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

Limiting Factors and Threats

The introduction and spread of the exotic zebra mussel (Dreissena polymorpha) throughout the Great Lakes has destroyed native freshwater mussel populations in infested areas (Schloesser et al. 1996). Zebra mussels attach to a unionid’s shell, where they interfere with activities such as feeding, respiration, excretion and locomotion -- effectively starving it to death (Haag et al. 1993; Baker and Hornbach 1997). Approximately 64% of the sites where O. subrotunda was historically collected in Ontario are in Great Lakes waters that are now heavily colonized by zebra mussels. It is not known at present why O. subrotunda and other species of unionids living in the shallow waters of the St. Clair delta have thus far survived the zebra mussel invasion. We speculate that the numbers of veligers reaching and/or settling in the area may vary from year to year depending on wind direction, currents and water levels (Zanatta et al. 2002). Several studies have shown that temporal variation in densities and colonization rates of zebra mussels can influence zebra mussel-induced mortality of unionids (Schloesser et al. 1997). We must caution, however, that there is no guarantee that the St. Clair “refuge” will persist. In fact, several of the species known to be most susceptible to the zebra mussel have declined, and overall infestation rates are higher than in other refuge sites in Lake Erie. If the Lake St. Clair population of O. subrotunda eventually succumbs to the zebra mussel, this will likely mean that the species has been extirpated from Canada.

Obovaria subrotunda has severely declined throughout the Tennessee River drainage (see Population Sizes and Trends), along with many other mussel species. Threats known to have caused the decline of mussels in this system include: dams (which cause altered temperature regimes, fluctuating water levels, seasonal oxygen deficits, and separate mussels from their hosts); channelization (for flood control); municipal sewage; silts and coal fines from strip-mining and coal washing facilities; silt from mica and feldspar mining; and runoff of silt from agricultural lands, as well as chemicals used on cotton and bean crops (Ahlstedt 1991). According to Strayer and Fetterman (1999), the main threats to mussels today are high loads of sediment, nutrients, and toxic chemicals from non-point sources, especially agriculture. Agriculture is the primary land use in the Sydenham River basin, with 85% of the land in agricultural use (mainly row crops) and 60% of the watershed in tile drainage (Staton et al. 2002). Only 17% of the original forest cover remains, and there are long reaches of the river with little or no riparian vegetation. Sediment loadings from overland runoff and tile drainage are high. Sediments originating from tile drainage tend to be fine-grained (Grass et al. 1979). Fine sediments are known to adversely affect mussels in many ways, e.g., they can clog the gills, thereby reducing respiration rates, feeding efficiency, and growth; they can affect their food source by reducing the amount of light available for photosynthesis; and they can affect mussels indirectly by impacting on their host fishes (see Brim-Box and Mossa 1999 for a review). Nutrient loadings are also high in the Sydenham River, and total phosphorus levels have consistently exceeded the provincial water quality objective over the past 30 years; chloride levels are slowly rising due to the increased use of road salt (Staton et al. 2002). Despite these threats, the unionid fauna of the Sydenham River remains remarkably intact -- 30 of the 34 species still survive in the system. However, O. subrotunda is one of three species that have shown a statistically significant decline in frequency of occurrence over time (Metcalfe-Smith et al. 2001).

The most significant natural controls on the size and distribution of mussel populations are the distribution and abundance of their host fishes, and predation. Unionids cannot complete their life cycle without access to their proper glochidial host. If host fish populations disappear, or decline in abundance to levels below that which can sustain a mussel population, recruitment will no longer occur and the mussel species may become functionally extinct (Bogan 1993). The host fish for O. subrotunda is unknown, although the eastern sand darter has been suggested as a possibility (see Biology). The eastern sand darter was designated as threatened in Canada in 1994, and many Canadian populations have declined or been extirpated (Holm and Mandrak 1996). Determination of the host(s) of the round hickorynut in the Sydenham Rivers and Lake St. Clair is crucial to understanding its chances for survival in these systems. There have been significant advances in the methodology for laboratory identification of glochidial hosts of freshwater mussels in recent years (e.g., Hove et al. 2000), and a testing facility has now been established at the University of Guelph, Guelph, Ontario (Woolnough and Mackie 2002).

Freshwater mussels are known to be food sources for a variety of mammals and fish (Fuller 1974). Predation by muskrats (Ondatra zibenthicus), in particular, may be a limiting factor for some mussel species. Tyrrell and Hornbach (1998) and others have shown that muskrats are both size- and species-selective in their foraging, and can significantly affect both the size structure and species composition of mussel communities. We are aware of only one study on the effects of muskrat predation on mussels that reported data for O. subrotunda. Watters (1993-94) compared the composition of the mussel community at two sites in the lower Muskingham River in Ohio with the composition of shells in nearby muskrat middens. He found that muskrats neither favoured nor avoided the round hickorynut, which represented 0.28-0.53% of the mussel community and 0.07-2.53% of the shells in middens. The authors of the present report found several fresh O. subrotunda shells in muskrat (or raccoon) middens along the banks of the East Sydenham River, even though only three live specimens were observed during mussel surveys. Although predation is a natural control on mussel populations, we must recognize that land use practices can significantly influence the distribution and density of predators. We are not aware of any studies on raccoon predation; however, we have observed raccoons feeding on mussels in the field, and there is anecdotal information from the farming community in the Sydenham River watershed that the recent adoption of conservation tillage practices has led to an explosion in the raccoon population. It is therefore possible that predation represents a significant threat to the population of O. subrotunda in this river.