Fawnsfoot (Truncilla donaciformis) COSEWIC assessment and status report: chapter 9

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Limiting Factors and Threats

The single most significant factor in the recent declines of the Fawnsfoot is the introduction in the late 1980s and subsequent spread of dreissenid mussels (D. bugensis and D. polymorpha). The attachment of dreissenid mussels to the shells of native mussels can interfere with normal metabolic functions, and impede feeding, respiration, reproduction and locomotion (Mackie 1991; Haag et al. 1993; Baker and Hornbach 1997). Eighty-six percent of the historical records in the Lower Great Lakes Unionid Database for the Fawnsfoot in Canada are from areas that have been heavily impacted by dreissenids. Many of the areas are now completely devoid of unionids (Western Lake Erie - Schloesser and Nalepa 1994; Lake St. Clair - Nalepa et al. 1996; Detroit River - Schloesser et al. 2006). With the exception of the Saugeen River (Muskrat Creek) and Sydenham River, all known remaining Fawnsfoot populations occur in areas where dreissenid mussels are established. Morris and Edwards (2007) surveyed 37 sites in the Thames River in 2004-2005 and found live dreissenid mussels and/or evidence of attachment to unionids (live animals or byssal threads attached to the unionid shells) at nearly every site between Fanshawe Reservoir in the city of London and the mouth. Although no dreissenids were observed attached to individual Fawnsfoot, this area overlaps completely with its known distribution in the Thames River. Likewise, dreissenid mussels are present in the St. Clair River delta (Metcalfe-Smith et al. 2005b) and Grand River up to the Dunnville dam (pers. obs. Morris, 2005).

Damming of rivers has been shown to detrimentally affect mussels in many ways. Reservoirs alter downstream flow patterns, completely disrupting the natural hydrograph, and have been shown to change the natural thermal profiles of the watercourse (Vaughn and Taylor 1999). Impoundments also act as physical barriers to host movement making large areas of potential habitat completely unavailable to some mussel species. Watters (1996) observed that the distributions of two mussel species (Leptodea fragilis and Potamilus alatus) in Indiana, Ohio and West Virginia were limited by the presence of small dams that acted to restrict movement of their host, the freshwater drum. Tiemann et al. (2007) have also shown that the Fawnsfoot is one of five species in the Fox River system of Illinois that appears to have an upstream distribution limited by the presence of a low-head dam. Freshwater drum, the reported glochidial host of the Fawnsfoot, is a species adapted to shallow lacustrine waters that uses the lower portions of rivers for spawning (Scott and Crossman 1973). What limits the upstream distribution of the freshwater drum in southern Ontario rivers is not known. However, Watters (1996) suggests that the limiting factor in some U.S. rivers may be the presence of dams. He supports this notion by reporting that freshwater drum were once present in unimpounded river reaches in the Ohio River but disappeared after dams were constructed.

There are no historical data for southern Ontario that could be used to directly evaluate the impact of dam construction on fish and mussel populations, although the distributions presented in Figure 5 for the freshwater drum, sauger and Fawnsfoot show some relation to the placement of dams. In the Grand River, the Fawnsfoot has only been collected up to the dam in Dunnsville even though freshwater drum are able to pass this dam and have been found almost as far as the next dam in Caledonia which they are unable to pass (MacDougall pers. comm. 2007) (Figure 5). The Thames River is a unique river in southern Ontario as it is unregulated for nearly 200 km from the mouth to the first dam at Springbank Park in the city of London. This dam represents a barrier to fish passage from mid-May until early November (Reid and Mandrak 2006). Both the Fawnsfoot and the freshwater drum have distributions that extend nearly as far as this dam but there are no records of either species beyond the dam (Figure 5). A second dam above the city of London represents a permanent barrier to fish passage. The first major barrier on the Sydenham River is located well above the distribution of both species. In the Saugeen River, freshwater drum are not found above Denny’s Dam; however, the only Fawnsfoot collected from this watershed was collected well above the dam. There are at least five additional barriers to fish passage between Denny’s Dam and Muskrat Creek (Nichols pers. comm. 2007) (Figure 5). It is highly unlikely that the Fawnsfoot collected from Muskrat Creek arrived at the site while encysted on a freshwater drum or sauger.

Evidence suggests that freshwater mussels are sensitive to PCBs, DDT, Malathion and Rotenone, all of which can inhibit respiration and accumulate in mussel tissue (USFWS 1994). The glochidial stage appears to be particularly sensitive to heavy metals (Keller and Zam 1990), acidity (Huebner and Pynnonen 1992) and salinity (Liquori and Insler, as cited in USFWS 1994). It has recently been reported that juvenile freshwater mussels are among the most sensitive aquatic organisms to un-ionized ammonia toxicity, typically showing adverse responses at levels well below those used as guidelines for aquatic safety in U.S. waterways (Newton 2003; Newton et al. 2003). High inputs of silt from agricultural lands may adversely affect mussels by clogging gill structures and negatively impacting feeding, respiration and reproduction (Strayer and Fetterman 1996). The primary land use in the Sydenham River basin is agriculture. Dextrase et al. (2003) report suspended solids levels as high as 900 mg/L leading to the conclusion that siltation and turbidity are the predominant threat to species at risk in this watershed. In the Grand River, clearing of riparian vegetation and allowing livestock to access the river has resulted in poor water quality with increased sediment loads (WQB 1989a). Agricultural activity is expected to increase in the Grand River basin over the next 25 years leading to a predicted increase in sediment, pesticide, fertilizer, and manure runoff. Water quality in the Thames River basin has historically suffered greatly from agricultural activities. Tile drainage, wastewater drains, manure storage and spreading, and insufficient soil conservation have all contributed to poor water quality within the Thames basin (Taylor et al. 2004). Phosphorus and nitrogen loadings have increased steadily and some of the highest inputs of livestock waste for the entire Great Lakes basin have been reported for the Thames River watershed (WQB 1989b). Mean ammonia concentrations in all sub-basins of the Thames River exceed the federal freshwater aquatic life guidelines (Taylor et al. 2004). Given the general sensitivity of freshwater mussels, particularly glochidia and juveniles, to aquatic pollutants, the levels of pollution observed in these watersheds may be negatively impacting the remaining riverine populations of the Fawnsfoot.

Natural levels of predation by species identified earlier (see Predation) are not likely to adversely impact healthy populations of the Fawnsfoot; however, if the range of the mussel continues to shrink then these influences may become disproportionately large.

In summary, the major factors shaping the current distribution of the Fawnsfoot in Canada are the establishment of dreissenid mussels, which has resulted in large portions of historical habitat being rendered unsuitable, and a limited availability of riverine habitat primarily restricted by the distribution of its presumed glochidial host, the freshwater drum. Except for a small population in the St. Clair delta, remaining Fawnsfoot populations are limited to relatively small sections of the lower reaches of rivers where they are subjected to declining water quality resulting from agricultural and urban influences in the upper portions of the watersheds.