Fawnsfoot (Truncilla donaciformis) COSEWIC assessment and status report: chapter 8

Population Sizes and Trends (2)

Abundance

To the best of our knowledge, the Fawnsfoot no longer occurs in the Detroit River (Schloesser et al. 2006), Lake Erie (Schloesser and Nalepa 1994; D. McGoldrick, Environment Canada, unpublished data) or the Niagara River (New York Power Authority 2003). Extant occurrences are restricted to the St. Clair River delta area of Lake St. Clair, the Saugeen River (Muskrat Creek), the lower Sydenham River, the lower Thames River, and the lower Grand River.

Lake St. Clair

Relative abundance of the Fawnsfoot in the delta area of Lake St. Clair can be estimated from the work of Zanatta et al. (2002) and Metcalfe-Smith et al. (2004). It is clear from these studies that the Fawnsfoot currently represents a very small component of the mussel fauna in this region of the lake. Zanatta et al. (2002) found 1356 live unionids at 33 different sites between 1998 and 2001 and did not find a single Fawnsfoot. Similarly Metcalfe-Smith et al. (2004) found 1778 live unionids at 18 sites in the delta and only reported a single live Fawnsfoot for a total relative abundance of 0.00024% of the unionid community. Focusing only on the Canadian waters of the delta where the single specimen was located still yields a relative abundance of only 0.12% of the total unionid community. Metcalfe-Smith et al. (2004) calculated a density of 6.86 x 10-5/m² (one Fawnsfoot found in 14,577 m² searched) for the entire delta. The one animal was collected from three circle plots of 650 m² each, yielding a site-specific density of 0.000513/m² (SE = 0.000513). It is not possible to estimate the size of the Fawnsfoot population in the delta based on information from a single individual.

Inland Rivers

Thirty-six hours of sampling at eight stations in the Saugeen River watershed in 2006 produced 1064 mussels of eight species but no Fawnsfoot specimens, either live or dead (Morris et al. 2007). The only record of the Fawnsfoot from this watershed or the entire Canadian portion of the Lake Huron watershed is from a benthic sample collected from Muskrat Creek upstream of Teeswater. It is not possible to estimate abundance in the Saugeen River with the available information.

The Fawnsfoot was only found at one of the 22 sites sampled in the Sydenham River between 1997 and 2003 (Metcalfe-Smith et al. 2007). Seven individuals were collected from the 78.1 m² quadrats excavated at this site, yielding a density estimate of 0.089/m² (SE = 0.0348). It is not possible to estimate the population size in the Sydenham River as the species was only found at a single site. Although the sample size is small (n = 20 and includes individuals collected during timed-searches as well as those collected during the quadrat excavations), the size frequency distribution presented for the Sydenham River (Figure 6) provides evidence of recruitment and representation from multiple size classes.

Figure 6. Size distribution of the Fawnsfoot collected from the Sydenham River using timed-search methods in 1998 (n = 13) and quadrat excavations in 1999 (n = 7) (J. Metcalfe-Smith, Environment Canada, unpublished data).

Figure 6. Size distribution of the Fawnsfoot collected from the Sydenham River using timed-search methods in 1998 (n = 13) and quadrat excavations in 1999 (n = 7) (J. Metcalfe-Smith, Environment Canada, unpublished data).

A total of 23 live specimens of the Fawnsfoot were collected from seven of 48 sites sampled in the Thames River between 1997 and 2005. All seven sites were located contiguously over a 112 km stretch of the lower Thames River between London and Chatham. One site within the range of the Fawnsfoot was chosen for detailed quadrat excavation work. Three Fawnsfoot specimens were recorded during these excavations giving a density estimate for the Thames River of 0.043/m² (SE = 0.0237). At sites where this species was found it ranged from 0.55 – 4.0% of the total mussel community. Figure 7 represents the size distribution for the 23 animals collected in the Thames River. Despite the small sample size there is again evidence for multiple size classes suggesting reproduction over several years (Figure 7).

Figure 7. Size distribution of the Fawnsfoot collected from the ThamesRiverusing timed-search methods (n = 20) and quadrat excavation (n = 3) (T. Morris, Fisheries and Oceans Canada, unpublished data).

Figure 7. Size distribution of the Fawnsfoot collected from the ThamesRiverusing timed-search methods (n = 20) and quadrat excavation (n = 3) (T. Morris, Fisheries and Oceans Canada, unpublished data).

Metcalfe-Smith et al. (2000b) surveyed 24 sites in the Grand River for mussels in 1997-1998, and found the Fawnsfoot at only one of these sites. At this one site, immediately below the dam in Dunnville, 11 individuals were collected representing 21.1% of all live mussels at the site. The Fawnsfoot was not collected during quadrat sampling at four sites in the Grand River in 2007, making an estimate of population abundance impossible. The size distribution of specimens from the Grand River (Figure 8) indicates a much smaller range of sizes in this river in comparison with the Sydenham and Thames rivers. It is not clear whether the truncated size distribution is indicative of limited reproduction or simply a consequence of the small sample size.

Fluctuations and trends

It is very difficult to evaluate population fluctuations or trends in Fawnsfoot numbers over time as there are very few records available. There are only 58 Fawnsfoot records in the Lower Great Lakes Unionid Database and only nine of these records represent collections of more than a single live animal.

Figure 8. Size distribution of the Fawnsfoot collected from the Grand River using timed-search methods in 1997 (n = 11) (J. Metcalfe-Smith, Environment Canada, unpublished data).

Figure 8. Size distribution of the Fawnsfoot collected from the Grand River using timed-search methods in 1997 (n = 11) (J. Metcalfe-Smith, Environment Canada, unpublished data).

The Fawnsfoot appears to be extirpated from the offshore waters of lakes St. Clair and Erie and the Detroit and Niagara rivers as a result of the invasion of dreissenid mussels. Even prior to this invasion, the Fawnsfoot was never a large part of the native mussel fauna. In Lake St. Clair, Nalepa et al. (1996) report that the Fawnsfoot represented 0.35% (1 of 281 animals), 2.4% (6 of 248 animals) and 1.0% (1 of 99 animals) of the total mussel fauna captured in 1986, 1990 and 1992 respectively. By the time Zanatta et al. (2002) sampled 95 sites between 1998 and 2001, no Fawnsfoot were among the 2356 live animals detected. The only record of the Fawnsfoot from Lake St. Clair in the last 15 years was the single live animal collected by Metcalfe-Smith et al. (2004) from the delta area in 2003.

Similarly, in the western basin of Lake Erie, Nalepa et al. (1991) reported that the Fawnsfoot represented 2.4% of the fauna in 1951-52 and 2.6% in 1961. Specific numbers of unionids captured were not reported by Nalepa et al. (1991); however, they did report a total average density of 10/m² in 1961. The Fawnsfoot has not been reported from Lake Erie since 1961.

Schloesser et al. (1998) summarized mussel survey efforts in the Detroit River between 1982 and 1994. Along the southeastern (Canadian) shore the Fawnsfoot represented 0.23% (1 of 422 animals) of the mussel fauna in 1982-83 and was not detected alive after that. Along the northwestern (American) shore the species was more abundant but still only represented 4.24% (7 of 165 animals) in 1982-83 and 0.31% (5 of 1592 animals) in 1992, with no animals detected in 1994. Additional survey effort since 1994 has produced no records and all freshwater mussels are now believed to be extirpated from the Detroit River (Schloesser et al. 2006).

The only historical record for the Fawnsfoot in the Sydenham River consists of a single specimen collected by Clarke from a site near Croton in 1991. Metcalfe-Smith et al. (2003) found the Fawnsfoot in almost the same location in 1998-1999. This site is the only place in the Sydenham River where the species has ever been recorded despite considerable recent sampling effort. This location has been revisited annually since 2005 as part of the Ontario Freshwater Mussel Identification Workshop and the report writer has observed live Fawnsfoot on each occasion. There are no data to estimate trends in abundance for riverine populations; however, there is no evidence of a change in the range of the Fawnsfoot in the Sydenham River.

The Fawnsfoot was first reported from the Thames River in 1997 as a single fresh valve from the area of Big Bend (Metcalfe-Smith et al., unpublished data). Morris and Edwards (2007) revisited this site in 2005 and found one weathered and four fresh shells in 4.5 person hours. An additional three live specimens were recorded during quadrat excavations at this site. The Fawnsfoot was recorded at six other sites over a 112 km reach of the lower Thames River making this likely the largest and healthiest remaining population in Canada. The lack of any time series makes evaluation of trends or fluctuations in this population impossible.

Historically the Fawnsfoot was known from six records in the lower Grand River between Dunnville and Port Maitland. Five of these records are identified as being in the Dunnville area where recent efforts have produced the only live specimens for the watershed. Kidd reported finding 10 shells but no live animals in this area in 1972, while Metcalfe-Smith et al. (2000b) found 11 live animals in 1997-98. There are no data to estimate trends in abundance for riverine populations; however, there is no evidence of a change in the range of the Fawnsfoot in the Grand River.

It is not possible to assess fluctuations or trends in Fawnsfoot numbers in Muskrat Creek of the Saugeen River drainage as this record consists only of a single animal collected on a single occasion. Fawnsfoot populations in the Lake St. Clair-Lake Erie corridor have undergone substantial declines over the last 20-40 years. Only one live specimen has been found in Lake St. Clair in the past 15 years and none have been found in the Detroit River. Fawnsfoot populations appear to have crashed earlier in the western basin of Lake Erie, with no specimens recorded since 1961. There are insufficient data available to estimate trends in abundance for the inland rivers, although there is no indication of a range reduction in any river.

Rescue effect

All Canadian populations of the Fawnsfoot are isolated from one another and from American populations by large areas of unsuitable habitat, making the likelihood of re-establishment of extirpated populations by immigration small. The two potential hosts of the Fawnsfoot, freshwater drum and sauger, are capable of the large-scale movements required to connect these populations. However, movements of this magnitude are typically associated with spring spawning behaviour in these fishes (Funk 1957; Pegg et al. 1997) and would likely occur at a time when they are not bearing glochidia. Furthermore, Fawnsfoot populations in adjacent U.S. states that could act as sources are not considered stable. In the four U.S. states of the Lake Huron-Lake St. Clair-Lake Erie corridor, populations in two are considered at greater conservation risk than in Ontario (New York – SH, possibly extirpated; Pennsylvania – S1, critically imperiled) while one is considered on par (Ohio – S2, Imperiled). The Fawnsfoot has not been ranked in Michigan.

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