Westslope cutthroat trout COSEWIC assessment and status report: chapter 11
Technical Summary: British ColumbiaPopulation
Oncorhynchus clarkii lewisi
Common name: westslope cutthroat trout Common name (French): truite fardée
Range of Occurrence in Canada: British Columbia
Extent and Area information
· extent of occurrence (EO) [from text and Figure 5] ~ 150,000 km²
· specify trend (decline, stable, increasing, unknown) Generally stable
· are there extreme fluctuations in EO (> 1 order of magnitude)? Unlikely
· area of occupancy (AO) [Populations are structured over small distances (1-2 km) coinciding with small watershed groups and different stream sections in larger systems] <30,000 km²
· specify trend (decline, stable, increasing, unknown) Generally stable
· are there extreme fluctuations in AO (> 1 order magnitude)? Unknown
· number of extant locations (includes lakes and streams, 301 of which have been stocked with WCT) ~ 928 (1229 including stocked waterbodies)
· specify trend in # locations (decline, stable, increasing, unknown) Decline
· are there extreme fluctuations in # locations (>1 order of magnitude)? Unlikely
· habitat trend: specify declining, stable, increasing or unknown trend in area, extent or quality of habitat Decline
Population information
· generation time (average age of parents in the population) (indicate years, months, days, etc.) 3-5 years
· number of mature individuals (capable of reproduction) in the Canadian population (or, specify a range of plausible values) (based on 30-100 adults/population and whether or not stocked systems are included) 29,400 – 122,900
· total population trend: specify declining, stable, increasing or unknown trend in number of mature individuals [No province-wide surveys have been conducted to accurately determine the number of pure populations remaining] Hybrid data suggests total number of pure individuals declining
· if decline, % decline over the last/next 10 years or 3 generations, whichever is greater (or specify if for shorter time period) Unknown
· are there extreme fluctuations in number of mature individuals (> 1 order of magnitude)? Likely
· is the total population severely fragmented (most individuals found within small and relatively isolated (geographically or otherwise) populations between which there is little exchange, i.e., < 1 successful migrant / year)? Unknown, suspected that many isolated in headwater streams, or geographically isolated by falls and rapids from hybrid populations
· list each population and the number of mature individuals in each As many as 928 -1229 populations each containing 30-100 mature individuals
· specify trend in number of populations (decline, stable, increasing, unknown) Number of pure populations declining due to introgression; headwater populations of unknown status
· are there extreme fluctuations in number of populations (>1 order of magnitude)? Unlikely
Threats (actual or imminent threats to populations or habitats)
- Hybridization with introduced rainbow trout, other CT subspp.
- Competition with introduced salmonids (rainbow trout, brook trout), and non-salmonids (e.g., yellow perch, bass species, walleye)
- Habitat loss/dewatering (damming, urbanization, irrigation withdrawals)
- Habitat degradation (road construction, agriculture, mining)
- Over-harvesting
- Climate change
Rescue Effect (immigration from an outside source) Low
· does species exist elsewhere (in Canada or outside)? Yes
· status of the outside population(s)? Pure populations severely isolated and depressed
· is immigration known or possible? Possible, but unlikely except from Flathead
· would immigrants be adapted to survive here? Unknown
· is there sufficient habitat for immigrants here? Likely yes
Quantitative Analysis Not available
Existing Status
Nature Conservancy Ranks (Naturserve 2004)
Global– T3
National
US – N2
Canada NNR
Regional
US – Colorado – SNA, Idaho – S2, Montana – S3, Oregon – S3, Washington – SNR, Wyoming S1
Canada – BC - S3
Wild Species 2005 (Canadian Endangered Species Council 2006)
National – 4
Regional – BC – 4
COSEWIC
Special Concern May 2005
Status and Reasons for Designation
Status: Special Concern
Alpha-numeric code: Not Applicable
Reasons for Designation: Populations are stressed by hybridization and competition with introduced species. Furthermore, expanding urban development, agricultural activities and resource-based industries are expected to lead to additional stresses associated with habitat loss and degradation, as well as increased exploitation. It should be noted that this assessment includes only genetically pure, native populations of the species occurring within their native historical range. Any populations known to be hybridized significantly (i.e., > 1%) with other trout species, or to have been introduced into a system previously free of natve populations, were not assessed.
Applicability of Criteria
Criterion A: (Declining Total Population): Not Applicable – Rate of decline is unknown; however, in 1999, in the upper Kootenay, of the 928-1229 streams known to harbour WCT, hybridization was found at 18 of 23 streams tested, an increase over previous sampling in 1986.
Criterion B: (Small Distribution, and Decline or Fluctuation): Not applicable – threshold values are exceeded.
Criterion C: (Small Total Population Size and Decline): Not applicable – Number of mature individuals exceeds threshold levels; however, the population structure is highly fragmented and no population contains more than 100 mature individuals. The rate of population decline is unknown, but more than 57% of sites sampled indicate that hybridization is occurring at a frequency >5% and the spread of hybridization is increasing.
Criterion D: (Very Small Population or Restricted Distribution): Not Applicable – Number of mature individuals, as well as EO and EO exceed the threshold values.
Criterion E: (Quantitative Analysis): Not Applicable – Data not available.
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