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COSEWIC assessment and status report on the Rocky mountain ridged mussel Gonidea angulata in Canada

Technical Summary


Gonidea angulata
Rocky Mountain Ridged Mussel        Gonidée des Rocheuses
Range of occurrence in Canada: Southern British Columbia, from Penticton South in the Columbia River System. Known only from Okanagan Falls, Osoyoos Lake, Vaseaux Lake, Skaha Lake, Okanagan River, and Park Rill Creek.

Extent and Area Information

·        Extent of occurrence (EO)(km²)        210 km2
·        Specify trend in EO        Unknown
·        Are there extreme fluctuations in EO ?        Unknown
·        Area of occupancy (AO) (km²)        Less than EO
·        Specify trend in AO        Unknown
·        Are there extreme fluctuations in AO ?        Unlikely
·        Number of known or inferred locations        Unknown, based on ½ shell. See Population Information
·        Specify trend in #        Probably declining
·        Are there extreme fluctuations in # locations ?        Unknown
·        Specify trend in area, extent or quality of habitat        Probably declining in Okanagan due to habitat alteration (past and present)

Population information

·        Generation time (average age of parents in the population)        Unknown
·        Number of mature individuals        Unknown
·        Total population trend:        Unknown
·        % decline over the last/next 10 years or 3 generations.        Unknown
·        are there extreme fluctuations in number of mature individuals (> 1 order of magnitude)?        Unknown
·        Are there extreme fluctuations in number of mature individuals?         Unknown
·        Is the total population severely fragmented?        Yes
·        Specify trend in number of populations        N/A
·        Are there extreme fluctuations in number of populations?        N/A
·        List populations with number of mature individuals in each:        N/A

Threats (actual or imminent threats to populations or habitats)

Water removal, changing water levels and general habitat degradation due to pollution and agriculture in Okanagan River poses immediate threat to species. 

Rescue Effect (immigration from an outside source)        Low

Status of the outside population(s)?        USA:        Rare in U.S.A.; ranked N3
·        Is immigration known or possible?        Given the state of the Okanagan River (i.e., multiple dams and instream structures) it is unlikely that the fish host(s) from U.S. populations could migrate far enough upstream to effectively disperse G. angulata from U.S. to Canadian waters.
·        Would immigrants be adapted to survive in Canada?        Yes
·        Is there sufficient habitat for immigrants in Canada?        Yes
·        Is rescue from outside population likely?        No

Quantitative Analysis        Unavailable

Status and reasons for designation

Status: Special Concern
Alpha-numeric code:N/A
Reasons for Designation: The distribution of this species is limited to southern British Columbia in the Okanagan and Kootenay River systems. This species has likely been impacted by the damming of the Kootenay, Columbia and Okanagan rivers and the channelization of the Okanagan River which resulted in loss or alteration of the mussel’s habitat quality and extent.

Applicability of Criteria

Criterion A (Declining Total Population): No data available to use A criteria.
Criterion B (Small Distribution, and Decline or Fluctuation): Uncertainties in size and trends of EO and AO and in extent and quality of habitat.
Criterion C (Small Total Population Size and Decline): No population size data available to use C criteria.
Criterion D (Very Small Population or Restricted Distribution): No D criteria apply and/or uncertainty in AO.
Criterion E (Quantitative Analysis): No data available to use E criteria.