8

Is it unlikely that bull trout are present now in the recipient habitat? .......................8

Is the recipient habitat currently suitable for supporting spawning and early 

rearing of at least one or more self-sustaining local populations of bull 

trout? .......................................................................................................................9

Have past, present, and potential future threats in the recipient habitat been 

mitigated sufficiently to justify a reintroduction? ..........................................9

Is natural recolonization of the recipient habitat unlikely in the short term?.........10

Assessment of donor populations ...................................................................................................10

Is there at least one available donor population that is a sufficient evolutionary 

match to the recipient? ......................................................................................10

Within this pool of potential donor populations is there at least one donor that 

could provide a sufficient number of propagules?........................................10

Results of the Clackamas River application...................................................................................11

Conclusions...................................................................................................................................................11

Acknowledgements.....................................................................................................................................11

References....................................................................................................................................................12
iv Report Title

Tables

Table 1. An example of scoring for a particular question within a component of the reintroduction feasibility assessment..........................................................................................................5

Table 2. Overview of the system of scores used to assess the feasibility of a bull trout reintroduction............................................................................................................................................6

Figures

Figure 1. Hierarchical series of evaluations used to assess the feasibility of a bull trout reintroduction............................................................................................................................................3

Figure 2. Current distribution of bull trout within the Willamette River recovery unit.......................7

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

°F=(1.8×°C)+32

Multiply By To obtain

centimeter (cm) 0.3937 inch (in.)

kilometer (km) 0.6214 mile (mi)

meter (m) 3.281 foot (ft)

Conversion Factors
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Assessing the Feasibility of Native Fish Reintroductions: 

A Framework and Example Applied to Bull Trout in the 

Clackamas River, Oregon

Jason Dunham1, Kirsten Gallo2

Abstract

In a species conservation context, translocations can 

be an important tool, but they frequently fail to successfully 

establish new populations. We consider the case of reintroductions for bull trout (Salvelinus confluentus), a federally-listed 

threatened species with a widespread but declining distribution in western North America. Our specific objectives in this 

work were to: 1) develop a general framework for assessing 

the feasibility of reintroduction for bull trout, 2) provide a 

detailed example of implementing this framework to assess the 

feasibility of reintroducing bull trout in the Clackamas River, 

Oregon, and 3) discuss the implications of this effort in the 

more general context of fish reintroductions as a conservation 

tool. Review of several case histories and our assessment of 

the Clackamas River suggest that an attempt to reintroduce 

bull trout could be successful, assuming adequate resources 

are committed to the subsequent stages of implementation, 

monitoring, and evaluation.

Keywords 

Reintroductions, translocations, decision support, decision 

framework, bull trout, Salvelinus confluentus

Introduction

Translocations are a common activity in species conservation (Griffith et al. 1989, Seddon et al. 2007). Among 

vertebrates, published studies of translocations for conservation are best represented by cases involving mammals and 

birds, whereas fishes are strongly underrepresented (Fischer 

and Lindenmayer 2000, Seddon et al. 2005, 2007). This 

relative dearth of documentation for translocations of fishes 

stands in contrast to the large number of actual translocations 

that have been attempted (e.g., Welcomme 1988, Lever 1996), 

including >400 translocations in efforts to conserve a single 

species (Gila topminnow Poeciliopsis occidentalis, Hendricksen and Brooks 2001, Sheller et al. 2006). In a species 

conservation context, translocations can be an important tool, 

but the efficacy of translocations is often in question because 

they frequently fail to successfully establish new populations 

(Minckley 1995). Here we focus on the problem of translocation of fish from a wild source to re-establish populations in 

formerly occupied habitat, namely “reintroduction.” 

There are readily available guidelines for translocations 

of fishes (e.g., Williams et al. 1988, Minckley 1995, IUCN 

1998), which identify several steps in species reintroductions 

including: 1) an initial assessment of the feasibility of such an 

effort, 2) actual implementation if an effort is deemed feasible, 

and 3) monitoring and evaluation to determine if the reintroduction was successful or to determine the reasons for failure. 

Most of the existing peer-reviewed literature on fishes has 

retrospectively evaluated the success of reintroductions from 

genetic (e.g., Stockwell et al. 1996, Stockwell and Leberg) or 

ecological perspectives (Harig and Fausch 2002, Shute et al.

2005, Sheller et al. 2006). In contrast, examples of feasibility assessments for specific reintroductions are still lacking in 

the literature on fish or for any species (Seddon et al. 2007). 

Feasibility assessments can be a critical step, and the general 

lack of success of reintroductions cited for many fishes could 

be attributed to inadequate feasibility assessments or a general 

lack of understanding of factors likely to contribute to the success of reintroductions (Minckley 1995).

Here we consider the case of reintroductions for bull trout 

(Salvelinus confluentus), a federally-listed threatened species 

with a widespread, but declining distribution in western North 

America (USFWS 2002). A major reason for declines in this 

species is habitat loss and fragmentation, but many potentially 

suitable unoccupied habitats may exist (e.g., Dunham and 

Rieman 1999). Given this possibility, the question of reintroductions has arisen in several situations (e.g., Epifanio et al.

2003), and reintroductions of bull trout have been attempted in 

at least two cases, including the McCloud River in California 

(Buchanan et al. 1997) and Middle Fork Willamette River in 

Oregon (M. Wade, Oregon Department of Fish and Wildlife, 

1

U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, 3200 SW Jefferson Way, Corvallis, OR 97331; jdunham@usgs.gov 

2

U.S. Forest Service, Pacific Northwest Region, 4077 Research Way, Corvallis, OR 97333
 Report Title

pers. comm.). The former case did not result in successful 

establishment of bull trout, and the latter effort is still in progress. Our specific objectives in this work were to: 1) develop a 

general framework for assessing the feasibility of reintroduction for bull trout, 2) provide a detailed example of implementing this framework to assess the feasibility of reintroducing 

bull trout in the Clackamas River, Oregon, and 3) discuss the 

implications of this effort in the more general context of fish 

reintroductions as a conservation tool. 

Methods

Framework for assessing the feasibility of a 

reintroduction

We began the assessment by consulting existing guidelines (Williams et al. 1988, Minckely 1995, IUCN 1998, 

Epifanio et al. 2003) to develop a general framework we could 

apply to assess the feasibility of a bull trout reintroduction. 

Assuming that a particular recipient habitat has been identified (e.g., priorities among alternative recipient habitats have 

been considered), we identified two major components of the 

feasibility assessment: 

The potential for a given recipient habitat to support a 

reintroduction, and 

The potential of available donor populations to support a 

reintroduction. 

Within each component of the assessment, we developed 

a list of key questions to address. For recipient habitats, questions included the following: 

Was the recipient habitat likely historically occupied by a 

self-sustaining population of bull trout?

Is it unlikely that bull trout are present now in the recipient habitat? 

Is the recipient habitat currently suitable for supporting 

spawning and early rearing of at least one or more selfsustaining local populations of bull trout?

Have past, present, and potential future threats in the 

recipient habitat been mitigated sufficiently to justify a 

reintroduction?

Is natural recolonization of the recipient habitat unlikely 

in the short term?

To address the second major component of the assessment involving potential donor populations of bull trout to be 

used in a reintroduction, we identified two key questions: 

Is there at least one available donor population that is a 

sufficient evolutionary match to the recipient?

1.

2.

1.

2.

3.

4.

5.

1.

Within this pool of potential donor populations is there at 

least one donor that could provide a sufficient number of 

propagules without damaging the donor population itself? 

The overall assessment was structured as a hierarchical series of evaluations (Figure 1). For each question listed 

above, we identified major types of evidence needed to answer 

them. Each major type of evidence was evaluated with available information. 

Assessment of the recipient habitat for a 

possible reintroduction

Was the recipient habitat likely historically occupied by a 

self-sustaining population of bull trout? 

To address this question in the Clackamas effort, we 

considered three types of evidence: 1) historical and current 

evidence for presence of any life stage, 2) the current presence of specific life stages, and 3) the likelihood of presence 

inferred from currently suitable habitat. In this study, the presence of any life stage of bull trout was evaluated by considering several sources of information: the presence of an archived 

specimen in a scientific collection, written documentation by a 

professional biologist, verbal accounts by a professional biologist, and anecdotal accounts. The number of accounts was also 

taken into consideration in the case of verbal and anecdotal 

accounts. Presence of specific life stages was considered in 

terms of documented spawning by adults or presence of small 

individual bull trout (<150 mm, FL), providing evidence of 

local reproduction, or presence of larger fish only (>150 mm) 

which do not as reliably indicate local reproduction. This is 

because bull trout can move extensive distances through river 

networks (>100 km, Swanberg 1997, Baxter 2000) and can be 

found in a broad diversity of habitats (Muhlfield and Marotz 

2005, Monnot et al., in press). To account for cases in which 

direct historical observations of bull trout are lacking, we also 

considered indirect evidence for habitats supporting bull trout, 

such as a habitat model to predict occurrence (Peterson and 

Dunham 2003) or simple comparisons of habitat conditions in 

a potential recipient habitat to those already supporting extant 

bull trout populations. 

Is it unlikely that bull trout are present now in the 

recipient habitat? 

A reintroduction effort cannot proceed unless it is reasonably certain that the species in question is not present. We 

considered several sources of information for evaluating this 

possibility. The currently available protocol for estimating 

the probability of presence for small (<150 mm) bull trout 

(Peterson et al. 2002) was considered to provide the best information. The probability (p) of presence determined by using 

this protocol or a statistical equivalent (Peterson and Dunham 

2003, Mackenzie et al. 2006) was deemed unlikely if p ≤ 0.10. 

2.
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Other sources of information included efforts to detect bull 

trout in targeted surveys using other protocols, efforts to detect 

bull trout during sampling or collection efforts targeting other 

salmonids, and other professional fisheries activities that could 

document occurrence of bull trout.

Is the recipient habitat currently suitable for supporting 

spawning and early rearing of at least one or more selfsustaining local populations of bull trout? 

In addressing this question we considered criteria indicating both the quality and quantity of potentially suitable 

habitat. Habitat quality for bull trout, at least in terms of the 

distribution of habitats used for spawning and early rearing in the field, is primarily a function of water temperature 

(McPhail and Murray 1979, Dunham et al. 2003a). Therefore, 

we binned habitats into broad categories representing different 

levels of thermal suitability for spawning and early rearing. 

Habitats with water temperatures <9 o

C during the spawning 

period and <16 oC summer maximum were classified as highly

suitable, whereas those with water temperatures >9 oC during 

the spawning period and <16 oC summer maximum were 

deemed moderately suitable, and those where both temperatures were exceeded were considered unsuitable.

The quantity of thermally suitable habitat is also widely 

recognized as important for persistence of local populations 

of bull trout (Dunham and Rieman 1999) and other closely 

related charrs (Morita and Yamamoto 2002, Koizumi and 

Maekawa 2004). We considered habitat size in terms of the 

area of watersheds or total length of streams supporting 

potentially suitable thermal habitat for bull trout. These areas 

of suitable habitat were designated as “patches” similar to the 

logic of past work (Dunham et al. 2002, Rieman et al. 2007), 

but using measured local water temperatures in place of elevation as a surrogate, since water temperature data were available. If at least one patch supported conditions similar to those

Feasibility

Recipient

habitat

Donor

population

Historical

Presence?

Contemporary

Presence?

Habitat

Suitability? Threats? Recolonization

Potential? Suitability? Threat to

Donor?

Any life stage

Sources of

information

Specific life

stage

Inferred

indirectly

Major Components

Questions

Types of evidence

Figure 1. Diagram depicting the hierarchical series of evaluations used to assess the feasibility of a bull trout 

reintroduction, including the overall assessment of feasibility, its two major components, key questions within 

each component, types of evidence used to evaluate each question (shown only for “Historical Presence?”), and 

information used to evaluate each type of evidence (not shown). Downward arrows from other questions point to 

types of evidence (not shown). Each of these components of the assessment is described completely in the text 

narrative.
 Report Title

sustaining bull trout in nearby watersheds (i.e., similar area or 

length of suitable streams), habitat quantity was scored as suitable, otherwise it was considered unsuitable. 

Have past, present, and potential future threats in the 

recipient habitat been mitigated sufficiently to justify a 

reintroduction? 

As with any species, assessing threats is an uncertain and 

difficult task. To consider threats, we adopted an existing protocol (Master et al. 2003) that was used by the U.S. Fish and 

Wildlife Service in a formal review of the status of bull trout 

(W. Fredenberg and J. Chan, U.S. Fish and Wildlife Service, 

pers. comm.). For the purposes of evaluating threats over a 

limited area (e.g., within a single river basin) we limited our 

assessment to the severity and immediacy of threats and did 

not consider scope as described by Master et al. (2003).

The severity of potential threats was classified into four 

categories: insignificant, low, moderate, and high. The severity 

of threats was classified as “insignificant” if the following 

conditions were satisfied: essentially no reduction of population or degradation of habitat or ecological community due to 

threats, or populations, habitats, or ecological communities 

able to recover quickly (within 10 years) from minor temporary loss. The severity of threats was considered to be “low” 

if the following were deemed likely: reductions of species 

populations or reversible degradation or reductions of habitat 

or ecological community in area affected were likely, with 

habitat or population recovery expected in 10-50 years. The 

severity of threats was considered “moderate” if the following 

were deemed likely: a major reduction of species population 

or long-term degradation or reduction of habitat or ecological community in area affected, requiring 50-100 years for 

recovery. The severity of threats was considered to be “high” 

if the following was deemed likely: loss of species population 

(all individuals) or destruction of species habitat or ecological 

community in area affected, with effects essentially irreversible or requiring long-term recovery (>100 years).

The immediacy of threats was considered “insignificant” 

if threats were not likely to be operational within 20 years, 

“low” if threats were likely to be operational within 5-20 

years, “unknown” if little available information exists to evaluate the immediacy of threat effect, “moderate” if threats were 

likely to be operational within 2-5 years, and “high” if threat 

effects were operational (happening now) or imminent (within 

a year). Our characterization of threats was admittedly simple 

and subjective, but no other method was readily available to 

meet our needs.

Is natural recolonization of the recipient habitat unlikely in 

the short term? 

The ability of bull trout to move extensively through 

stream networks raises the possibility of natural recolonization as an important mechanism in maintaining habitat 

occupancy (Rieman and McIntyre 1993, Dunham and Rieman 1999, Whiteley et al. 2007). If natural recolonization is 

likely within a given habitat, then a reintroduction effort may 

not be warranted, or may even be detrimental if interbreeding 

between naturally isolated populations is promoted. To determine whether recolonization was likely in the short term, we 

addressed four types of evidence: 1) influence of fish movement barriers, 2) distance to habitats currently occupied by 

bull trout, 3) abundance of bull trout in adjacent habitats, and 

4) migratory life history of bull trout in adjacent habitats. 

In the first of these, fish movement barriers were classified into four broad categories: 1) complete barriers resulting 

from blockage of both up- and downstream movement, 2) limited connectivity resulting from up- or downstream restrictions 

on movement (e.g., limited downstream passage via entrainment through dams) or from severely degraded migratory corridors (e.g., limited flows, degraded water quality), 3) partial 

connectivity as related to two-way fish passage structures or 

human-assisted transport of fish over barriers or moderately 

degraded migratory corridors, and 4) complete up- and downstream connectivity (no substantial barriers present). 

Distance is an important and distinct component of connectivity. We considered distance at three arbitrary levels (<20 

km, >20-100km, >100 km), based on distances that bull trout 

are found to move through stream networks (e.g., Swanberg 

1997, Baxter 2000). We did not have information to consider 

how natal homing may condition the influence of distance, 

nor did we attempt to account for this. The degree of fidelity of bull trout in terms of migrating back to their habitats of 

natal origin is unknown (Whiteley et al. 2007). In addition to 

distance, the number of individuals should also influence the 

probability of recolonization. For lack of reliable information, 

we used three subjectively determined levels of abundance 

(>500 adults, 100-500 adults, <100 adults) representing the 

abundance of bull trout in adjacent occupied habitats that 

could potentially recolonize the recipient habitat. A fourth 

possible influence is development of migratory life history, 

with an assumption that adjacent populations having stronger 

representation of migratory life histories should be more likely 

to supply individuals that could recolonize a recipient habitat. 

These were classified into three subjective levels (“strong,” 

“depressed,” and “absent,” Rieman et al. 1997). Overall our 

consideration of recolonization potential was subjective for 

lack of quantitative information on local populations of bull 

trout, but we attempted to consider all of the major factors that 

could influence the potential for recolonization over relatively 

short (20 yr) time frames.

Assessment of donor populations to be used for 

a possible reintroduction

Assessment of donor populations represented a series 

of considerations separate from assessing the suitability of 

a receiving habitat for a reintroduction. We focused on two 

basic questions about donor populations, including: 1) Is 

there at least one available donor population that is a suf-
Heading 1 

ficient evolutionary match to the recipient? and 2) “Within 

this pool of potential donor populations is there at least one 

donor that could provide a sufficient number of propagules 

without damaging the donor population itself?” To address 

the first question, we considered information on how historical populations of bull trout within the Willamette River basin 

may have shared a common evolutionary history with other 

extant populations of bull trout (e.g., Haas and McPhail 1991, 

Taylor et al. 1999, Spruell et al. 2003). We assumed that any 

population within a well-defined lineage could serve as a 

donor, unless there was compelling evidence to suggest that 

local environmental conditions may have selected for local 

population characteristics that were not compatible with the 

receiving environment. To address the second question, we 

evaluated information on numbers of spawning adult bull trout 

in potential donor populations. Our primary concern was to 

avoid adverse impacts to the viability of donor populations 

from removals of individuals for a potential reintroduction. 

To maintain sufficiently large effective sizes of donor populations, we considered only donors supporting greater than 

1000 spawning adults per year (Rieman and Allendorf 2001). 

Potential donors with fewer than 1000 spawning adults per 

year were considered too small for removal of adults or the 

demographic equivalent of adults (e.g., eggs, juveniles) without risking adverse genetic or demographic influences.

System for evaluating the evidence

Questions within each component of the assessment 

(recipient habitat and donor populations) were evaluated 

based on the types of evidence specified above. Scores ranged 

between -1 and 1 to represent our degree of belief in the affirmative (maximum score =1 or “yes”) or negative (minimum 

score = -1 or “no”). A score of zero was used when available 

information provided no basis for an evaluation. In most cases, 

a score of zero should be interpreted as a sign that more information is needed before proceeding further with a feasibility 

assessment. 

Scores for each type of evidence were summed across all 

types (if there was more than one type considered) to produce 

an overall value for each question ranging between -1 

(= “no”) and 1 (= “yes”) to allow an evaluation of each question. In some cases particular scores served as a “trump” value 

(see below). A score equal or close to zero represented cases 

where uncertainty is high, and in some cases, where additional 

information may be needed for an assessment to proceed 

(Appendix). The system of scoring provided a simple and 

transparent summary of the logic for addressing each question 

about the feasibility of a reintroduction. To illustrate, consider 

the question about suitability of the recipient habitat (Table 1). 

For this question we considered two types of evidence (quality and quantity of habitat), each with different information 

indicating different aspects of suitability. A simple average 

was used to combine scores for each question, except in cases 

where quantity of habitat was not sufficient or habitat quality was unsuitable. In these cases, a trump score of -1 was 

assigned overall, rather than an average. Similar scoring was 

used for each question within each of the two components of 

the assessment (Appendix).

Scores for each question were combined to yield an overall combined score for each component that ranged between 1 

Table 1. An example of scoring for a particular question within a component of the reintroduction feasibility assessment (Figure 1). 

For each of the two types of evidence there are several sources of information to be evaluated. The score for each type of evidence 

is summed across all types of evidence to produce an overall score for the question resulting in a value ranging from 1 (answer = 

“yes”) to -1 (answer = “no”). In this case a score of zero should indicate the need to suspend a feasibility assessment and to gather 

the necessary information. Specific descriptions of sources of information indicated here are given in the text.

Question Type of evidence Sources of information Score

Is the recipient habitat Sufficient quality of habitat? Highly suitable for both incubation 1

potentially suitable? and early rearing 

 

 Moderately suitable for egg 0.75

incubation, suitable for rearing 

 

No information 0

Not suitable for egg incubation or -1

 rearing

 Sufficient quantity of Yes 1

 habitat? 

 No information 0

 

Not enough habitat -1
 Report Title

and -1. To develop a final overall score for assessing the feasibility of reintroduction, scores for each component were combined in a similar fashion. Thus, a score near 1 would indicate 

a strong belief in the feasibility of a potential reintroduction, 

and scores near -1 would indicate that such an effort is not 

considered feasible (Table 2). A spreadsheet that combines the 

scores for all components of the feasibility assessment is available from the authors.

It is important to recognize that this system was not created to justify our interpretation of the evidence. Rather, we 

adopted it as a formal and transparent approach for articulating 

our logic in arriving at an overall assessment. In the case of 

the Clackamas River basin, the technical team chose to adopt 

a consensus approach in terms of arriving at an overall score 

for the feasibility assessment. The system is very flexible, 

however, and could be adapted to incorporate uncertainty in 

the form of differences in the interpretation of individuals or 

degrees of belief in the evidence supporting the assessment, 

rather than using discrete scores as applied here. 

Applying the Framework to the 

Clackamas River Basin

Study system 

The Clackamas River is a major tributary of the lower 

Willamette River in western Oregon (Figure 2). Bull trout are 

extirpated from most formerly occupied habitat in Willamette 

River basin. Extant populations are confined to several isolated 

enclaves persisting in the McKenzie River (Buchanan et al.

1997). Up until the 1990s, bull trout were also present in the 

upper Middle Fork of the Willamette River (Buchanan et al.

1997), which is currently the focus of reintroduction efforts 

involving donor populations from the adjacent McKenzie 

River (M. Wade, Oregon Department of Fish and Wildlife, 

pers. comm.). 

The draft recovery plan for bull trout (USFWS 2002) 

identified the Willamette River basin as one of 27 “recovery” 

or “management” units for the species within the conterminous United States. Draft recovery criteria for this unit address 

goals for population distribution, population abundance, population trends, and habitat connectivity (USFWS 2002). In this 

draft, reintroduction of bull trout in the Clackamas River was 

stated specifically as an action that could meet the draft recovery goal of establishing bull trout in a third local population 

in the Willamette River basin. It is important to note here that 

we do not consider how to prioritize among potential recipient habitats for a reintroduction. Some potential guidelines for 

classifying bull trout habitat suitability, which may apply to 

reintroduction efforts, are provided elsewhere (Peterson and 

Dunham 2003)

We considered the feasibility of reintroducing bull trout 

into the headwaters of the Clackamas River and its tributaries 

upstream of North Fork Dam (Shively et al. 2007). Historical observations of bull trout in the Clackamas River include 

this portion of the basin, as well as the Oak Grove Fork and 

Collawash Rivers (Buchanan et al. 1997). At present, habitats 

in the Oak Grove Fork are extensively altered for hydropower 

generation, and restoration of habitat conditions that may 

be suitable for bull trout is unlikely within a short (20 year) 

time frame. Within the Collawash River, summer maximum 

temperatures are generally too warm to support spawning 

and early rearing of bull trout (Shively et al. 2007). For these 

Table 2. Overview of the system of scores used to assess the feasibility of a bull trout reintroduction. Shown are levels 

of the assessment (e.g., Figure 1) and specific actions taken at each to ultimately produce a score corresponding to the 

feasibility of a reintroduction.

Level Action Example 

Type of evidence Determine score based on available information. Evidence for historical presence 

 Consider terminating assessment when key of any life stage

 information is lacking (score = 0).

Key questions Combine scores for each type of evidence. Historical presence of bull trout?

 Includes “trump” values (occurs when a score for 

 a given type of evidence is allowed to “trump” 

 other scores).

Components Use average of scores for each question to produce Recipient habitat

 an overall score for each component.

Feasibility Use average between components to produce an 

 overall assessment of feasibility.
Heading 1 

reasons, we excluded these major tributary basins from our 

feasibility assessment. 

The upper Clackamas River is characterized by a mixed 

flow regime that is strongly influenced by discontinuities 

in geology, landform, and climate in the western Oregon 

Cascade Mountains (O’Connor and Grant 2003, Tague and 

Grant 2004). In higher elevations the combination of snowdominated precipitation, lower elevation relief, and catchment 

geology that is dominated by volcanic bedrock leads to a flow 

regime that is characterized by strong influences from subsurface flow. In lower elevations, rain-dominated precipitation, 

steeper topography, and relatively impermeable substrates lead 

to flow regimes that are more strongly responsive to seasonal 

patterns of precipitation (e.g., “flashy perennial” flow regimes, 

Poff 1996). In general, streams dominated by volcanic bedrock 

geology yield discharges that are more constant or moderated 

through time, with less flooding in winter and greater stream 

flow during seasonal droughts that typically occur in summer 

and autumn (e.g., “stable and superstable groundwater” flow 

regime, Poff 1996). Stream temperatures also follow this disFigure . Current distribution of bull trout within the Willamette River recovery unit 

(inset, USFWS 00) showing historical distribution of all life stages of bull trout 

(spawning, rearing, migration) and last dates that bull trout was verified in major 

river basins (from Shively et al. 00)

Basin Location Map

Washington

Oregon

California Nevada

Idaho

Pacific Ocean

Columbia River

Clackamas River

(last verified 1963)

North Santiam River

(last verified 1945)

South Santiam River

(last verified 1953)

McKenzie River

Middle Fork Williamette River

(last verified 1990) re-introduction 

underway since 1998 using fry transplants

from Anderson Creek, McKenzie River

Willamette River

Historical Distribution

Current Distribution

0 5 10 20 30 40

1: 1,200,000

Miles
 Report Title

charge regime, with the influence of groundwater in volcanic 

geologies leading to relatively cool (<10 oC) summer water 

temperatures and moderate winter temperatures (O’Connor 

and Grant 2003, Jefferson 2006). Although bull trout occupy 

habitats with a wide range of geologies and flow regimes 

throughout its broad geographic range, the contemporary 

distribution of the species within southern Cascade Mountains 

is tied strongly to the influence of volcanic geology (Goetz 

1989).

Assessment of the recipient habitat

Evidence supporting the potential feasibility of the 

Clackamas River for a bull trout reintroduction 

A technical team was assembled in 2003 to address the 

feasibility of reintroducing bull trout into the Clackamas 

River. This team represented major regulatory agencies and 

private stakeholders charged with fisheries, water, and land 

management responsibilities and interests within the Clackamas River (Shively et al. 2007). To illustrate an application 

of the above framework for assessing the feasibility of a bull 

trout reintroduction, we draw upon the most relevant findings 

in this assessment, which should be consulted for additional 

supporting details beyond the scope of this paper. We begin by 

summarizing evidence relevant to addressing each major question concerning suitability of the receiving habitat and donor 

populations of bull trout and then provide a brief overview 

of results from applying the framework (see Appendix for 

complete results).

Was the recipient habitat likely historically occupied by a 

self-sustaining population of bull trout? 

Bull trout were first collected from the Clackamas River 

by Livingston Stone in 1878, and a specimen is archived in 

the Smithsonian Institution (museum catalog number 22355, 

www.mnh.si.edu/). It was 100 years later that the bull trout 

was described as a unique species separate from Dolly Varden 

(Cavendar 1978), so records of bull trout before this time were 

referred to as Dolly Varden or a related synonym. During 

the early 20th century, bull trout were commonly reported in 

creel surveys in the upper Clackamas River conducted by the 

Oregon Department of Fisheries and Wildlife and in accounts 

by local fisherman (Shively et al. 2007). After the early 1960s, 

bull trout were no longer regularly observed, and there are no 

reports of any kind after the early 1970s in spite of numerous 

surveys for other salmon and trout, continued angler surveys, 

and informal targeted surveys for bull trout (Buchanan et al.

1997, Zimmerman 1999). In reference to the levels of evidence needed to address this question (Appendix), we can 

confidently say that bull trout were present in the Clackamas 

River, but from direct observations alone, we cannot determine 

if there were smaller (<150 mm) bull trout present historically. 

From an assessment of the suitability of habitat (see below), 

however, we determined it was very possible that the Clackamas River could have supported all life stages of bull trout.

Is it unlikely that bull trout are present now in the 

recipient habitat?

Whereas historical surveys of many types failed to detect 

bull trout in the Clackamas River, we were concerned that 

sampling designs and low sampling efficiencies of many methods may have been biased against detection of small (<150 

mm) bull trout. To provide a final confirmation of the low 

probability of occurrence of bull trout in the upper Clackamas 

River, we employed formal surveys in 2004 (Shively et al.

2007). These surveys followed guidelines in Peterson et al.

(2002) with the following specifications:

• The sampling frame was restricted to habitats with 

access to other migratory salmonids (salmon and 

steelhead trout and presumably bull trout) and with 

measured summer maximum temperatures < 16 o

C, 

which are more likely to support spawning and early 

rearing of bull trout (Dunham et al. 2003a).

• Streams less than 2 m in base flow (summer) wetted 

width were considered unlikely to support bull trout 

relative to larger streams (Dunham and Rieman 1999) 

and were excluded from the sampling frame. Although 

bull trout can occur in streams smaller than 2 m in 

width, we assumed our likelihood of actually encountering bull trout was greater in streams with larger 

widths.

• Some sites that were unsafe to sample were excluded 

from consideration. We assumed this did not bias our 

detection probability.

• In place of block nets recommended by the protocol 

(Peterson et al. 2002), sampling units were enlarged to 

200 m in length to compensate for the effects of fish 

disturbed from snorkeling and potentially leaving the 

sample site (Peterson et al. 2005). It was assumed that 

detection probabilities were at least as high as those 

with 100 m sites with block nets in place to prevent 

fish escape during sampling.

Surveys in 2004 to detect bull trout were conducted by 

snorkeling at night with a single pass in an upstream direction, and completed for 21 sampling units equaling 4.2 km 

of stream or 9.3 percent of the sampling frame in the upper 

Clackamas River. In addition to sampling to detect bull trout in 

the field, we also used a Bayesian method proposed by Peterson and Dunham (2003) that allows combined information 

from sampling effort and prior information (e.g., habitat model 

predictions, professional opinion, or other forms of prior 

knowledge) to estimate the probability of presence for bull 

trout. From the 2004 sampling effort, a probability of presence of 0.12 was estimated based on Peterson et al. (2002). 

Using a prior probability of presence based on expert opinion of 0.10 and the method of Peterson and Dunham (2003) 
Heading 1 

yielded a combined posterior probability of 0.01 for presence 

of bull trout in the upper Clackamas River. Prior probabilities 

estimated from local experts ranged from 0.01 to 0.05, but we 

used a higher prior (0.10) in our application as a precautionary measure to account for potential biases in underestimating 

the prior probability. From these results, we determined that it 

was very unlikely that bull trout were still present in the upper 

Clackamas River (Appendix).

Is the recipient habitat currently suitable for supporting 

spawning and early rearing of at least one or more selfsustaining local populations of bull trout? 

To measure the quality of habitats in the upper Clackamas River, we focused on water temperature, which is a major 

factor influencing spawning and early rearing by bull trout 

within river networks (McPhail and Murray 1979, Dunham et 

al. 2003a). A combination of existing data from past monitoring efforts (1996-2003; n=12) and deployments of water 

temperature data loggers in 2004 (n=14) was used to identify 

coldwater habitats during summer (July through September) 

throughout the upper Clackamas River. Sampling was focused 

near the mouths of major tributaries and at several points along 

the main upper Clackamas River. Overall a total of 139.5 km 

of habitat was identified with suitably cold (< 16 oC) water 

for occurrence, and temperatures exceeded a summer maximum of 16 o

C in only one location on the main Clackamas 

River at river kilometer 92 (16.6 oC). Sampled stream reaches 

to exceed a mean of 9 oC in September (the expected time of 

spawning) included two reaches of the main Clackamas River 

(9.1-9.4 oC), Lowe Creek (9.8 oC), Hunter Creek (9.3 oC), and 

the lower portion of Cub Creek (9.2 oC). The degrees to which 

temperatures exceeding a summer maximum of 16 oC or a 

mean September temperature of 9 oC were within the range of 

measurement error for temperature data loggers (Dunham et 

al. 2005). 

In terms of hydrological processes (Tague and Grant 

2004, Tague et al. 2007) and species composition in the Willamette River basin, the most comparable system to the Clackamas River is the McKenzie River. The McKenzie River supports the only known self-sustaining populations of bull trout 

left in the Willamette River basin. The total amount of habitat 

that is potentially suitable for spawning and early rearing of 

bull trout in the upper Clackamas River compares favorably 

to the McKenzie River, where an estimated 170 km of habitat 

is used by all life stages of bull trout, including habitats used 

only for migration (USFWS 2002). Furthermore, the McKenzie River bull trout population is fragmented into three isolated 

enclaves, with two of them occupying only a small fraction of 

this total habitat. Within the largest enclave in the McKenzie 

River, spawning occurs over about 4 km of the tributary stream 

supporting the greatest numbers of spawning bull trout, and 

juveniles have been found to occur within the tributary and 

extending to about 13 km downstream within the main stem 

McKenzie River (USFWS 2002). If the McKenzie River is 

accepted as a valid baseline in terms of a minimal amount of 

habitat required to support a self-sustaining population of bull 

trout, it compares very well to the Clackamas River in terms 

of the amount of habitat available for spawning and early rearing. We did not consider habitat availability for migratory bull 

trout in the Clackamas River, but the river network is presently 

accessible and used by other migratory salmon and trout, and 

we assume the same would be true of bull trout, as is the case 

in other systems where they coexist with other salmonids.

Have past, present, and potential future threats in the 

recipient habitat been mitigated sufficiently to justify a 

reintroduction? 

A detailed review of the history of fishery, river, and land 

management in the Clackamas River (Shively et al. 2007) 

provided ample evidence to demonstrate that many significant 

threats to bull trout from past human influences have been 

mitigated. A full review of this history is beyond the scope 

of this paper (Shively et al. 2007), but the following major 

changes have been implemented to mitigate threats:

Stocking of nonnative trout (e.g., brook trout and 

hatchery origin rainbow trout) and angling regulations 

have been modified to benefit native fishes. 

Historical barriers to fish movement have been 

removed or modified to permit fish passage. 

Land management has been modified to minimize 

adverse impacts to native salmon and trout, including 

other federally listed salmonid species in the upper 

Clackamas River. Lands within the assessment area are 

primarily under federal ownership and management 

(U.S. Forest Service and Bureau of Land Management), with regulatory oversight by National Marine 

Fisheries Service.

All of these factors were likely contributors to the historical extirpation of bull trout from the Clackamas River. With 

such limited information it is difficult to construct a likely 

scenario for exactly why the bull trout was extirpated. Here we 

offer a tentative hypothesis. 

Due to strong geological influences on the hydrology of 

the upper Clackamas River (Tague and Grant 2004), many 

streams have likely remained consistently suitable for spawning and early rearing of bull trout. If bull trout used these 

habitats, which seems likely (Goetz 1989), then loss and degradation of habitats in downstream areas may have been more 

important. Historically, downstream portions of the Clackamas 

River were strongly impacted by barriers to fish movement. 

Given that bull trout in similar habitats are strongly migratory (e.g., the McKenzie River), it follows that bull trout in 

the Clackamas River should have exhibited similar behaviors. 

Therefore, downstream movement barriers could have substantially impacted populations in the Clackamas River. This 

loss of migratory connectivity may have been compounded 

by impacts of intense recreational angling (Post et al. 2003) 

from the nearby Portland metropolitan area, including active 

•

•

•
10 Report Title

efforts to remove bull trout, which were historically reviled by 

many anglers and fishery managers. Historical introductions 

of nonnative brook trout are another potential threat (USFWS 

2002) that is difficult to quantify. Documented impacts of this 

species on bull trout vary widely from strong to non-detectable 

(e.g., Rieman et al. 2006). Nonnative brook trout are present in 

comparable habitats with extant populations of bull trout in the 

Willamette River basin (e.g., the McKenzie River) and is not 

widespread within the Clackamas River. This could be due to 

the fact that many habitats within the Clackamas River support 

water temperatures that may be too cold to sustain brook trout 

(Adams 2000, Rieman et al. 2006, Benjamin et al. 2007), thus 

permitting coexistence with bull trout, which can use much 

colder water temperatures (Dunham et al. 2003a).

Loss of cold water and changes to flow regimes in the 

future could substantially impact many fishes in rivers of the 

Pacific Northwest if anticipated climate change scenarios are 

realized over longer (>20 yr) time frames (Battin et al. 2007). 

Assessments of changes in the distribution of bull trout under 

climate change scenarios are suggestive of major losses of 

habitat across the species’ range (Rieman et al. 2007), but 

these models are based on generalized assumptions about relationships between water temperatures and elevation gradients. 

At a local scale it is known that these generalized relationships 

do not apply (Dunham and Chandler 2001). This is due to the 

fact that elevation is not a process directly influencing heat 

budgets and resulting water temperatures of streams (Johnson 

2003, Moore et al. 2005). Thus, the likely impacts of climate 

change on local water temperatures and flow regimes in the 

upper Clackamas River are highly uncertain. What is known is 

that many habitats in the upper Clackamas River support water 

temperatures that would have to increase by several degrees 

to become unsuitable for bull trout, at least according to our 

current understanding of thermal habitat requirements for this 

species (Dunham et al. 2003a). Further work is clearly needed 

to better understand local hydrological processes and how 

these may influence water temperatures in the future (Jefferson 2006).

Is natural recolonization of the recipient habitat unlikely in 

the short term? 

Perhaps the strongest evidence to address this question 

is lack of recolonization of the Clackamas River by bull trout 

in nearby populations in the >40 yr since bull trout were last 

known to occur. Fish movement barriers between the Clackamas River and other localities supporting extant bull trout 

do not prevent the possibility of recolonization, but seasonal 

conditions in movement corridors (e.g., the lower mainstem 

Willamette and Columbia Rivers) can become very unsuitable for bull trout (e.g., water temperatures >20 o

C), which 

may limit potential movement. The nearest population of 

bull trout that could serve as a possible source of immigrants 

(North Fork of the Lewis River, Washington) is over 100 km 

distant. Nearby populations are dominated by individuals with 

migratory life histories and have relatively large numbers of 

individuals (>500 adults), which may increase the probability 

of recolonization. However, we viewed the large distance from 

the nearest potential source of immigrants as an overriding 

factor in the present isolation of the Clackamas River. 

Assessment of donor populations

Is there at least one available donor population that is a 

sufficient evolutionary match to the recipient? 

The Clackamas River falls within the bounds of a distinct 

lineage of bull trout (Spruell et al. 2003) that was proposed 

by the U.S. Fish and Wildlife Service to serve as a distinct 

population segment for recovery planning (Whitesel et al.

2004). Within this lineage, we considered donor populations 

with a direct hydrographic connection to the Clackamas River, 

including tributaries of the Columbia River and within the 

Willamette River basin. Within a distinct population segment, 

the draft bull trout recovery plan (USFWS 2002) recognizes 

core areas, which consist of one or more interacting local 

populations. We identified six core areas with extant local 

populations that could potentially provide donors for a reintroduction, including the upper Willamette River, Lewis River, 

Klickitat River, lower Deschutes River, Odell Lake, and Hood 

River.

Within this pool of potential donor populations is there at 

least one donor that could provide a sufficient number of 

propagules? 

Numbers of adults in core areas and local populations have been determined by survey designs and sampling 

methods with variable degrees of accuracy or precision (e.g., 

Dunham et al. 2001, Peterson et al. 2004, Muhlfeld et al.

2006, Thurow et al. 2006). Accordingly, as a precautionary 

measure, we wished to identify potential donor populations 

with abundances well above a minimum of 1000 spawning 

adults per year (Rieman and Allendorf 2001). Within the six 

core areas considered as potential donors, only one (lower 

Deschutes River) exceeded an abundance of 1000 spawning 

adults per year from 2001-2005, and abundance for a single 

local population within the core area, the Metolius River, is 

estimated at nearly double this minimum level within this time 

frame (Shively et al. 2007). Bull trout within the Metolius 

River are distributed across several tributaries used for spawning, but recent data from DNA microsatellites indicate there is 

considerable gene flow among them (W. Ardren and Patrick 

DeHaan, U.S. Fish and Wildlife Service, pers. comm.). Thus, 

following our conservative interpretation of minimum numbers of spawning adults per year, we find good evidence for 

the existence of at least one potential donor population with 

abundance levels that could support removal of a relatively 

large number of adults (e.g., >200 individuals) or other life 

stages (e.g., juveniles) to support a reintroduction effort. 
Heading 1 11

Results of the Clackamas River application 

With the evidence summarized above, we applied our 

framework for a feasibility assessment of a bull trout reintroduction to the Clackamas River (Appendix). An overall score 

of 0.95 was given to the Clackamas River, indicating a strong 

belief in the potential of this recipient habitat to support a reintroduction effort. Other biological factors that could influence 

a final decision on whether to continue with a reintroduction 

include a risk assessment of the potential for disease transmission by the most likely donor population (Metolius River, S. 

Gutenberger, U.S. Geological Survey, pers. comm.) and a regulatory consultation to be conducted by the National Marine 

Fisheries Service on the effects of reintroduction of bull trout 

on other listed salmon and steelhead in the Clackamas River. 

Conclusions

Reintroductions are a potentially powerful tool in recovery of imperiled native fishes, but experience has shown they 

often fail. Whereas the reasons for failure are not always clear, 

it is obvious that conducting a feasibility assessment before 

implementing a reintroduction will increase the chances of 

success. Ironically, translocations of trout and salmon outside 

of their native range ranges have been successful in freshwaters worldwide, mostly without the benefit of detailed 

feasibility assessments (Welcomme 1988). In the case of bull 

trout, targeted reintroduction attempts have either failed (e.g., 

McCloud River, CA) or shown evidence of limited local reproduction (Middle Fork Willamette River, OR). Translocations 

have established locally reproducing populations in several 

cases, however. In at least one example, human-assisted 

transfer of adults over an impassible waterfall (Sunset Falls, 

South Fork Skykomish River, WA) has led to establishment of 

upstream populations, and migratory adults entrained below 

dams without upstream passage have established local populations in downstream tributaries (e.g., Cougar Creek, North 

Fork Lewis River, WA, Moore Creek, Boise River, ID, Twin 

Creek, Clark Fork River, MT, USFWS 2002). These case 

histories and our assessment of the Clackamas River suggest 

that an attempt to reintroduce bull trout could be successful, 

assuming adequate resources are committed to the subsequent 

stages of implementation, monitoring, and evaluation.

Acknowledgements

Members of the Clackamas Bull Trout Working Group 

(Shively et al. 2007) were critical in helping to develop the 

framework. The patient efforts and leadership of Dan Shively 

(U.S. Forest Service, Mount Hood National Forest), Chris 

Allen, and Brad Goehring (U.S. Fish and Wildlife Service, 

Oregon Fish and Wildlife Office) were especially important. 

Rollie White (U.S. Fish and Wildlife Service, Oregon Fish and 

Wildlife Office) also provided great feedback and perspective 

on certain elements of this work. Steve Lanigan (U.S. Forest Service, Pacific Northwest Region) was very generous in 

allowing time for his program to assist in this effort. Rollie 

White and Steve Jacobs provided constructive reviews of an 

early draft of this manuscript. This report will be part of a 

manuscript to be submitted for review and publication in a 

scientific journal with additional authors to be determined 

based on consultation with the Clackamas Bull Trout Working 

Group. 
12  Report Title

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Appendix – Application of assessment process to the Clackamas River. Shown are the scores assigned to differe

information used in the assessment. Equal weights were assigned to each type of evidence. Information representRiver is shown in bold. 

Question Type of evidence Information 

Was the recipient habitat historically occupied? 

a. Presence of any life stage Confirmed record 

• Archived specimen • Professional biologist documented 

Biologist verbal accounts 

• >10 Accounts • 5-10 Accounts • < 5 Accounts 

Anecdotal record; verbal accounts by public 

• >10 Accounts • 5-10 Accounts • < 5 Accounts 

No evidence 

b. Presence of different life stages 

Presence of confirmed spawning or bull trout fry Presence of juvenile (<150 mm) fish 

Presence of larger (>150 mm) fish only 

c. Presence inferred from suitable habitat 

Historical habitat believed to support bull trout Limited connectivity OR no information1 

1 If information documenting species presence or that infers presence from habitat is not available, we recommend holding off on the re

information is available to determine whether the habitat was historically occupied. 
Question Type of evidence Information 

Is bull trout unlikelypresent now? 

Probability of presence Low probability of presence estimated by a formal 

protocol Failure to detect during recent (last 10 yr) using other protocols Failure to detect during recent (last 10 yr) sampling for other salmonids No accounts of bull trout from any source within the la10 yr No information or effort in the last 10 yr2 Documented presence (any life stage) within the last 1yr. 

Is the recipient habitat potentially suitable? 

a. Sufficient quality of habitat? 

Highly suitable for both incubation and early rearin

Moderately suitable for egg incubation, suitable for rearing No information3 Not suitable for egg incubation or rearing 

b. Sufficient quantity of habitat? 

Yes No information 

Not enough habitat 

Threats a. Severity Insignificant 

Low Unknown Moderate 

High 

b. Immediacy Insignificant 

2 If species presence surveys have not been conducted, we suggest conducting surveys using a statistically valid sampling protocol befo

reintroduction. 

3 If habitat surveys have not been conducted, we suggest conducting surveys before proceeding with the reintroduction. 
Question Type of evidence Information 

Low Unknown Moderate 

High 

Short-term recolonization potential 

a. Intervening passage barriers? 

Complete up- and downstream connectivity Partial connectivity Limited connectivity Complete two-way movement barrier 

b. Distance to nearest occupied habitat 

< 20 km >20-100km >100km4 

c. Abundance in adjacent occupied habitat 

>500 adults 100-500 adults 

<100 adults 

d. Migratory life history in adjacent core area 

Strong Depressed Absent 

4 Distance to nearest occupied habitat >100km is an overriding value, meaning it “trumps” other information given for this particular ty

distance to nearest occupied habitat >100km, then score short-term recolonization potential as a 1 regardless of other types of evidence
Question Type of evidence Information Is there at least one available donor stock? Yes 

Unknown No 

Is there at least one donor that could provide propagules without damaging the donor stock? 

Donor population > 1,000 spawning adults per year 

Donor population <1000 spawning adults per year 
Clackamas River assessment. Clackamas River information are presented in bold font. All questions and types of 

weighted. All question scores and the feasibility score are calculated using simple average, unless otherwise spec

Question Type of 

evidence 

Information Score Type of 

evidence score 

Question Score5

CompScore6

Was the recipient habitat historically occupied? 

a. Presence of any life stage 

Confirmed record 

• Archived specimen • Professional biologist 

documented 

1 0.75 0.758 0.9 

Biologist verbal accounts 

• >10 Accounts 1 • 5-10 Accounts 0.75 • < 5 Accounts 0.5 

Anecdotal record; verbal accounts by public 

• >10 Accounts 0.5 • 5-10 Accounts 0.25 • < 5 Accounts 0 

No evidence 0 Presence of confirmed spawning or bull trout fry 

b. Presence of 1 different life stages Presence of juvenile (<150 mm) 

fish 

1 

5 Average of type of evidence scores for the question 6 Average of question scores for the component 7 Average of the two component scores 8 Average scores from types of evidence a and b, then take the maximum of the a-b average or c. 
Question Type of 

evidence 

Information Score Type of 

evidence score 

Question Score5

CompScore6

Presence of larger (>150 mm) fish only 

0.5 

Historical habitat believed to support bull trout 

c. Presence 0.75 inferred from suitable habitat 

Limited connectivity OR no information

0 

Is bull trout unlikelypresent now? 

Probability of presence 

Low probability of presence estimated by a formal protocol 

1 1 1 

Failure to detect during recent (last 10 yr) using other protocols 

0.5 

Failure to detect during recent (last 10 yr) sampling for other salmonids 

0.25 

No accounts of bull trout from any source within the last 10 yr 

0.1 

No information or effort in the last 10 yr 

0 

Documented presence (any life stage) within the last 10 yr. 

-1 

Is the recipient habitat potentially suitable? 

a. Sufficient quality of habitat? 

Highly suitable for both incubation and early rearing 

1 1 1 

Moderately suitable for egg incubation, suitable for rearing 

0.75 

No information 0 
Question Type of 

evidence 

Information Score Type of 

evidence score 

Question Score5

CompScore6

Not suitable for egg incubation or rearing 

-1 

b. Sufficient Yes 1 1 quantity of habitat? 

No information 0 

Not enough habitat -1 

Threats a. Severity Insignificant 1 1 0.75 

Low 0.5 Unknown 0 Moderate -0.5 High -1 

b. Immediacy Insignificant 1 0.5 

Low 0.5 Unknown 0 Moderate -0.5 High -1 

a. Intervening passage barriers? 

Complete up- and downstream connectivity 

-1 0.5 19

Partial connectivity -0.5 

Short-term recolonization potential is unlikely 

Limited connectivity 0 

Complete two-way movement 

barrier 

1 

9 Distance to nearest occupied habitat is an overriding or “trump” value, meaning that it can override the other types of evidence in the 

nearest occupied habitat is >100km, score short-term recolonization potential as a 1 regardless of other types of evidence. 
Question Type of 

evidence 

Information Score Type of 

evidence score 

Question Score5

CompScore6

b. Distance to nearest occupied habitat 

< 20 km -1 0 

>20-100km 0 >100km 1 

c. Abundance in adjacent occupied habitat 

>500 adults -1 -1 100-500 adults 0 <100 adults 1 

d. Migratory life history in adjacent core area 

Strong -1 -1 Depressed 0 Absent 1 

Is there at least one available donor stock? 

Yes 1 1 1 

Unknown 0 No -1 

Is there at least one donor that could provide propagules without damaging the donor stock? 

Donor population > 1,000 spawning adults per year 

1 1 
Question Type of 

evidence 

Information Score Type of 

evidence score 

Question Score5

CompScore6

Donor population <1000 spawning adults per year 

-1