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2.5 Species’ Needs

2.5.1 Biology and Life History

Until recently, very little was known of the biology and life history of the western silvery minnow.  Consequently information from the eastern silvery minnow in New York (Raney 1939) was often cited instead.  This may not have been appropriate, since the eastern silvery minnow lives in lakes and the western silvery minnow inhabits rivers (D. Watkinson, pers. comm.).  Fortunately, since it was listed by COSEWIC in 2003, ongoing studies by T. Clayton (Alberta Sustainable Resource Development) and D. Watkinson (Fisheries and Oceans Canada) have filled some important gaps in knowledge of the western silvery minnow. Where gaps remain, information from other minnow species that inhabit similar Great Plains river habitats has been cited, rather than that from the eastern silvery minnow.

Growth

Western silvery minnows in the Milk River can grow to at least 140 mm in fork length (FL) (D. Watkinson, pers. comm.).  Both sexes mature at age 2+ years, and can live to at least age 4+ years (Sikina and Clayton 2006).  In Missouri, adult western silvery minnows of ages 3 to 5 years are common, with a maximum age of 5.5 years (Pflieger 1997).

Reproduction

Species within the genus Hybognathus exhibit a range of spawning strategies, and the strategy used by the western silvery minnow is unknown.  It is likely a broadcast spawner, like other Great Plains stream minnows such as the central silvery minnow, Rio Grande silvery minnow (H. amarus), and plains minnow (H. placitus) which release non-adhesive, semi-buoyantpelagic eggs into open water to develop as they drift downstream in the current (Platania and Altenbach 1998; Cowley 2002; R. Bramblett, pers. comm.). Sediment-laden waters keep the eggs of the Rio Grande silvery minnow afloat and modest currents transport them downstream (Cowley 2002).  The embryos develop quickly as they drift in the current, hatching within 24 to 48 hours depending upon the water temperature. These broadcast spawners require extensive stretches of connected habitat to enable the fish born from eggs that drift downstream to return upstream to suitable habitats.

Western silvery minnows in the Milk River spawn in June or July (D. Watkinson, pers. comm.).  Their fecundity increases with size, from less than 2,000 eggs in an 80 mm (FL) female to 19,500 eggs in one that is 130 mm (FL). Fish caught during an extremely warm spring had not spawned in late May, and some still contained mature eggs in mid-July. Adult western silvery minnows in Missouri have been observed in breeding condition in late June (Pflieger 1997). 

Ecological Role

During the summer, western silvery minnows in the Milk River consume diatoms, higher plant material, blue-green algae, green algae, cyanobacteria, fungus, pollen, protozoa, dinoflagellates, zooplankton, cryptophyceae, and rotifers (D. Watkinson, pers. comm.).  Given the paucity of aquatic vegetation, the higher plant material may have come from the leaves of trees or the undigested feces of herbivores.  Charcoal, likely from bottom sediments, and a sponge spicule were also found in their stomachs.

Sauger (Sander canadensis), northern pike (Esox lucius) and burbot (Lota lota) are likely the major predators of all life stages of the minnow, while other species may opportunistically consume eggs and larvae. Twenty-two fish species, including the western silvery minnow, have been documented in the Milk River mainstem and tributaries (Table 1) (Alberta Sustainable Resource Development 2003; T. Clayton, pers. comm.).  Seventeen of these species occur within the western silvery minnow’s range in the Milk River.  The MULTISAR (Multi-Species at Risk) Program, a basin-wide terrestrial and aquatic species identification and stewardship program,

Table 1. Fish species that occur in the Milk River watershed.

Common NameScientific NameOccurs within minnow’s range?
Brassy minnowHybognathus hankinsoniY
Brook sticklebackCulaea inconstansY
BurbotLota lotaY
Fathead minnowPimephales promelasY
Flathead chubHybopsis gracilisY
Iowa darterEtheostoma exileN
Lake chubCouesius plumbeusY
Lake whitefishCoregonus clupeaformisN
Longnose daceRhinichthys cataractaeY
Longnose suckerCatostomus catastomusY
Mountain suckerCatostomus platyrhynchusY
Mountain whitefishProsopium williamsoniN
Northern pikeEsox luciusY
Northern redbelly dacePhoxinus eosN
SaugerSander canadensisY
East slope sculpin (or St. Mary sculpin)Cottus sp.Y
StonecatNoturus flavusY
Trout-perchPercopsis omiscomaycusY
White suckerCatostomus commersoniiY
WalleyeSander vitreusY
Yellow perchPerca flavescensY

recently identified trout-perch (Percopsis omiscomaycus), a yellow perch (Perca flavescens), a walleye (Sander vitreus), and lake whitefish (Coregonus clupeaformis) in the Milk River system (T. Clayton, pers. comm.), suggesting movement from Montana or illegal introductions.

2.5.2 Habitat

The western silvery minnow is most commonly found in large, silty prairie streams, generally in areas with little or no current and sandy, muddy or debris-covered bottom (Pflieger 1980; Trautman 1957, Missouri Fish and Wildlife Information System 2002).  Within these systems, gradient, bottom type and turbidity appear to be strongly associated with minnow presence.  In North Dakota, 98% of all western silvery minnows were captured in water less than 1 m deep and current velocities of less than 0.5 m/s (Welker and Scarnecchia 2004).  Eighty-five percent of these fish were in areas of relatively low turbidity (<250 NTU; nephelometric turbidity units), where summer temperatures were relatively high (18°-22°C). A habitat model (using logistic regression) that incorporated water velocity, depth, and percentage sand predicted minnow presence in river segments during the open water period in North Dakota with 97% accuracy (Welker and Scarnecchia 2004), indicating that these habitat variables are key determinants of the species’ presence.

In the Milk, Missouri and Mississippi river mainstems the western silvery minnow occurs in transitional areas characterized by elevated velocity and turbidity, an unstable streambed with shifting sand and silt substrates, and flows that fluctuate through the year (Burr and Page 1986, Alberta Sustainable Resource Development 2003).  Welker and Scarnecchia (2004) referred to the species’ preferred habitat as channel border habitat.  These minnows tolerate a wide range in turbidity (Missouri Fish and Wildlife Information System 2002).  They occur in areas that are rich in phytoplankton (Trautman 1957) and in streams devoid of aquatic vegetation, such as the lower Missouri River (Cross et al. 1986) and the Milk River (D. Watkinson, pers. comm.). 

The open water distribution of western silvery minnow in the Milk River is strongly correlated with gradient and substrate type (Figure 3).  During the summer in the lower Milk River, the species shows preference for water velocities less than 0.3 m/s, depths of less than 0.3 m, and silt bottom substrate (R.L.&L. 2002; D. Watkinson, pers. comm.).  However, it also occurs in water velocities of at least 1.2 m/s, at depths of at least 1.4 m, and over sand and gravel substrates. Upstream of the confluence with Police Creek, where the species is present in lower abundance, there is an abrupt increase in both gradient and the size of substrate.  The species winter distribution is unknown.  Some fish likely overwinter in the same areas they occupy in summer, while others may move elsewhere to find suitable habitat that does not freeze or become anoxic.

Key Habitat

For the purposes of this document, we define “key habitat” as the habitat believed to be important to the survival of specific life stages of the western silvery minnow based on knowledge of the minnow’s distribution in the Milk River and the physical state of the river at certain periods in the year. The following sections describe potential key overwintering, spawning, and rearing habitat.

Little is known about the characteristics or availability of overwintering habitat for the western silvery minnow in the Milk River.  When diversion from the St. Mary River ceases in the fall, the river reverts back to its natural flow conditions until spring.  In normal years, flow is maintained within a reduced channel.  Under severe drought conditions, such as those in 2001, the river may be reduced to a series of isolated pools suggesting that these may be important to the species survival.  While, previous winter sampling efforts have not documented western silvery minnow from such pools (R.L.&.L. 2002), this may be an artefact of limited sampling effort. Alternatively, the species may seek refuge in areas where flowing water is still available.

Small areas of open water along the shoreline of the lower Milk River during the winter months may be maintained in part by small springs or re-emerging subsurface flows (R. Audet, pers. comm.).  Minnows (species unknown) have been observed at these sites, which may provide winter refugia for the western silvery minnow.

Periodic re-colonization of western silvery minnow from downstream habitats is also a possibility, although dam construction on the lower reaches of the Milk River may limit that option.  Elsewhere in the United States, the western silvery minnow persists in the upstream portions of many small intermittent streams where it may find overwintering refuges rather than re-colonize annually (R. Bramblett, pers. comm.)  More detailed studies are required to characterize and evaluate overwintering habitat in the Milk River, as this habitat is likely to be important to the species’ survival and may be vulnerable to human disturbance.

Spawning habitat of the western silvery minnow has not been described.  If the species is a pelagic-broadcast spawner (pelagophil), like other minnow species found in Great Plains streams (see Section 2.5.1), it may require significant stretches of connected habitat with turbid, sediment-laden water of moderate flow velocity for spawning (Cowey 2002, Platania and Altenbrach 1998).  The distance that larvae are displaced, the habitat where displaced larvae are deposited, and their ability to move unimpeded to upstream reaches of sustained flow are important determinants of spawning success in these species (Platania and Altenbrach 1998).

In the Milk River, rearing and feeding habitat is probably not a limiting habitat feature for western silvery minnow under the current flow regime (R.L.&L. 2002).  After flows peak, usually in June at the Town of Milk River, the water level drops, providing backwater areas in the main channel of the river where minnows may seek refuge (T. Clayton, pers. comm.).  A fish habitat survey in June 2004 noted significant erosion and inter-annual movement of sandbars in the lower Milk River in response to changes in flow conditions (T. Clayton, internal memorandum).  This variation may benefit the species by providing the necessary dynamic habitats that result from constant erosion and deposition processes, provided that quiet backwater habitats persist. Sustained, increased discharges resulting in bank to bank flows, on the other hand, could be energetically costly to the species and limit its available habitat (D. Watkinson, pers. comm.).

Habitat Trends and Limitations

While the channel pattern and character of the river have remained essentially unchanged since 1917, the augmented flows have widened the channel and increased cutoff activity and sediment yield (McLean and Beckstead 1980). These effects are most prominent in the North Milk River, where the flood frequency has also doubled since diversion and the magnitude of the flood flows has increased. Flow augmentation continues to erode river banks and reduce fine-sediment bottom habitats in the Milk River (McLean and Beckstead 1980; D. Watkinson, pers. comm.). Habitat availability varies from year to year depending on flow, particularly in late summer, fall, and winter. Drought and premature or temporary canal closure for emergency maintenance work during the augmentation period can have a significant impact on flows and water levels in the Milk River. Potential changes for the future include a water storage dam 30 km upstream of the Town of Milk River and altered flow regimes through the St. Mary Canal (Alberta Environment 2004; U.S. Bureau of Reclamation 2004).

Habitat Protection

The western silvery minnow is afforded varying degrees of direct or indirect habitat protection through existing statutes and programs.

Federally, the Fisheries Act (R.S. 1985, c. F-14) prohibits the harmful alteration, disruption or destruction of fish habitat  except as authorized by the Minister (S. 35) and similarly prohibits the deposit of deleterious substances into waters frequented by fish (i.e. fish habitat) (Ss.36.3).  TheCanadian 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.  The Canadian Environmental Assessment Act (1992, c.37) ensures that prescribed federal regulatory actions including the authorized destruction of fish habitat are subjected to an environmental review process.  The Species at Risk Act (2002, c.29) prohibits the destruction of any part of critical habitat once it has been identified in a recovery strategy or action plan for any listed endangered, threatened or extirpated wildlife species (Ss.58.1).

At the provincial level, Alberta’s Wildlife Act(R.S.A. 2000, W-10), requires that the Minister (responsible for this act) establish an Endangered Species Conservation Committee that will advise on issues relating to species at risk in Alberta, such as assigning status and preparing and adopting recovery plans. The Environmental Protection and Enhancement Act(Chapter/Regulation: E-12 RSA 2000) protects land, water, and air by requiring those operating or proposing developments to meet their environmental responsibilities.  It includes a legislated environmental assessment process.  The Alberta Public Lands Act (R.S.A. 2000, c. P-40) enables the designation of different types of Crown land use including agricultural, oil and gas and other resource uses.  The Alberta Water Act(Chapter/Regulation: W-3 RSA 2000) focuses on managing and protecting the province’s water, and regulates the allocation of water resources. 

Under the “Water for Life” strategy, Alberta supports the formation of Watershed Planning and Advisory Councils and the development of Watershed Management Plans.  These plans identify water needs, including those of fish, and may influence the licensing of water diversions by the Government of Alberta.  The Milk River Watershed Council of Canada plans to complete a State of the Watershed Report in 2007, and the Basin Management Plan 2 years later (K. Miller, pers. comm.).

At writing, 56% of the land bordering the Milk River mainstem and North Milk River was publicly owned; the rest was held privately. Only 11% of the public and 14% of the private lands had conservation plans associated with them that included riparian protection (T. Clayton, pers. comm.). The remaining land was 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 was designated park land, for public use and access during the summer but with restrictions on development.  Municipal approval is required for shoreline development on any municipal environmental easements. Other initiatives or agencies that make recommendations affecting water quality and/or water flows, management of shorelines, and other 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.

2.5.3 Limiting Factors

Too little is known of the western silvery minnow’s physiology or ability to adapt to different conditions to identify factors that might limit population survival and maintenance. The minnow is typical of many large plains streams fish species in that it has adapted to a system with a high sediment load and naturally fluctuating flow conditions.  While these river conditions may seem harsh, species that have evolved under them may only survive if these conditions persist.  Changes such as flow regulation or increased water clarity might, for example, cause them to lose their advantage to competitors or increase their vulnerability to sight-dependent predators (e.g., sauger and northern pike).  Flow changes might also alter downstream drift by western silvery minnow eggs and fry, decreasing their viability or increasing their risk of predation.