Western silvery minnow (Hybognathus argyritis) COSEWIC assessment and status report: chapter 3

• Previous page
• Table of contents
• Next page

Species Information

Name and classification

Common names:

The western silvery minnow (Hybognathus argyritis Girard, 1856) is a small cyprinid of the genus Hybognathus first discovered in Canada by Grant Campbell in 1961 (UAMZ 5320, University of Alberta Museum of Zoology) (Figure 1). The Hybognathus genus contains seven species in North America, of which four are found in Canada, i.e., the western silvery minnow, the eastern silvery minnow (H. regius Girard, 1856), the brassy minnow (H. hankinsoni Hubbs, 1929), and the plains minnow (H. placitus Girard, 1856) (Robins et al. 1991; Schmidt 1994). Along with the eastern silvery minnow, the western silvery minnow was formerly treated as a synonym of the central silvery minnow (H. nuchalis Agassiz, 1855) (Pflieger 1980a, b). Scott and Crossman (1973) treated them as subspecies, namely H. nuchalis nuchalis in the west and H. nuchalis regius in the east. Pflieger (1971) recommended that H. nuchalis, H. argyritis, and H. regius be considered distinct species based on the differences in the shape of the basioccipital process (Pflieger 1971). This decision has been accepted by the American Fisheries Society (Nelson et al. 2004).

Figure 1. Western silvery minnow, Hybognathus argyritis (92 mm FL) male, collected May 28, 2006 (49.00537, -110.58744).

Morphological description

The average size of the western silvery minnow in Canada is approximately 86 mm fork length (FL) with a maximum FL of 140 mm (Watkinson et al. MS 2007). The body is elongate, moderately compressed laterally, and has a stout caudal peduncle. The head is short, bluntly triangular with a moderately large eye; the snout is rounded and overhangs the mouth, which is subterminal. The distance between eyes is about 2 times the eye diameter. The isthmus is very narrow, less than a ¼ width of the head. The pharyngeal teeth (0, 4-4, 0) are not hooked and have a distinct grinding surface. The dorsal fin has eight rays and originates slightly in advance of the origin of the pelvic fins, which have eight rays, but sometimes seven. The caudal fin is forked; the anal fin originates behind the posterior margin of the depressed dorsal fin and usually has eight rays, sometimes nine [eight in Alberta (Nelson and Paetz 1992)]; the pectoral fins are relatively short with 15 or 16 rays. The anterior tips of the dorsal and pectoral fins are pointed. The lateral line is complete and decurved. Lateral-line scales are 36−40. The scales are cycloid with 8 to 11 long radii (Nelson and Paetz 1992). The peritoneum is black and the elongate intestine is coiled on the right side; vertebrae number 36−38 (see Scott and Crossman 1973; Trautman 1957), but 39−41 in Alberta specimens (Nelson and Paetz 1992).

Small nuptial tubercles may be found on the head, back, sides, and on the fins of breeding individuals (sparse on females, more numerous on males). Both sexes are silvery in colour, hence the common name, with a broad, slaty mid-dorsal stripe. Alberta specimens are brownish-yellow dorsally and silver laterally, no lateral band is obvious, but dusky spots may be present (Nelson and Paetz 1992). During spawning the males are light yellow along the sides and the lower fins (Scott and Crossman 1973; Trautman 1957).

Live specimens of the western silvery minnow can be distinguished from the sympatric brassy minnow by a pointed dorsal fin and silvery colour (Scott and Crossman 1973; Nelson and Paetz 1992), a larger body size, four scales between the lateral line and the pelvic fins, a thin black line along the side of the body partly over the dark lateral band, and 5 to 12 radiating grooves on scales lateroventral to the dorsal fin (McAllister and Coad 1974).Close examination of the scales shows the circuli to be much more sharply angulated at the basal corner of the scales in the western silvery minnow than in the brassy minnow.

Genetic description

The genetic population structure of the western silvery minnow in Canada is unknown. Given the lack of obvious barriers between the lowest section of the Milk River in Alberta and the section immediately south of the United States border the potential for gene flow throughout this entire section in most years could be high, likely preventing the development of genetically distinct subpopulations. Thus, the Alberta population of western silvery minnows is a part of the genetic population found in Montana upstream of the Fresno Reservoir. However, this potential is limited; in drought years, such as 2001–2002, the river has been known to be completely dry from the Fresno Reservoir north to the international border and the reservoir itself, at low (< 4%) capacity (K. Gilge, pers. comm. 2002).

Designatable units

This report deals with the species; there is no evidence to support the existence of units below the species level in Canada.

Eligibility

The western silvery minnow is a recognized species (Nelson et al. 2004) that is considered to be native to Alberta, although the earliest record dates from 1961 (Nelson and Paetz 1992). It has undoubtedly been there for some time; unnoticed because of the lack of earlier sampling effort, or perhaps misidentified since the species has been known from the Milk River in Montana since 1856. There is no evidence to suggest that the species was introduced.