Western silvery minnow (Hybognathus argyritis) COSEWIC assessment and status report: chapter 5

Habitat

Habitat requirements

Adult western silvery minnows inhabit the lower sections of the Milk River where the river is described as low gradient and even-flowing, with many backwater areas and shallow flat and run habitats. Within this section of river, adult presence was strongly associated with backwaters and pool habitats where velocities are minimal, ranging from 0−1.1 m/s with a mean velocity of 0.22 m/s, temperatures ranging from 13.6°C in May to 27.2°C in July, average depths of 0.32 m with a maximum depth of 1.0 m, and silt as the dominant substrate (Watkinson et al. MS 2007). Western silvery minnows were captured in habitats with high turbidity, with Secchi disk transparency ranging from 0.13−0.16 m in May 2006, and 0.12−0.18 m in July 2005, downstream of Aden Bridge (Watkinson et al. MS 2007). Watkinson et al. (MS 2007), and Quist et al. (2004) found that western silvery minnows are positively correlated with percentage of fine substrate in reaches of the Missouri River drainage and favour habitats with increased turbidity and silt deposition. In addition, given the lack of other refugia in the lower Milk River, Sikina and Clayton (2005) suggest western silvery minnows utilize turbidity as a means of protection and cover.

In total, only five specimens have been collected from three sites in the Milk River upstream of the Town of Milk River (208, 213 and 223 km upstream of the U.S. border) (Watkinson et al. MS 2007). At these sites, the river flows through erosion-resistant sandstone formations and is characterized by increased runs, riffles and rapids (RL&L 2001). This data suggests only limited or marginal use of such habitats. Velocities ranged from 0.41−0.65 m/s, depth ranged from 0.42−1.2 m, Secchi disk transparency was 0.63 m, and June water temperature was 17.7°C (Watkinson et al. MS 2007). Sand was the dominant substrate at two of the sites while the third site was dominated by gravel (Watkinson et al. MS 2007).

In the United States the presence and abundance of the western silvery minnow is strongly associated with a number of habitat features including bottom type, gradient, and turbidity (Quist et al. 2004). The western silvery minnow occurs in the Mississippi River proper only below the mouth of the Missouri River, a transitional area with increased turbidity, increased velocity, shifting sands, and silty substrates providing suitable habitat for the western silvery minnow (Burr and Page 1986). These characteristics also are common in the Missouri River, where western silvery minnow is common to dominant throughout the system (Cross et al. 1986). In particular, the lower Missouri River has extreme fluctuations in water flow throughout the year, high silt loads, and unstable streambeds devoid of vegetation (Cross et al. 1980). These same conditions generally occur in the lower reaches of the Milk River.

Abundant rearing and feeding habitat for the western silvery minnow are thought to be present in the lower Milk River in Alberta (RL&L 2001). With the exception of extreme drought conditions, such as occurred between 1998 and 2004, quiet waters with low to moderate velocities are usually prevalent (RL&L 2002a).

Overwintering requirements of the western silvery minnow are unknown (Clayton and Ash 1980). Limited data from the Milk River suggest water depths and oxygen levels would not appear to limit overwinter use (Clayton and Ash 1980).

Spawning habitat of western silvery minnows has not been determined.Areas rich in aquatic vegetation have been listed as a key feature for spawning habitat for the Mississippisilvery minnow and the eastern silvery minnow (Scott and Crossman 1973; Ramshaw and Mandrak 1997).Western silvery minnows must utilize different spawning habitat or strategy, as the Milk River is devoid of aquatic vegetation due to high silt loads and unstable stream beds. Although flooded quiet backwaters were proposed as possible spawning habitat in the Milk River, recent sampling efforts in these areas failed to observe any eggs, larvae or fecund females (Clayton and Pollard pers comms. 2008). It is more likely that they are pelagic broadcast spawners similar to the Rio Grande silvery minnow (Hybognathus amarus (Girard, 1856)) and plains minnow with semi-buoyant eggs (Platania and Altenbach 1998). These species rely on adequate water flows and intact stretches of river to passively disperse eggs to downstream habitats. As such, impoundment and changes to hydrology seriously undermine this spawning strategy.

The greatest changes to western silvery minnow habitat in Alberta have been associated with irrigation needs. In 1917, the St. Mary Canal (Figure 5) was completed in Montana to divert water from the St. Mary River to the North Milk River for irrigation purposes. In most years, the canal diverts water from March to October, increasing the water volume in the North Milk River and the Milk River proper. The water in the Milk River (and St. Mary River) is shared by Canada and the United States via the order in the Boundary Waters Treaty. During the augmentation period in the Milk River in Canada (March to October), Canada must leave the majority of that water for the U.S., so it is not available irrigation water. According to the agreement, the U.S. is able to use the Milk River in Canada simply for conveyance of water (Petry, pers comm. 2008).

Figure 5. Milk River system, upstream of the Fresno Reservoir.

Figure 5.  MilkRiversystem, upstream of the Fresno Reservoir.

Before the construction of the diversion, the Milk River was probably a typical small prairie stream, possibly intermittent in times of drought, and generally less turbid (Willock 1969b). The even-flowing waters now observed in the lower Milk River in Alberta were probably mainly restricted to downstream of the international border before the diversion was constructed (Willock 1969b). The significant increase in water volume since the canal went into use is believed to have extensively altered the ecological regime of the Milk River (with the exception of the Milk River upstream of its confluence with the North Milk River. The result has been the creation of a more turbid, higher-flow system in the North Milk and Milk rivers in Alberta (Willock 1969b).

Since the construction of the St. Mary Canal, no major losses or changes in habitat have occurred. Rather, the availability of habitat is highly variable from year to year, and mainly dependent on adequate water flows, particularly in the late summer and fall, as well as for overwintering. During periods of very low flows, the western silvery minnow may experience temporary reductions in available habitat and under extreme drought conditions, such as those of fall and winter 2001/2002, temporary habitat fragmentation. The extent of the drought during this period was such that the lower section of the Milk River in Alberta, where most minnows have been documented, was reduced to a series of isolated pools, many of which were not deep enough to support overwintering fish (RL&L 2002a). A winter survey of a subset of these pools did not find any minnows present (RL&L 2002a). Furthermore, south of the international border, the Milk River was completely dry to the Fresno Reservoir from September 2001 to February 2002, and the reservoir was only at 4% of its capacity (K. Gilge, pers. comm.). Western silvery minnows may also be present in the Fresno Reservoir but this has not been confirmed by surveys (K. Gilge, pers. comm.). Therefore, limited re-colonization potential from upstream and downstream sections in the system exists. Downstream of the Fresno Reservoir in Montana, six more impassable dams upstream of the confluence with the Missouri River prevent any upstream dispersal of western silvery minnow (Stash 2001, K. Gilge, pers. comm.).

Southern Alberta is susceptible to extreme drought conditions during the summer, and naturally low flows at this time of year may be exacerbated by the seasonal operation of the St. Mary Canal, and by water removal for irrigation (Pollard 2003). In 2001, the August, October and December discharges were 50%, 7% and 6% of historic values, and the October, December rates in 2002 were 11% and 20%. Such low flows could seriously limit overwintering habitat, and in fact, during the late fall and winter of 2001/2002 the lower Milk River dried up completely, except for a number of isolated pools (R.L. & L. 2002a,b). This severity of drought conditions in southern Alberta is not uncommon (Pollard 2003) and may be more common given predicted changes in aquatic ecosystems associated with global climate change (Poff et al. 2002). This may prevent populations from expanding, and even more significantly, the higher temperatures that accompany the summer drought would expose all fish species, including the silvery minnow, to increased risk which may be exacerbated by ongoing maintenance of the St. Mary Canal that results in closures of the canal for extended periods.

Conserving western silvery minnows in Canada is likely dependent on maintaining flows and sediment erosion and deposition within the Milk River. While rearing and feeding habitat for the western silvery minnow in Alberta appears to be abundant in most years, the availability of overwintering habitat may be limited in some years, depending on flow conditions. In particular, the combination of extreme drought conditions and water removal could severely reduce or even eliminate winter refugia for the western silvery minnow in the lower Milk River.

Habitat protection/ownership

The federal Fisheries Act of Canada (R.S. 1985, c, F-14) provides protection for the habitat of western silvery minnow by prohibiting the harmful alteration, disruption or destruction of fish habitat unless authorized by the Minister (S 35). It also prohibits the deposit of deleterious substances into waters frequented by fish (Ss. 36.3).

The Canadian Environmental Protection Act (1999, c. 33), which is in place to prevent pollution and protect the environment and human health, focuses on regulating and eliminating the use of substances harmful to the environment. In addition, habitat of the western silvery minnow receives further protection via the provisions in the Canadian Environmental Assessment Act (1992, c.37). When certain regulatory duties are exercised under the Fisheries Act, a mandatory environmental review is undertaken that considers a broader scope of environmental effects including species at risk. The Species at Risk Act (SARA) [2002, c.29] makes it an offence in section 33 to damage or destroy the residence of one or more individuals of a listed endangered or threatened species (Ss.58.1; SARA 2007).

In Alberta, the western silvery minnow is currently ranked as “At Risk” according to The General Status of Alberta Wild Species 2005. Although this listing does not provide additional protection, priority may be given to this species in order to conduct additional research to obtain a more detailed status determination. In 2003, the species was approved for listing as “Threatened” provincially, and since 2002 the species is no longer allowed for use as live or dead bait in Alberta. Western silvery minnow are currently in the Alberta Wildlife Act under "Endangered Fish". The Act lists both Threatened and Endangered species as "Endangered Fish" – but such listed species can be further defined in law as E or T (Court, pers. comm. 2008). Despite being listed as Threatened, there exist no prohibitions/protections for silvery minnows in Alberta. A draft set of regulations has been prepared, but have yet to become law.

Provincially, as federally, various legislation and regulations provide protection for species at risk. The Alberta Wildlife Act (R.S.A. 2000, W-10) requires the responsible Minister to establish an Endangered Species Conservation Committee that will advise on issues relating to species at risk in Alberta. The Environmental Protection and Enhancement Act (Chapter/Regulation: E-12 RSA 2000) protects land, water, and air through a legislated environmental assessment process. The Alberta Public Lands Act (R.S.A. 2000, c. P-40) enables the regulation of the use of Crown Lands, and the Alberta Water Act (Chapter/Regulation: W-3 RSA 2000) provides for the management, protection and allocation of provincial water resources.

Currently, approximately 56% of the land bordering the Milk River mainstem and North Milk River is publicly owned; the rest is held privately. Only 11% of the public and 14% of the private lands have conservation plans that include riparian protection (Milk River Species at Risk Recovery Team 2007). The remaining land has been traditionally used mainly for grazing, or for small areas of municipal development (e.g., Town of Milk River). Six percent of the public land along the river has been designated park land, for public use and access during the summer months.

Other agencies that may be associated with aspects of watershed conservation include: Environmental Farm Planning, Alberta Riparian Habitat Management Society (Cows and Fish), Operation Grassland Community, Ducks Unlimited, MULTISAR, Nature Conservancy, Agriculture Canada, and Alberta Agriculture (Milk River Species at Risk Recovery Team 2007).

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