Science Advisory Report 2023/046

Recovery Potential Assessment of Purple Wartyback (Cyclonaias tuberculata)

Summary

• Purple Wartyback is a long-lived (maximum age > 90 years) freshwater mussel (Unionidae). Age-at-maturity is estimated to be 6-10 years and generation time at 26 years in Canada.
• The current distribution of Purple Wartyback in Canada is limited to three rivers in southwestern Ontario, including the Ausable River (Lake Huron drainage), and the Sydenham and Thames rivers (Lake St. Clair drainage). Populations in the Sydenham and Thames rivers are showing positive population growth while the Ausable River is likely stable. It is extirpated from the Detroit River and Lake Erie around Pelee Island.
• In Canada, Purple Wartyback occupies medium to large rivers. Occurrence is associated with a range of habitat conditions including variable substrate types, depth, and water velocity.
• Purple Wartyback glochidia must encyst on the gills of an appropriate host fish to survive and metamorphose. The potential host fishes for Purple Wartyback in Canada include Black and Yellow bullheads (Ameiurus melas, A. natalis), Channel Catfish (Ictalurus punctatus) and possibly Flathead Catfish (Pylodictis olivaris). This is based on laboratory infestations in the United States (as infestation experiments have not occurred with Canadian Purple Wartyback), and distributional overlap of known ranges in Canadian waters.
• To achieve ~99% probability of persistence over 250 years requires ~2,800 (CI: 1,900-4,000) adult Purple Wartyback. A minimum of 623.3 m² (CI: 251.9-1,396.9) and 2,900 m² (CI: 301.5-17,166.3) of suitable habitat in the Sydenham and Thames rivers, respectively, is required to support a minimum viable population (MVP); there is sufficient habitat available in both rivers. The spatial configuration of populations and habitat is important and has not been considered.
• Population projections were completed for the quadrat-sampled area of the Sydenham, Thames, and Ausable rivers. Estimated abundance in the sampled habitats in the Sydenham River exceeded the estimated MVP upper confidence interval. Positive population growth rates were estimated in the Sydenham and Thames rivers, and the size distribution of Purple Wartyback suggests successful recruitment is occurring in both systems. The Ausable River populations have not exhibited positive population growth and remain at low density. Results are considered conservative because quadrat surveys have not been completed across all occupied habitat, and a substantial amount of habitat has not been surveyed.
• Purple Wartyback populations are generally most sensitive to perturbations in adult survival; however, they become more sensitive to perturbations to juvenile survival when experiencing significant population growth. Uncertainty in age-at-maturity and the proportion of reproduction that occurs later in life (relative fertility) affect how sensitive populations are to perturbations in adult and juvenile survival.
• Dreissenid mussels are the cause of extirpation of Purple Wartyback from the Detroit River and Lake Erie around Pelee Island. The greatest threats to extant populations are pollution from agricultural and urban sources, and road runoff; extreme weather events associated with climate change; aquatic invasive species (dreissenid mussels and Round Goby Neogobius melanostomus); and bridge or culvert construction/maintenance works.
• Key knowledge gaps remain regarding vital rates, glochidia-host relationships, and threat mechanisms, magnitude of impact, and interactions. Knowledge gaps resulting in uncertainties related to how Purple Wartyback will respond to threats and recovery actions should be addressed with additional research.

This Science Advisory Report is from the October 25–27, 2022 regional peer-review meeting on the Recovery Potential Assessment of Purple Wartyback (Cyclonaias tuberculata). Additional publications from this meeting will be posted on the Fisheries and Oceans Canada (DFO) Science Advisory Schedule as they become available.

Science Advisory Report 2023/046 (PDF, 803 KB)

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