Hickorynut and kidneyshell (Ptychobranchus fasciolaris) final recovery strategy: chapter 7
Habitat Trend
Habitats for the Round Hickorynut, Kidneyshell and other unionids in Lake Erie and Lake St. Clair have been largely destroyed by dreissenid mussels. Native mussel communities were virtually extirpated from the offshore waters of western Lake Erie by 1990 (Schloesser and Nalepa 1994) and the offshore waters of Lake St. Clair by 1994 (Nalepa et al. 1996). The mussel communities of Lake Erie were already in decline, probably due to a general decline in water quality over the past 40 years (Nalepa et al. 1991), but Lake St. Clair still supported an abundant and diverse mussel assemblage as recently as 1986 (Nalepa and Gauvin 1988). Unionids continue to survive in some nearshore areas with very shallow water, a high degree of connectivity to the lake (which ensures access to host fishes), and harsh conditions for dreissenid mussels (high water temperatures and considerable wave action in summer; ice scour in winter). However, such “refugia” are rare, and most of the unionid habitat in the Great Lakes has been permanently lost (COSEWIC 2003a).
The Round Hickorynut and Kidneyshell have apparently been lost from the Thames and Grand rivers, and the Round Hickorynut has declined significantly in the Sydenham River. Agriculture is believed to be the main cause of the destruction of mussel habitat across North America (Strayer and Fetterman 1999) and southwestern Ontario is no exception. Since agriculture accounts for 75-85% of land use in the Grand, Thames and Sydenham River basins, it is likely that agricultural impacts (e.g., runoff of sediment, nutrients and pesticides, increased water temperatures due to loss of riparian vegetation, destruction of habitat by tractor crossings and cattle) are primarily responsible for the loss of mussel habitat in these rivers (COSEWIC 2003a).
The federal Species at Risk Act (SARA) was proclaimed in June of 2003. Under SARA there are general prohibitions against killing, harming, taking, possessing, capturing, and collecting the Round Hickorynut or Kidneyshell and against damaging or destroying their residences, as well as prohibitions on the destruction of Critical Habitat. The Federal Fisheries Act represents another significant piece of legislation protecting freshwater mussels and their habitat in Canada since fish are broadly defined under the Act to include shellfish. The collection of live mussels is considered fishing and would fall under the Ontario Fishery Regulations that are made under the Fisheries Act. The protection of other fish and fish habitat under the Fisheries Act may indirectly protect the habitat of the Round Hickorynut or Kidneyshell and other species of freshwater mussels. The Provincial Policy Statement under Section 3 of the Planning Act provides for protection from development and site alteration in the significant habitats of threatened and endangered species. Other mechanisms for protecting mussels and their habitat in Ontario include the Ontario Lakes and Rivers Improvement Act, which prohibits the impoundment or diversion of a watercourse if it would lead to siltation; and the voluntary Land Stewardship II program of the Ontario Ministry of Agriculture, Food, and Rural Affairs, which is designed to reduce erosion on agricultural lands. Stream-side development in Ontario is managed through flood plain regulations enforced by local Conservation Authorities. The majority of land in the Sydenham and Ausable rivers where these mussels are found is privately owned while the land in the St. Clair delta is controlled by the Walpole Island First Nation.
Freshwater mussels play an integral role in the functioning of aquatic ecosystems. Vaughn and Hakenkamp (2001) have summarized much of the literature relating to the role of unionids and identified numerous water column (size-selective filter-feeding; species-specific phytoplankton selection; nutrient cycling; control of phospohorus abundance) and sediment processes (deposit feeding decreasing sediment organic matter; biodeposition of feces and pseudofeces; epizoic invertebrates and epiphytic algae colonize shells; benthic invertebrate densities positively correlated with mussel density) mediated by the presence of mussel beds. Welker and Walz (1998) have demonstrated that freshwater mussels are capable of limiting plankton in European rivers while Neves and Odom (1989) reported that mussels also play a role in the transfer of energy to the terrestrial environment through predation by muskrats and raccoons.
Although these species have no apparent economic significance, freshwater mussels are sensitive to environmental pollution and a diverse mussel community indicates a healthy ecosystem. Besides decreased biodiversity in Canada, the loss of the Round Hickorynut or Kidneyshell may indicate further environmental degradation of southwestern Ontario watercourses which would adversely affect those people who depend on surface water for drinking, recreation or watering livestock.
· What is the Canadian host for the Round Hickorynut?
Although the host for the Round Hickorynut has been identified in the United States as the greenside darter, host specificity has been reported at the watershed scale for some species and this identification should be verified for Canadian populations if possible.
· What are the habitat requirements of the Round Hickorynut and Kidneyshell?
Habitat use must be quantified for all life-stages with particular attention to the glochidial, encysted and juvenile stages when mortality is high.
· Are the Round Hickorynut and Kidneyshell host-limited?
Host fish distributions for both mussel species need to be mapped in high detail. Host fish may be functionally unavailable to mussels if their distributions do not overlap at times when female mussels are releasing mature glochidia.
· Are there life-stage specific threats?
The relative importance of each identified threat to each distinct life-stage (glochidium, larva, adult) must be identified.
· Can the St. Clair refuge sites be maintained?
It must be determined if these sites represent permanent refugia or whether the mussels at these sites will eventually succumb to the harmful effects of dreissenid mussels. If these sites can not be naturally maintained then the feasibility of actively managing these sites to reduce the effects of dreissenid mussels must be investigated.
· Can these species be relocated from other jurisdictions or artificially propagated for reintroduction?
Conservation genetics need to be assessed as they relate to relocations/reintroductions and the technical feasibility of artificial propagation should be examined.
11. Biological and Technical Feasibility of Recovery
Recovery of the Round Hickorynut and Kidneyshell is believed to be both biologically and technically feasible as reproducing populations still exist as potential sources to support recovery, suitable habitat can be made available through recovery actions, threats can be mitigated and proposed recovery techniques are anticipated to be effective. Although recovery at the species level is believed to be feasible the effort required to achieve recovery will not be uniform across all populations.
· Mussels are slow growing and sedentary animals dependant upon their host fishes for the survival and dispersal of their young. The slow rate of population growth of freshwater mussels makes the natural recovery of decimated populations extremely difficult.
· The habitat in the Sydenham and Ausable rivers could be improved significantly with proper stewardship of both agricultural and urban lands in the watershed.
· Reductions in soil erosion and turbidity in all the watersheds can be achieved but would be challenging due to the number and intensity of the impacts.
· Complete removal of the impacts of dreissenid mussels to the Lake St. Clair populations is not possible at this time however it may be possible to establish managed refuge sites to reduce the impacts of dreissenid mussels on Round Hickorynuts and Kidneyshells.
A high level of effort will be required to recover the Sydenham and Lake St. Clair populations of the Round Hickorynut. There is little evidence of natural reproduction within these populations and recovery may require captive breeding and/or relocations from U.S. populations.
A low to moderate level of effort will be required to recover the Sydenham and Ausable river Kidneyshell populations. These populations are believed to be threatened by general habitat loss resulting from characteristic land-use practices within the basin. A general suite of ecosystem recovery actions such as those proposed by Dextrase et al. (2003) will assist with the recovery of this population.
Recovery of the Lake St. Clair populations of both species will require a higher degree of effort. Active management of selected refuge sites including the regular cleaning of dreissenid mussel infested individuals will be required to maintain and recover this population. Long term population augmentation and/or translocations may also be required to return the Round Hickorynut and Kidneyshell to healthy self-sustaining levels in Canada.
Page details
- Date modified: