Bull trout (Salvelinus confluentus) COSEWIC assessment and status report 2012: chapter 5

  • South Coast British Columbia populations
  • Western Arctic populations
  • Upper Yukon Watershed populations
  • Saskatchewan - Nelson Rivers populations
  • Pacific populations

Wildlife Species Description and Significance

Name and Classification

Phylum: Chordata

Class: Actinopterygii

Order: Salmoniformes

Family: Salmonidae

Subfamily: Salmoninae

Genus: Salvelinus

Species: Salvelinus confluentus (Suckley 1859)

English common name: Bull Trout

French common name: Omble à tête plate

The taxonomy of North American char (Salvelinus), to which Bull Trout (Salvelinus confluentus) belongs, has a tangled history. Many of the systematic uncertainties stem from limitations of morphological analyses in a group of fishes with extensive phenotypic plasticity. This Holarctic genus has been heavily influenced by Pleistocene glaciations, with periodic episodes of range fragmentation also confounding its complex intraspecific relationships. Historical processes, including fragmentation within refugia (Taylor et al. 1999; Brunner et al. 2001), as well as introgression between species within refugia or in subsequently recolonized deglaciated areas (Bernatchez et al. 1995; Wilson and Bernatchez 1998; Phillips et al. 1999; Redenbach and Taylor 2002), have likely contributed to conflicting phylogenies from morphological, mitochondrial DNA and nuclear markers (Grewe et al. 1990; Phillips et al. 1999; Redenbach and Taylor 2002; Crespi and Fulton 2004).

For many years, Dolly Varden (Salvelinus malma) and Bull Trout were considered to be geographic variants within the Arctic Char (Salvelinus alpinus) species complex. Even when morphological analysis showed them to be sufficiently divergent from Arctic Char to be designated a separate species, S. confluentus remained part of the S. malma ‘species complex’ (McPhail 1961). Once often assumed to be the land-locked form of Dolly Varden, subsequent analysis revealed Bull Trout to be sufficiently morphologically diverged from Dolly Varden to warrant designation as an individual species in 1978 (Cavender 1978; Haas and McPhail 1991). Molecular phylogenies now reveal Bull Trout and Dolly Varden are, in fact, not even sister species; the two species probably last shared a common ancestor more than 1 million years ago (Grewe et al. 1990; Crane et al. 1994; Phillips et al. 1994). Subsequent genetic evidence of these two char maintaining distinct gene pools in sympatry, despite some ongoing hybridization and gene flow (Baxter et al. 1997; Taylor et al. 2001; Redenbach and Taylor 2003), provides the most compelling evidence yet that Dolly Varden and Bull Trout are distinct biological species.

Morphological Description

Bull Trout is a long slender fish with a comparatively large head and jaws (Figure 1), hence the derivation of its common name “bull”. Their body size at maturity depends on life history strategy (average length and range (mm) of: resident is 250 [140-410]; fluvial is >400 [240-730]; and adfluvial >400 [330-900+]; reviewed in Pollard and Down 2001; Rodtka 2009; Mochnacz et al. submitted). Although under-reported in the literature, it may be that anadromous Bull Trout attain the largest sizes of all (Brenkman et al. 2007).


Figure 1. Bull Trout (Salvelinus confluentus)

Illustration of the Bull trout, lateral view (see long description below).

Picture courtesy of J.D. McPhail and D.L. McPhail.

Description of Figure 1

Illustration of the Bull trout, lateral view. This long slender fish has a comparatively large head and jaws. There are pale round spots along the flank and back, and the belly is pale. The tail fin is slightly forked.

Bull trout are olive-green to blue-grey in colour, with adfluvial fish often displaying silvery sides (Nelson and Paetz 1992). Pale round spots along their flanks and backs that are pink, lilac, yellow-orange or red distinguish them from others: Brook Trout (Salvelinus fontinalis) has distinct, light-coloured, worm-like markings on top of the head, back and dorsal fin, while Rainbow Trout (Oncorhynchus mykiss), Cutthroat Trout (O. clarkii) and Brown Trout (Salmo trutta) have dark spots (Nelson and Paetz 1992; McPhail 2007). Bull Trout usually have pale bellies, which may turn red or orange in spawning males (Nelson and Paetz 1992). Their tail fin is slightly forked, and pelvic or anal fins may have a leading white edge, but this is not followed by black as it is in Brook Trout (Nelson and Paetz 1992). Bull Trout larvae may be distinguished from other larval char by the presence of a prominent fleshy ridge underneath the chin (Gould 1987).

Bull Trout are morphologically very similar to Dolly Varden. Although no single character can consistently distinguish between them, the two species do differ across a suite of several characters. Generally, Bull Trout have larger, broader, and flatter heads than Dolly Varden, with bodies that are more slender and ventrally flattened (Cavender 1978; Haas and McPhail 1991). Together, branchiostegal ray number, anal fin ray number, and the ratio of total upper jaw length to standard body length consistently distinguish between the two species. Bull Trout tend to have larger upper jaws in proportion to their body lengths compared with Dolly Varden. They also have more anal fin and branchiostegal rays (Haas and McPhail 1991). A morphometric identification protocol utilizing these four variables is presented in Haas and McPhail (1991).

Population Spatial Structure and Variability

The phylogeography of Bull Trout has been well studied and provides strong evidence for two major genetic lineages of Bull Trout in northwestern North America: a southern coastal group (henceforth called Genetic Lineage 1) and an interior group (henceforth called Genetic Lineage 2). The first genetic evidence came from mitochondrial DNA (mtDNA); a survey of mtDNA variation (115 restriction sites over 410 base pairs) in 47 populations (N = 348) spanning the geographical range revealed a sharp discontinuity in the geographical distribution of haplotypes (sets of alleles of closely linked loci; Taylor et al. 1999). While most of Genetic Lineage 1 based on mtDNA occurs at or west of the Coast and Cascade mountain crests, most of Genetic Lineage 2 based on mtDNA is found east of these (Figure 2). The sequence divergence (d) between these lineages is comparable to that found in other northern Holarctic fishes (d = 0.8% [Taylor et al. 1999, 2001] compared to average maximum intraspecific d of ~1.2% from 25 other species [Bernatchez and Wilson 1998]). Subsequent surveys of nuclear DNA (microsatellites) across Bull Trout’s geographical range have consistently corroborated the presence and distribution of these groupings (Spruell et al. 2003; Taylor and Costello 2006). Morphological and comparative life-history (Haas and McPhail 2001; see ‘Dispersal and Migration’ section) evidence has also substantiated this major subdivision of Bull Trout into Genetic Lineage 1 and 2.


Figure 2. Distribution of two major Bull Trout mitochondrial DNA lineages identified by restriction fragment length polymorphism of 47 Bull Trout populations (N = 348)

Map showing the distribution in northwestern North America of two major Bull Trout mitochondrial DNA lineages (see long description below).

The solid black line dividing groups A (Genetic Lineage 1) and B (Genetic Lineage 2) is the approximate location of the Cascade/Coast Mountain crest. Sourced from Taylor et al. 1999.

Description of Figure 2

Map showing the distribution in northwestern North America of two major Bull Trout mitochondrial DNA lineages. These are identified by restriction fragment length polymorphism of 47 Bull Trout populations.

This pattern of an inland/coastal genetic split corresponding to the Coast and Cascade mountain ranges is one that is repeated in other northwestern fishes (e.g., Rainbow Trout: McCusker et al. 2000; Cutthroat Trout: Allendorf and Leary 1988; Chinook Salmon, Oncorhynchus tshawytscha: Teel et al. 2000; Coho Salmon, Oncorhynchus kisutch: Small et al. 1998; and Longnose Suckers, Catostomus catostomus: McPhail and Taylor 1999), as well as other taxa (e.g., amphibians: Carstens et al. 2005). It is likely explained by the Bull Trout’s historical isolation in, and subsequent post-glacial dispersal from, two distinct glacial refugia at the southern edges of the Cordilleran ice sheet during the late Pleistocene: the Chehalis Refuge and the Columbia Refuge (Taylor et al. 1999).

The Chehalis Refuge is a region dominated by drainages of the Chehalis River between the Columbia River and Puget Sound that was ice-free during much of the Pleistocene. Based on the distribution of endemic species and differentiated populations in fishes and plants, it was likely independent from the nearby Columbia Refuge (see Taylor et al. 1999). It was the probable refuge for Genetic Lineage 1 Bull Trout, given the localization of this lineage around the southern region of British Columbia (the lower Fraser below Hell’s Gate Canyon and Squamish systems), Puget Sound and the Olympic Peninsula in western Washington, the lower Columbia River, and the Klamath River in southwestern Oregon (Figure 2). Postglacial dispersal from this refuge into the lower Fraser or Columbia rivers or adjacent coastal systems may have occurred via freshwater connections through the Puget lowlands (McPhail 1967; Thorson 1980), or even via the sea given this group’s anadromous behaviour (see ‘Dispersal and Migration’ section). The Columbia Refuge probably served as the source of Bull Trout’s Genetic Lineage 2 postglacial colonists. Well-documented postglacial connections among the upper Columbia in the USA and Canada right through to more northern and eastern draining systems (i.e. Liard River in British Columbia, lower Peace, Athabasca, and South Saskatchewan rivers in Alberta) would have aided the dispersal of this group across the Continental Divide into interior regions (Lindsey and McPhail 1986; McPhail and Lindsey 1986).

Patterns of postglacial dispersal from these refugia can also account for peculiarities in the geographical distribution of the two lineages. For example, headwater faunal exchanges between interior and coastal drainages likely explain why all large coastal-draining systems north of the Squamish River (e.g., Skeena, Stikine, Nass, Klinaklini) carry Genetic Lineage 2 Bull Trout mtDNA and microsatellite DNA alleles (Figure 3; Figure 4). Interdigitation of these rivers’ extensive headwater tributaries is strongly suspected to be the route of past faunal exchanges (Lindsey and McPhail 1986; McPhail and Lindsey 1986) and was the likely conduit for the expansion of Genetic Lineage 2 west of the Coast mountains’ divide at its mid-northern end (Taylor et al. 1999; Taylor and Costello 2006).


Figure 3. UPGMA dendogram of pairwise sequence divergence estimates from 21 restriction fragment length polymorphism mitochondrial DNA haplotypes

UPGMA dendogram of pairwise sequence divergence estimates from 21 restriction fragment length polymorphism mitochondrial DNA haplotypes (see long description below).

Includes 348 Bull Trout samples analyzed from 47 populations. For each haplotype, geographical locations in which it occurred are listed. Sourced from Taylor et al. 1999. Genetic lineages and probable anadromous populations (*) indicated.

Description of Figure 3

UPGMA (Unweighted Pair Group Method with Arithmetic Mean) dendogram of pairwise sequence divergence estimates from 21 restriction fragment length polymorphism mitochondrial DNA haplotypes. It includes 348 Bull Trout samples analyzed from 47 populations. The geographical locations where each haplotype occurred are listed. Genetic lineages and probable anadromous populations are indicated.


Figure 4. UPGMA dendogram of genetic similarity among 373 samples of Bull Trout from 20 populations estimated from variation across 7 microsatellite loci

UPGMA dendogram of genetic similarity among 373 samples of Bull Trout from 20 populations estimated from variation across 7 microsatellite loci (see long description below).

Numbers along branches represent bootstrap scores from 1000 pseudoreplicate analyses. Sourced from Taylor and Costello 2006. Genetic lineages and probable anadromous populations (*) indicated.

Description of Figure 4

UPGMA (Unweighted Pair Group Method with Arithmetic Mean) dendogram of genetic similarity among 373 samples of Bull Trout from 20 populations estimated from variation across 7 microsatellite loci. Genetic lineages and probable anadromous populations are indicated.

Another anomaly occurs at the southern end of the Bull Trout’s range. Here, Genetic Lineage 1 predominates in the lower Columbia area at or west of the Cascade Crest (Figure 2; Taylor et al. 1999, Spruell et al. 2003) despite the presumed role of the Lower Columbia River valley as a glacialrefuge for the Genetic Lineage 2 lineage. Fish in the Columbia refuge likely concentrated east of this divide and dispersed mostly inland into the upper Columbia, Fraser and other northern interior drainages, while Bull Trout from the Chehalis Refuge went on to colonize the lower reaches of the Columbia River valley (Taylor et al. 1999). The hypothesis of the lower Columbia River not being a single faunal unit in terms of postglacial dispersal of freshwater fish was, in fact, postulated to account for the curious absence of several other species that occur widely elsewhere in this river system (McPhail and Lindsey 1986).

A further transition between Bull Trout Genetic Lineage 1 and 2 occurs abruptly in the Fraser River at an area known to be difficult for fish passage, the Fraser Canyon (Figure 3). The Fraser Canyon is associated with abrupt shifts in the distribution of genetic variation within some other fish species (see Taylor et al. 1999), as well as changes in the geographical distribution of several others (McPhail and Lindsey 1986). Evidently, this point of biogeoclimatic change from coastal wetlands to dry interior represents a strong natural barrier to fish dispersal and has maintained a bimodal contact zone between the two Bull Trout lineages, which have colonized this river from opposite directions.

In addition to the major division of Bull Trout into two evolutionary lineages, the hierarchical division of genetic variation among local populations contributes to our understanding the extent and origin of diversity within Bull Trout. Throughout Bull Trout’s range, most genetic variation resides at the interpopulation and inter-region level. For example, a mtDNA survey (115 restriction sites over 410 base pairs) of 47 populations (N = 348) sampled from across its geographical range revealed that 55% of the variation was found between Genetic Lineage 1 and 2, 33% between populations within these groups and only 12% within them (P < 0.00005; Taylor et al. 1999). Similarly, a comprehensive survey of microsatellites (N = 7) among 57 populations (N = 1561) sampled from across its range found most variation (46%) between the two lineages, 21% among populations within groups and 33% within them (P < 0.001; Taylor and Costello 2006).

Not surprisingly, therefore, there is a high degree of substructure within geographical lineages; overall FST among populations (N = 8-37) within lineages but spanning many hundreds of kilometres have been consistently estimated as lying between 0.30 and 0.39 (P < 0.005) in microsatellite (N ≥ 5) studies (Taylor et al. 2001; Costello et al. 2003; Whiteley et al. 2004; Taylor and Costello 2006). Significant microsatellite differentiation among populations (P < 0.05) within localized areas is even common (Spruell et al. 1999; Taylor et al. 2001; Costello et al. 2003; Taylor and Costello 2006). However, caution is warranted in defining Bull Trout populations according to a priori stream-of-origin designations. As for other stream-spawning fishes, fine-scale population structure in Bull Trout has traditionally been explored by designating genetic populations according to where individuals were captured. However, not all streams-of-origin may represent genetically distinguishable units and, even though levels of gene flow are considered to be low amongst Bull Trout populations, we cannot assume that each individual sampled at a site was born there.

Rather than assume a certain geographic population structure prior to analysis, a more appropriate approach in systems showing low levels of gene flow may be to define genetic populations statistically, independent of capture location, using model-based genetic clustering methods. A comparison of genetic clustering methods and a traditional stream-of-origin approach applied to Bull Trout in southwestern Alberta found the stream-of-origin approach was prone to overestimating population structure due to genetic and statistical effects (Warnock et al. 2010). In contrast, the genetic clustering methods are less likely to generate spurious groupings and define them within a hierarchical structure (Warnock et al. 2010). Because the designation of populations has strong implications for management decisions, future genetic studies on Bull Trout should be based on this more objective approach.

The restricted gene flow suggested by the high degree of substructure found within geographical lineages of Bull Trout will favour divergence among different selective environments (Lenormand 2002). Given empirical evidence that estimates of neutral genetic divergence provide conservative estimates of adaptive divergence (Pfrender et al. 2000; Morgan et al. 2001), microsatellite assays of neutral genetic variation are likely to be conservative estimates of Bull Trout biodiversity. As is common among salmonids (Quinn and Dittman 1990), Bull Trout most likely diverge in quantitative traits important to population persistence in specific environments. Local adaptation will likely be most evident at larger scales, for example among populations inhabiting the four different National Freshwater Biogeographic Zones that the Bull Trout’s range straddles (NFBZ 4 [Saskatchewan-Nelson Rivers Watershed], 6 [Yukon River Watershed], 11 [Pacific] and 13 [Western Arctic]; Figure 5). The disjunction between two groupings of these ecozones (Areas 4 and 13, and 11 and 6) by the Rocky Mountains, in particular, is likely to foster adaptive divergence.


Figure 5. Canadian distribution of Bull Trout

Map of the Canadian distribution of the Bull Trout (see long description below).

Data from: Province of British Columbia (2007); Rodtka 2009; Laframboise (pers. comm. 2010); Parkinson (pers. comm. 2010); Mochnacz et al. (submitted); Reist and Sawatzky (in prep.); Hagen and Decker (2011).

Description of Figure 5

Map of the Canadian distribution of the Bull Trout. The areas occupied by the five designatable units of the Bull Trout (South Coast B.C. populations, Western Arctic populations, Upper Yukon Watershed populations, Saskatchewan - Nelson populations, and Pacific populations) are indicated.

Although the concentration of genetic variation among populations and geographical regions is commonly observed in freshwater fish species (e.g., Ward et al. 1994), this pattern is pronounced in Bull Trout, and perhaps char in general (Wilson et al. 1996; Angers and Bernatchez 1998) relative to many other salmonids (e.g., Bernatchez and Osinov 1995; Whiteley et al. 2004; Harris and Taylor 2010). On the other hand, genetic variability within Bull Trout populations is typically lower than that of many other freshwater salmonids, including other char. Average expected heterozygosity (HE) from a microsatellite (N = 7) survey of 20 populations (N = 373) spanning the coastal range of Bull Trout in northwestern Washington and southern B.C. (but encompassing both Genetic Lineages 1 and 2) was 0.35 (Taylor and Costello 2006). Another survey of the same loci over 37 Genetic Lineage 2 Canadian populations (N = 1188) found an even lower average HE of 0.24 (Costello et al. 2003). This compares to an average HE of 0.62 among five other freshwater salmonid species (see Costello et al. 2003). This pattern of low genetic diversity is consistently found within Bull Trout populations across its range using other independent genetic markers (allozymes: Leary et al. 1993; mtDNA: Taylor et al. 1999), as well as microsatellites (Spruell et al. 1999, 2003; Taylor et al. 2001; Whiteley et al. 2006).

While depauperate neutral genetic variation within populations does not necessarily imply low variability at fitness-related traits (Armbruster et al. 1998; Pfrender et al. 2000), this coupled with high differentiation between populations strongly suggests that Bull Trout have been subjected to large and repeated reductions in effective population size. This will have resulted in part from the influence of postglacial dispersal on stochastic demographic processes such as founder events, bottlenecks, and genetic drift (Hewitt 1996). The influence of historical postglacial recolonization on genetic variation is illustrated by significant reductions (P < 0.05) in microsatellite diversity (HE and number of alleles) in populations that are peripheral to the putative refugia (Costello et al. 2003; Whiteley et al. 2006). Contemporary factors will also influence these demographic processes, modifying historical patterns of intraspecific genetic variation. For example, microsatellite surveys have shown that migration barriers (both human-constructed and natural) influence distribution of genetic variation among Bull Trout populations (Costello et al. 2003; Whiteley et al. 2006). The extent of their impact varies spatially, however, and interacts with other important influential factors, such as watershed area and habitat complexity (Costello et al. 2003; Whiteley et al. 2006).

Life-history characteristics will strongly affect the impact of these demographic processes. As a long-lived, late-maturing top aquatic predator, Bull Trout populations tend to be relatively small (see ‘Population and Sizes’ section). This makes them especially vulnerable to the effects of founder events and bottlenecking (Avise 2004). Radiotelemetry shows this largely migratory species displays strong site fidelity to spawning area and overwintering habitat (Swanberg 1997a; Bahr and Shrimpton 2004); a characteristic that is linked to increased population differentiation (Quinn and Dittman 1990). Other intrinsic barriers, such as avoidance of marine waters by most populations, could also constrain gene flow between local populations. Salmonid fishes that make migrations to sea are usually less genetically subdivided than those that are freshwater bound (Ward et al. 1994). Nevertheless, there is no compelling evidence to suggest that sea migration affects genetic structure in Bull Trout (although it awaits closer scrutiny); anadromous (see ‘Dispersal and Migration’ section) Genetic Lineage 1 populations (FST = 0.33, P < 0.001; Taylor and Costello 2006) are no less structured than non-anadromous Genetic Lineage 2 ones (FST = 0.33-0.39, P < 0.005; Taylor et al. 2001; Costello et al. 2003).

Given the plethora of historical, contemporary landscape and biological influences, it is no surprise that there is considerable variation in genetic structure across the range of Bull Trout at the fine scale. Within the broad pattern of low genetic diversity within and high differentiation between populations, there are significant differences in mean HE, number of alleles, and pairwise FST among river basins (Whiteley et al. 2006). This indicates the varying roles of genetic drift and gene flow at this scale.

Designatable Units

Designatable units (DUs) in Bull Trout within Canada were evaluated in light of the discreteness and significance criteria of COSEWIC (2009). In terms of discreteness, Bull Trout occupy four of Canada’s fourteen National Freshwater Biogeographic Zones (NFBZs; Zones 11 [Pacific], 4 [Saskatchewan-Nelson River], 13 [Western Arctic] and 6 [Yukon River Watershed]), resulting in several putative DUs. Recognition of these putative DUs is further supported by various aspects of the zoogeography, ecology, and evolutionary history of Bull Trout.

First, the Pacific NFBZ (Figure 5) encompasses in part, Bull Trout populations east of the Coastal-Cascade Mountain crest that are tributary to the North Pacific Ocean. Their extinction would constitute a loss of approximately 50% of the range of Bull Trout, and the vast majority (> 90%) of the range west of the Continental Divide. This assemblage of populations is also the only one to contain representatives of Genetic Lineage 1, the major evolutionary Bull Trout lineage which dominates populations south of about 50 degrees north latitude. Genetic Lineage 1 contains the only anadromous (sea-going) Bull Trout, a major life history difference with attendant adaptations for survival in marine waters relative to inland populations. Although it awaits closer scrutiny, there is no compelling evidence to suggest that sea migration affects genetic distinctness in Bull Trout (Taylor et al. 2001; Costello et al. 2003).

The Pacific NFBZ also harbours representatives of the other major evolutionary Bull Trout lineage, Genetic Lineage 2. While most river systems within this NFBZ harbour populations belonging to just one of these, one major river system, the Fraser River, holds populations from both. These lineages are distinguished by mtDNA, and corroborated by a diverse and independent set of traits (neutral nuclear DNA markers, other inherited traits and biogeographical patterns). Two putative DUs for the Pacific NFBZ are, therefore, proposed: Genetic Lineage 1: Southcoast BC populations, and Genetic Lineage 2: Pacific populations). All other putative DUs contain only representatives of Genetic Lineage 2.

Second, the Yukon River Watershed NFBZ (Figure 5) encompasses a proposed DU (Genetic Lineage 2: Upper Yukon Watershed populations) whose populations are tributary to the Yukon River drainage. They represent the only assemblage of Bull Trout populations west of the Continental Divide in a system that is tributary to the Bering Sea. The Yukon River watershed in British Columbia (where Bull Trout occur) has a distinctive freshwater fauna (e.g., many species were derived from the Bering Glacial Refuge (Lindsey and McPhail 1986) such that these populations of Bull Trout exist in an ecological setting that is very unusual for the species as a whole.

Third, the Western Arctic NFBZ (Figure 5) encompasses a proposed DU (Genetic Lineage 2: Western Arctic populations) whose populations are from the Mackenzie River system (and major tributaries such as the Liard, Peace and Athabasca rivers). These rivers have a distinctive zoogeographic assemblage of fishes (being a variable mix of largely Bering and Great Plains species), and loss of these populations would eliminate approximately 30% of the range of Bull Trout and the few that occur north of the Arctic Circle.

Finally, the Saskatchewan-Nelson Rivers Watershed NFBZ (Figure 5) consists of a proposed DU (Genetic Lineage 2: Saskatchewan-Nelson populations) whose populations are tributary to the western headwaters of the North and South Saskatchewan rivers. These systems, particularly the latter are dominated by a Great Plains fish fauna within an environmental setting that is quite distinct compared to other northern-flowing Arctic drainages (which also flow east of the Continental Divide). Loss of these populations would eliminate the only component of the Bull Trout assemblage in Canadian watersheds that are tributary to the Hudson Bay drainage.

In summary, recognition of five DUs in Bull Trout is based on the obvious discreteness inherent in two phylogenetic lineages occupying four NFBZs. Each of these DUs is also significant in terms of the distinctive ecological and zoogeographic settings that they represent (and the realized and inferred attendant phylogeographic and adaptive differences associated with such distinctions), their current demographic independence (all are and have been historically separated by natural watershed divides, and the major gaps in distribution of Bull Trout that would be created should any DU become extinct. Consequently, this report recognizes five DUs for Bull Trout in Canada (Figure 5):

DU1[Genetic Lineage 1: Southcoast BC populations]
DU2 [Genetic Lineage 2: Western Arctic populations]
DU3 [Genetic Lineage 2: Upper Yukon Watershed populations]
DU4 [Genetic Lineage 2: Saskatchewan-Nelson River populations]
DU5 [Genetic Lineage 2: Pacific populations]

Special Significance

Bull Trout’s worldwide Vulnerable status (NatureServe 2009; IUCN 2010) reflects its moderate risk of extinction or elimination. Although there has been a general decline throughout its range during the last century, the Canadian range of Bull Trout is considered to be its stronghold; a general north to south trend in the status of populations describes increasing imperilment near its southern margins (Haas and McPhail 1991). Bull Trout is listed as Threatened under the Endangered Species Act (USFWS 1999) in the USA, with many of its populations having become extinct or isolated (Rieman and McIntyre 1993). The Bull Trout’s narrow tolerance to environmental conditions, combined with its broad distribution in British Columbia, leads it to be used as an indicator species in this province, whose population status may be representative of the health of the watersheds in which it occurs (BCMWLAP 2002).

Although there has been little investigation into Bull Trout’s ecological role, its voracious, piscivorous appetite is probably a strong influence on community structure, and ecosystem energy and nutrient flows. This supposition is supported by studies on other piscivorous fishes, including char, which demonstrate their capacity to indirectly regulate organisms at lower trophic levels (e.g., Dolly Varden: Nakano et al. 1999; Baxter et al. 2004; Brook Trout: Bechara et al. 1992). Coupled with their migratory life histories, this characteristic likely leads Bull Trout to link food webs, as well as the flow of energy and nutrients, between different habitats. Again, support for this role comes from descriptions of other migratory fishes (e.g., Gende et al. 2002).

Considerable life history diversity characterizes this species. Anadromous Genetic Lineage 1 populations in the south west of British Columbia (Fraser and Squamish drainages) and northwest of Washington are of particular interest for their unique migratory behaviour (Cavender 1978; Haas and McPhail 1991; Brenkman and Corbett 2005; Brenkman et al. 2007). In addition, high levels of genetic diversity residing at the interpopulation and inter-region level typify Bull Trout. A growing understanding of this phylogeography contributes to a broader understanding of the biogeography of northwestern North America (e.g., Taylor et al. 1999). Contact sites between Bull Trout and Dolly Varden are of particular scientific interest, where sympatric populations persist as genetically distinct species in the face of ongoing hybridization. This contact zone provides important opportunities for biogeography and evolutionary research, such as:

  1. The historical and geographic contexts of past introgression and current hybridization (Rieseberg 1998);
  2. The potential role of ecology and genetics in structuring hybrid zones and influencing the evolution of reproductive isolation itself (Jiggins and Mallet 2000);
  3. The concordance between aquatic and terrestrial “suture zones”, broad areas of contact and hybridization between formally isolated species that are roughly coincident across a broad range of taxa (Remington 1968).

Once considered ‘junk’ fish because of their tendency to prey on other salmonids (McPhail 2007; Dunham et al. 2008), Bull Trout are now valued as sportfish. For example, there are locally important recreational fisheries in the lower Fraser River (especially between New Westminster and Vancouver, Chilliwack Lake, Squamish River, Pitt Lake, and upper Pitt River, Taylor and Costello 2006), as well as in the upper Columbia Basin (Hagen 2008). Misidentification of Bull Trout with other trout and char species (Rodtka 2009) increases the fishing pressure on this species that is particularly sensitive to overharvesting (Paul et al. 2003; Post et al. 2003).

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