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Recovery Strategy for the Northern Riffleshell, Snuffbox, Round Pigtoe, Mudpuppy Mussel and Rayed Bean in Canada (Final)

6. Threats

All five mussel species are exposed to a wide range of stresses throughout their range. In the Sydenham River watershed, Jacques Whitford Environment Ltd. (2001) determined the principal anthropogenic stresses affecting populations of species at risk to be loadings of suspended solids, causing turbidity and siltation, nutrient loads, contaminants, thermal effects, and exotic species. These likely represent the most significant threats to these species across their entire Canadian range. The following discussion emphasizes threats in the Sydenham and Ausable rivers and St. Clair delta; areas where extant reproducing populations can still be found. Remnant populations of these five species, existing in the Detroit, Thames, Grand, and Niagara rivers, as well as the offshore waters of Lake Erie and Lake St. Clair are discussed under Threats in Historically Occupied Habitats.

Threats to Extant Populations

Sydenham, Ausable and St. Clair Delta Populations


Loading of suspended solids causing turbidity and siltation is presumed to be the primary limiting factor for most species at risk in the Sydenham and Ausable rivers. The majority of rare mussel species depend on clean gravel and sand riffles and are particularly vulnerable to siltation. Siltation can bury and smother mussels as well as interfere with feeding and successful reproduction. Clarke (1992) noted that all of the species missing from the Sydenham River during his 1991 survey were riffle-dwelling species such as the Northern Riffleshell and Snuffbox. The Snuffbox and Rayed Bean are the only two species in Ontario that burrow completely in the substrate. These species may be more sensitive to sedimentation than most other mussel species because an accumulation of silt on the streambed would reduce flow rates and dissolved oxygen concentrations below the surface (Watson et al. 2001b). Although the Mudpuppy Mussel may be directly impacted by siltation due to silt settling around the flat rocks, logs and other debris under which it is found, it is more likely indirectly impacted as there is some evidence that siltation has extirpated the mudpuppy from some areas by reducing its access to nesting sites and hiding places (Gendron 1999).

Nutrient loads

Phosphorus and nitrogen compounds, primarily from agriculture, are at high levels within these watersheds and represent potential risks to aquatic fauna. Mean levels of total phosphorus at sites on the East Sydenham River ranged from 0.125 to 0.147 mg/L, with levels as high as 2.9 mg/L reported; mean total phosphorus levels for sites in the North Sydenham basin were about three times higher. Not surprisingly, nitrogen has replaced phosphorus as the limiting nutrient in the system. Although there has been no evidence of blooms of blue-green algae, which can occur when nitrogen is limiting, there is still potential for significant reductions in dissolved oxygen at night. Nutrients enter the system from several sources and long-term water quality monitoring data indicate that much of the nutrient load is bound to suspended solids and thus likely originates from farmland. Manure spills also occur and can have significant nutrient-enriching effects, as well as being acutely toxic to fish and invertebrates. Urban areas are not extensive in the watersheds but contribute to total nutrient loadings through municipal wastewater discharges. Loadings from domestic septic systems may also be significant.

Nutrient concentrations within the Ausable River typically exceed provincial water quality objectives with mean nitrate concentrations at eight stations within the watershed ranging between 3.5 and 5.6 mg/L between 1965 and 2002 (Ausable River Recovery Team 2005). Phosphorus concentrations are also high within the Ausable River watershed with large proportions (30-58% occurring in the dissolved fraction (Veliz 2003).


Herbicides and insecticides associated with agricultural practices and urban areas run off into the Sydenham River watershed and could have a significant impact on species at risk. Roads and urban areas can also contribute significant contaminants to waterways, including oil and grease, heavy metals, and chlorides. Until about 1990, chloride levels in particular were high enough in the North Sydenham River to cause significant biological impairment. Chloride concentrations at all three monitoring sites in the north branch were as high as 1000 mg/L between 1967 and 1990, often exceeding 200 mg/L, which is the concentration estimated to cause long-term toxicity to some freshwater organisms (Evans and Frick 2002). Prior to 1990, saline formation waters produced from local oil wells were released to surface waters in the North Sydenham watershed. Since then, these waters have been injected back into the ground, and chloride concentrations have declined to levels similar to those in the East Sydenham River (10–50 mg/L). The impacts of high chloride concentrations on species at risk in the North Sydenham watershed are unknown.

Pesticide runoff (e.g., herbicides and insecticides) associated with agricultural practices and urban areas enter the Ausable River basin and could have a significant impact on species at risk. For example, tributary monitoring at the mouth of the Ausable River for currently used pesticides in 2002 indicated that both atrazine and des-ethyl atrazine were found to exceed federal guidelines for the protection of aquatic life (J. Struger, Environment Canada cited in Ausable River Recovery Team 2005). The extent and impact of these and other toxic contaminants (e.g., chloride) to species at risk have not been assessed and thus, the significance of their threat is unknown. It is likely that this threat is widespread as the primary source of pesticides is from agricultural land. The risks from toxic contaminants to some species may be heightened at juvenile life stages (particularly for mussels) and at times of increased stress.

Thermal effects

The loss of riparian zones in agricultural lands increases solar radiation reaching the stream surface. Although there are riparian corridors along the Sydenham River and its tributaries, these vary in width and quality, and there are extensive reaches lacking riparian zones. Reservoirs also increase temperatures by increasing surface area and by water holding. There are six significant reservoirs in the Sydenham watershed at conservation areas in Strathroy, Coldstream, Petrolia, Alvinston, Henderson, and Warwick. Finally, global climate change is expected (among other disruptions) to cause an increase in surface water temperatures in southern Ontario. Although the Sydenham River supports a warm-water environment, and many species are tolerant of warm water, higher water temperatures may be an added stress for some. Increased water temperatures may also increase algal growth, which could result in reductions in dissolved oxygen levels at night.

Exotic species

The introduction and spread of the Dreissenid mussels throughout the Great Lakes in the late 1980s have decimated native mussel populations in the Lower Great lakes region of Ontario (Schloesser et al. 2006; Schloesser et al. 1996; Schloesser and Nalepa 1994). Zebra and quagga mussels attach to a mussel's shell and interfere with feeding, respiration, excretion ad locomotion (Haag et al. 1993, Baker and Hornback 1997). The recent discovery of a refuge for native mussels including the Round Pigtoe in the delta region of lake St. Clair raises hope for their continued coexistence with Dreissenid mussels however, it is not known if this native mussel community is stable or simply in a slower decline than other Great Lakes communities (Zanatta et al. 2002). It is clear that Dreissenid mussels pose the most significant threat to all native mussels within the St. Clair delta.

Currently, Dreissenid mussels are found only in the lower reaches of the Sydenham River. It does not threaten existing populations of these five mussel species as the river is not navigable by boats and has no significant impoundments that could support a permanent colony (Dextrase et al. 2003). However, the reservoirs at Coldstream and Strathroy in the East Sydenham River headwaters are of some concern.

Dreissenid mussels are not currently found within the Ausable River or its reservoirs however, should they become established within the river or reservoirs (e.g., Morrison Dam reservoir) they will likely represent a significant threat to these species.

Another exotic species which may currently be exerting negative effects in the Sydenham River is the common carp (Cyprinus carpio). This species is abundant throughout the watershed and is likely to be adversely affecting sensitive species. Although they can potentially consume juvenile mussels, their uprooting of plants and feeding on sediment-associated fauna can significantly increase turbidity, which is likely a far greater impact (Dextrase et al. 2003).

The round goby (Neogobius melanostomus) has decimated populations of mottled sculpins and possibly logperch in the St. Clair River (French and Jude 2001). This species may pose a direct threat to fish species at risk and an indirect threat to mussel species if host fish populations are affected. The round goby has not yet been documented from the reaches of the Sydenham and Ausable rivers where these mussels occur, but it is abundant in Lake St. Clair and its connecting channels (Ray and Corkum 2001). This species has recently been confirmed from Running Creek in Wallaceburg near the mouth of the Sydenham River (E. Holm, Royal Ontario Museum, personal communication). Additional introductions of exotic species into these waters are most likely to occur through the movement of boats from infested areas, the use of live bait fish, or the natural invasion of species introduced into the Great Lakes basin.

Table 1. Assessment of threats to the extant populations of the Northern Riffleshell, Mudpuppy Mussel, Round Pigtoe, Rayed Bean and Snuffbox in the Sydenham and Ausable rivers.
ThreatRelative ImpactSpatial NatureTemporal NatureCertainty of Effect
Siltation and turbidityPredominantWidespreadChronic, episodicProbable
Nutrient loadsContributingWidespreadChronic, episodicProbable
Toxic compoundsContributingWidespreadChronic, episodicProbable
Thermal effectsContributingWidespreadChronicProbable
Exotic speciesContributingWidespreadChronicProbable


Host Fish Species

Due to the parasitic stages in their life cycle, the Northern Riffleshell, Mudpuppy Mussel, Round Pigtoe, Rayed Bean and Snuffbox are sensitive not only to environmental factors that limit them directly, but also to factors that affect their hosts (Burky 1983; Bogan 1993). Therefore, any factor that changes the abundance or species composition of host fauna may have detrimental effects on the mussel populations.

Until recently, the glochidial hosts for the Northern Riffleshell were completely unknown in Canada. Host fish determination studies at the University of Guelph (McNichols and Mackie 2002; McNichols and Mackie 2003; McNichols et al. 2004) have found that the Northern Riffleshell may have seven host species, including the blackside darter, fantail, Iowa darter, Johnny darter, logperch, mottled sculpin and rainbow darter. Of these five host species, only the blackside darter, johnny darter and logperch are common in the Sydenham River. The mottled sculpin may have served as a host historically, but now is likely restricted to the colder headwater regions where the Northern Riffleshell does not occur (Staton et al. 2000).

The Snuffbox was thought to have had two host species in Ontario, namely the blackside darter and logperch. Historical data of the distribution of these two species indicated that the logperch was likely the primary host as its distribution is more similar to that of the Snuffbox (Watson et al. 2001a). Recent records of the blackside darter show that it presently occupies the same reach of the Sydenham River as the Snuffbox. However, it is a less likely host since it was never found in the reaches of the Grand and Thames rivers where the Snuffbox historically occurred. Host fish determination studies at the University of Guelph (McNichols and Mackie 2002; McNichols and Mackie 2003; McNichols et al. 2004) have found that the Snuffbox successfully transformed on six host species including the brook stickleback, Iowa darter, logperch, mottled sculpin, largemouth bass and rainbow darter. The logperch was confirmed during repetitive studies, while the three other species still require further studies. The blackside darter has been repeatedly tested at the University of Guelph but has never lead to the successful development of juvenile Snuffbox.

In the United States, the glochidial hosts of the Round Pigtoe are known to be the bluegill, spotfin shiner, bluntnose minnow, northern redbelly dace and southern redbelly dace. All but the southern redbelly dace are known to occur in the Sydenham River and are likely hosts for the Round Pigtoe. However, laboratory testing and field confirmation is required to identify the functional host(s) with certainty.

The only known host for the Mudpuppy Mussel is the mudpuppy (Necturus maculosus). The status of the mudpuppy in Canada is considered “Not at Risk” (Gendron 1999). Significant limiting factors for the mudpuppy include habitat loss as a result of severe siltation and environmental contamination, particularly the use of the lampricide TFM. Indications of extirpations from formerly occupied habitats are relatively few, although Gendron (1999) did report the loss of the species from the highly impacted Hamilton Harbour and low capture rates at several localities in Lakes Ontario, Erie and St. Clair in 1995. McDaniel and Martin (2003) conducted surveys of mudpuppies in the Sydenham River in 2002-2003 and found a total of 61 animals with densities estimated at between 13-22 animals per 100m2. The highest densities were observed between Dawn Mills and Shetland with no records above Alvinston.

Until recently, the glochidial hosts for the Rayed Bean were completely unknown in Canada. Host fish determination studies at the University of Guelph (Woolnough 2002; McNichols and Mackie 2002; McNichols and Mackie 2003; McNichols et al. 2004) have found that seven host species served as successful hosts for the Rayed Bean including the brook stickleback, greenside darter Johnny darter, logperch, rainbow darter, mottled sculpin, largemouth bass. The greenside and rainbow darters have been confirmed as hosts during repetitive studies. All of these species, except the brook stickleback, have been confirmed to inhabit the Sydenham River.

Threats in Historically Occupied Habitats

Grand and Thames rivers

The Northern Riffleshell, Snuffbox and Mudpuppy Mussel historically occurred in Thames River and the Round Pigtoe is still represented by small isolated non-reproducing populations in both the Thames and Grand rivers. The Rayed Bean was once distributed throughout the South Thames River in the area near Dorchester however this population is no longer believed to exist. Although not historically known from the North Thames River, a single live Rayed Bean was found in this river in 2004. It is difficult to attribute a cause to the historic loss of mussel populations in the Grand River although untreated wastewater inputs from major urban centres likely contributed to the declines. Aquatic species at risk in the Thames River are threatened by the highly-developed urban and rural portions of the upper watershed. The watershed is also intensively used for both livestock and row crop agriculture. The main threats faced by aquatic species at risk within the Thames River ecosystem include siltation and turbidity, nutrient loading, toxic compounds, altered water flow, barriers to movement, non-native species, disturbance and thermal pollution (Thames River Recovery Team 2004). While single threats may be associated with the decline of certain populations of species at risk, in most cases, population declines are likely a result of the cumulative effect of multiple widespread and chronic stresses. Potential colonization of these rivers with zebra mussels is a concern as large sections are impounded. Zebra mussels have recently been found in the Fanshawe and Springbank reservoirs on the Thames River (S. Hohn, Upper Thames River Conservation Authority, September 2003) and have been found and very low densities throughout the Thames River from Fanshawe reservoir downstream to Wardsville. In the lower Thames River near Big Bend zebra mussels have been found attached to adult unionids (Todd J. Morris, Fisheries and Oceans Canada, unpublished data). Round gobies have been detected in the lower Thames River as far upstream as Thamesville (pers. comm. A. Dextrase, Ontario Ministry of Natural resources).

Lake St. Clair, Detroit River, Lake Erie and Niagara River

The loss of the Northern Riffleshell, Mudpuppy Mussel, Round Pigtoe, Rayed Bean and Snuffbox from historical habitat in these water bodies can be largely attributed to the detrimental effects of zebra mussels. Mussel species that are long-term brooders, such as the Northern Riffleshell, Mudpuppy Mussel, Rayed Bean and Snuffbox, are generally more sensitive than short-term brooders, as they tend to have greater energy requirements for growth and reproduction and may be more vulnerable to energy depletion caused by the zebra mussel (Strayer 1999). The Round Pigtoe is a short-term brooder and may be less susceptible to the harmful effects of the zebra mussel. The Rayed Bean and Snuffbox are the only two species in Ontario that burrow completely in the substrate and may escape serious infestation due to their preferred habitat. The Mudpuppy Mussel has several traits that suggest it may be very sensitive to zebra mussels, however, it also tends to burrow under rocks and mud which may help it escape from infestation. In the Detroit River, populations of the Northern Riffleshell and Round Pigtoe have been extirpated due to the zebra mussel.