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2.4 Distribution and Population

 

Canadian distribution of the western silvery minnow showing key habitat features of the Milk River, Alberta

 

Figure 3. Canadian distribution of the western silvery minnow showing key habitat features of the Milk River, Alberta. Distribution records are from the ASRD Fisheries Management Information System as of January 2005.

2.4.1 Distribution

The western silvery minnow is only found in North America, where it occurs in large lowland plain streams of the Mississippi River system, from the mouth of the Ohio River north to the Missouri River basin and the Milk River in Alberta. It has adapted to the highly variable spring-summer flows characteristic of plains streams and occurs in small, naturally-intermittent streams far upstream of confluences with larger rivers, where it has likely adapted to access habitat under limited flow conditions (R. Bramblett, pers. comm.). In the Mississippi River, it has only been found downstream of the confluence with the Missouri River.  The species’ distribution in the Milk River appears to be continuous from Writing-on-Stone Provincial Park in Alberta downstream to the confluence with Missouri River, although fragmentation has occurred with the construction of seven storage and diversion dams in Montana (Stash 2001; T. Clayton and D. Watkinson, pers. comm.).  Upstream movement past these dams is not possible.  There are no barriers to fish movement above the Fresno Dam, which is located approximately 80 km downstream of the eastern border crossing.  

The distribution of the western silvery minnow has declined significantly in extensive areas in the United States over the past century (Willock 1969).  The species is listed by most Missouri River basin states, including North Dakota, South Dakota, Iowa, Kansas and Missouri, as “Threatened” or as a “Species of Concern” (Welker and Scarnecchia 2004).  Its Canadian range represents <1% of the species’ global range.

In Canada, the distribution of western silvery minnow has only been confirmed in the mainstem of the Milk River in southern Alberta (Figure 3).  This is the northwestern limit of the species’ known range.  A single specimen was also documented in the South Saskatchewan River near Medicine Hat in 1963 (Henderson and Peter 1969). This fish may have been a western silvery minnow introduced as bait, since a series of recent surveys have not found additional specimens that would support the existence of a resident population in the system (Alberta Sustainable Resource Development 2003). 

Within Alberta, the western silvery minnow distribution appears to be limited to the lower 220 km of the mainstem Milk River, from about 20 km upstream of the Town of Milk River downstream to the Alberta/Montana border (Figure 3).  Within this stretch of river, the species’ distribution appears to be continuous downstream of the confluence with Police Creek (Willock 1969; P&E 2002; T. Clayton and D. Watkinson, pers. comm.).  Recent upstream range extensions likely reflect improved sampling techniques rather than a recent change in the species’ distribution.  The use of Milk River tributaries has not been documented despite numerous surveys of many tributaries (Alberta Sustainable Resource Development 2003). 

The distribution of the western silvery minnow may have been significantly different prior to 1917, when the Saint Mary Canal was constructed (Willock 1969).  However, the effects of increased seasonal flows on its distribution and abundance are unknown.  On the one hand they may have enabled the minnow to expand upstream into the Alberta portion of the Milk River; on the other they may be limiting its abundance and distribution in Alberta (R. Bramblett, pers. comm.). 

2.4.2 Population Size and Trends

Very little information is available on population size or trends in abundance of the western silvery minnow in Alberta.  Since it was first identified in the Milk River in 1961, the species has remained common in small, local areas of the river from downstream of Writing-on-Stone Provincial Park to the Montana border (T. Clayton, pers. comm.). In 2000 and 2001, it was one of the more abundant species caught in fall surveys of the Milk River, where its abundance was highest downstream of Pinhorn Ranch, probably reflecting the increased availability of preferred habitat.  It was the second most abundant fish species taken in mid-July 2005 by a DFO survey of the Milk River downstream of its confluence with Breed Creek (D. Watkinson, pers. comm.).

2.4.3 Nationally Significant Populations

The western silvery minnow has no direct economic importance and limited importance as a forage species (Scott and Crossman 1973). However, it does have intrinsic value as a contributor to Canada’s biodiversity and as a forage fish.