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Recovery Strategy for the Dwarf Wedgemussel (proposed)
2. Recovery Feasibility
- 2.1 Ability to improve population abundance
- 2.2 Habitat availability and ability to restore
- 2.3 Recovery feasibility conclusion
The only possible approach to recovery the dwarf wedgemussel would be the re-establishment of a viable population based on reintroduction from another population. The recovery of the dwarf wedgemussel is not considered feasible at this time, based on examination of a series of issues in the following sections.
2.1 Ability to improve population abundance
Techniques for re-establishment of extirpated freshwater mussel populations are currently in development and are showing promise. Three approaches to re-establishment are being tested: propagation of mussels in captivity to support outplanting of individuals to the wild; infecting fish hosts with larvae and releasing the fish, allowing larvae to settle in the wild; and direct translocations of adults from viable populations to areas where mussels were formerly found. All have been shown to result in successful introduction of viable individuals to the wild, and in the case of one species (the Neosho mucket, Lampsilis rafinesqueana) the progeny were produced in captivity using controlled propagation methods and then subsequently released to the wild to create a new population (Jess Jones, pers. comm.).
Although there are no established guidelines on numbers of individuals required to ensure a demographically viable population, realistically several thousand adult individuals would probably be required. Effective population sizes of the order of 250 (to be above the “endangered” threshold) to 1000 (to be above the “threatened” threshold) would be required to ensure genetic viability, based on International Union for the Conservation of Nature and Natural Resources (IUCN) assessment criteria for species at risk. Effective population sizes, however, are typically a small fraction of census sizes (5-10%), therefore census sizes of several thousand would be required to ensure that these minima were met (Jess Jones, pers. comm.).
Achieving this level of abundance in a re-established dwarf wedgemussel population would require either imports of several thousand viable adults from populations in the USA, or substantial propagation work to build up seed stock of thousands of individuals from a smaller number of imported individuals. Dwarf wedgemussel juveniles have been successfully produced in captivity (Richard Neves, pers. comm.) which suggests that an approach to re-establishment based on propagated individuals could be successful.
The species is listed as endangered in the USA. Although it is relatively abundant at one US site (Nedeau 2005) it is uncertain that individuals could be made available in sufficient numbers to re-establish a viable Canadian population, given the potential impact that these removals might have on recovery efforts in the USA. Propagation in captivity based on limited numbers of spawners from the wild might be possible and would minimize impact of removals on wild populations.
It is not certain that individuals from US populations would have the same genetic characteristics as the extirpated Canadian population. In light of the significant range disjunction with US populations, the possibly different post-glacial origin and lengthy genetic separation of US and Canadian populations (Nedeau 2005), the Canadian population may have had unique genetic characteristics which would not be replaced by reintroduced US mussels.
2.2 Habitat availability and ability to restore habitat
Suitable habitat for adult and juvenile dwarf wedgemussels appears to be present in the Petitcodiac system in the Little, Petitcodiac, North, and Anagance Rivers. A portion of the former range (North River from Fawcett to the route 112 bridge) has been degraded by agricultural development (chemical runoff, bank destruction by cattle, and low oxygen conditions), and is therefore no longer suitable. This habitat could probably be restored.
The feasibility of dwarf wedgemussel recovery also depends on whether there is sufficient suitable habitat available for its fish host. The dwarf wedgemussel fish host, thought to be American shad, disappeared from the Petitcodiac River following the construction of the causeway between Moncton and Riverview. Pre-conditions for ensuring adequate habitat for dwarf wedgemussel therefore include the restoration of fish passage for the fish host by removing or re-engineering the causeway, and the re-establishment of a fish host population.
The New Brunswick Government has conducted an environmental impact assessment on options for re-engineering the causeway (Government of New Brunswick 2005), which concluded that adding a bridge section to the causeway would remove or mitigate the impacts on aquatic fauna. Specifically, such modifications would allow passage of fish species which historically were present in the upper Petitcodiac, and would remove negative environmental impacts on fishes (silting of the passage, sediment conditions in water). Although there are currently no specific plans to re-engineer the causeway, such construction could be undertaken in the future.
If the causeway were re-engineered to allow for fish passage, American shad populations might be re-established either by straying from nearby populations, or from hatchery supplementation. American shad are present in other river systems in the upper Bay of Fundy (Chaput and Bradford 2003), and individuals from all river systems of Atlantic North America mix in the upper Bay of Fundy during their at-sea residence. Existing spawning runs are known from rivers draining into the lower Saint John River system, and the Shubenacadie and Stewiacke Rivers draining into Minas Basin (Chaput and Bradford 2003). Experience in the USA has shown that hatchery supplementation can support rebuilding of American shad populations from low levels (Olney et al. 2003; Interstate Commission on the Potomac River, 2004), although this has required significant investment in hatchery production and a time period of the order of at least a decade. Experience with re-establishing populations in areas where they have been completely extirpated is not well documented, but is presumed to be potentially feasible.
Although American shad is presumed to be the primary host, this is not known with certainty. Accordingly, efforts to re-establish American shad might not be sufficient to lay the groundwork for dwarf wedgemussel re-establishment; therefore the re-establishment of the pre-extirpation fish community might be required.
In summary, while some suitable mussel habitat remains in the Petitcodiac River, considerable effort would be required to restore habitat for the fish host, and to reestablish the fish host population.
2.3 Recovery feasibility conclusion
The re-establishment of dwarf wedge mussel would require a series of steps (re-engineering the causeway, re-establishing the fish host population, re-establishing a mussel population) each with its respective difficulties and uncertainties. Based on the above evaluation, the recovery of dwarf wedgemussel is not considered technically or biologically feasible at this time, although changes in the factors leading to this determination might justify reconsideration in the future. The following conditions would have to be met to lead to a determination that recovery might be feasible:
- changes to the Petitcodiac causeway which would allow adequate fish passage to occur, and which would remove or mitigate negative environmental impacts on fish populations;
- re-establishment of populations of American shad, and possibly other fish species, either through straying from adjacent populations or through hatchery supplementation;
- availability of adequate numbers of dwarf wedgemussel, either adults from US populations, or adults from propagation programs based on US populations, to support re-establishment. Several thousand individuals would be required to ensure a demographically viable population.
The question of whether recovery efforts based on US individuals would lead to a re-established population genetically equivalent to the extirpated Canadian population remains uncertain. In the absence of clear information to the contrary, it would appear appropriate to base reintroduction on US material if the pre-conditions above were met.
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