Westslope cutthroat trout COSEWIC assessment and status report: chapter 7

Population Sizes and Trends

Few quantitative estimates of population size exist for WCT in Canada, especially for British Columbia. Because they tend to occupy colder, less productive habitats, population sizes for WCT are expected to be smaller than for other sympatric salmonids. Exact numbers are likely a function of stream size but are typically on the order of tens to hundreds in even the largest systems (Trotter 1987, Behnke 1992). Age structure may be highly variable as juveniles may reside in natal streams for up to five years before becoming sexually mature (Behnke 1992). Westslope cutthroat trout are iteroparous (repeat spawning) so that several age classes may contribute to annual spawning efforts. The actual number of spawners appears to vary from year to year, but effective population sizes (the percentage actually contributing to reproduction) may be on the order of only 8 - 10% of the total census population so that the number of adult spawners supporting population growth may be quite small (Frankham 1996; Brown and MacKay 1995b; A. Costello unpublished data).

 

British Columbia population

We suggest a probable range of values of between 30 and 100 mature individuals per population (these values will be used to give rough estimates of total population size in the Technical Review section). These values are in keeping with what is known about cutthroat trout biology and the nature of the smaller, less productive streams they inhabit. “Best guess” population estimates for coastal cutthroat trout streams on the east coast of Vancouver Island, for example, average between 30 and 50 adults (Scholten 1997) and in the absence of better data for inland populations, we will assume similar values for WCT.  We will further assume that there is a minimum of one “population” per stream or lake. For small systems (i.e., first to third order streams), this assumption can be supported by genetic data which suggests that the majority of WCT populations are structured over small geographic distances (e.g., Taylor et al. 2003) as all life history forms of WCT return to small tributary streams to spawn (see Carl and Stelfox (1989) for a rare exception). For larger river systems, it is likely that this assumption will be violated and that several independent component stocks (from isolated stream reaches or tributary streams) will be unaccounted for. As noted earlier, Brown and Mackay (1995b) found that fluvial WCT in the Ram River, Alberta maintained breeding territories of ~ 400 m in their natal creeks. For this reason, we extend the upper limit of probable population size to 100 to allow for any missed population subdivision in larger systems. GIS-based analysis of the number and size of the individual systems inhabited by WCT, and the informed, “best-guess” approach of regional fisheries biologists, could be used in future to generate more accurate estimates.

Within the native range of WCT in BC, WCT have been reported in 928 water bodies (including creeks, rivers and lakes) where no record of WCT hatchery releases exists, based on survey data.  The majority of these streams are in the Kootenay River and Flathead watersheds but a number of disjunct systems containing WCT also occur in the Upper Columbia and South Thompson watersheds. Of these 928 waterbodies, 38 have received hatchery rainbow trout at least once, and 31 have received hatchery coastal cutthroat trout at least once.  Another 301 waterbodies within the native range of WCT have been stocked with WCT at least once since 1923, but many of these may have originally contained native WCT populations as well (S. Pollard, BC Ministry of Water, Land and Air Protection, Victoria, BC, personal communication 2006; Table 2).

Fluvial populations in large rivers appear to be stable, based on creel surveys (J. Baxter, Fisheries Biologist, BC Hydro, Castlegar, BC, personal communication, 2004) but are clearly subject to increasing fishing pressure and hybridization (Rubidge et al. 2001; Bill Westover, BC Ministry of Water, Land and Air Protection, Cranbrook, BC, personal communication, 2003). Many WCT populations were overexploited in BC from the 1960s to the 1980s leading to dramatic declines (Heidt 2002). River closures and restrictive sport fishing regulations were implemented in the 1980s and 1990s and have been somewhat effective in stabilizing or restoring WCT populations in large rivers such as the Elk, Skookumchuck, and St. Mary’s rivers. However, fishing pressure in the East Kootenay region continues to increase annually.  During 1991, for example, just 81-guided days were recorded on the Elk River. By 2000, that number had jumped to 1458 (Westover, pers.comm. 2003). In a recent creel survey in the Elk River, WCT made up 94.5% of a total catch estimate of 98,031 fish (~ 1.48 fish/rod hour; Heidt 2002). Similar increases have occurred in other major WCT fishing rivers. As noted, this could simply reflect their greater relative abundance. However, it is likely that fish are caught numerous times during a season, which would lead to creel survey estimates that are biased upwards. Furthermore, creel surveys do not account for a typical salmonid hooking mortality of 3-5% (Marnell and Hunsaker 1970), which suggests approximately 3000-5000 cutthroat caught in the Elk River may have died after being released.

In the upper Kootenay River watershed, it is evident that many populations have become adversely impacted by hybridization with non-native rainbow trout introduced to supply sport-fishing demand (Figure 8). Evidence of hybridization with introduced rainbow trout has been reported in 78% of the streams genetically tested in the area (ntotal=23; Rubidge 2003). The Lodgepole Creek population in particular (tributary of the Wigwam River in the Elk River drainage) has experienced advanced hybridization (37.5% heterospecific alleles) and appears to be forming a hybrid swarm. Hybrid swarms have been shown to form between cutthroat and rainbow trout in as little as five generations (Hitt 2002) and pose a critical risk to the remaining WCT populations throughout their range (see LIMITING FACTORS). Increasingly, the introgression appears to be spreading throughout the lower reaches of systems nearest the Koocanusa Reservoir, where a rainbow trout stocking program existed from 1986 to 1998 (Rubidge et al. 2001; Westover, pers. comm. 2003). In fact, recent survey work in the upper Kootenay watershed suggests that any lower tributary reaches accessible from the Kookanusa Reservoir are likely to contain some level of hybridization (S. Bennett, Aquatic, Watershed, and Earth Sciences, Utah State University, Logan, Utah; personal communication 2006), Rubidge and Taylor 2004, Fig. 8).  To date, no systematic surveys of BC waters have been conducted that would permit a description of the amount of introgression existing throughout the B.C. range (Pollard, pers. comm. 2006).  Thus, the existing hybrid data can only be used to assess threats to populations, not the number of WCT populations remaining.  It is critical that systematic survey work be completed to establish the extent of introgression in BC to better quantify this threat

Figure 8.  Summary of levels of hybridization in selectedAlbertadrainages (modified from Janowicz 2004).

Figure 8.  Summary of levels of hybridization in selectedAlbertadrainages (modified from Janowicz 2004).

and level of impact this DU has already experienced.  Regardless, existing data suggests that these populations may become increasingly restricted to isolated headwater streams where they are subject to stochastic extinction events such as rockslides or drought.

Currently, an estimated 928 to 1229 (if we include stocked systems) streams and lakes in the native range may contain WCT populations (Pollard, pers. comm. 2006).  Applying an average of between 30 – 100 individuals per stream/lake, we get an estimate for the total British Columbia population ranging from 29,400 to 122,900 mature individuals.  The situation may be considerably worse depending on level of introgression; a significant portion of these estimates could be introgressed individuals depending on the spread of rainbow trout genes from the original site of introduction.

 

Alberta population

A total of 274 streams in Alberta are believed to have contained native populations of cutthroat trout; of these only 61 (22%) are now known or suspected to still have pure strains of westslope cutthroat trout (Alberta Fisheries Management Information System 2004; Stelfox pers. comm. 2006).  Stream census data (Stelfox, unpubl. data) indicates that the majority of these streams average about 8 km in length and contain from 30 to 200 (mean = 100) adults.  Stelfox (pers. comm. 2006) suggests that the population size for most streams is probably closer to 30 than to 100, but a few larger steams result in the higher mean.  Applying the average number of individuals per stream (n=100) from Table 3, the native Alberta population is probably less than 6100 mature individuals.  Of these, 29 (48%) are deemed to be at risk of extirpation, primarily due to hybridization and/or competition with exotic salmonids.  Wherever cutthroat trout and rainbow trout co-exist it is only a matter of time before pure cutthroat trout are extirpated (Stelfox, pers. comm. 2006).  Recent rates of decline (since the 1990s, i.e., within the last three generations) are not known, but the story has been one of progressive decline since the early decades of the twentieth century.  Initially the declines were largely due to exploitation, but more recently they are a result of competition and introgressive hybridization with introduced species, particularly rainbow trout.

Habitat degradation and the stocking of non-native species in Alberta have led to the displacement/replacement of WCT from many areas and the hybridization of several of the remaining native populations (Carl and Stelfox 1989; Strobeck 1994). Westslope cutthroat trout are known to have disappeared from an estimated 30% of their historic range in Banff National Park (Schindler and Pacas 1996) and now occupy less than 5% of the native range in the Bow River drainage. Several WCT populations are considered to be severely depressed or extirpated [e.g., Quirk, Bragg, Lesueur, Meadow, Sullivan, Loomis, Flat, Odlum, McPhail, Carnarvon, Pekisko, Ware, Threepoint, Fisher, Fish, and Jumpingpound creeks; (Stelfox, pers. comm. 2003)].

Quirk Creek provides one example of these trends.Quirk Creek is a small creek in the Elbow River drainage (Bow River drainage) in southwestern Alberta that was the subject of a WCT population study between 1995 and 2002.  This creek supported only native bull trout and WCT prior to the introduction of brook trout to the Elbow River watershed in 1940 (Stelfox et al. 2001).  A fisheries survey in 1948 found no brook trout in Quirk Creek, but by 1978, they had managed to colonize the lower 3 km of the creek and comprised 35% of the fish population (Tripp et al. 1979).  Electrofishing surveys in 1987 showed catches were still dominated by native WCT and bull trout, but by 1995, brook trout had spread throughout the entire creek and comprised ~ 92% of the fish population. Despite the selective harvest of brook trout since 1998 (Stelfox et al. 2001) the relative composition of fishes in Quirk Creek remained fairly stable from 1995 - 2002 with an average relative composition of 83% brook trout, 15% westslope cutthroat trout and 2% bull trout (Paul 2003). A similar trend is evident in Fish Creek (also in the Bow River drainage). Historically, Fish Creek supported a significant WCT fishery. In 1915, the Department of Naval Science reported that the value of Fish Creek’s native trout fishery was nearly eight times that of the Bow River (reported in Baayens and Brewin 1999). More recent surveys reveal that the WCT population has declined greatly since that time.  Baayens and Brewin (1999), reported maximum likelihood population estimates for introduced brook trout at 211 fish/km, introduced RBT at 59 fish/km and native WCT at only 4 fish/km in the spring of 1993. It is a pattern common throughout the region. 

In areas stocked with RBT, WCT are more subject to hybridization than to displacement. For example, population estimates of the Gorge Creek WCT population (Sheep River drainage) were approximately 800 fish/mile in 1949 (Andrekson 1949). Rainbow trout were introduced into Gorge Creek in 1941 and hybrids are now present in that population (Janowicz 2004).  Hybridization between RBT and cutthroat subspecies is widespread throughout Alberta (Mayhood 2000; Potvin et al. 2003; Janowicz 2004). Again, introduced brook trout and RBT appear to prefer lower elevation mainstem stream reaches (Paul and Post 2001). For this reason, many remaining genetically pure WCT populations are present in small, isolated headwater populations (Donald 1987; Hilderbrand and Kershner 2000b).

The current status of many populations in Banff National Park is unresolved. Early in the last century, WCT were noted to be plentiful in the area of Banff National Park and were recorded in a number of systems in that area. Surveys of the Bow River mainstem through Banff National Park performed during the 1990s, however, found very few WCT between Redearth Creek and Forty Mile Creek; brook trout are now common in the area and the few WCT that were seen appeared to be WCT x RBT hybrids (C. Pacas, Aquatic Ecosystems Specialist, Banff, AB, Parks Canada, personal communication, 2003). There is also evidence of WCT x RBT hybridization in several lakes (Landry et al. 2000; Potvin et al. 2003; see Table 3). It has been suggested that fluvial populations no longer exist in Banff National Park, although at least a small portion of lacustrine populations (WCT were stocked into 64 lakes in the park) appear to be stable. At least one lake (Baker Lake) currently only supports brook trout, and it is believed that the WCT may have been fished out (Pacas, pers. comm. 2003).

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