Western silvery minnow (Hybognathus argyritis) COSEWIC assessment and status report: chapter 2

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COSEWIC Executive Summary

Species information

The western silvery minnow (Hybognathus argyritisGirard, 1856) is a small cyprinid and is one of the four species of the genus Hybognathus occurring in Canada. It was first discovered in the Milk River in Montana by Girard in 1856. Scott and Crossman later treated the species as a subspecies (H. nuchalis nuchalis) of the central silvery minnow (H. nuchalis Agassiz, 1855). Currently, the species has again been recognized as the western silvery minnow (H. argyritis). This decision was made based on differences in the shape of the basioccipital process between species and has been accepted by the American Fisheries Society.

Distribution

The western silvery minnow is found throughout the Missouri River basin in the United States and in the Mississippi River as far south as the confluence with the Ohio River. In Canada its most northerly distribution occurs only in the Milk River in southern Alberta. A single specimen reported from the South Saskatchewan River near Medicine Hat in 1963 is the only record of a western silvery minnow in the Saskatchewan River system and the only occurrence outside the Missouri/Mississippi drainage system. Additional samplings have not confirmed its presence there.

Habitat

Western silvery minnows occur primarily in slow, turbid habitats commonly found in backwaters and pools of large, silty, plains streams. Spawning habitats and the habitats of the young of the year western silvery minnows are not known.

Biology

Of the Hybognathus species found in Canada, the western silvery minnow has the longest life span, reaches the greatest lengths, and is the most fecund. This cyprinid species lives for approximately 4+ years, obtains lengths of 140 mm FL (fork length), and reaches sexual maturity during its third year of life. The spawning period begins in late May and continues until early July. Large fecund females can produce up to 20 000 eggs. The diet of the western silvery minnow is primarily algae, diatoms, and organic matter filtered out from the sediments they ingest.

Population sizes and trends

The population size of western silvery minnows in Canada is unknown. Abundance may have declined in the early 1900s in the Milk River as a result of the combination of water removal for irrigation and extreme drought conditions after the construction of the St. Mary Canal. Since the first collection in 1961 by Grant Campbell there is no evidence that the western silvery minnow population has declined. In 2003, Pollard estimated the population to be no more than a few thousand individuals. This estimate was based on limited records with only 192 specimens sampled. Recent surveys collected a total of 2232 western silvery minnows at several new sites; however, due to its limited distribution the species may be sensitive to future anthropogenic and environmental disturbances.

Limiting factors and threats

The Milk River is unique in that it receives significant flow augmentation in the summer from the St. Mary River, a South Saskatchewan River tributary (Hudson Bay drainage). Water is diverted from the Lake Sherburne Reservoir on the Swift Current Creek in Montana via two siphons and a canal to the North Milk River in Montana. The North Milk then flows into Alberta and joins with the Milk River. The Milk River flows out of Alberta back into Montana and eventually into the Fresno Reservoir in Montana. The Fresno Reservoir restricts upstream fish movement.

Western silvery minnow persistence within the Milk River is dependent on maintaining flow and sediment transport. Current water management infrastructure supplements flow and increases available habitat from March to October. During the shut down period western silvery minnow are exposed to natural low flow amplified by water withdrawals, low dissolved oxygen, and extreme low (freezing) water temperatures. Historically, the siphon has been shut down for repairs in the summer months.

Special significance of the species

The western silvery minnow is probably an important forage species where abundant. The species has economic importance in the United States as a valuable bait fish. In Alberta the species has been banned as live and dead bait. The western silvery minnow also has special significance and interest to the scientific community in relation to the zoogeographic history and distribution of the species subsequent to the Wisconsin Period of glaciation.

Existing protection or other status designations 

In Canada, the western silvery minnow was designated as “Threatened” in 2001 by COSEWIC and is listed as such under the Species at Risk Act (SARA).In Alberta,the western silvery minnow is currently ranked as “At Risk”. Although this listing does not provide additional protection, it does increase the awareness of the species as possibly threatened and may result in additional research with the goal of obtaining a more detailed status determination.

COSEWIC History

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2009)

Wildlife Species

A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)

A wildlife species that no longer exists.

Extirpated (XT)

A wildlife species no longer existing in the wild in Canada, but occurring elsewhere..

Endangered (E)

A wildlife species facing imminent extirpation or extinction..

Threatened (T)

A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC)*

A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR)**

A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD) ***

A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.

* Formerly described as “Vulnerable” from 1990 to 1999, or “Rare” prior to 1990.
** Formerly described as “Not In Any Category”, or “No Designation Required.”
*** Formerly described as “Indeterminate” from 1994 to 1999 or “ISIBD” (insufficient scientific information on which to base a designation) prior to 1994. Definition of the (DD) category revised in 2006.

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.