Fawnsfoot (Truncilla donaciformis) COSEWIC assessment and status report: chapter 12
Technical Summary
Truncilla donaciformis
Fawnsfoot – Troncille pied-de-faon
Range of Occurrence in Canada:
Ontario
Demographic Information
Population trend and dynamics
Number of mature individuals in each population
- Lake St. Clair: Unknown
- Muskrat Creek: Unknown
- Sydenham River: Unknown
- Thames River: Unknown
- Grand River: Unknown
Extent and Area Information
Quantitative Analysis
Not available.
Threats (actual or imminent, to populations or habitats)
- Dreissenid mussels have rendered much of the historical habitat in Lake St. Clair, the DetroitRiver and Lake Erie unavailable and pose a continuing threat to remaining populations in the St. Clair delta, Thames River and Grand River.
- River fragmentation resulting from instream barriers and dams may restrict the upstream movement of the host species (believed to be freshwater drum), thereby limiting the available habitat.
- Poor water quality resulting from rural and urban influences in the watersheds poses a threat as mussels in general are known to be sensitive to many aquatic pollutants.
Rescue Effect (immigration from an outside source)
Current Status
COSEWIC: Endangered, April 2008
Status and Reasons for Designation
Status: Endangered
Alpha-numeric code: A2ce; B2ab(i,ii,iii,iv,v)
Reasons for Designation: This freshwater mussel is widely distributed in central North America, with the northern portion of its range extending into the Lake Erie, Lake St. Clair and lower Lake Huron drainages of southwestern Ontario. It appears to have always been a rare species in Canada, representing < 5% of the freshwater mussel community in terms of abundance wherever it occurs. Approximately 86% of historical records are in waters that are now infested with zebra mussels and therefore uninhabitable. Zebra mussels, which were accidentally introduced into the Great Lakes, attach to the shells of native freshwater mussels, causing them to suffocate or die from lack of food. The species has declined dramatically and has been lost from four historical locations resulting in a 51% reduction in its range. It is now found in only five widely separated locations, two of which represent single specimens. In two locations, the species’ distribution may be limited by the presence of dams that restrict the movements of the frehswater drum, the presumed fish host of the juvenile mussels. Poor water quality resulting from rural and urban influences poses an additional continuing threat.
Applicability of Criteria
- Criterion A: (Decline in Total Number of Mature Individuals): Meets Endangered A2ce: A(2) Estimated population size reduction of >50% over the past 3 generations (~30 years, given that the average age of maturity for unionids is 6-12 years), where the main cause (impacts of zebra mussels, which began in 1986) may not have ceased and may not be reversible based on:
- (c) a decline in EO of ~ 51% from 51,238 km² historically to 24,952 km² at present and a decline in the quality/availability of habitat, and
- (e) the effects of introduced taxa (zebra mussels).
- Criterion B: (Small Distribution Range and Decline or Fluctuation): Meets Endangered B2ab(i,ii,iii,iv,v): B(2) AO < 500 km² (AO ~ 128 km²) and:
- (a) severely fragmented and known from < or = 5 locations (known from 5 widely separated locations), and
- (b) (i,ii,iii,iv,v) continuing decline projected in EO, AO, area, extent and quality of habitat, number of locations or populations and number of mature individuals (2 locations are based on single specimens; 3 locations, including the largest remaining population, are in areas where zebra mussels are established and represent a continuing threat).
- Criterion C: (Small and Declining Number of Mature Individuals): Does not apply because the number of mature individuals is unknown.
- Criterion D: (Very Small Population or Restricted Distribution):
- Meets D2 for Threatened: Known from only 5 locations, 2 of which are based on single specimens, 3 of which (including the largest remaining population) are in areas where zebra mussels are established and represent a continuing threat, and 2 of which may be limited in distribution by the presence of dams that restrict the movements of the presumed fish host.
- Criterion E : (Quantitative Analysis): Does not apply (no data available).