Eastern pondmussel (Ligumia nasuta) COSEWIC assessment and status report: chapter 7

Population sizes and trends

Search effort

Historical surveys

Approximately 85% of the historical records (1860-1996) for Ligumia nasuta in Canada are based on either museum specimens or occurrence data. For most of these records, there is little if any information on sampling method, search effort, numbers of sites visited where the species did not occur, or even if the animals were dead or alive when collected. Data on relative abundance or density of unionids are available for the remaining 15% of records from the western basin of Lake Erie (Nalepa et al. 1991; Schloesser and Nalepa 1994; Schloesser et al. 1997), the Detroit River (Schloesser et al. 1998), Lake St. Clair (Nalepa et al. 1996) and East Lake and Consecon Lake in Prince Edward County (Metcalfe-Smith et al. unpublished data). Sampling techniques and efforts for these surveys are described below.

Nalepa et al. (1991) reported data from unionid surveys conducted in soft substrates at 17 offshore sites (3.0 - 20.0 km from shore) in the western basin of Lake Erie in 1961, 1972 and 1982 using either a Peterson or Ponar grab (3 - 5 replicate grabs/site). Data from an additional 23 offshore sites surveyed in 1961 and 1982 and 40 offshore sites surveyed in 1930 (all of the above sites) were also discussed, as were data from another survey conducted in 1951-52 using a drag-dredge that was repeated in 1973-74. The 17 offshore sites were re-surveyed by Schloesser and Nalepa (1994) in 1991 after the zebra mussel invasion using a 46 × 26 cm epibenthic sled (one 5-minute tow per site) and 3 replicate 0.05 Ponar grabs. Schloesser et al. (1997) surveyed 15 nearshore sites (max. 1.5 km from shore) with firm substrates in the western basin in 1983 (pre-zebra mussels), 1991 and 1993 using a 30-minute SCUBA search within a 50 m diameter circle at each site. Schloesser et al. (1998) conducted surveys along the length of the Detroit River in 1982-83 (pre-zebra mussels, 13 sites), 1992 (17 sites) and 1994 (9 sites). Sampling effort was a 60-minute search of a 500 m² area by one SCUBA diver. If live unionids were found, an additional 15 to 30-minute search was conducted in an area adjacent to the site. Nalepa et al. (1996) surveyed 29 sites in the offshore waters of Lake St. Clair in 1986 (pre-zebra mussels), 1990, 1992 and 1994; ten 0.5 m²quadrats were sampled at each site. Gillis and Mackie (1994) conducted intensive surveys at two of these sites in 1990-92. They sampled 20-1m²quadrats at depths of 1, 2, 3 and 4 m once a month between June and September at Puce, ON in 1990 and 1991 and at Grosse Pointe, MI in 1991. They also sampled both sites in July 1992. Metcalfe-Smith et al. (unpublished data) conducted 4.5 person-hour (p-h) timed-search surveys using waders, polarized sunglasses and underwater viewers at 7 sites in the Moira River, Moira Lake, Skootamatta River and Salmon River, one site in East Lake and one site in Consecon Lake in 1996. The sampling technique is described in detail in Metcalfe-Smith et al. (2000a).

Recent surveys

In contrast to the historical data, almost all of the recent records (1997-2005) for Ligumia nasuta in Canada include information on sampling method and sampling effort, data on both presence and absence, and descriptions of the condition of the specimens collected (i.e., live animals, fresh shells or weathered shells). Surveys conducted within the range of the species during this time period used either semi-quantitative (timed-search) or quantitative sampling methods, thus providing data on relative abundance or density, respectively. Sampling techniques and efforts for these surveys are described below.

Lake St. Clair:

Zanatta et al. (2002) surveyed 95 sites in various nearshore areas around Lake St. Clair between 1998 and 2001. In 1998, 3 sites were surveyed at depths of 1, 2.5 and 4 m along 10 transects in the vicinity of Puce and Belle River, ON. Five 1 m² quadrats and 20 Ekman grabs were taken at each of the 30 sites. Ten of these sites (depths of 2.5 and 4 m along 5 transects) were re-surveyed in 1999 along with 12 new sites (depths of <1, 2-3 and 4 m along 4 transects) near Grosse Pointe, MI. Another 48 sites at depths of <1 to >3 m along the eastern shore of the lake and in the Canadian waters of the delta were also surveyed in 1999. Searches at water depths greater than 2 m were conducted by two SCUBA divers for a total effort of 0.5 p-h whereas searches at depths less than 2 m were conducted by three people using mask and snorkel for a total of 0.75 p-h. At sites where live mussels were found (all were ≤ 1.5 m deep), snorkel searches were extended to a total of 1.5 p-h. Ten of the sites where unionids were most abundant were re-surveyed in 2000 using 1.5 p-h timed-searches. Four of the best sites from 2000 and 5 new sites, including 4 in U.S. waters, were surveyed quantitatively in 2001 using the following technique: each of two snorkelers searched the area until a live unionid was found, and then surveyed a 65 m²circular area around the animal and collected any other live unionids encountered. Ten such “circle plots” (total area = 650 m²) were searched at 7 of the 9 sites, 5 were searched at one site, and 21 were searched at the most productive site.

Metcalfe-Smith et al. (2004) surveyed 18 sites throughout the St. Clair delta in 2003, including 9 sites in Canadian waters and 9 sites in U.S. waters using the circle plot technique described above, except that three divers were used and each diver searched 10 plots for a total search area of 1950 m²/site (fewer plots were searched at a few sites). Nine of these sites had been surveyed in 2001. Timed-search surveys were also conducted at 10 sites in 2003, including 8 sites in U.S. waters and 2 sites in Canadian waters. Sampling effort was ~ 1.0 p-h/site (Metcalfe-Smith et al. 2004). Four additional sites in Canadian waters were surveyed in 2005 using 3-4 p-h search effort/site (Metcalfe-Smith et al. 2005b).

In 2004, Metcalfe-Smith et al. (2005a) conducted intensive quantitative surveys at two sites in the Canadian waters of the delta that were found to support the richest and most abundant unionid communities in the area. The circle plot technique was used to survey 29 points along 10 transects in Pocket Bay (covering ~ 1% of the 170,000 m²area of the bay), and 138 points along 16 transects in Bass Bay (covering ~ 2% of the 370,000 m² area).

In all of the surveys described above, all live native mussels found were identified, counted, measured, sexed (if sexually dimorphic) and returned to the lake bottom.

Detroit River:

In 1998, Schloesser et al. (2006) re-surveyed four sites in the upper Detroit River where live unionids were observed in 1992 and 1994. They searched a 500 m² area for 60 minutes and a second 500 m² area for 25 minutes at each site. Excavated quadrat searches consisting of 10-1 m²quadrats within a 10 m × 10 m grid were also conducted at one site in 1998 for comparison with pre-zebra mussel data available from 1987, and at a second site in 1998 only. Line-transect searches were conducted at one site in 1997 for comparison with data from 1990. A total of 480 m² along four 120-m transects were sampled in 1997 and 180 m² along three 60-m transects were sampled in 1990.

Lake Erie:

Metcalfe-Smith et al. (2000b) surveyed two sites near the mouth of the Grand River in 1997 using 4.5 p-h timed searches. Zanatta and Woolnough (unpublished data) surveyed 6 sites in Rondeau Bay in 2001 using mask and snorkel for ~ 2 p-h/site. The authors of the present report surveyed 17 historical sites in the western basin of Lake Erie, including sites around Point Pelee and Pelee Island, in 2005. Snorkel searches (1.5 p-h) were conducted at depths of < 3 m at 12 sites. At 5 sites where the water was too rough or turbid for snorkeling, the beach was searched for shells instead.

Lake Ontario/Eastern Ontario:

The authors of the present report surveyed 15 historical sites in East Lake, Consecon Lake, the Bay of Quinte and a few other nearby locations in 2005 using 0.75 to 2 p-h snorkel searches at 5 sites, <0.5 to 1.5 p-h visual searches while wading at 8 sites and a beach search for shells at the remaining sites. Frederick W. Schueler of the Bishops Mills Natural History Centre in Bishops Mills, Ontario, surveyed 2 sites in Lyn Creek in 2006 and one site in Golden Creek, a tributary of Lyn Creek, in 2005. Search effort ranged from 1 to 2.8 p-h/site and searches were tactile, as water clarity was greatly reduced when the soft, muddy substrate was disturbed by the searchers (Schueler pers. comm. 2006).

Abundance

To the best of our knowledge, the only locations in Canada where Ligumia nasuta still survives are the delta area of Lake St. Clair and Lyn Creek near Brockville, Ontario.

Lake St. Clair

Information on relative abundance is available from timed-search surveys conducted between 1999 and 2005. In 1999-2001, Ligumia nasuta was found alive at 48% of 33 sites (29 in Canadian waters) that supported unionid communities, accounting for 3% of the 2356 live unionids collected (Zanatta et al. 2002). Overall catch per unit effort (CPUE) at the 28 sites surveyed in 2000 (all in Canadian waters) was 43 unionids/p-h for all species and about 1.5/p-h for L. nasuta. Fourteen additional sites were surveyed in 2003 and 2005, including 6 sites in Canadian waters (Metcalfe-Smith et al. 2004, 2005b). The Eastern Pondmussel was found alive at 3 or 21% of these sites (2 in Canadian waters) where it accounted for 2% of the 367 live unionids collected. Overall CPUE for this species was 0.3/p-h. These data suggest that the Eastern Pondmussel is widely distributed throughout the delta but sparse in numbers.

Density estimates for the St. Clair delta population of Ligumia nasuta are available from quantitative surveys conducted between 2001 and 2004. Nine sites were surveyed in 2001 and L. nasuta was found at 2 sites, both in Canadian waters, at low densities of 0.001 and 0.002 individuals/m² (Zanatta et al. 2002). Eighteen sites were surveyed in 2003 – nine sites in Canadian waters and nine in U.S. waters (Metcalfe-Smith et al. 2004). Ligumia nasuta was found at 3 Canadian sites at a mean density of 0.014/m² and at 3 U.S. sites at a mean density of 0.0008/m². These results suggest that U.S. waters of the delta support a smaller population of L. nasuta than Canadian waters. Two of the sites in Canadian waters, Bass Bay and Pocket Bay, supported the richest and most abundant unionid communities in the delta. Intensive quantitative surveys were conducted at these sites in 2005 (Metcalfe-Smith et al. 2005b), allowing the calculation of precise estimates of density and abundance for the Eastern Pondmussel in these two bays (Table 1). The estimate of abundance for L. nasuta in the remainder of the delta was based on an occupied area of 17 km², which is the proportion of the 44 km² AO that represents actual mussel habitat (~ 27 km² of the AO is either on dry land or in water deeper than 1.5 m, which does not support mussels). The total size of the population of L. nasuta in the St. Clair delta is estimated to be 22,000 – 44,000 individuals (Table 1).

Table 1. Estimates of population size for the Eastern Pondmussel (Ligumia nasuta) in the Canadian waters of the delta area of Lake St. Clair, based on surveys conducted at 15 sites between 2003 and 2005.
Area Mean density
(±SE)
(# individuals/m²)
Occupied area (m²) Correction factor for occupied area Estimated abundance
(# individuals)
Pocket Bay
0.001 ± 0.0006
170,000
20 – 82
Bass Bay
0.020 ± 0.003
370,000
6,290 – 8,510
Remainder of the delta
0.005 ± 0.002
17,000,000
15,300 – 35,700
Total
 
 
 
21,610 – 44,292

Shell lengths of all live mussels collected during timed-search and quantitative surveys in the Canadian waters of the delta in 2003, 2004 and 2005 were recorded. A total of 235 live Ligumia nasuta were captured, 209 of which were in Bass Bay. The size frequency distribution of the Bass Bay specimens is shown in Figure 5. Lengths of these specimens ranged from 49-99 mm with good representation in several size classes, suggesting that regular recruitment is occurring. Very small specimens were not encountered, but this is probably because juveniles tend to burrow deeply in the substrate where they would not be detected using visual searches. Size class distributions could not be determined for other sites in the delta because too few specimens were collected. Ligumia nasuta exhibits subtle sexual dimorphism, with the female shell being more rounded or swollen along the posterior ventral margin than the male shell. In the authors’ experience most specimens cannot be reliably sexed on the basis of shell shape; therefore, sex ratios have not been presented.

Figure 5. Size frequency distribution of 209 live Ligumia nasuta collected from Bass Bay in the Canadian waters of the Lake St. Clair delta in 2003 and 2004.

Figure 5.  Size frequency distribution of 209 live Ligumia nasuta collected from BassBay in the Canadian waters of the Lake St. Clair delta in 2003 and 2004.
Lyn Creek

Several fresh shells of Ligumia nasuta were discovered in Golden Creek, a tributary to Lyn Creek near Lyn, Ontario in September 2005 (Schueler pers. comm. 2005). In 2006, 4 live specimens and several fresh shells were found in Lyn Creek at a site just downstream of the confluence with Golden Creek, and a single fresh valve was found in a muskrat midden at another site ~ 5 km downstream. Based on information provided by F.W. Schueler, the Eastern Pondmussel is thought to occupy a 6 km stretch of Lyn Creek from the town of Lyn to Younge Mills. Live specimens have only been found at one site to date; thus, the current AO for the Lyn Creek population is estimated at 1 km². As there are no density estimates for L. nasuta in Lyn Creek, the total size of the population cannot be determined.

Fluctuations and trends

Ligumia nasuta was one of the most common species of freshwater mussel in the lower Great Lakes and connecting channels prior to the invasion of dreissenid mussels in the late 1980s. The National Water Research Institute’s Lower Great Lakes Unionid Database consists of over 8700 occurrence records for 40 species of unionids collected from over 2500 sites in the lower Great Lakes drainage basin between 1860 and 2006. A query of this database showed that L. nasuta was the 4th most common species in Lake St. Clair, Lake Erie, Lake Ontario and the Detroit and Niagara Rivers prior to 1990, accounting for 7.5% of the 1591 records for 39 species. The Eastern Pondmussel is now found only in a small portion of Lake St. Clair, where it accounted for ~ 6% of the 5359 live unionids collected between 1999 and 2005. Thus, the current population of L. nasuta in Canada is undoubtedly many orders of magnitude smaller than it once was.

Ligumia nasuta was the second most abundant species (after Lampsilis siliquoidea, the Fatmucket) in surveys conducted in the offshore waters of the western basin of Lake Erie in 1930, 1951-52, 1961, 1972, and 1982 and the 3rd most abundant in 1973-74 (Nalepa 1994). Mean density of all unionids at the 17 sites surveyed in 1961, 1972 and 1982 declined from 9.8 to 5.6 to 4.1/m², respectively, most likely due to a general decline in water quality. Ligumia nasuta accounted for 25.6% of the community in 1961 and the overall density of unionids was 9.8/m². The authors of the present report used these data to calculate a density of 2.5 individuals/m² for L. nasuta in 1961, 0.46/m² in 1972 and 0.95/m² in 1982. It follows that the abundance of L. nasuta declined by about 60% between 1961 and 1982. As the area of the western basin is approximately 1354 km², abundance of L. nasuta in the western basin in 1982 was calculated to be 1,286,550,000 individuals. Schloesser and Nalepa (1994) surveyed the same 17 sites in 1991 after the dreissenid invasion and found only four live animals, none of which was L. nasuta. Schloesser et al. (1997) reported a decline in unionids at 15 nearshore sites in the western basin from 85 specimens and 12 species in 1983 to 97 specimens and 9 species in 1991 and 5 specimens of 4 species in 1993; however, the Eastern Pondmussel was not found during any of these surveys.

Schloesser et al. (1998) surveyed 13 sites in the Canadian and U.S. waters of the Detroit River in 1982-83 before the dreissenid invasion, 17 sites in 1992, and 9 sites in 1994. Ligumia nasuta accounted for 3.7% of 1279 live unionids of 23 species collected in 1982-83, 0.2% of 1653 unionids of 25 species collected in 1992 and none of the 58 live animals of 13 species collected in 1994. Abundance of L. nasuta at the 9 sites sampled on all three occasions declined from 37 to 4 to 0. Schloesser et al. (2006) surveyed four of these sites again in 1998; only 4 unionids of 4 species were found alive and none were L. nasuta.

Nalepa et al. (1996) surveyed 29 sites in the offshore waters of Lake St. Clair in 1986 (pre-zebra mussels), 1990, 1992 and 1994 using the same method in all years; 18 sites were in Canadian waters. Unionid abundance declined from 281 specimens of 18 species in 1986 to 6 specimens of 5 species in 1994. Relative abundance of Ligumia nasuta in these years was 2.8%, 2.0%, 5.1% and 0%, respectively. Mean density of all unionids in 1986 was 1.9/m², and since L. nasuta represented 2.8% of specimens it follows that the mean density of L. nasuta was probably ~ 0.05/m². As the area of Lake St. Clair is 1110 km², abundance of L. nasuta in the offshore waters of Lake St. Clair in 1986 was calculated by the authors of the present report to be 55,500,000 individuals. There are virtually none left. Gillis and Mackie (1994) conducted intensive surveys at two of Nalepa et al.’s (1996) sites that were relatively close to shore in 1990-92. Density of L. nasuta at the site in Canadian waters near Puce, Ontario, declined from 0.01/m²to 0.004/m²to 0/m²over this period. No live unionids of any species were found in 1992. Nearshore waters of the Lake St. Clair delta were not surveyed prior to 1999, so there is little information on changes in population size over time for L. nasuta in the delta area. However, a decline in unionids, including L. nastuta, is known to have occurred at one site in Johnston Bay. Zanatta et al. (2002) collected 137 live unionids of 7 species from the site in 1999 using 1.5 p-h survey effort; 19 specimens were L. nasuta. They resurveyed the site in 2000 using the same effort and found only 12 live unionids of 4 species, including one L. nasuta. Metcalfe-Smith et al. (2004) surveyed the site again in 2003 using 1.3 p-h survey effort and found only 10 live unionids, 2 of which were L. nasuta. Zanatta et al. (2002) noted that zebra mussel infestation rates in Johnston Bay and nearby Goose Lake (mean = 177 zebra mussels/unioinid) were higher than elsewhere in the delta in 1999.

Timed-search surveys were conducted at one site in each of East Lake and Consecon Lake in Prince Edward County (Bay of Quinte area, Lake Ontario) in 1996. A total of 167 live unionids were collected from Consecon Lake, 14 of which were Ligumia nasuta, whereas East Lake yielded 16 live unionids, including 2 L. nasuta (Metcalfe-Smith et al. unpublished data). The authors of the present report re-surveyed these sites in 2005 and searched an additional site in Consecon Lake and four sites in nearby West Lake. No live unionids of any species were found. Similarly, Schueler (pers. comm. 2006) visited Consecon Lake and Beaver Lake in the Salmon River watershed in 2006 and found both lakes to be heavily infested with zebra mussels and no longer likely to support live unionids. As there are no historical records for Ligumia nasuta in Lyn Creek, changes over time in the size of the Lyn Creek population are unknown.

Rescue effect

The Eastern Pondmussel occurs in four Great Lakes states (Michigan, New York, Ohio and Pennsylvania) that are connected to Ontario waterways via Lakes Ontario, Erie, St. Clair and Huron. Populations of Ligumia nasuta are generally not doing well in these jurisdictions. The species is listed as Endangered in Ohio and a Species of Conservation Concern in New York and Pennsylvania. Its status in Michigan is unknown. In New York, Strayer and Jirka (1997) describe the Eastern Pondmussel as not very widespread but still encountered regularly and sometimes abundant, being most common in the western part of the state where it is found at scattered locations in the Erie-Niagara drainage, central New York and the St. Lawrence drainage. The only location in the U.S. waters of the Great Lakes where L. nasuta is known to persist is Thompson Bay, a small (~ 1 km²) bay just outside of Presque Isle Bay along the south shore of Lake Erie near Erie, PA. There were an estimated 20,000 unionids of 22 species, including L. nasuta, living in the larger (15 km²) Presque Isle Bay before the dreissenids arrived, but by 1995 all were believed to have died (Masteller pers. comm. 2002).

It is unlikely that extirpated populations of L. nasuta in Canada could become re-established by natural immigration of animals from the United States because there are few potential source populations and the distance is vast. The St. Clair delta population may be an exception: if the population in the Canadian portion of the delta were to disappear, it is possible that the species could return naturally through the movement of host fishes from U.S. waters of the delta, especially if the yellow perch is found to be one of the hosts. Yellow perch are shallow water fish that are not usually found at depths below 9.2 m (Scott and Crossman 1973). As the navigation channel that bisects the delta along the Canada/U.S.A. border is only 8.3 m deep (Edsall et al. 1988), yellow perch should be able to move freely throughout the delta. It should be noted, however, that the U.S. population of L. nasuta is smaller than the Canadian population and that unionid communities in U.S. waters are not as healthy (less diverse and abundant; individuals have lower energy reserves) as those in Canadian waters (Metcalfe-Smith et al. 2005a).

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