Bull trout (Salvelinus confluentus) COSEWIC assessment and status report 2012: chapter 6

Distribution

Global Range

Bull Trout are endemic to western Canada and the U.S. Pacific Northwest and, like many other taxa that have recolonized formerly glaciated areas, Bull Trout occupy a large geographic range (Figure 5; Figure 6). Their current distribution extends from the Oregon-California border and northern Nevada (42 °N) north to southern Yukon and southwestern Northwest Territories (65 °N; Haas and McPhail 1991; Mochnacz et al. 2009). Although Bull Trout do reach the Pacific Coast in southwestern British Columbia (Fraser and Squamish drainages) and north west Washington (Skagit drainage and the Olympic Peninsula; Cavender 1978; Haas and McPhail 1991), and extend to approximately 113 °W, they are generally restricted to interior drainages (Haas and McPhail 1991). Concentrated west of the Continental Divide, Bull Trout extend across the eastern slope of the Continental Divide to their eastern edge (114 °W); from the upper Columbia and South Saskatchewan systems in western Montana and Alberta north to the Mackenzie River system in the Northwest Territories (Haas and McPhail 1991; Reist et al. 2002).


Figure 6. Approximate current and historical global range of Bull Trout

Map of the approximate current and historical global range of the Bull Trout. Distribution is not continuous throughout the range.

Distribution is not continuous throughout range. Historical range sourced from McPhail and Baxter 1996; current range modified from Figure 5, USFWS 2008, Rodtka 2009).

Their range in the contiguous US, however, has become greatly restricted in recent times. Historically, they were considerably more widespread (Rieman et al. 1997; USFWS 1999; Figure 6). Originally found in northern California (41°N), they have been extirpated from all but northern parts of Nevada, although they are still to be found in Washington, Idaho, Montana, and Oregon, with the southern extent of their range now lying at the Oregon-California border (42 °N; Haas and McPhail 1991; USFWS 2008). Populations have been historically fragmented but remnant Bull Trout populations have now likely become even more isolated (Rieman et al. 1997). Temperature appears to be an important determinant of the southern limit of cold water fish (Dunham et al. 2003) and, as one moves north through its range, Bull Trout appear to increase in the number of sites where they occur (Haas and McPhail 1991; McPhail 2007). This trend is likely due, at least in part, to the more pristine and suitable environments in northerly regions (Haas and McPhail 1991). In recent decades, the distribution of Bull Trout has also declined in eastern parts of its range in Alberta (Rodtka 2009; see below).

Canadian Range

The largest portion of Bull Trout’s global range occurs in western Canada (about 80% of its global range; Rieman et al.1997), occurring in British Columbia, Alberta, Yukon and the Northwest Territories across four NFBZs (Figure 5): Zones 4 (Saskatchewan-Nelson River Watershed), 6 (Yukon River Watershed), 11 (Pacific), and 13 (Western Arctic). In fact, the majority of the land base for extant populations of Bull Trout is in British Columbia (Pollard and Down 2001). It is considered the last remaining jurisdiction with wide distribution of Bull Trout (Pollard and Down 2001; McPhail 2007); it is known to occur in 26 of 36 Ecological Drainage Units (EDUs) that have been defined in British Columbia (Hagen and Decker 2011). This classification level represents distinct major water drainages that contain unique fish assemblages based on broad zoogreographic, physiographic and climate patterns (Ciruna et al. 2007). Bull Trout are found in the cool waters of most major mainland drainages of this province; they are distributed throughout interior river drainages (e.g., upper Columbia, Peace, Liard, and Yukon River drainages) and in those major coastal drainages that penetrate the Coast Mountains into the interior of the province (e.g., the Fraser, Homathko, Klinaklini, Skeena, Nass, Iskut-Stikine, and Taku rivers (Haas and McPhail 1991; McPhail 2007; Hagen and Decker 2011). They are absent, however, from Vancouver Island, the Queen Charlotte Archipelago, and three adjacent, warm water drainages (Okanagan, Kettle and Similkameen) of the southern interior (Figure 5; Haas and McPhail 1991; McPhail 2007; Hagen and Decker 2011). Coastal populations of Genetic Lineage 1 found on the British Columbia’s south coast (e.g., Squamish River, Lower Fraser River) are isolated from the coastal lineage of Genetic Lineage 2 by an extensive area of coastline between the Squamish watershed on the south coast and the Homathko watershed on the central coast (McPhail 2007; Hagen and Decker 2011). In the shorter coastal mainland rivers between these rivers, Dolly Varden are thought to be the only native char species present (McPhail 2007; Hagen and Decker 2011).

Detailed knowledge of Bull Trout distribution has been considered inadequate for most areas of British Columbia (Cannings and Ptolemy 1998), and expert opinion has often been relied upon to provide best estimates. Increased efforts in reconnaissance level inventories over the past 15 years have improved our understanding of general Bull Trout distribution in this province (Pollard and Down 2001), although some gaps in Bull Trout records most likely still exist, particularly in remote or pristine areas with low levels of industrial activity e.g., broad areas of the Middle Fraser, and the northernmost parts of BC (reviewed in Hagen and Decker 2011). Gaps in knowledge also occur in coastal headwater streams where recent sampling has identified Bull Trout in systems thought to be fishless (E. Stoddard, pers comm. 2009) In contrast to these sources of error that likely result in underestimation bias, a key source of overestimation bias that needs to be mitigated is the broad zone of Bull Trout sympatry with Dolly Varden, where positive identification is difficult (Hagen and Decker 2011).This issue represents a major source of uncertainty with respect to the distribution and status of both species throughout a large portion of British Columbia (Hagen and Decker 2011).

Bull Trout is the only native char species that can be found in all of the major eastern slope drainages in Alberta (Peace, Athabasca, South Saskatchewan and North Saskatchewan River drainages [Figure 5; Haas and McPhail 2001; Rodtka 2009]). Historically, Bull Trout was even more widely distributed in this province (Figure 6), with anecdotal information and limited historical records suggesting a large decline in distribution in all the river systems occupied in Alberta since the early 1900s. Where Bull Trout were once to be found further downstream, they now tend to occupy only the upstream reaches of the major drainages. Most populations are now found within the Rocky Mountain and Foothills natural regions, as well as a small portion of the Peace River Parkland and Dry Mixedwood subregions (Rodtka 2009). They can, however, be found further inland in the more northerly Peace and Athabasca drainages, albeit in lower abundance (Berry 1994).

Previous taxonomic confusion, combined with generally poor sampling of northern areas (> 60°N), has led to uncertainty in the northern limit of Bull Trout’s range. While taxonomic resolution and identifications between Dolly Varden and Bull Trout were addressed for the other provinces and territories in which Bull Trout is found by Haas and McPhail (1991), the situation remained unresolved in the Northwest Territories until 2002 (Reist et al. 2002). Their review of historical records and new specimens revealed Bull Trout occurs in the western portion of the Northwest Territories, in Mackenzie River drainages north to the central Sahtu Settlement Area (Reist et al. 2002). Work since then has continued to strengthen our knowledge about the northern extent of this species: e.g., Mochnacz et al. (2006), Mochnacz and Reist (2007) and Mochnacz et al. (submitted) have confirmed that Bull Trout is widely but sparsely distributed throughout much of southern (Deh Cho) and central (Sahtu) Northwest Territories in drainages west of the Mackenzie River (Figure 5). To date the northernmost location known is the Gayna River (Mochnacz et al. 2009). While this summarizes our most up-to-date understanding of Bull Trout’s distribution in the Northwest Territories, new information from this area will continue to refine our knowledge as it becomes available (Reist and Sawatzky 2010).

In Yukon, Bull Trout occur mainly in the Liard River drainage basin but are also thought to occupy the upper Yukon River watershed (Figure 5). Since a Bull Trout sample from the Liard River (which drains into the Mackenzie River) corroborated its presence in the southeast of this territory (Haas and McPhail 1991), Bull Trout has been confirmed in numerous drainages and lakes of the Liard River watershed in southeast Yukon (Can-nic-a-nick Environmental Sciences 2004). Although the overall distribution of Bull Trout in this remote area remains somewhat unclear, a recent modeling exercise and site visits reveal that Bull Trout is likely widespread in this drainage basin (Miller pers. comm. 2010). On the other hand, little is known about the distribution of Bull Trout in the upper Yukon River watershed drainage. Bull Trout have been found in the extreme headwaters of this drainage in northwestern British Columbia (Haas and McPhail 1991) and a traditional knowledge study undertaken by the Teslin Tlingit Council in the late 1990s indicated that fish of the Dolly Varden/Bull Trout complex could be found in rivers of the Yukon River drainages within their Traditional Territory (Connor et al. 1999). Anecdotal reports also report char from this area. However, a thorough survey failed to capture any from this vicinity (Connor et al. 1999).

The extent of occurrence (EO) and index of area of occupancy (IAO) were estimated for each DU according to the COSEWIC guidelines (i.e. using the minimum convex polygon method for EO, and using an overlaid grid of cells 2kmX2km for IAO). All index of area of occupancy (IAO) calculations are minimum estimates based on confirmed Bull Trout observations. Although many smaller streams are known to support seasonal adult populations and/or juvenile or resident populations of Bull Trout (Christiansen pers. comm. 2010), the Bull Trout observation data used here is limited to larger order rivers and streams. The IAO estimates are, therefore, likely to be underestimates. In every instance, recorded observations are insufficient to accurately calculate IAO and estimates are provided only as rough guidelines for comparison with threshold values:

  • DU1[Genetic Lineage 1: Southcoast BC populations] range includes the Skagit River, Squamish River, Ryan River, Ure Creek, Lillooet River, Pitt Lake/River, Lower Fraser River (below Hell’s Gate Canyon), Chilliwack Lake, Phelix Creek, Birkenhead Lake, and Chehalis Lake (Taylor et al. 1999; Taylor and Costello 2006). Its EO is estimated to be 32,053 km2. The IAO is in excess of the Threatened threshold of 2000 km2.
  • DU2 [Genetic Lineage 2: Western Arctic populations]range includes Mackenzie River drainages including the Liard, Peace and Athabasca River basins. Its EO is estimated to be greater than 20,000 km2. The IAO is in excess of the Threatened threshold of 2000 km2.
  • DU3 [Genetic Lineage 2: Yukon River Watershed populations]range includes the upper Yukon River basin. There is a lack of information about Bull Trout distribution for this DU. EO and IAO are unknown for this DU.
  • DU4 [Genetic Lineage 2: Saskatchewan-Nelson Rivers populations] range includes North and South Saskatchewan River drainages. Its EO is estimated to be greater than 20,000 km2. The IAO is in excess of the Threatened threshold of 2000 km2.
  • DU5 [Genetic Lineage 2: Pacific populations]range includes the Upper Columbia, Fraser above Hell’s Gate Canyon, Homathko, Klinaklini, Skeena, Nass, Iskut-Stikine and Taku rivers. Its EO is estimated to be greater than 20,000 km2. The IAO is in excess of the Threatened threshold of 2000 km2.

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