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Recovery Strategy for the Western Silvery Minnow (Hypognathus argyritis) in Canada [Proposed]

1. Introduction

Figure 1. Western Silvery Mennow (Photo credit: Karen Scott, DFO).

Figure 1. Western Silvery Mennow (Photo credit: Karen Scott, DFO).

The western silvery minnow (Hybognathus argyritis) is a small cyprinid1 species native to large plains streams in northwestern North America.  It was first documented in Canada in 1961 from the lower Milk River, Alberta (UAMZ 5320, University of Alberta Museum of Zoology), and its presence has not been verified in any other Canadian river systems since (Alberta Sustainable Resource Development 2003) (Figure 1-facing page).  There is very little historical information on the western silvery minnow in the Milk River, but this fish has probably persisted at naturally low numbers without significant changes in abundance or range since it was first observed in Alberta (Alberta Sustainable Development 2003).  Natural rarity in terms of both distribution and abundance in Canada makes the minnow vulnerable to extirpation, so it requires protection.

In June 2003, the western silvery minnow was listed as “Threatened” under Schedule 1 of the Species at Risk Act (SARA), which required its immediate protection and the development of a recovery strategy within four years.  Also in 2003, the species was approved for listing as “Threatened” provincially by Alberta’s Minister of Sustainable Development.

In 2004, a joint federal/provincial recovery team was established for the western silvery minnow to produce a recovery strategy that would meet the needs of both Canada and Alberta.  Membership on the Milk River Fish Species at Risk Recovery Team (the Recovery Team) includes representatives from each of the responsible jurisdictions (Fisheries and Oceans Canada, Alberta Environment, Alberta Sustainable Resource Development) and from key stakeholders including local municipalities, the Milk River Ranchers’ Association, the Milk River Watershed Council of Canada, and the Southern Alberta Environmental Group. The first recovery team meeting was held in March 2004, in Lethbridge, Alberta. 

This document presents the recovery strategy for the western silvery minnow in Canada in fulfillment of the SARA requirements. It proposes a maintenance and protection approach for the species and its habitat, and follows the two-step model developed by the National Recovery Working Group (2004).  Development of the recovery strategy is the first step, followed by the development of an action plan to implement its recommendations.

2. Background

Recovery efforts should be based on a sound understanding of the species, including its biology, ecology, and the environmental conditions under which it exists.  The following sections describe the environmental setting of the Milk River, what is known about the western silvery minnow, and what can be inferred from other closely related species.

2.1 Species Assessment Information from COSEWIC and Alberta

COSEWIC Assessment Summary

Common Name: Western Silvery Minnow
Scientific Name:  Hybognathus argyritis
Status: Threatened Last Examination: November 2001 (In a higher risk category)
Canadian Occurrence: AB
Reason for Designation: This species is known in Canada from two rivers in Alberta, one of which flows through short-grass prairie that is subject to continuous erosion leading to increased siltation.
Status History: Designated Special Concern in April 1997. Status re-examined and designated Threatened in November 2001. Last assessment based on an existing status report with an addendum.

Note:  The above summary is based on information available to COSEWIC at the time of the initial species assessment and is included for reference purposes. The identified threats must be considered in the development of a recovery strategy under the Species at Risk Act (SARA). However, subsequent review and analysis of all available information by the Recovery Team has led to different conclusions regarding the species’ distribution (i.e., occurs only in the Milk River) and some of the identified threats.

Alberta Summary

Common Name:  Western Silvery Minnow
Scientific Name:  Hybognathus argyritis
Rank:  Threatened
Designated: 2003
Reason for Designation:  This species is moderately abundant, but its distribution is very restricted.  The only location in Canada where this species is found is in the Milk River of southern Alberta.Status History:Designated “May be at Risk” in 2000.  Upgraded to Threatened in 2003 based on a new status report (Alberta Sustainable Resource

2.2 Environmental setting

The Milk River is the northernmost tributary of the Missouri River and the only watershed in Canada where populations of the western silvery minnow have been found (Figure 1).  The Milk River is situated in the Dry Mixedgrass Natural Subregion of Alberta (Natural Regions Committee 2006), where it flows within the confines of a defined valley with limited road access.  The surrounding land is semi-arid, short grass prairie that is used primarily for cattle grazing.  The river is shallow and turbid, with a dynamic hydrology and poorly developed riparian zone that lacks higher aquatic plants due to the highly mobile stream bed (D. Watkinson, pers. com.).  Rainfall in the Milk River basin averages only 333 mm annually, 72% of which falls during the growing season (Natural Regions Committee 2006).  Periods of high runoff occur briefly in late March and April due to snowmelt and in June and July due to intensive, localized rain storms (McLean and Beckstead 1980).

The Milk River has been severely impacted by changes in its seasonal flow regimes. Since 1917, Montana has diverted water from St. Mary River in northwestern Montana via the St. Mary Canal into the North Milk River (ISMMRAMTF 2006).  This water flows eastward through southern Alberta before entering northeastern Montana, where it is used for irrigation. These augmented flows occur in the Alberta portion of the Milk River from late March or early April through late September or mid-October.  During the rest of the year natural flows prevail within a somewhat modified river channel (McLean and Beckstead 1980). Flow augmentation of the Milk River is actively managed at the Saint Mary Diversion Dam in Montana in response to major runoff events to prevent or reduce erosion, scouring and risk of canal failure, and to optimize use of the water for irrigation.

Since 1917, the diversion of flow from the St. Mary River has augmented summer flows in the Milk River.  Under natural conditions summer flows in Canada ranged from 1 to 2 m³/s in the North Milk River to between 2 and 10 m³/s at the Milk River’s eastern crossing of the international border.  Since the diversion, flows in the Milk River at the Town of Milk River have ranged from 10 to 20 m³/s from May to September, and have averaged 15 m³/s between June and August.  The flow augmentation is much greater in relative terms in the North Milk River, which has a relatively small drainage area (238 km² at the North Milk River gauge 11AA001), than it is further downstream at the eastern crossing of the international border, where the river receives runoff from a much larger area (6,800 km² at gauge 11AA031) (McLean and Beckstead 1980). 

When the diversion of water from the St. Mary River is terminated in late September to mid-October, the river reverts to natural flow conditions for the remainder of the winter season (ISMMRAMTF 2006). Ramping down of the diverted flow occurs over about a week, and flows in the river decline over the next several weeks. The decline is most rapid in upstream reaches of the river. Under severe drought conditions, such as those of 2001-2002, there may be little or no surface flow and the lower Milk River can be reduced to a series of isolated pools until spring, although subsurface flows may continue (K. Miller, pers. com.). Indeed, during much of the non-augmented fall and winter period the natural flow at the Town of Milk River is low enough to flow through a 4 foot diameter culvert--a 2 foot culvert in dry years (K. Miller, pers. com.).  At the Town, the average flow rate over the period 1912 to 2005 was less than 2 m³/s (cubic metres per second) in November and February, and less than 1 m³/s in December and January (WSC 2006). 

Water management within the Milk and St. Mary rivers is governed by the 1909 Boundary Waters Treaty (the Treaty) between the United States and Canada, which is administered by the International Joint Commission (IJC) (ISMMRAMTF 2006).  Over the past two decades, the St. Mary Canal has transported an average of about 208 ha³ (cubic hectares; 169,000 acre-feet) of water annually into the North Fork of the Milk River (U.S. Bureau of Reclamation 2004).  In 2003, Montana requested that the Treaty be re-opened to reconsider how the diverted water is apportioned. However, at the time of writing, this issue had not yet been resolved. At present the operating capacity of the St. Mary Canal is about 18.4 m³/s (650 cfs), significantly less than its original design capacity of 24.1 m³/s (850 cfs). Montana is considering whether to rehabilitate the aging canal infrastructure and return the canal to its original capacity, or whether to increase its capacity to 28.3 m³/s (1000 cfs) (Alberta Environment 2004; U.S. Bureau of Reclamation 2004).

2.3 Species Description

The western silvery minnow belongs to the minnow family (F. Cyprinidae).  It is a small fish native to the large plains streams of the Missouri and Mississippi drainages in midwestern North America.  The head is characterized by a blunt snout with a subterminal mouth and relatively large eyes (Scott and Crossman 1973).  Specimens in Alberta tend to be brownish-yellow on the back with silvery sides (Nelson and Paetz 1992) (Figure 2).   Fork lengths (tip of snout to fork of tail) of up to 140 mm have been recorded in the Milk River (R L&L 2002)

Originally, the western silvery minnow and eastern silvery minnow (Hybognathus regius) were considered subspecies of the central silvery minnow (H. nuchalis) (Scott and Crossman 1973), but they are now considered distinct species based on morphological differences (Hlohowskyj et al. 1989, Schmidt 1994, Pfliefer 1997).  This distinction was accepted by the American Fisheries Society in 1991 (Robins et al.1991).  Recent taxonomic studies have verified that fish in the Canadian reaches of the Milk River are western silvery minnows (D. Watkinson, pers. com.).