Western silvery minnow (Hybognathus argyritis) COSEWIC assessment and status report: chapter 7
Population Sizes and Trends
Fluctuations and trends
There is little information available to establish population size or trends (Houston 1998a). The western silvery minnow appears to be native to the Alberta fish fauna and has been there for some time; unnoticed or perhaps misidentified. The species was first described as Hybognathus argyritis by C.P. Girard in 1856 from specimens collected in the Milk River in Montana (Nelson and Paetz 1992). Grant Campbell collected the first western silvery minnow in Canada in 1961 (UAMZ 5320, University of Alberta Museum of Zoology). Over the next four decades sporadic collections were made verifying the presence of the western silvery minnow in the Milk River. Watkinson et al. (MS 2007) reported the furthest upstream western silvery minnow distribution in the Milk River approximately 15 km downstream of the Milk River and North Milk River confluence, where five western silvery minnows were collected.
It is suspected that both the extent of distribution and abundance of western silvery minnow within the Milk River may have been altered significantly prior to this time (in the early 1900s), when the St. Mary Canal was constructed to divert irrigation water from the St. Mary River to the Milk River. Since it was first identified in the early 1960s the known western silvery minnow distribution in the Milk River has been expanded with additional sampling effort.
It is uncertain if abundance in the Milk River has changed since it was first identified in Alberta in the 1960s as limited sampling has been conducted in the Milk River. Based on historical records from 1961−2003, Pollard (2003) estimated the western silvery minnow population to be no more than a few thousand individuals. Recent findings suggest that the populations of western silvery minnows in the Milk River are much higher than was previously believed (Sikina and Clayton 2005; Watkinson et al. MS 2007). Watkinson et al. (MS 2007) and Sikina and Clayton (2005) caught a total of 2232 western silvery minnows in the Milk River in 2005 and 2006. Sikina and Clayton (2005) caught 88 western silvery minnows seine netting with a catch per unit effort (CPUE) of 0.4 and 0.9 fish/100 m² in the summer and fall of 2005, respectively. Total CPUE was 0.6 fish/100 m². Minnows were caught at all sample sites (N = 12) with the largest sample consisting of 16 individuals. They also witnessed a school of approximately 150 western silvery minnows near Deer Creek confluence with the Milk River in October of 2004 (Sikina and Clayton 2005). Watkinson et al. (MS 2007) caught an additional 2 144 western silvery minnows from the backwaters and pools in the lower section of the Milk River downstream of Aden Bridge to the United States Border in 2005 and 2006. Catch per unit effort (CPUE) electroshocking produced 0.004 fish/s in July 2005 and 0.007 fish/s in May 2006 (Watkinson et al. MS 2007). While seine netting, total CPUE was 19.59 fish/100 m² in July 2005 and 72.15 fish/100m² in May 2006 (Watkinson et al. MS 2007). Within these samples, large numbers of 1+ fish were caught suggesting the Milk River western silvery minnow population is stable. The largest catch, 578 western silvery minnows was sampled near the mouth of Deer Creek in October 2006.
Watkinson et al. (MS 2007) found western silvery minnows comprising 29% of the total catch in the Milk River. Stash (2001) reported that all Hybognathus species combined comprised a large portion of the fish community, making up 5.78% of the total fish species composition, but made no attempt to identify the Hybognathus specimens to species level. The western silvery minnow is common throughout the Missouri River, where it is a dominant species in the lower reaches (Cross et al. 1986). Pflieger (1980b) stated that where they still occur in the United States, the western silvery minnow is considered common. Unfortunately, no specific information was available to establish population sizes or trends for any of these areas (Houston 1998a).
In the absence of historical information, it is not clear what impact increased flows associated with the diversion of the St. Mary River after 1917 have had on western silvery minnow populations in the Milk River. Although the diversion has significantly increased summer flows, baseline conditions are restored during the late fall and winter months after the diversion has been terminated possibly limiting any benefit that might be derived from increased summer habitats.
Rescue effect
The available habitat for the western silvery minnow in Alberta is restricted to the portions of the Milk River in Canada. Re-colonization is only possible from that portion of the Milk River in Montana upstream of the Fresno reservoir where there are no impediments to migration or exchange with Canadian populations. Any populations downstream of this have been effectively isolated. However, re-colonization may be temporarily restricted by drought conditions (see Habitat Trends).
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