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Recovery Strategy for the Dwarf Wedgemussel

Executive Summary

The dwarf wedgemussel (Alasmidonta heterodonta) was designated extirpated in Canada by the Committee on Status of Endangered Wildlife in Canada (COSEWIC) in 2000.  

The dwarf wedgemussel was historically found at about 70 locations in 15 major drainages from North Carolina to New Brunswick, and has always been considered naturally uncommon or rare throughout this range.  In Canada the dwarf wedgemussel was restricted to several locations in the Petitcodiac River of New Brunswick, where it was described as “common” based on a survey in 1960, but where subsequent surveys in 1984 and 1997/98 failed to locate specimens.  There is a substantial range disjunction between the northern United States (US) occurrence in Vermont, and the single historic occurrence in Canada, possibly due to origin of the Canadian population in a different glacial refugium (ice-free areas that provided habitat for species during glacial periods) from US populations.  If so, the Canadian population may have been genetically isolated from US populations for as long as 50,000 years and may have developed unique genetic characteristics.  The species is listed as endangered in the US and now only occurs in 20 of its historical locations.

The dwarf wedgemussel has two critical early life history stages: the parasitic larval stage requiring a specific fish host, and the settlement stage where specific microhabitat conditions are required for survival.  The fish host for this species is not known with certainty, but evidence suggests American shad as the probable host in the Petitcodiac system.  Sand or fine gravel bottom is required for settlement and adult survival.  Juveniles and adults apparently require flowing waters and are sensitive to low oxygen, siltation and chemical pollution.

The primary cause of the extirpation of dwarf wedgemussel is believed to be the elimination of the fish host due to a lack of fish passage at the Moncton-Riverview causeway constructed in 1968.   Substantial changes in fish communities followed the construction of the causeway, with the disappearance of several species including the presumed host and significant reductions in others.  Available information suggests that suitable habitat for the species remains in the Petitcodiac system, despite degradation in some areas.  Several other freshwater mussel species, including two species from the same genus as the dwarf wedgemussel, (A. undulata and A. varicosa) have been recently collected in the system.

Recovery of the dwarf wedgemussel in Canada is considered not feasible at this time.  For recovery to be feasible (1) the causeway would have to be re-engineered to permit fish passage (2) the fish host species would have to be re-established in the Petitcodiac system, either by natural processes (American shad and other coastal species are present in the upper Bay of Fundy), or by hatchery supplementation and (3) dwarf wedgemussels from another population or from captivity would have to be reintroduced in numbers adequate to permit establishment of a viable population.  It is probable that several thousand adult individuals would be required in the re-established population to ensure demographic viability.  Although each of these steps is possible, there are difficulties and uncertainties at each.  The species is listed as endangered in the USA, which might constrain numbers of individuals available to support reintroduction.