Westslope cutthroat trout COSEWIC assessment and status report: chapter 12

Technical Summary: Alberta Population

 

Oncorhynchus clarkii lewisi
Common name:  westslope cutthroat trout      Common name (French): truite fardée
Range of Occurrence in Canada: Alberta

Extent and Area information

·        extent of occurrence (EO) [from text and Figure 6]      ~ 20,000 km²
·        specify trend (decline, stable, increasing, unknown)      Decreasing
·        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}      <2000 km2
·        specify trend (decline, stable, increasing, unknown)
            Declining
            213 of 274 locations lost
·        are there extreme fluctuations in AO (> 1 order magnitude)?      Unknown
·        number of extant locations      61 named streams in 2 river systems
·        specify trend in # locations (decline, stable, increasing, unknown)      Declining
·        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      Declining

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)      61 streams 100 mature individuals/stream) = 6100 (Table 3)
·        total population trend:  specify declining, stable, increasing or unknown trend in number of mature individuals      Decline
·        if decline, % decline over the last/next 10 years or 3 generations, whichever is greater (or specify if for shorter time period [Although 78% of the historical populations have been lost, the trend started in the early 1900s due to over- harvesting; the % decline during the last 3 generations is unknown]      Unknown
·        are there extreme fluctuations in number of mature individuals (> 1 order of magnitude)?      Unknown
·        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)?      Yes, many populations isolated in headwater systems
·        list each population and the number of mature individuals in each      An average of 100/stream in each of 61 streams in 2 river systems
·        specify trend in number of populations (decline, stable, increasing, unknown)      Pure populations declining
·        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 non-native salmonids (rainbow trout, brook trout, Yellowstone CT, coarse fish)
-         Habitat loss/dewatering (damming, urbanization, irrigation withdrawals)
-         Habitat Degradation (Road construction, agriculture, mining)
-         Over-harvesting

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?      Unlikely
·        would immigrants be adapted to survive here?      Unknown
·        is there sufficient habitat for immigrants here?      Unknown

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 – AB – S1
            Wild Species 2005 (Canadian Endangered Species Council 2006)
                                   National – 4
                                   Regional – AB – 2
           COSEWIC
                       
Threatened May 2005

Status and Reasons for Designation

Status: Threatened
Alpha-numeric code: B1+2ab(i,ii,ii,iv,v)+ 2ab(i,ii,iii,iv,v); C2a(i)
Reasons for Designation: Native populations have been reduced by almost 80% through over-exploitation, habitat degradation, and hybridization / competition with introduced, non-native trout.  Remaining genetically pure, individuals persist as mainly severely fragmented, remnant headwater populations.  It should be noted that this assessment includes only genetically pure, native populations of the species occurring within their historical range.  Any populations known either to be hybridized significantly (i.e. >1%) with other trout species, or to have been introduced into a system previously free of native populations were not assessed.

Applicability of Criteria

Criterion A (Declining Total Population): Not Applicable – decline rate in the last 3 generations is not known.

Criterion B (Small distribution, and Decline or Fluctuation): Meets criterion 1 and 2 for Threatened (EO < 20,000 Km2, AO < 2000 km2), and the population structure is one of extreme fragmentation (criterion a).  Continuing decline in extent of occurrence, area of occupancy, area and extent of habitat, number of locations and number of mature individuals (criterion b(i,ii,ii,iv,v.)

Criterion C (Small Total Population Size and Decline):  Meets criteria 2a(i) for Threatened.  The total is probably less than 6100 mature individuals and is declining due to over-harvest, habitat loss and degradation, and competition and hybridization with other introduced salmonids. The population structure is extremely fragmented and no population appears to contain more than 200 individuals.  The rate of decline is not known, but 59% of sampled steams provide evidence of hybridization at a mean frequency of 34% (see Table 4).
Criterion D (Very Small Population or Restricted Distribution: Not Applicable – Number of individuals and area of occupancy exceed threshold values.
Criterion E (Quantitative Analysis): Not applicable – Data not available.

 

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