Bull trout (Salvelinus confluentus) COSEWIC assessment and status report 2012: chapter 10

Threats and Limiting Factors

A number of factors combine to limit the abundance of Bull Trout in Canada. Some of these are naturally occurring limiting factors but the most serious threats to Bull Trout come from anthropogenic disturbance.

Naturally Occurring Limiting Factors

The natural limiting factors for Bull Trout discussed herein are universal across their range and, therefore, relevant to all DUs. Any geographical trends in the extent of their influence are highlighted in the following DU-specific subsections.

Bull Trout’s specific habitat requirements are its most significant natural limiting factor (reviewed in Rieman and McIntyre 1993; Dunham et al. 2003). Its need for cold water (most commonly less than 12°C) in particular, as well as the very specific habitat required for spawning and rearing, strongly influence its occurrence and result in its characteristic patchy distribution (Rieman and McIntyre 1993; Dunham et al. 2003; see ‘Habitat Requirements’ section). A warmer climate in the southern margins of its global range influences Bull Trout’s spotty distribution here (Dunham et al. 2003). This sensitivity makes it an excellent indicator of environmental disturbance. Interactions with other fish species are likely another important determinant of Bull Trout distribution and abundance; interference competition from other species, such as Rainbow or Cutthroat Trout, also appears to be mediated by water temperature, while the abundance of prey species, such as Kokanee, likely also impacts Bull Trout growth and survival (see ‘Interspecific Interactions’ section).

Bull Trout are also limited by their low reproductive potential. Within suitable reaches, density-dependent survival appears to limit production of age-1+ Bull Trout parr to mean densities of about 8 fish/100 m2or less (Hagen 2008 and references therein). This density-dependent survival at the juvenile life stage can be an important determinant of abundance at later life stages (Johnston et al. 2007). Other life history attributes, such as it being a top aquatic predator and showing high site fidelity, can contribute to relatively low densities (see ‘Population and Sizes’ section). Together with its restricted gene flow (Taylor et al.2001; Taylor and Costello 2006) and natural pattern of fragmentation, these factors make Bull Trout vulnerable to local extinctions through stochastic processes. Such natural extinctions may even be common (Rieman and McIntyre 1993, 1995). The pattern of depauperate neutral genetic variation within Bull Trout populations and high differentiation between them (see ‘Population Spatial Structure and Variability’ section) indicates a historical demographic pattern of bottlenecks and local extinctions.

These limiting factors render Bull Trout vulnerable to human activities and their impacts (Rieman and McIntyre 1993, 1995). On the other hand, strategies that Bull Trout has evolved to persist in the face of variable environmental conditions may also offer some compensation when dealing with human-induced changes. For example, phenotypic plasticity and density dependent changes in life history traits, such as faster maturation and more frequent reproductive events at lower density, may offer some resilience to perturbations (Johnston and Post 2009).

Anthropogenic Threats

While the gradual demise of Bull Trout in developed areas over the last century (Rieman et al. 1997; USFWS 1999, 2008; Rodtka 2009) clearly indicates their environmental sensitivity, the reasons underlying this vulnerability are not clearly understood. Most evidence is correlative in nature and identification of causal mechanisms is needed. Nevertheless, three main anthropogenic factors are likely responsible for their decline: loss of habitat network through degradation and fragmentation, interaction (hybridization and competition) with introduced species and overexploitation (Rieman and McIntyre 1993; BCMWLAP 2004; Brewin 2004; Rodtka 2009). These broad categories apply to Bull Trout across its range, and the descriptions given in each category’s subsection are relevant to all Canadian Bull Trout DUs. However, the type and extent of specific threats will vary at regional and local scales; information that is available for individual Bull Trout DUs is outlined in the subsequent DU-specific subsections.

It can be extremely difficult to predict and quantify the influences of anthropogenic specific threats, and their interactions with other threats and natural limiting factors. For example, increasing connectivity in landscapes that have become fragmented through human disturbance may reduce extinction risk by facilitating movement. However, it may simultaneously foster invasion by other non-native species (Fausch et al. 2008) or threaten previously isolated resident populations with replacement by larger, migratory ones (Hagen 2008). In another example, the Bull Trout’s ability to resist invasion and persist in watersheds may be strengthened where intact habitat allows the expression of a full range of life histories, including large, highly fecund, migratory individuals (Nelson et al. 2002). When these migratory individuals are lost (e.g., through habitat loss or fragmentation, or overfishing), non-native fishes may be better able to displace or replace remaining resident Bull Trout (Dunham et al. 2008). Although we have a limited understanding of such interactions, it is undisputed that this battery of anthropogenic threats forms a formidable obstacle to the persistence of many Bull Trout populations (Rieman and McIntyre 1993; BCMWLAP 2004; Brewin 2004; Rodtka 2009).

Loss of Habitat Network

The degradation and fragmentation of freshwater habitat associated with disruptive land use practices, such as commercial forestry, hydroelectric, oil, gas and mining development, agriculture, urbanization, and all of their associated road development has been widely documented (reviewed in Rieman and McIntyre 1993; Ripley et al. 2005; Rodtka 2009). The gradual demise of Bull Trout in developed areas over the last century (Rieman et al. 1997; USFWS 1999, 2008; Rodtka 2009) suggests a trend of negative biological response to this environmental disruption. Indeed, road density, as a general, indirect measure of habitat disturbance, has frequently been found to significantly negatively correlate (P < 0.05) with Bull Trout occurrence (Rieman et al. 1997; Baxter et al. 1999; Dunham and Rieman 1999; Ripley et al. 2005; Scrimgeour et al. 2008).

Habitat degradation:

The environmental sensitivity of Bull Trout should come as no surprise, given their very specific habitat requirements. Variables such as temperature, depth, velocity, substrate and cover are critical to the persistence of this cold water specialist (see ‘Habitat Requirements’ section). The Bull Trout’s long overwinter incubation and rearing phase make these particularly vulnerable stages during Bull Trout’s development. For example, the occurrence of Bull Trout is negatively correlated to the percentage of fine sediment filling interstitial spaces (Weaver and White 1985; Ripley et al.2005). Groundwater is key to providing the high quality habitat required for this stage, as well as overwintering, in many Bull Trout populations (Baxter 1997; Baxter and McPhail 1999; Baxter and Hauer 2000; Ripley et al.2005). As well as direct impacts, habitat degradation that impacts the availability and abundance of prey species will also likely have a trickle-up effect on this top aquatic predator.

The exact mechanisms by which disruptive land use practices adversely affect the occurrence and abundance of Bull Trout are not well understood. Their impacts on habitat quality are likely related to changes in forest composition and age that alter the input of groundwater and woody debris, loss of deep pools, channel simplification, decreased vegetation cover, and increase surface runoff, sediment inputs and nutrient pulses. These effects can lead to diminished water quality, reduced cover, increased thermal and light regimes, increased sedimentation, and altered flow regimes that destabilize streambeds (reviewed in Rieman and McIntyre 1993; Ripley et al. 2005; Rodtka 2009). For example, increased stream temperatures are a common result of watershed developments when they result in loss of riparian vegetation (Holtby 1988; Johnson and Jones 2000; Post and Johnston 2002).

Bull Trout’s susceptibility to increasing water temperatures extends beyond the localized effects of altered patterns of forest cover, to global climate change (Rieman and McIntyre 1993; Rieman et al. 1997, 2007). Climate change and associated global warming in North America is likely to exceed global means in most areas, with mean projected warming ranges lying between 3°C and 5°C over most of the continent (Christensen et al. 2007). Such temperature changes would limit the availability of suitable Bull Trout habitat, and increase the risk of invasion, and displacement, by other species that require warmer water (Kelehar and Rahel 1996; Rahel et al. 1996; Porter and Neritz 2009). An increase in winter precipitation and a decrease in summer rainfall are also expected in western regions (Christensen et al. 2007). Subsequent winter flooding caused by heavy precipitation or glacial floods could damage Bull Trout spawning and rearing habitat. Changes like these are likely to have their biggest impact on Bull Trout populations in the south of its range, where temperature already defines its southern limit (Dunham et al. 2003). Here, simulations of predicted 5°Cwarming result in a 69% decrease in the length of streams having thermally suitable habitat for cold water salmonids in a Wyoming drainage of the Rocky Mountains (Rahel et al. 1996), and a loss of 92% of thermally suitable Bull Trout natal habitat area over 50 years in the interior Columbia River basin of the USA (Rieman et al. 2007). There has been no consideration of potential impacts, including potential range extensions, at the northern limits of the species’ range.

Habitat fragmentation:

As well as having very specific habitat requirements, migratory populations need uninterrupted migratory corridors that connect spawning grounds with feeding and overwintering habitats. The viability of these populations, therefore, is linked to their need to access this diversity of habitat at different stages throughout their life cycle (Rieman and McIntyre 1993). Several activities can fragment Bull Trout’s habitat. Hydroelectric dams are obvious barriers to movement that can threaten the viability of Bull Trout populations across their range (USA: Neraas and Spruell 2001; BC: Decker and Hagen 2008; Hagen 2008; AB: reviewed in Rodtka 2009). They can isolate populations and prevent migration between productive juvenile and adult rearing environments (Swanberg 1997b; Neraas and Spruell 2001; Decker and Hagen 2008; Hagen 2008), as well as alter and degrade Bull Trout habitat (Brown 1995; Decker and Hagen 2008; Hagen 2008).

Road construction can also lead to fragmentation of Bull Trout habitat via numerous smaller blockages and hanging culverts (reviewed in Rieman and McIntyre 1993; Ripley et al. 2005; Rodtka 2009). Other obstructions to movement can be more subtle than these obvious physical impacts; degraded habitat resulting from, for example, increased water temperatures and velocities, can also ruin and fragment suitable habitat patches (Rieman and McIntyre 1993; BCMWLAP 2004; Hagen 2008).

Existing fragmentation restricts gene flow, making isolated populations more susceptible to local extinction from stochastic and deterministic risks (Lande 1993; Dunham and Rieman 1999). With less chance of recolonization through regional connectivity, extinction at the regional scale becomes more likely (Rieman et al. 1997). As a result of such fragmentation, Bull Trout’s distribution may diminish in a way that is not directly proportional to the loss of habitat area. Rather, rates of extinction may accelerate beyond rates of habitat loss (Rieman and McIntyre 1995).

Interaction with Introduced Species

Although population declines may be largely attributed to the effects of land management and development (reviewed in reviewed in Rieman and McIntyre 1993; BCMWLAP 2004; Rodtka 2009), the expansion of introduced fish species also poses a significant threat to Bull Trout (Donald and Alger 1993; Leary et al. 1993). Introduced species, such as Lake Trout, Yellow Perch (Perka flavescens), Smallmouth Bass (Micropterus dolomieu), Largemouth Bass (Micropterus salmoides), Walleye (Sander vitreus) and Northern Pike (Esox Lucius), may pose a threat to Bull Trout populations. The greatest threat, however, may come from non-native Brook Trout populations, given the known potential negative consequences of their direct interactions with Bull Trout (see Interspecific Interactions’ section), and their widely overlapping range. Introduction of this recreational fish across the Pacific Northwest from its native eastern North America range began in the late 1800s. Ongoing introductions and its subsequent invasion have led to its wide establishment throughout much of Bull Trout’s range (Fuller et al. 1999), and its presence in many of the same basins (Rieman and McIntyre 1993).

Anecdotal evidence of Bull Trout’s occurrence being negatively associated with the presence of Brook Trout strongly implicates this non-native fish in the decline in Bull Trout populations across much of its range (Paul and Post 2001; Rich et al. 2003; Rieman et al. 2006; McCleary and Hassan 2008). Hierarchical analysis confirms that Brook Trout can influence upstream displacement of Bull Trout, although the extent of displacement is strongly influenced by environmental conditions (including elevation and temperature; Rieman et al. 2006). While complete elimination of Bull Trout is not a foregone conclusion of Brook Trout invasion, even partial upstream displacement of Bull Trout by Brook Trout may pose a serious threat to these low density fish. Bull Trout occurrence decreases with stream width (Rieman and McIntyre 1995; Earle et al. 2007; McCleary and Hassan 2008) so, as Bull Trout are displaced upstream, smaller and more isolated Bull Trout populations will become more vulnerable to local extinction through other causes (Lande 1993; Dunham and Rieman 1999).

The potentially devastating and unpredictable impact of non-native species on Bull Trout is illustrated by the crash in the early 1990s of Bull Trout in Flathead Lake and the Flathead River system in northwest Montana. The collapse of these Bull Trout populations that were previously considered to be abundant and secure resulted from the introduction of the combination of Lake Trout and the non-native invertebrate, the Opossum Shrimp (Mysis relicta;Spencer et al. 1991). These species caused major ecosystem changes and cascading food web interactions (Spencer et al. 1991).

Overexploitation

Bull Trout were once considered ‘junk’ fish because of their tendency to prey on other salmonids (McPhail 2007; Dunham et al. 2008). Active eradication plans combined with easy road access resulted in Bull Trout being “fished out” of some areas, including parts of southern Alberta and British Columbia (McPhail 2007; Dunham et al. 2008). Changing attitudes and management practices (see ‘Legal Protection and Status’ section), however, mean that the threat of extirpation from overharvesting has been reduced for many Canadian Bull Trout populations (McPhail 2007). Nevertheless, not all populations that have been subject to strict angling regulations have shown signs of recovery (reviewed in Rodtka 2009; Hagen and Decker 2011). The lack of change in some systems may be partly attributed to Bull Trout’s high catchability. Angler-mediated mortality from hooking, poaching and non-compliance to fishing regulations still poses a significant threat in some areas (Post et al. 2003; Earle et al. 2007; Rodtka 2009; Hagen and Decker 2011). The infrastructure of road networks developed to support urban and industrial activities can exacerbate this threat by increasing accessibility (reviewed in Rieman and McIntyre 1993; Ripley et al. 2005; Rodtka 2009). Simulations using reasonable estimates of fishing effort, mortality from catch-and-release, and illegal harvest, demonstrate that many Bull Trout populations will continue to require restrictive angling regulations if they are to be sustained (Post et al. 2003).

Although there is no published information on the extent of mortality of Bull Trout in rivers where intensive fisheries exist for other Pacific salmonids, incidental by-catch mortality from commercial and recreational fisheries directed at these other fish poses a risk to Bull Trout. This may be borne out not just through increased hooking mortalities (Paul et al. 2003), but also through misidentification with other char and trout species (Rodtka 2009); many anglers remain unaware of a key distinguishing morphological feature in Bull Trout, the absence of spotting on the dorsal fin (Rodtka 2009). The introduction of sport fish, such as Brook Trout, adds to this threat (Paul et al.2003).

Features of Bull Trout life history, including late age-at-maturity, low fecundity and a tendency towards non-consecutive year spawning, will hamper recovery from anthropogenic disturbances (Paul et al. 2003; Post et al. 2003; Johnston et al. 2007; Johnston amd Post 2009). Its high catchability also renders Bull Trout particularly vulnerable to overharvesting, even when angling effort and harvest limits are low (Paul et al. 2003; Post et al. 2003; Brenkman et al.2007).

DU1 [Genetic Lineage 1: Southcoast BCpopulations]

The assigned overall threat impact to this DU is High-Low (IUCN Threats Calculator - Table 2). The lack of a general trend among populations in this DU is reflected in inconsistent designations of conservation status to provisional core areas; while one is considered to be ‘At Risk’ and another as “Low Risk’ of extirpation, three others remain ‘Unranked’ (Appendix 2). Considerable gaps in our knowledge about Bull Trout populations in this area make it challenging to identify threats in a DU where potential threats are diverse and location-specific (Hagen and Decker 2011). The most significant threats that have been identified include:

Table 2. Summary of threats assessment for Bull Trout within each designated unit (DU). Threats recorded according to the IUCN classification system. Impacts calculated from recorded scope and severity values (‘Not Calc.’ refers to values not calculated because they lay outside of the assessment timeframe). Assigned overall threat impact may vary from the calculated value based on best professional judgment. Accessible version of Table 2
Threat Impact
  DU1 DU2 DU3 DU4 DU5
1.Residential & commercial dev. Medium Low Unknown Low Low
2.Agriculture & aquaculture Medium Unknown Unknown Low Unknown
3.Energy production & mining Not Calc. Medium Unknown Low Low
4.Transportation & service corridors Medium Low Unknown Low Not Calc.
5.Biological resource use Low Low Unknown Low Low
6.Human intrusions & disturbance Not Calc. Medium Unknown Low Low
7.Natural system modifications Medium Low Unknown Low Low
8.Invasive & other problematic species & genes Medium Not Calc. Unknown High Medium
9.Pollution Unknown Unknown Unknown Unknown Unknown
10.Geological events Not Calc. Not Calc. Not Calc. Not Calc. Not Calc.
11.Climate change & severe weather Medium Not Calc. Not Calc. Medium Medium
Calculated Overall Threat Impact High High Low High High
Assigned Overall Threat Impact High-Low High-Low Low High-Medium High-Low

Loss of habitat network:

The numerous hydroelectric projects and their associated dams in the Lower mainland (BCME 2011), as well as extensive urbanization, agricultural, and transportation system development (and, to a lesser extent, forestry) may degrade and/or fragment Bull Trout habitat within this DU (Hagen and Decker 2011).

Introduced species:

Brook Trout in Canada are concentrated in south-eastern British Columbia, as well as southwestern Alberta (Fuller et al. 1999; McPhail 2007). British Columbia’s Brook Trout Stocking Program supplies these fish to less than 100 lakes lakes (as of 2001; Pollard and Down 2001). Several initiatives in British Columbia attempt to address concerns about the threat Brook Trout pose to Bull Trout. For example, BC’s draft Brook Trout Stocking Policy, developed in 1998, calls on no further expansion of its stocking program, sterilization of all stocked fish, and pilot projects investigating their replacement with less risky stocking practices (Pollard and Down 2001).

Overexploitation:

Anadromous Bull Trout may be particularly susceptible to incidental by-catch, given their multiple migrations between freshwater and salt water, and their tendency to congregate in estuaries (Taylor and Costello 2006; Brenkman et al. 2007). Incidental by-catch of anadromous Bull Trout has been documented in terminal gill-net fisheries directed at Pacific salmon in north-west Washington State (Brenkman et al. 2007). Although protective regulations are in place, illegal harvest is thought to be a potential threat to Bull Trout populations in the Lillooet provisional core area in particular (Hagen and Decker 2011).

DU2 [Genetic Lineage 2: Western Arctic populations]

The assigned overall threat impact to this DU is High-Low (Table 2). The general trend of decline among Albertan populations in this DU is reflected in the designation of 11 (73%) of these core units as ‘High Risk’ or ‘At Risk’ of extirpation (Figure 11, Appendix 1). Ripley et al. (2005) also identified a significant threat of extirpation to Albertan Bull Trout populations in this DU; using road density and levels of commercial foresting as indirect measures of habitat disturbance, they forecast the local extirpation of Bull Trout from 24% to 43% of stream reaches that currently support Bull Trout in the Kakwa River basin over the next 20 years. Due to the limited information available on British Columbian Bull Trout populations within this DU, the majority of its provisional core units (n = 26, 87%) remain ‘Unranked’ for conservation status (Appendix 2). Three of the four remaining provisional core areas are from the Lower Peace EDU, and are all considered to be ‘At Risk’ of extirpation (Appendix 2). The fourth one from the Upper Peace EDU has been assessed as being at ‘Potential Risk’ (Appendix 2). As in other DUs, threats are location-specific, and vary here depending on major watershed. For example, much of the Upper Liard is considered remote and pristine, whereas the Lower Peace faces considerable pressure from rapid development (Hagen and Decker 2011). Significant threats that have been identified include:

Naturally occurring limiting factors:

The lower productivity of the colder waters in Bull Trout’s northern extent likely limits its population density (Mochnacz and Reist 2007; Mochnacz et al. 2009). In addition, the more northerly populations within this DU may recover more slowly from adverse impacts compared to their more southerly counterparts, given their tendency for slower growth and less frequent mating (Stewart et al. 2007a; Mochnacz et al. in review). Given this likely susceptibility to perturbations, there is concern about the potential impact of development activities (Cott et al. 2008) on Bull Trout habitat in the Northwest Territories (Mochnacz et al. in review).

Loss of habitat network:

Habitat disturbances from intense development pressure in the Lower Peace River basin within British Columbia and Alberta warrant particular attention for this DU. Exploration for and extraction of oil and gas, as well as mining developments and timber harvesting, and their accompanying developments (e.g., roads, urbanization) are of the most concern (Rodtka 2009; Hagen and Decker 2011). To a lesser extent, similar concerns extend to the Lower Liard River basin within British Columbia (Hagen and Decker 2011) and Yukon (Connor et al. 1999). The proposed Site C dam on the Peace River if developed can be included as a threat to the populations in the Halfway-Peace, Murray, Moberly, and Pine/sukunka core areas. Conversion of river to reservoir habitas and associated changes in species assemblages, and changes to life history strategies are likely. Fish passage facilities at the dam site may not be built.

Despite their significant potential to be detrimental to Bull Trout populations, however, little evidence of this has been documented. Scrimgeour et al. (2008) is an exception to this; they found the occurrence of Bull Trout in the Kakwa and Simonette watersheds of west central Alberta negatively related to percent disturbance from exploration and extraction of oil and gas resources, as well as forest harvesting. Ripley et al. (2005) also found the level of commercial foresting (cumulative area of the subbasin harvested) in the Kakwa River Basin negatively correlated to Bull Trout occurrence. Both of these studies also found that road density acted as a general, indirect measure of habitat disturbance that significantly negatively correlated (P < 0.05) with Bull Trout occurrence (Ripley et al. 2005; Scrimgeour et al. 2008).

The susceptibility of Bull Trout to detrimental changes in water quality from heavy metal contaminants released from mining activities is also poorly understood (but see Hansen et al. 2002a, b, c). There is, however, concern about the contribution of mining activity in Alberta’s northeast slopes region to declining Bull Trout stocks in the area. Elevated levels of selenium, which can reduce recruitment in fish populations by increasing rates of deformities during early development (Hodson et al. 1980; Hodson and Hilton 1983), occur in the region (Casey and Siwik 2000). Muscle biopsies indicate that selenium concentrations do, in fact, exceed toxicity threshold values for negatively impacting reproductive success in most Bull Trout captured downstream of coal mining activity (Palace et al. 2004). However, further analysis of Bull Trout eggs is needed to understand the impact of selenium on Bull Trout survival and recruitment in these coal impacted waters (Palace et al. 2004). Coal mine development planned for the Murray river area in the lower Peace River watershed may pose a risk to Bull Trout spawning in this area.

Although hydroelectric dams can pose a risk to Bull Trout populations, there are relatively few such developments in Northern British Columbia or in Alberta. Those that exist within this DU are clustered around the Upper Peace River (BCME 2011; Hagen and Decker 2011), although the proposed Site C Dam on the Peace River has the potential to profoundly affect Bull Trout populations in the Lower Peace EDU (Hagen and Decker 2011).

Introduced species:

Although Brook Trout in Canada are most prevalent in southern British Columbia and southwestern Alberta (Fuller et al. 1999; McPhail 2007), Bull Trout’s occurrence has been negatively associated with the presence of Brook Trout within this DU(McCleary and Hassan 2008). While most Brook Trout stocking within Bull Trout’s range in Alberta has stopped (see ‘Protection, Status, and Ranks’ section), an ongoing Provincial Brook Trout Stocking Program continues to supply these fish to less than 100 lakes across British Columbia (as of 2001; Pollard and Down 2001). As listed under DU1 [Genetic Lineage 1: Southcoast BCpopulations]subsection, several initiatives attempt to address concerns about the threat to Bull Trout from this continuing Brook Trout stocking program (Pollard and Down 2001). An increasing abundance of Lake Trout in Williston Reservoir (Upper Peace EDU) is also a growing but low severity threat at present (Hagen and Decker 2011).

Overexploitation:

There is a curious pattern of increases in some Bull Trout populations within this DU(e.g., Pinto Lake) but no change in others (e.g., Kakwa River) that have been subject to strict angling regulations (reviewed in Rodtka 2009). The lack of change in some systems may be partly attributed to Bull Trout’s high catchability, with hooking mortality, poaching and non-compliance to fishing regulations still posing a significant threat in some areas (reviewed in Rodtka 2009). The potential for overexploitation of Bull Trout is recognized as a moderately severe threat in specific locations in the Upper Peace EDU (Hagen and Decker 2011).In addition, the increase in angler-mediated mortality that may be associated with increased accessibility (Ripley et al. 2005) will likely be a threat in remote areas of this DU that have experienced recent increases in road development for primary resource extraction but where enforcement remains difficult.

DU3 [Genetic Lineage 2: Yukon populations]

The assigned overall threat impact to this DU is Low (Table 2). Very little is known about the distribution of Bull Trout in this DU, let alone abundance and trends for this species (Appendix 2). Despite their expected vulnerability, very few anthropogenic threats exist in this remote area. Their estimated threat level is, therefore, assumed to be low (Hagen and Decker 2011). This suggests a relatively low level of conservation concern for this DU.

Naturally occurring limiting factors:

As for the northerly populations of DU2 [Genetic Lineage 2: Western Arctic populations], Bull Trout populations within this northerly DU are likely to have lower population densities and exhibit slower recovery from adverse impacts compared to their more southerly counterparts.

Loss of habitat network:

Unlike the other DUs, there are no hydroelectric dams within this DU that threaten Bull Trout habitat (BCME2011). Furthermore, there is very minimal road access and little (historical) mining activity (Hagen and Decker 2011).

DU4 [Genetic Lineage 2: Saskatchewan-Nelson Rivers populations]

The assigned overall threat impact to this DU is High-Medium (Table 2). The general trend of population decline identified in this DU is reflected in the designation of 30 (91%) of its extant core units as ‘High Risk’ or ‘At Risk’ of extirpation (Figure 11, Appendix 1). Significant threats that have been identified include:

Loss of habitat network:

All of the land use practices that have been identified as general threats to the integrity of Bull Trout habitat throughout their Canadian range have been associated with the demise of Bull Trout in southwestern Alberta during the mid-20thcentury (e.g., commercial forestry, hydroelectric, oil, gas and mining development, agriculture, urbanization, their associated road development, and climate change; see Appendix 3). However, little quantitative evidence of their impact on Bull Trout populations has been documented within this DU.

Although hydroelectric dams can pose a risk to Bull Trout populations, there are few such developments in Alberta compared to British Columbia. Nevertheless, the potential for the developments that do exist within this DUto fragment Bull Trout habitat is illustrated by the congregation of Bull Trout attempting spawning migration below Oldman Dam, which has no provision for fish passage (Fernet and O’Neil 1997).

The anticipated effects of global climate change (Christensen et al. 2007) on Bull Trout habitat within its Canadian range can be expected to be exacerbated in the rain-dominated habitat of this DU, although there are currently no modeling simulations to support this.

Introduced species:

Brook Trout are particularly prevalent in southwestern Alberta, (Fuller et al. 1999; McPhail 2007). Brook Trout introductions in southwestern Alberta are thought to have contributed to the historical pattern of decline in Bull Trout populations in this DU (Paul and Post 2001; Fitch 1997). In recognition of this, most Brook Trout (as well as Brown Trout) stocking within Bull Trout’s range in Alberta has either stopped for more than 8 years or, in a few cases, been replaced by stocking of only sterile, triploid fish (see ‘Protection, Status, and Ranks’ section).

A Brook Trout removal research project in Quirk Creek, southwestern Alberta (Paul et al. 2003; Earle et al. 2007; see ‘Protection, Status, and Ranks’ section) provides a cautionary note on the difficulty of removing or suppressing introduced species to promote Bull Trout recovery. Here, Brook Trout have been found to be relatively resilient to even selective harvesting, thanks to their fast growth and early maturation, and their lower catchability (i.e., proportion of vulnerable population caught per unit of angling effort) compared to native salmonids, including Bull Trout (Paul et al.2003; Earle et al. 2007). On the other hand, Bull Trout, with their higher catchability, slower growth and later maturity, are extremely sensitive to overexploitation, and may even be negatively impacted from incidental mortalities resulting from such initiatives (Paul et al. 2003; Earle et al. 2007).

Overexploitation:

The diminished threat of extirpation from overharvesting within this DU is reflected in the expansion of some previously exploited Bull Trout populations since the introduction of strict angling regulations (e.g., Lower Kananaskis, Jacques and Harrison lakes, and Clearwater and Sheep rivers; Johnston et al. 2007; and reviewed in Rodtka 2009). Nevertheless, not all Bull Trout populations in southwestern Alberta that have been subject to strict angling regulations have shown change (e.g., Elbow and Highwood rivers, and Quirk Creek; reviewed in Rodtka 2009). The lack of change in some systems may be partly attributed to Bull Trout’s high catchability, with hooking mortality, poaching and non-compliance to fishing regulations still posing a significant threat in some areas (reviewed in Rodtka 2009).

DU5 [Genetic Lineage 2: Pacific populations]

The assigned overall threat impact to this DU is High-Low (Table 2). As for most of the other Bull Trout DUs, considerable gaps in our knowledge about Bull Trout populations make it difficult to fully assess threats in this DU; the majority of its provisional core units (n = 52, 67%) and many of its EDUs (n = 7, 41%) remain ‘Unranked’ for conservation status (Appendix 2). Nevertheless, threats are known to vary by major watershed in this very broadly distributed DU(Hagen and Decker 2011) and, not surprisingly, those provisional core areas that have been designated a conservation status range widely from ‘High Risk’ (n = 4) and ‘At Risk’ (n = 7) of extirpation to ‘Potential Risk’ (n = 3) and ‘Low Risk’ (n = 12, Appendix 2). One EDU, the Upper Kootenays, is considered to be ‘Low Risk’, seven other EDUs are thought to be at ‘Potential Risk’, while the greatest concerns occur in the Flathead and Upper Skeena EDUs, which have core areas listed as ‘At Risk’ of extirpation (Appendix 2). The most significant threats that have been identified include:

Loss of habitat network:

Hydroelectric dams within this DU are concentrated in a southern central area that covers the Upper Columbia basin, the Thompson-Okanagan region, and the interior of the Cariboo-Chilcotin region (BCME2011; Hagen and Decker 2011). Evidence from this DU indicates that hydroelectric dam projects can degrade Bull Trout habitat, as well as potentially isolating resident populations and preventing migratory fishes from moving between their spawning and feeding grounds. The inundation of streams and lakes can ruin spawning and rearing grounds, and adult habitat can be degraded, and the reduced flow can degrade adult habitat downstream through sedimentation (Brown 1995; Decker and Hagen 2008; Hagen 2008). The spawning preference of Bull Trout for colder, higher elevation headwaters will, however, reduce this impact relative to other salmonids (Hagen 2008). While riparian restoration along streams and the removal of migration barriers can correct for these losses in habitat and connectivity, care must be taken to not create other negative impacts, such as threatening previously isolated resident populations with replacement by larger, migratory ones (Hagen 2008).

While risks to Bull Trout associated with dam developments should not be underplayed, the reservoirs that they hold may positively impact adfluvial Bull Trout populations that can readily shift from a fluvial to adfluvial life history; in the headwaters of the Kootenay and Columbia Rivers, reservoirs have supported the large expansion of Kokanee populations over the last 30 years, with subsequent increases in the abundance of Kokanee’s predators, including Bull Trout (Jamieson pers. comm. 2010).

Another serious threat to Bull Trout across parts of this DU (especially the Middle Fraser EDU but also including parts of the Homathko-Klinaklini, Bella Coola-Dean, and Thompson EDUs) is the recent massive loss of pine forest cover to the mountain pine beetle, which could lead to significantly warmer thermal regimes (Hagen and Decker 2011) While these impacts will likely be lessened in the long term as forests regenerate, climate change will likely exert an increasingly negative influence on thermal regimes for Bull Trout. Although detrimental habitat changes associated with global warming are likely to have their biggest impact on Bull Trout populations in the US, where temperature already defines its southern limit (Dunham et al. 2003), an assessment of the snowmelt-dominated watersheds in the Cariboo-Chilcotin region of the Middle Fraser EDU in British Columbia suggests that the thermal and precipitation effects of global warming will produce a long-term pattern of considerably decreased cold water stream habitat by the 2080s (Porter and Neritz 2009). Indeed, the potential of climate change to be a major threat to the long-term persistence of Bull Trout is recognized for a number of provisional core areas in the Middle Fraser, Thompson and Columbia-Arrow Lakes EDUs (Hagen and Decker 2011). It is also recognized as a potential threat to areas of the Upper and Lower Kootenays, Bella Coola-Dean, and Upper Fraser EDUs (Hagen and Decker 2011). Bull Trout streams downstream of heavily glaciated headwaters that are found in some areas of this DU (e.g., some areas in the Homathko-Klinaklini, Thompson,Columbia-Arrow Lakes, and Middle Fraser EDUs) will likely be buffered against such degradation of thermal regimes (Hagen and Decker 2011).

Habitat threats related to other watershed development are also recognized in EDUsacross this DU (Hagen and Decker 2011). In places these threats are potentially widespread e.g., mining in the Upper Kootenays, and forestry in the Upper Columbia and Lower Kootenays EDUs (Hagen and Decker 2011). Potentially significant threats to Bull Trout populations posed by some proposed watershed developments (e.g., hydroelectric projects in the Homathko-Klinaklini EDU; mining in the Upper Nass, Upper Stikine, and Nakina and Taku EDUs, and; recreation resort in the Thompson EDU) are recognized as requiring immediate attention (Hagen and Decker 2011).

Introduced species:

Brook Trout in this DU are concentrated in southeastern British Columbia (Fuller et al. 1999; McPhail 2007). The potential threat posed by this species is recognized for several areas in this DU(Upper Columbia, Columbia-Arrow Lakes, and Upper Fraser EDUs, Hagen and Decker 2011). While British Columbia’s ongoing Provincial Brook Trout Stocking Program supplies these fish to less than 100 lakes (as of 2001; Pollard and Down 2001), several initiatives attempt to address concerns about the threat to Bull Trout from this continuing Brook Trout stocking program (listed under DU1 [Genetic Lineage 1: Southcoast BCpopulations]subsection,Pollard and Down 2001). Lake Trout incursion in the Flathead EDU is considered to be a major threat (Hagen and Decker 2011).

Overexploitation:

Overexploitation is likely the most significant historical impact on Bull Trout in the Middle Fraser EDU, alongside hydroelectric development (Hagen and Decker 2011). As is the case elsewhere (e.g., Wigwam River, Pollard and Down 2001), at least some Bull Trout populations within this EDU have recovered from past exploitation following stricter angling regulations (e.g., Quesnel Lake, Porter and Nelitz 2009). Nevertheless, concern about localized overharvest still exists for some provisional core areas in this and other EDUs (e.g., Thompson, Lower and Upper Skeena, Upper Nass, Iskut-Lower Stikine and Upper Stikine EDUs, Hagen and Decker 2011).

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