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COSEWIC assessment and update status on the round hickorynut Obovaria subrotunda in Canada - 2003

Technical Summary

Obovaria subrotunda

Round Hickorynut

Obovarie ronde

Southwestern Ontario


Extent and Area Information

Extent of occurrence (EO) (km2)East Sydenham River and portion of the Lake St. Clair delta ~1750 km2
specify trend (decline, stable, increasing, unknown)Decline, estimate 90% (likely extirpated from offshore waters of Lake St. Clair, Lake Erie, Detroit River, Thames River, and Grand River)
Are there extreme fluctuations in EO (> 1 order of magnitude)?No
Area of occupancy (AO) (km2)Scattered specimens in the East Sydenham River (no area estimate possible); 8 km2 in a portion of the Lake St. Clair delta
specify trend (decline, stable, increasing, unknown)Decline
Are there extreme fluctuations in AO (> 1 order magnitude)?No
number of extant locations2
specify trend in # locations (decline, stable, increasing, unknown)Decline
are there extreme fluctuations in # locations (>1 order of magnitude)?No
habitat trend: specify declining, stable, increasing or unknown trend in area, extent or quality of habitatDeclining


Population Information

generation time (average age of parents in the population)
(indicate years, months, days, etc.)
Unknown (estimate 10 years)
number of mature individuals (capable of reproduction) in the Canadian population (or, specify a range of plausible values)Unknown (estimate 55 000)
total population trend: specify declining, stable, increasing or unknown trend in number of mature individualsDeclining
if decline, % decline over the last/next 10 years or 3 generations, whichever is greater (or specify if for shorter time period)90% over last 10 years due to zebra mussel infestation of Lake Erie, Lake St. Clair, Detroit River
are there extreme fluctuations in number of mature individuals (> 1 order of magnitude)?No
is the total population severely fragmented (most individuals found within small and relatively isolated (geographically or otherwise) populations between which there is little exchange, i.e., ≤ 1 successful migrant / year)?Yes, only 2 isolated populations
list each population and the number of mature individuals in eachLake St. Clair: Estimate 55 000
Sydenham River: Unknown (very few)
specify trend in number of populations(decline, stable, increasing, unknown)Decline
are there extreme fluctuations in number of populations (>1 order of magnitude)?No


Threats (actual or imminent threats to populations or habitats)

- Zebra mussels (invasive species)
- Habitat loss and degradation:
   - Siltation
   - Nutrient loading
   - Loss of riparian vegetation
- Potentially declining host fish species
- Predation by muskrats and raccoons


Rescue Effect (immigration from an outside source)

does species exist elsewhere (in Canada or outside)?Yes (United States)
status of the outside population(s)?Declining
is immigration known or possible?No
would immigrants be adapted to survive here?Likely (genetic testing required)
is there sufficient habitat for immigrants here?No


Quantitative Analysis