Fawnsfoot (Truncilla donaciformis) COSEWIC assessment and status report: chapter 5
Habitat
Habitat requirements
The Fawnsfoot is known to occur in areas of moderate to low flows in medium and large rivers at depths ranging from less than 1 m to greater than 5 m, although it can adapt to low flow environments such as lakes and reservoirs (Clark 1981; Parmalee and Bogan 1998). The Fawnsfoot is usually associated with substrates of sand or mud (Clark 1981; Parmalee and Bogan 1998), but can be found in areas with coarser substrate (Howells et al. 1996). Remaining populations in Canada are usually found in the lower portions of larger rivers on fine sand or gravel substrates.
Habitat trends
The most significant change in habitat for the Fawnsfoot is associated with the invasion of the dreissenid mussels (Dreissenapolymorpha and D. bugensis) in the mid-1980s. Dreissenid mussels compete with native unionids for space and food and, by attaching directly to native mussel shells, impair the ability of the native mussels to feed, respire and move normally (see Limiting factors and threats). Within about a decade of the first invasion, native unionids had been almost completely eradicated from Lake St. Clair, Lake Erie and the Detroit and Niagara rivers (Schloesser and Nalepa 1994; Nalepa et al. 1996; Schloesser et al. 2006). Over 80% of historical Fawnsfoot records in the Lower Great Lakes Unionid Database were from areas that have been negatively impacted by dreissenid mussels and represent areas where unionids are now considered essentially extirpated. Despite these catastrophic effects, there are still areas where dreissenid mussels occur in sufficiently low densities to allow coexistence with unionids, such as the St. Clair delta (Zanatta et al. 2002). Recent work by Strayer and Malcom (2007) suggests the potential for continued coexistence in areas where the impacts of dreissenids are more related to competition for food (e.g., the Hudson River in New York) than to biofouling.
Habitat trends for riverine populations are difficult to assess as there are very few historical records available. The following discussion summarizes general conditions for unionids in these watersheds.
The Sydenham River flows through an area of prime agricultural land in southwestern Ontario. Over 85% of the land in the watershed is in agricultural use, with 60% of land in tile drainage (Dextrase et al. 2003). Large areas of the river have little to no riparian vegetation as only 12% of the original forest cover remains. Strayer and Fetterman (1999) identified high sediment and nutrient loads and toxic chemicals from non-point sources, especially agricultural activities, as the primary threat to riverine mussels. Agricultural lands, particularly those with little riparian vegetation and large amounts of tile drain, allow large inputs of sediments to the watercourse. In the case of tile drained land, the sediment is often of a very fine grain that can clog the gill structures of mussels and result in decreased feeding and respiration rates and reductions in growth efficiency. The Sydenham River has had high nutrient levels with total phosphorus levels consistently exceeding provincial water quality levels over the last 30 years, while chloride levels have shown recent increases due to the increased use of road salt (Dextrase et al. 2003). Human population pressure within the watershed is low as the total population is less than 90,000 with roughly half occurring in urban settings. Although the watershed is not highly populated, the lower portion of the river is subject to commercial shipping activities that tend to fluctuate in response to economic conditions.
The lower Thames River watershed in the area where the Fawnsfoot occurs is subjected to intense agricultural pressure. Eighty-eight percent of the land in the lower watershed is in agricultural use and less than 5% of the historical forest cover still remains (Taylor et al. 2004). Despite these land-use pressures, the lower Thames River remains one of the largest free-flowing river systems in southern Ontario. No major barriers or dams exist for approximately 200 km from the mouth of the river upstream to the city of London, the largest urban centre in the watershed with a population of approximately 330,000. The city of London has undergone a 10-fold population expansion in the last century. Although this city is located upstream of both the known historical and current ranges of the Fawnsfoot, the impacts of such a large urban centre (e.g., wastewater treatment outflows) and its expansion are likely to be observed downstream in the areas where the species occurs.
Mussel communities in the Grand River are among the best studied in Canada and there is abundant evidence to indicate that these communities have undergone a significant decline and subsequent recovery over the last 35 years (Kidd 1973; Mackie 1996; Metcalfe-Smith et al. 2000b). When Kidd (1973) sampled the river (115 sites between 1970-72) he reported only 17 of the 31 species historically known from the river. He attributed much of this loss to impaired water quality related to agricultural activity and habitat fragmentation resulting from dam construction. Mackie (1996) indicated that anthropogenic stressors, particularly below urban centres, were likely driving the species declines. Metcalfe-Smith et al. (2000b) surveyed 94 sites over a four-year period and found 25 species, representing a 50% increase in species richness compared with Kidd’s (1973) results from 25 years earlier. Morris (2006b) reports that the Grand River is now believed to be home to one of the two largest populations of the Endangered Wavyrayed Lampmussel (Lampsilis fasciola) in Canada. When Mackie (1996) surveyed 70 sites ten years earlier, no live Wavyrayed Lampmussels were found. Metcalfe-Smith et al. (2000b) relate much of the improvement in mussel communities of the Grand River to improved water quality and the addition of fish ladders promoting fish movement (allowing dispersal through host activity) and reconnection of formerly fragmented habitat.
Habitat protection/ownership
The Fisheries Act represents an important tool for habitat protection for aquatic species. Under the federal Fisheries Act mussels are considered shellfish, falling under the definition of ‘fish’, and their habitat is, therefore, protected from harmful alteration, disruption or destruction unless authorized by the Minister of Fisheries and Oceans, or his/her delegate. The Ontario Lakes and Rivers Improvement Act prohibits the impoundment or diversion of a watercourse if siltation will result, while the voluntary Land Stewardship II program of the Ontario Ministry of Agriculture, Food and Rural Affairs is designed to reduce erosion on agricultural lands. Stream-side development in Ontario is managed through floodplain regulations enforced by local conservation authorities. A majority of the land adjacent to the rivers where the Fawnsfoot is found is privately owned; however, the river bottom is generally owned by the Crown. The uppermost portion of the Thames River population occurs adjacent to the Munsee-Delaware First Nations whereas the upstream extent of the Grand River population extends to the Byng Conservation Area owned by the Grand River Conservation Authority. The only remaining lake population is found in the St. Clair delta in waters within the boundaries of the Walpole Island First Nation.