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

Population Sizes and Trends

United States

Obovaria subrotunda is a very uncommon species that is clearly declining throughout much of its range in the U.S. It is extremely rare in the southeastern states. The only extant population in Alabama is in the Paint Rock River (J. Garner, Alabama Division of Wildlife and Freshwater Fisheries, pers. comm. October 2001), where Ahlstedt (1995-96) found only 9 live specimens during surveys in 1991. In Georgia, it may occur over an extremely limited range in the northern parts of the state in the Tennessee River drainage (P. Hartfield, U.S. Fish and Wildlife, pers. comm. July 2001). In Mississippi, it occurs only in the Big Black and Big Sunflower rivers (B. Jones, Mississippi Museum of Natural Science, pers. comm. October 2001). According to Parmalee and Bogan (1998), O. subrotunda was historically found throughout the Tennessee and Cumberland River systems, but “has disappeared, or nearly so, from most of these rivers.” In the Cumberland system, it occurs in the Obey, Stones, Harpeth and Red rivers and in the mainstem Cumberland. In the Tennessee system, it was found in the main channel of the Tennessee River and in the Clinch, Pigeon, Little Tennessee, Sequatchie, Powell, Holston, Buffalo, Duck and Elk rivers. Ahlstedt (1983, 1991) conducted extensive surveys in the latter five rivers, plus the Nolichucky River, in 1979-80 and found a significant population of round hickorynuts in the Duck River and just a few live specimens in the Elk River. The Duck River population had also been noted in 1933 (van der Schalie 1938). Obovaria subrotunda is one of 103 species of mussels known from Kentucky. Based on a compilation of unionid records dating back to 1818, Cicerello et al. (1991) determined that O. subrotunda occurred in 12 of the 19 major river systems in the state. It is presently described as occasional in the upper Cumberland and Big Sandy rivers, sporadic in the upper Green, Kentucky and Licking rivers and Tygarts Creek, and rare in the Ohio River, lower Cumberland River and Little Sandy River.

The round hickorynut once occurred in four of the 25 drainages in Illinois, i.e., the Embarras River and Wabash River tributaries, the Vermilion and Little Vermilion River drainages, the Ohio River, and the Wabash River. It may have been fairly abundant in the past; for example, 131 live O. subrotunda were collected from the Embarras River by M.R. Matteson in 1956 (K. Cummings, Illinois Natural History Survey, pers. comm. August 2001). Since 1969, it has only been found alive in the Vermilon system (Cummings and Mayer 1997). In Indiana, O. subrotunda was most common in streams flowing to the Ohio River such as the Wabash and White rivers, and was rare in the Maumee system (Goodrich and van der Schalie 1944). Watters (1996) observed the species in Fish Creek, a tributary of the St. Joseph River in the Maumee system, in both 1988 and 1996. Although 17 live specimens were collected during surveys of the Tippecanoe River in 1987, the round hickorynut is generally quite rare in the state (K. Cummings, Illinois Natural History Survey, pers. comm. August 2001). Obovaria subrotunda is found throughout the state of Ohio, but rarely in any numbers (G.T. Watters, Ohio Biological Survey, pers. comm. July 2001). In the St. Joseph River, which seems to have received the most attention, Clark and Wilson (1912, in Clark 1977) found it to be “…fairly common in the feeder canal where 16 live specimens were secured, and in the St. Joseph River near its mouth, where we obtained 10.” Clark (1977) reported finding it at seven sites in the mainstem or tributaries during collecting trips between 1938 and 1975. Way and Shelton (1997) recorded the round hickorynut from a site in the Ohio River in 1995, and Watters (1993-94) found it alive in the Muskingham River in 1992. In West Virginia, O. subrotunda is found throughout the interior basin of the state, though never in large numbers (J. Clayton, West Virginia Department of Natural Resources, pers. comm. August 2001).

Van der Schalie (1938) studied the mussel fauna of the Huron River in southeastern Michigan in the early 1930s. He only found shells of O. subrotunda, and these were at a site near the mouth. Strayer (1980) compared the historical distribution of mussels in the Clinton River (1870-1933), a tributary to the St. Clair River just north of Detroit, with the results of his own surveys of 76 sites in 1977-78. The round hickorynut had been reported from two sites in the lower reaches of the river prior to 1935, but he did not find it during his surveys and declared it “extinct” in the Clinton. He also stated that the species has been eliminated from most of its former range in the Erie-St. Clair system. Hoeh and Trdan (1985) surveyed 27 sites in the Black, Pine and Belle rivers (also Michigan tributaries to the St. Clair River), in 1982-83 and found the round hickorynut at one site in the Pine River where it was described as “rare” (<1 mussel found per man-hour of search effort). Strayer et al. (1991) surveyed 52 sites in rivers and streams of western New York in 1987-90, and found a single subfossil shell of O. subrotunda in Conewango Creek in the Allegheny River system. According to Strayer and Jirka (1997), the species is likely extirpated from the state of New York “although it may turn up in the Allegheny or Erie-Niagara basins.” In Pennsylvania, it historically occurred in the Ohio, Beaver and Monongahela River drainages (Ortmann 1919); however, it has severely declined in recent years (A. Shiels, Pennsylvania Nongame and Endangered Species Unit, pers. comm. Sept. 2001).


Great Lakes Waters

Lake Erie

There is evidence that O. subrotunda may have been extirpated from Lake Erie by as early as 1950. The species was reported from all but one of seven surveys conducted between 1910 and 1942 but not in 1960 or 1993, nor was it found during surveys of 33 sites along the southwest shore and around the Bass Islands in 1998 (Ecological Specialists 1999). Sixteen species of unionids were collected from the western basin of Lake Erie between 1930 and 1982 (Nalepa et al. 1991). Obovaria subrotunda was present in 1930 and 1951-52, but not in 1961, 1972, 1973-74, or 1982. By 1991, the community had been virtually eliminated by the zebra mussel (Dreissena polymorpha) -- only four specimens of two species were found alive (Schloesser and Nalepa 1994). Schloesser et al. (1997) sampled 15 sites along the Michigan shoreline both before (1983) and after (1990 and 1993) the zebra mussel invasion, but did not find O. subrotunda on either occasion. The round hickorynut was also absent from the collections of Ortmann (1919) and Masteller et al. (1993) from Presque Isle Bay, Erie, PA, and was not among 20 species found alive in Metzger Marsh, near Toledo, Ohio in 1996 (Nichols and Amberg 1999). Surveys of seven other marshes near Metzger in 2000 produced few live mussels, and none were O. subrotunda (Ecological Specialists 2001).

Lake St. Clair and the Detroit River

The round hickorynut still persists in Lake St. Clair. Nalepa and Gauvin (1988) surveyed 29 sites throughout the lake in 1986 and collected 281 live unionids of 18 species, including one live specimen of O. subrotunda. Nalepa et al. (1996) re-surveyed these sites in 1990, 1992 and 1994, and reported finding two live round hickorynuts in 1990. By 1994, unionids had been virtually extirpated from the offshore waters of the lake by the zebra mussel. Similarly, Schloesser et al. (1998) sampled 17 sites in the Detroit River before (1982-83) and after (1992 and 1994) the zebra mussel invasion, and found two live O. subrotunda in 1982-83, three in 1992 and none in 1994. Near total mortality of the unionid community in the river had occurred by 1994. Gillis and Mackie (1994) sampled several nearshore sites in the southwestern portion of Lake St. Clair near Puce, Ontario, and Grosse Pointe, Michigan, between 1990 and 1992, and did not find any O. subrotunda. Richness and abundance of unionids had declined dramatically over the sampling period, due to impacts of the zebra mussel. In contrast, Zanatta et al. (2002) discovered a significant unionid community of 22 species surviving in shallow (<1 m) nearshore waters of the St. Clair delta in 1999. A total of 53 live O. subrotunda were collected from five of the 31 sites sampled between 1999 and 2001; all five sites were in Canadian waters.

Two good indicators of the overall health or “strength” of a mussel population are: (a) density, which can be compared with the densities of known healthy populations; and (b) size class frequencies of live animals, which provide a record of reproductive success. Density estimates for O. subrotunda in the St. Clair delta were found to be 0.014/m2, 0.005/m2 and 0.002/m2 at three sites sampled quantitatively in 2001 (Zanatta et al. 2002). These densities are one to two orders of magnitude lower than the densities of 0.10/m2 to 0.20/m2 reported in 1979 for the Duck River in Tennessee (Ahlstedt 1991), which may harbour the healthiest remaining populations of the round hickorynut in North America. Although current densities in Lake St. Clair are low, the area of occupancy is large (approximately 8 km2). Assuming that densities are consistent throughout the area of occupancy, an optimistic estimate of population size is 55 000 individuals. Size frequency distributions for the 53 live specimens collected from the St. Clair delta between 1999 and 2001 are presented in Figure 4. The population is dominated by animals from a few size classes, which may indicate frequent year-class failure. Mean shell length was 31.3 mm (±3.6 mm SE), which is probably typical for adult specimens in the Great Lakes. Various authors have noted that the “lake form” of O. subrotunda attains a much smaller size than the river form. For example, Ortmann (1919) reported that the largest specimen he had seen from Lake Erie was only 42 mm long.


Figure 4: Size Frequency Distribution for Live Specimens of Obovaria subrotunda Found in Lake St. Clairbetween 1999 and 2001 (n = 53)

Figure 4: Size frequency distribution for live specimens of Obovaria subrotunda found in Lake St. Clairbetween 1999 and 2001.

Table 1 summarizes the available information on frequency of occurrence and relative abundance of O. subrotunda in various locations in Alabama, Kentucky, Michigan, Ohio, Tennessee and Ontario, including the Great Lakes. The round hickorynut was found at about 10% of sites surveyed (range 3-24%), representing 0.7% on average (range 0.2-8.0%) of the total number of mussels collected. It appears that the healthiest remaining populations in North America are in the Duck River, Tennessee, and the Ontario waters of the St. Clair delta in Lake St. Clair. The Paint Rock River population was significant in 1980, but has since declined rather dramatically.


Canadian Rivers

Obovaria subrotunda has been reported from the Grand, Thames, Sydenham and Welland rivers in Ontario. There is only one record for the Welland River -- from 1931. To our knowledge, this river has not been surveyed for mussels in recent years. Metcalfe-Smith et al. (1998b, 1999) surveyed 66 sites on the Grand, Thames, Sydenham, Ausable and Maitland rivers in 1997-98 to determine the conservation status of rare species of freshwater mussels in southwestern Ontario. They used the timed-search technique, which they have shown to be the most efficient method for detecting rare species (Metcalfe-Smith et al. 2000a), and an intensive sampling effort of 4.5 person-hours (p-h)/site. Sites that were known to support rare species (including O. subrotunda) in the past were targeted. Results of these and other recent surveys were compared with the historical data to determine population trends for the round hickorynut. This species was not found in the Ausable or Maitland rivers in the lower Lake Huron drainage, nor did it occur there historically.

Table 1: Frequency of Occurrence and Relative Abundance of Obovaria subrotundain Various Locations in the United States and Ontario
River/LakeState/ProvinceFrequency of occurrence, as % of sites surveyed (# sites)Relative abundance, as % of communityYear of survey
Paint Rock RiverAL24% (25)3.0%1980a
Paint Rock RiverAL12% (25)0.7%1991b
Duck RiverTN18% (99)1.4%1979a
Elk RiverTN3.7% (108)0.2%1980c
Cumberland RiverTN, KY3.4% (29)0.3%1984-86d
Green RiverKY-0.1%1990-91e
St. Joseph RiverOH18% (40)-1938-75f
Muskingham RiverOH-0.3%; 0.5%1992g
Black, Pine and Belle riversMI3.7% (27)-1982-83h
Western basin Lake ErieOH, MI, ON-0.3%1951-52i
Detroit RiverMI, ON-0.2%1982-83j
Detroit RiverMI, ON-0.1%1992j
Lake St. ClairMI, ON3% (29)0.4%1986k
Lake St. ClairMI, ON16% (31)2.2%1999-2001l
a Ahlstedt (1991)
e Cochran and Layzer (1993)
i Nalepa et al. (1991)
b Ahlstedt (1995-96)
f Clark (1977)
j Schloesser et al. (1998)
c Ahlstedt (1983)
g Watters (1993-94)
k Nalepa and Gauvin (1988)
d Ahlstedt and Saylor (1995-96)
h Hoeh and Trdan (1985)
l Zanatta et al. (2002)


Small numbers of shells of the round hickorynut have appeared in several museum collections from the Grand and Thames rivers, dating back to the late 1800s. A fresh whole shell collected from the Thames River at Chatham in 1894 was most likely taken from a live animal, but the condition of another shell collected in 1930 is unknown. Metcalfe-Smith et al. (1998b and unpublished data) found a total of 13 weathered (subfossil) half shells at three sites in the middle reaches of the Thames River in 1997-98. Shells of the round hickorynut were collected from the lower Grand River in 1966 by John Oughton and in 1972 by Brian Kidd. After conducting an exhaustive search of the literature and examining many museum collections, Kidd (1973) concluded that the species had not been collected alive from the Grand River as far back as 1885. Metcalfe-Smith et al. (2000b) surveyed 95 sites throughout the Grand River and its tributaries between 1995 and 1998, and did not find a single shell of O. subrotunda. These results suggest that the Round Hickorynut may have already been extirpated from the Grand and Thames rivers by the turn of the century.

The round hickorynut has declined dramatically over time in the Sydenham River. Only three live individuals were collected over the past five years, despite at least 200 person-hours of survey effort, whereas 32 live specimens had been found at 11 different sites in the East Sydenham River between 1965 and 1991. The presence of O. subrotunda in the Sydenham River was first documented by Carol Stein and Joanne Stillwater (Ohio State University), who collected five live animals from a site near Florence in 1965. Stein and Karen Heffelfinger also collected one live specimen from a site near Alvinston in 1967. Stein revisited the Florence site in 1973, and found only two fresh whole shells; however, she also visited a site near Dawn Mills, where she found 18 live animals. Clarke (1973) surveyed 11 sites in the river in 1971 using an average sampling effort of 1.1 p-h/site and found 26 live species. He found one live Round Hickorynut at each of three sites. Mackie and Topping (1988) surveyed 22 sites in the system in 1985 using a sampling effort of 1.0 p-h/site and found only 13 species alive, not including O. subrotunda. Clarke (1992) surveyed 16 sites in 1991, using a greater sampling effort than in 1971 (mean = 2.4 p-h/site) and found five live Round Hickorynuts at four sites. Metcalfe-Smith et al. (1998b, 1999) surveyed 17 sites on the Sydenham River in 1997-98, with good coverage of the reach where O. subrotunda previously occurred, and found only 24 shells -- 70% of which were weathered, broken valves. Quantitative sampling was subsequently conducted at four of these sites (10-12 p-h search effort/site), and one live O. subrotunda was found at a site near Dawn Mills. Another live specimen was found at this site during other sampling in 2001 (Daelyn Woolnough, University of Guelph, pers. Comm..), and a third live specimen was found by the authors at a site near Alvinston in 2001 after 6.25 p-h of search effort. Since all live specimens and fresh whole shells collected in recent years were large, i.e., 48-61 mm (D.L. Strayer, Institute of Ecosystem Studies, Millbrook, NY, described the 61 mm shell as “huge” for this species), this leads us to believe that they may be old, non-reproducing, remnants of the original population.