Fawnsfoot (Truncilla donaciformis) COSEWIC assessment and status report: chapter 4

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Distribution

Global range

The global range of the Fawnsfoot is limited to central North Americawhere it is widely distributed, occurring in 23 American states and one Canadian province. Largely occurring in the Great Lakes and Mississippi drainages, the Fawnsfoot is also found in the Mobile basin and the Gulf Coastal region (NatureServe 2007).In the United States the Fawnsfoot has been reported from Alabama, Arkansas, Georgia, Iowa, Illinois, Indiana, Kansas, Kentucky, Louisiana, Michigan, Minnesota, Missouri, Massachusetts, Nebraska, New York, Ohio, Oklahoma, Pennsylvania, South Dakota, Tennessee, Texas, Wisconsin and West Virginia (Figure 2). The New York population is considered possibly extirpated (NatureServe 2007). Despite its widespread distribution, the Fawnsfoot is considered apparently stable or stable in only six jurisdictions (Alabama, Illinois, Kentucky, Mississippi, Oklahoma and Tennessee) and the overall global short-term trend is considered to be rapidly to very rapidly declining (NatureServe 2007).

Canadian range

In Canada, the Fawnsfoot is known only from the Great Lakes drainage of southern Ontario including lower Lake Huron, Lake St. Clair, Lake Erie and the Detroit and Niagara rivers. There are no records of the Fawnsfoot from any other Canadian province or territory (Metcalfe-Smith and Cudmore-Vokey 2004).

The Fawnsfoot has always been a rare species in the faunal record for Canada. Only 58 records exist for this species in the Lower Great Lakes Unionid Database (see Collections Examined for details), dating back to 1930 when Wright (1955) detected the species in the western basin of Lake Erie. By 1966, it was known from several locations in the western basin of Lake Erie including Pelee Island and East and Middle Sister Islands. J. P. Oughton and H. van der Schalie reported it from the lower Grand River (Royal Ontario Museum specimen # ROM23), Lake St. Clair (Royal Ontario Museum specimen # ROM43) and the Niagara River (University of Michigan Museum of Zoology specimen # MZUM444) in 1934. The first documented live collection of the Fawnsfoot was made in 1982 when D.W. Schloesser collected it from the Detroit River. The species was first collected from the Sydenham, Thames and Saugeen rivers in 1991, 1997 and 2005 respectively.

The historical distribution (1930-1996) shown in Figure 3 is based on 43 records of which eight are known live collections representing 17 individuals. The current distribution (1997-2007) as shown in Figure 4 is based on 15 records (11 records for live individuals) reporting 56 live animals. The year 1997 has been selected as the starting point for the current records as it marks the beginning of a more intensive, and ongoing, survey effort throughout the range of the Fawnsfoot. The following discussion contains a detailed description of historical and current distribution of the species throughout the Great Lakes basin, beginning with the Lake Huron drainage and moving downstream through the Great Lakes system.

The Fawnsfoot had never been recorded from the Lake Huron drainage until a live specimen was collected in 2005. During a benthic invertebrate assessment, a single live animal was collected in a kick net sample from Muskrat Creek on the Teeswater River in the Saugeen River watershed (Logan pers. comm. 2007; specimen verified by the report writer). Prior mussel sampling in this watershed in 1993 and 1994 (5 sites; effort = 1 person-hour per site) had not detected the presence of the Fawnsfoot (Morris and Di Maio 1998-1999). Eight additional sites were surveyed in the Saugeen watershed in 2006 including two sites on the Teeswater River, one just below the confluence with Muskrat Creek (Morris et al. 2007). Despite the greater effort expended (4.5 person-hours per site), no specimens of the Fawnsfoot were encountered. Additional sampling in the Bayfield (18 sites in 2007), Maitland (11 sites between 1998 and 2003) and Ausable (6 sites in 1993-1994; 15 sites between 1998 and 2002) rivers of the Lake Huron drainage did not produce any specimens. The Saugeen River record is unique not only in that it represents the only record from the Lake Huron drainage but also because it occurs far up in the watershed, well removed from the mouth of the river, outside the known range of its presumed host(s) (see Life cycle and reproduction) and upstream of the first six dams in this river (see Figure 5). All other Canadian Fawnsfoot records are associated with the lake proper or the lower portions of rivers below the first major instream barrier, possibly due to limitations on host dispersal abilities (see Limiting factors and threats).

The first extensive surveys for unionids in Lake St. Clair were undertaken in 1984 by Nalepa and Gauvin (1988) and in 1986, 1990 and 1992 by Nalepa et al. (1996). These efforts identified that the Fawnsfoot represented a small component of the lake’s sizable freshwater mussel community. Nalepa and Gauvin (1988) reported only a single individual from one of the 29 sites they sampled while Nalepa et al. (1996) reported that the Fawnsfoot represented between 0.35% - 2.4% of the total mussel fauna at the same sites. Recent surveys by Zanatta et al. (2002) and Metcalfe-Smith et al. (2004) have only produced a single individual.

The Fawnsfoot was first recorded from the Sydenham River of the Lake St. Clair drainage in 1991 when Clarke (1992) reported finding a shell near the town of Croton. Subsequent surveys by Metcalfe-Smith et al. (2003) in 1997-1998 (17 sites) and intensive quadrat excavations at 15 sites in 1999-2003 (Metcalfe-Smith et al. 2007) have confirmed the presence of the Fawnsfoot near the town of Dawn Mills but not at any other site in this watershed.

There are no historical records for the Fawnsfoot in the Thames River of the Lake St. Clair drainage (Figure 3), possibly because the lower portion of the watershed had not been systematically surveyed during the historical period. However, recent survey efforts by Environment Canada in 1997 (Metcalfe-Smith et al., unpublished data) and Fisheries and Oceans Canada in 2004-2005 (48 sites) (Morris and Edwards 2007) have shown that this species is relatively widespread, although at low densities, in the lower portion of the Thames River (see Abundance). The Thames River population likely represents the largest remaining known population in Canada.

The Fawnsfoot was first collected from the Detroit River in 1982 when a single animal was found at one of 13 sites sampled by Schloesser et al. (1998). This was also the last time the species was recorded from the Canadian waters of the Detroit River although Schloesser et aI. (1998) continued to find it in the U.S. waters through 1992. No live Fawnsfoot specimens have been found in the Detroit River since the 1992 survey and they, along with all other unionids, are now considered extirpated from the river (Schloesser et al. 2006).

Wright’s (1955) surveys of the Lake Erie benthos in 1930 produced some of the earliest Fawnsfoot specimens from the Great Lakes. Nalepa et al. (1991) summarized the decline of unionids in the western basin of Lake Erie during the 1951-1982 period and showed that the Fawnsfoot, while always rare (2.4 – 2.6% of the mussel community), apparently disappeared from the basin by 1961.

Metcalfe-Smith et al. (2000b) provide a summary of the historical and current data available regarding the freshwater mussel fauna of the Grand River of the Lake Erie drainage. They provide information for over 900 records from the Grand River between 1885 and 1998. Only eight of these records, dating between 1934 and 1997, are for the Fawnsfoot. All Fawnsfoot records are from the area defined by Metcalfe-Smith et al. (2000) as the Lower River and are in fact from the extreme lower portion between the mouth at Port Maitland and the Byng Conservation Area at Dunnville approximately 8 km upstream from the mouth. The most recent record for the Fawnsfoot from the Grand River is for the collection of 11 live individuals and many fresh dead valves from an area just below the dam in Dunnville in 1997 (Metcalfe-Smith et al. 2000b). This site was revisited, although not formally sampled, in 2005 by the report writer. No Fawnsfoot specimens were found, although shells of 10 other species were observed.

One of the first records for the Fawnsfoot in Canada was from the Niagara River in 1934. The only other record for the species from the Niagara River watershed is from 2002 when a single live specimen was collected from Lyons Creek in a kick net sample not directed at detecting unionids (Logan pers. comm. 2007). Although a voucher specimen was apparently collected, it was not verified and has since been misplaced. The report writer was not able to confirm this record. Thus, this occurrence must be regarded as suspect until a specimen can be properly confirmed and it is not considered further in this report. Limited mussel sampling has been conducted in the Niagara River in recent years. Riveredge Associates surveyed 15 sites for mussels in the area of Grand Island in 2001 and 2002 and found live specimens of six species, none of which was the Fawnsfoot (New York Power Authority 2003).

Extent of occurrence (EO) was calculated in ArcView GIS 3.3 using the maximum convex polygon constructed around historical and current distributions represented in Figures 3 and 4. Assuming the Muskrat Creek population has always been present but went undetected until 2005, a possibility given the low densities and low sampling effort in this watershed, the historical EO is 51,238 km². In contrast, the current EO is 24,952 km² and represents a reduction of 51%. The current area of occupancy (AO) is estimated to be 128 km²but has declined over the last twenty years as populations have been lost from the offshore waters of Lake St. Clair, the Detroit River and Lake Erie. AO does not appear to have changed in the Sydenham and Grand rivers, while new populations have been recently discovered in the Thames (1997) and Saugeen (2005) rivers (see Fluctuations and trends). For all populations except the Thames River, AO was calculated by applying a 2x2 km grid over each occurrence. For the Thames River population, AO was calculated by assuming a continuous distribution between the furthest upstream and downstream records in the river and applying a 1´1 km grid over this entire stretch of river (Filion pers. comm. 2007). The AO for each of the five extant populations was calculated as follows: 4 km² for each of the St. Clair delta, Muskrat Creek, Sydenham River and Grand River populations and 112 km² for the Thames River population.