Fawnsfoot (Truncilla donaciformis) COSEWIC assessment and status report: chapter 7

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Population Sizes and Trends (1)

Search effort

Historical surveys

Based on records from the Lower Great lakes Unionid Database, the Fawnsfoot was historically (1930-1996) found in Lake Erie, Lake St. Clair and the Detroit, Niagara, Sydenham and Grand rivers. Approximately two-thirds of the 43 historical records are from museum specimens that have no information on search effort associated with them. Information on sampling methods and search effort is available for the remaining records, which were taken from surveys published in the literature. Table 1 provides a summary of this information. Surveys conducted on the Saugeen and Thames rivers are included in Table 1 because the Fawnsfoot was recently recorded from these rivers. A variety of quantitative and semi-quantitative sampling techniques were used in the historical surveys.

Recent Surveys

The Lower Great Lakes Unionid Database contains 15 recent (1997-2007) records for the Fawnsfoot from the Grand, Thames and Sydenham rivers, Muskrat Creek in the Saugeen River watershed, and Lake St. Clair. All but one of these records is accompanied by information on sampling methodology and search effort. The remaining record, from Muskrat Creek, is from a survey targeting other benthic invertebrates. Southwestern Ontario, which encompasses the range of the Fawnsfoot, has been extensively surveyed for freshwater mussels over the past ten years. Surveys have been conducted at approximately 300 different sites in 11 systems. Table 2 provides a summary of the sampling methods and search effort used in these surveys. It should be noted that different sampling techniques may provide different types of data. Semi-quantitative methods (timed-search surveys while wading, snorkelling or using SCUBA gear) provide data on the presence/absence of species and their relative abundance. Quantitative methods, i.e., methods that sample a defined area of the substrate surface using quadrats, grabs, circular plots or line transects, provide additional data on mussel densities. The above-described methods detect only those mussels visible at the sediment surface, all of which would be adults. Quadrat surveys with excavation, i.e., where the substrate in each quadrat is searched to a depth of about 10 cm, have been used at ~ 10% of the sites surveyed to date. This very time-consuming technique detects both adult mussels occurring at the sediment surface and juvenile mussels that tend to burrow deeply in the substrate (see Biology). The latter method can therefore provide additional data on recruitment.