Rocky Mountain ridged mussel (Gonidea angulata) management plan: summary
Executive Summary
The Rocky Mountain Ridged Mussel is a freshwater mollusc and was listed as a species of special concern under the Species at Risk Act (SARA), in July 2005. This large freshwater bivalve has a trapezoidal shell. The most distinctive feature of the shell is a prominent posterior ridge running almost parallel with the anterior margin, from the umbo to the angular basal posterior margin of each valve. The posterior length of the shell exceeds the anterior length of the shell.
The life cycle of the Rocky Mountain Ridged Mussel is complex and poorly understood. After fertilization, the female incubates eggs within her shell; the eggs hatch into microscopic larvae (called glochidia). Planktonic glochidia are released from the female mussel and must attach to the gills of a suitable host fish (species unknown) within a few days of release. After this parasitic stage, the glochidia develop into a benthic free-living juvenile and drop off the host fish, where they settle into substrate and grow into an adult mussel. Adult mussels will live within suitable substrate in the littoral zone, siphoning water and growing in size. It is unknown to what water depths mussels live. The maximum age or age of maturity of adult mussels is also unknown.
In Canada, the Rocky Mountain Ridged Mussel is known to occur in British Columbia in the Okanagan River watershed, one historic occurrence from the ‘Kootenays’ (location unknown) collected in the early 1900’s and one historic occurrence labelled ‘Vancouver Island’ (location unknown).
Occurrence records from various sources yield eleven sites where live specimens have been found within the Okanagan River watershed (one location). It is difficult to define a location (as defined by the Committee on the Status of Endangered Wildlife in Canada) for the Rocky Mountain Ridged Mussel due to the lack of information on the dispersal of the species at all life stages.
The current range of the Rocky Mountain Ridged Mussel includes the Okanagan River watershed, from the northernmost record in Vernon to the southernmost record of a shell found in the Osoyoos area. The range extent will be expanded if occurrences are found in the Kootenay or Columbia River watersheds, or the historic record from Vancouver Island is confirmed.
Population information on the Rocky Mountain Ridged Mussel is unknown and densities likely vary depending on habitat suitability and quality, with lower quality habitat supporting lower mussel densities. Most records are from 1 – 10 individuals within a survey area. However, there are three sites in the Okanagan River watershed with colonies > 100 individuals.
Threats to the Rocky Mountain Ridged Mussel are not well understood. Potential threats include: 1) foreshore/riparian development; 2) historic riverbed channelization; 3) hydrograph modification and regulation; 4) aquatic introduced species; 5) host species availability; 6) watershed land-use related pollution; 7) disturbance or direct harm; and 8) climate change.
The management goal for the Rocky Mountain Ridged Mussel is to maintain viable, self-sustaining, ecologically functioning and broadly distributed populations within suitable habitats in its current distribution/range in B.C.
The management objectives for the Rocky Mountain Ridged Mussel are: 1) By 2015, address knowledge gaps about the life history, provincial range and threats to the Rocky Mountain Ridged Mussel; 2) By 2015 inventory 75% of potential littoral habitat within the Okanagan River watershed, with standardized protocol for habitat and threat information collected at each site searched; 3) By 2015, demonstrate an increased number of stewardship activities initiated and completed for land managers and public users of habitats occupied by the Rocky Mountain Ridged Mussel; and 4) As research and inventory results on Rocky Mountain Ridged Mussel become available, incorporate into land-use planning to inform future threat mitigation and land use protection.
The approach to implementing new actions is to focus on addressing the uncertainty related to the Rocky Mountain Ridged Mussel e.g. life history, limiting factors and threats, through research and assessment thereby strengthening the foundation for future management actions.
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