COSEWIC Status Appraisal Summary on the Dwarf Wedgemussel Alasmidonta heterodon in Canada – 2009

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Document Information

Document Information

Extirpated – 2009

COSEWIC status appraisal summaries are working documents used in assigning the status of wildlife species suspected of being at risk in Canada. This report may be cited as follows:

COSEWIC. 2009. COSEWIC status appraisal summary on the Dwarf Wedgemussel Alasmidonta heterodon in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. ix pp.

Production note:
This status appraisal summary constitutes a review of classification of the Dwarf Wedgemussel Alasmidonta heterodon in Canada which was last assessed by COSEWIC in 2000. The 2000 COSEWIC Status Report on the Dwarf Wedgemussel Alasmidonta heterodon in Canada is posted on the Species at Risk Public Registry link.

COSEWIC would like to acknowledge Janice L. Smith for writing the status appraisal summary on the Dwarf Wedgemussel Alasmidonta heterodon in Canada.  The status apppraisal summary was overseen and edited by Dwayne Lepitzki, co–chair of the Molluscs Specialist Subcommittee.

For additional copies contact:

COSEWIC Secretariat
c/o Canadian Wildlife Service
Environment Canada
Ottawa, ON
K1A 0H3

Tel.: 819–953–3215
Fax: 819–994–3684

Également disponible en français sous le titre Sommaire du statut de l’espèce du COSEPAC sur l'alasmidonte naine (Alasmidonta heterodon) au Canada.

© Her Majesty the Queen in Right of Canada, 2010.
Catalogue CW69–14/2–1–2010E–PDF
ISBN 978–1–100–16628–5

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COSEWIC Assessment Summary

Assessment Summary – April 2009

Common name
Dwarf Wedgemussel

Scientific name
Alasmidonta heterodon


Reason for designation
* A reason for designation is not specified when a review of classification is conducted by means of a status appraisal summary.

New Brunswick

Status history
Extirpated by 1968. Designated Extirpated in April 1999. Status re–examined and confirmed in May 2000 and November 2009.

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COSEWIC Status Appraisal Summary

Alasmidonta heterodon
Dwarf Wedgemussel - Alasmidonte naine
Jurisdictions: NB, DFO

Current COSEWIC Assessment:

Status category:

XT = Extirpated

Date of last assessment: May 2000

Reason for designation at last assessment:  This freshwater mussel was previously known in Canada from only one river drainage. It has disappeared subsequent to the building of a causeway across the river in 1967/68, and has not been found despite intensive systematic searches of its former habitat (COSEWIC In Press).

Criteria applied at last assessment: None (species designated before guidelines for Extirpated were available but conformed to definition of Extirpated: a species no longer existing in the wild in Canada, but occurring elsewhere).

If earlier version of criteria was applied[1] , provide correspondence to current criteria: Not applicable.

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Recommendation: Update to the status report NOT required (species’ status category remains unchanged)


sufficient information to conclude there has been no change in status category

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Evidence (indicate as applicable):

Wildlife species:

Change in eligibility, taxonomy or designatable units: no


No changes since last assessment



Change in Extent of Occurrence (EO): no

Change in Area of Occupancy (AO): no

Change in number of known or inferred current locations: no

Significant new survey information: yes


The species requires the presence of fish hosts, most likely American Shad (Alosa sapidissima), for the parasitic larval stage of development. In Canada, the Dwarf Wedgemussel was known from only the Petitcodiac River system in New Brunswick. In 1967/68, the Petitcodiac Causeway was built across the tidal portion of the Petitcodiac River, which has severely restricted the passage of anadromous fishes into the freshwater stretches of the river and its tributaries. This mussel was described as “common” in 1960 but subsequent surveys of 12 sites in 1984 and 66 sites from 1997 to 2000 failed to locate any specimens (COSEWIC In Press; Hanson and Locke 2001).

To date, no further surveys have been conducted in this river; however, it has always been considered possible that the species occurs in other river systems in New Brunswick but has escaped detection. This possibility becomes less likely as survey effort increases. Approximately 125 sites in nearly 20 rivers across NB have been surveyed for mussels since the 1997/98 surveys in the Petitcodiac (current to 2006), and no Dwarf Wedgemussels have been found.


Population Information:

Change in number of mature individuals: no

Change in total population trend: no

Change in severity of population fragmentation: no

Change in trend in area and/or quality of habitat: no

Significant new survey information: yes


No live specimens have been found since 1960. See above for new survey information.



Change in nature and/or severity of threats: no


Nothing has changed to date.

The extirpation of the Dwarf Wedgemussel was primarily caused by the elimination of its host fish due to a lack of fish passage at the Petitcodiac River causeway. Fish communities changed substantially following the construction of the causeway; populations of several species disappeared (American Shad; Atlantic Salmon, Salmo salar; Atlantic Tomcod, Microgadus tomcod; and Striped Bass, Morone saxatilis) and others were greatly reduced (Alewife, Alosa pseudoharengus; Blueback Herring, Alosa aestivalis; Rainbow Smelt, Osmerus mordax; and Sea–run Brook Trout, Salvelinus fontinalis) (Department of Fisheries and Oceans 2007).

The Recovery Strategy for the Dwarf Wedgemussel concluded that “Recovery of this species is considered not technically or biologically feasible at this time”, although suitable physical habitat (substrate, currents) appears to be available (Department of Fisheries and Oceans 2007). Three steps are required to re–establish the species (Department of Fisheries and Oceans 2007): (1) re–engineer the Petitcodiac Causeway to allow fish passage, (2) re–establish the host fish species (presumed to be American shad, but possibly other species as well), and (3) translocate several thousand Dwarf Wedgemussels from a population in the US or from propagation programs using US populations. Step 1 is already occurring. On 7 August 2007, New Brunswick announced its intention to replace the Petitcodiac Causeway with a 280 m–long bridge (Communications New Brunswick 2007). Preparatory work for opening the gates to allow fish passage has begun with the expectation that they will be opened in the spring of 2010 (Government of New Brunswick 2009). Step 2: re–establishment of American Shad in the Petitcodiac River, also is possible. American Shad are present in other rivers in the upper Bay of Fundy and might re–establish naturally. Hatchery supplementation has been used to successfully rebuild U.S. Shad populations and, although not well documented, it is potentially feasible to re–establish extirpated populations (Department of Fisheries and Oceans 2007). Lastly, microsatellite markers have now been developed for the Dwarf Wedgemussel (Shaw et al. 2005) allowing population genetic studies. Data from New York rivers and the Connecticut River depict strong phylogeographic structure throughout the species’ range, suggesting that each river system consists of genetically discrete and evolutionarily significant populations (T. King pers. comm. 2008). All Canadian specimens located to date are preserved in formalin and thus not useful for DNA analysis. In the absence of data on Canadian specimens, it is recommended that mussels from the most northerly US population be used to re–establish the Canadian population (D. Zanatta pers. comm. 2008 and T. King pers. comm. 2008).



Change in effective protection: no


Nothing has changed to date although the Petitcodiac watershed is a part of the Fundy Biosphere Reserve, designated by UNESCO in September 2007. The conservation benefits of such a designation may include: “helps to protect ecological integrity of protected areas; promotes restoration of watersheds and fragile habitats; supports protection of ecologically significant areas; aids in water quality conservation and promotes water quality improvements; advances new knowledge through research and education; contributes to national efforts to reduce and manage green house gas emissions; and contributes to global biodiversity monitoring programs.” (Fundy Biosphere Reserve 2008).


Rescue Effect:

Evidence of rescue effect: no


As noted above (Threats), re–establishment of the Dwarf Wedgemussel in Canada using specimens from northern US populations or from propagation programs based on US populations may be possible. However, this is not natural rescue. This mussel can not return to the Petitcodiac River without intervention by humans.


Quantitative Analysis:

Change in estimated probability of extirpation: no


No additional data since previous assessment


Summary and Additional Considerations:

While the Recovery Strategy for the Dwarf Wedgemussel concluded that recovery of the species was not possible (Department of Fisheries and Oceans 2007), significant steps have been made that may allow for recovery in the future. The status of this mussel in Canada is unlikely to change until fish hosts are re–established and Dwarf Wedgemussels (translocated directly from the U.S. or through propagation from U.S. populations) re–establish a self–sustaining population in the Petitcodiac River of New Brunswick.

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Alain Branchaud, Species at Risk Recovery Biologist, Canadian Wildlife Service, Environment Canada, 105 McGill Street, 7th Floor, Montreal QC H2Y 2E7.

Theresa Fowler, Scientific Authority, Species Assessment, Species Population and Standards Management, Canadian Wildlife Service, Environment Canada, 351 St. Joseph Boulevard, 4th Floor Place Vincent Massey Building, Gatineau QC J8Y 3Z5

Tim L. King, U.S. Geological Survey, Leetown Science Center, Aquatic Ecology Branch, 11649 Leetown Road, Kearneysville, WV 25430

Simon Nadeau, Senior Advisor, Fish Population Science, Fisheries and Oceans Canada, 200 Kent Street, Room 12S027, Mail Stop 12S032, Ottawa ON K1A 0E6

Patrick Nantel, Conservation Biologist, Species at Risk Program, Ecological Integrity Branch, Parks Canada, 25 Eddy Street, 4th Floor, 25–4–S, Gatineau QC K1A 0M5

Howard Powles, 53, rue Lortie, Gatineau, QC, J9H 4G6 (Author of the Recovery Strategy).

Maureen Toner, Biologist, Species at Risk Program, Fish and Wildlife Branch, Department of Natural Resources and Hugh John Flemming Forestry Centre, P.O. Box 6000, Fredericton, NB, E3B 5H1

Christie Whelan, Science Advisor, Fish Population Science, Fisheries and Oceans Canada, 200 Kent Street, Room 12S042, Mail Stop 12S032, Ottawa ON K1A 0E6

David T. Zanatta, Biology Department, Central Michigan University, 186 Brooks Hall, Mount Pleasant, MI 48859

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Sources of information:

COSEWIC. In Press. COSEWIC assessment and status report on the Dwarf Wedgemussel Alasmidonta heterodon in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 18 pp.

Communications New Brunswick. 2007. Option selected for restoration of Petitcodiac River. News Release, Department of Supply and Services, August 7, 2007.

Department of Fisheries and Oceans. 2007. Recovery Strategy for the Dwarf Wedgemussel (Alasmidonta heterodon) in Canada. Species at Risk Act Recovery Strategy Series. Department of Fisheries and Oceans, Ottawa. vi + 9 pp.

Fundy Biosphere Reserve. 2008. Fundy Biosphere Reserve, Université de Moncton, Moncton, NB. [Accessed October 2008]

Government of New Brunswick. 2009. Petitcodiac River project. [Accessed September 2009]

Hanson, J.M., and A. Locke. 2001. Survey of freshwater mussels in the Petitcodiac River drainage, New Brunswick. Canadian Field Naturalist 115(2):329–340.

Shaw, K.M., T.L. King, W.A. Lellis and M.S. Eackles. 2005. Primer Note: Isolation and characterization of microsatellite loci in Alasmidonta heterodon (Bivalvia: Unionidae). Blackwell Publishing Ltd, Molecular Ecology Notes, 10.1111/j.1471–8286.2005.01235.

Author of Status Appraisal Summary:

Janice L. Smith, October 2008; modified by Dwayne A.W. Lepitzki, May & September 2009.


The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal–Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.


The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non–government science members and the co–chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2010)

Wildlife Species
A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)
A wildlife species that no longer exists.

Extirpated (XT)
A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)
A wildlife species facing imminent extirpation or extinction.

Threatened (T)
A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)*
A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)**
A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD)***
A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.
**  Formerly described as “Not In Any Category”, or “No Designation Required.”
***  Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994. Definition of the (DD) category revised in 2006.

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

1 An earlier version of the quantitative criteria was used by COSEWIC from October 1999 to May 2001 and is available on the COSEWIC website.