Western silvery minnow (Hybognathus argyritis) COSEWIC assessment and status report: chapter 4

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Distribution

Global range

In North America, the western silvery minnow is distributed in large, lowland plains streams of the Mississippi River system, extending from the mouth of the Ohio River north to the Missouri River basin and the Milk River in Alberta, Canada (Pflieger 1980b) (Figure 2). Western silvery minnows are found in the mainstem of the Missouri River and in the mainstem of the Mississippi River only below the mouth of the Missouri River (Burr and Page 1986). Within these systems, distribution appears to be fairly continuous (Pflieger 1980b), although the creation of reservoirs and dams has fragmented some rivers. The distribution of the western silvery minnow in the Milk River extends from its northern limits within Alberta downstream to the Fresno Reservoir in Montana, located approximately 80 km downstream of the border (Stash 2001). Further downstream, populations are fragmented by a series of seven impassable irrigation diversions and dams (from Fresno Reservoir downstream to the Vandalia diversion dam in Montana) before the confluence with the Missouri River (K. Gilge, pers. comm.). Willock (1968 1969a) indicated that western silvery minnow distribution in the United States has declined from the extensive areas it once occupied, but no specific locations were provided. These older studies suggest that changes in extent of distribution likely occurred much earlier in the century, but no specific records for western silvery minnow were available.

Figure 2. The distribution of western silvery minnow in North America. From Pflieger (1980b), U.S. Fish and Wildlife Service (1995), Houston (1998b), and United States Geological Survey (2001).

Canadian range

The western silvery minnow’s range in Canada appears to be restricted to the Milk River, the northwestern most tributary of the Missouri River system (Figure 3). Although Henderson and Peter (1969) documented a single specimen of a western silvery minnow from the South Saskatchewan River within the city limits of Medicine Hat in 1963, a series of sampling efforts in 1974−1975 and during 1994−1996 in this area, as well as upstream and downstream of this section, did not detect any additional specimens (W. Roberts, pers. comm.). The specimen collected by Henderson and Peter (1969) is believed to be a spurious account and not representative of a breeding population. Its presence is thought to be the result of an accidental release of bait fish (Henderson and Peter 1969) rather than a misidentification, because its identification was confirmed by experts in the field (W. Roberts, pers. comm. 2002). If confirmed, this would be the only known occurrence of the species outside of the Missouri/Mississippi drainage. However, additional samplings have not confirmed its presence there (Clayton pers. comm. 2008).

Figure 3. Canadian distribution of the western silvery minnow, Hybognathus argyritis.

The previous distribution of western silvery minnow within the Milk River was thought to be limited to the lower section of the river from Police Creek (approximately 140 km upstream) to the United States border (Figure 4) (Scott and Crossman 1973; Pflieger 1980b, Sikina and Clayton 2005). Watkinson et al. (MS 2007) extended the western silvery minnow distribution to approximately 15 km downstream of the North Milk River and Milk River confluence (223 km upstream from the United States border). Western silvery minnow distribution downstream of Aden Bridge (approximately 100 river kilometres from the United States border) was confirmed and extended in 2005 and 2006 (Watkinson et al. MS 2007). The extent of occurrence of the western silvery minnow in Canada was estimated to be 1200 km² (estimated from a convex hull around the river stretch from the U.S. border to one km past the last distribution point upstream), the area of occupancy at 244 km² [based on overlaid grid of cells one km², total area of occupancy is the number of squares that are intersected by the river from the U.S. border to one km past the last upstream distribution point]. The biological area of occupancy was estimated at 13.4 km² [based on occupied riverine habitat assuming average river width of 60 m from the U.S. border to 1 km past the last distribution point upstream].

Figure 4. Canadian distribution of western silvery minnow (Hybognathus argyritis) in the Milk River, Alberta.

Occurrences are based on evidence of historic and/or current likely recurring presence at a given location. Occupied sites separated by a gap of 15 km or more of any aquatic habitat that is not known to be occupied are taken to represent different locations. Thus, western silvery minnow in Canada occur within one location, although the gap of approximately 42 km between sites above the Town of Milk River, and those downstream of the town, might imply two. Further sampling is required to confirm the lack of silvery minnow in the area. Dams, impassble falls and upland habitat constitute separation barriers (Hammerson 2004 as cited in NatureServe 2007). Data on dispersal and other movements are generally not available, and separation distances (in aquatic kilometres) for cyprinids are arbitrary, but do take into consideration that movements and separation distances generally increase with fish size.