Hybognathus nuchalis Agassiz, 1855
Mississippi Silvery Minnow
Basis for Listing
The Mississippi Silvery Minnow (Hybognathus nuchalis) is at the northern edge of its range in Minnesota (The Blufflands Subsection). The species' distribution and abundance has been drastically reduced since the completion of the lock and dam system on the Mississippi River. Surveys conducted from 1942-1949 collected Mississippi Silvery Minnows from Pools 3-9, and at many localities it was the dominant species in the catch (e.g., 1,981 specimens from Pool 4 on 25 July 1945). The species is still reported from Pool 8 of the Mississippi River. However, the population has exhibited extreme fluctuations from 1989-2004. In 1993, the species was present at 27 localities and consisted of a total catch of 450 specimens; in 1994 there were only two localities and two specimens (LTRMP 2013). Periods of low numbers occur more frequently than high numbers (Lyons et al. 2000). From 2006-2008, the Mississippi Silvery Minnow was one of several rare species targeted in a DNR State Wildlife Grant survey of the Mississippi River from the Coon Rapids Dam (Anoka County) to the Iowa border. However, only two specimens were collected in Pool 6 and one in Pool 9 (Schmidt and Proulx 2009). The species is apparently extirpated from Lake Shetek (Murray County), where specimens have not been collected since 1943.
Prior to the construction of dams in the 1940s, the Mississippi Silvery Minnow occurred throughout the entire Tennessee River system. Here too, they were frequently the dominant species in numbers and biomass; whereas, they are now extirpated from the system. The definitive cause of this sudden extirpation is unknown; however, the pre-impounded Tennessee River may have provided a vital aspect of the species life history (Etnier et al. 1979).
The extirpation of the Lake Shetek population and major loss of range in the Mississippi River prompted listing the Mississippi Silvery Minnow a special concern species in 2013.
The Mississippi Silvery Minnow is a medium-sized minnow that is usually 5.0-8.5 cm (2.0-3.3 in.) total length, with a maximum of 11 cm (4.3 in.). Distinguishing characteristics include a slightly subterminal mouth, round lateral line scales, straight lateral line, black peritoneum, and an elongated coiled intestine. It is easily confused with two other species: the Brassy Minnow (H. hankinsoni) has a rounded, instead of a pointed, tip on the dorsal fin and an eye diameter greater than the length from the corner of mouth to snout tip; the River Shiner (Notropis blennius) has a terminal mouth, silvery peritoneum, and short intestine (Lyons et al. 2006).
The Mississippi Silvery Minnow is most often found in lower reaches of tributary streams near confluences with large rivers. Water is usually clear up to 1.5 m (4.9 ft.) deep. Substrates are comprised primarily of sand and sparsely vegetated (Becker 1983). In Pool 8 of the Mississippi River, the species prefers borders along main and side channels, where it was sampled 30 times each; and backwaters, 24 times. Depths ranged from 0.3-2.1 m (1.0-6.9 ft.). Sand dominated the main channels; in the side channels and backwaters, there was a mix of silt, clay, and sand. Velocities ranged from 0.1-0.53 m/s (0.3-1.7 ft./s) in channels to 0.01-0.35 (0.03-1.15) in backwaters. The species was also found in the tailwaters of dams eight times and impounded areas seven times (LTRMP 2013).
Biology / Life History
Very little life history information is available on the Mississippi Silvery Minnow, and much of the published literature cites studies of surrogate species of the same genus. In southern Wisconsin, the species begins spawning in late April or early May and continues at least to the end of July. Spawning habitat includes the shallow water of creeks and rivers in or near riffles. Growth is rapid in young of the year, which ranged from 4.4-7.4 cm (1.7-2.9 in.) total length in two samples collected during September. Some females mature at 1-year, but usually maturity is two years for both sexes (Becker 1983).
Conservation / Management
Research needs include life history studies and habitat assessments of Pool 8 to determine why Mississippi Silvery Minnows have persisted here, and what is lacking in the other navigation pools. Analysis of Wisconsin DNR Pool 8 data may identify limiting factors for why the species is typically present at few localities in low numbers but occasionally exhibits brief periods of wide distribution and abundance.
Conservation Efforts in Minnesota
Large tracts of Pool 8 are protected within the boundaries of the Upper Mississippi River National Wildlife and Fish Refuge. The recent inception of Minnesota’s Clean Water Legacy Program will eventually yield benefits to Mississippi Silvery Minnow habitats through sediment load reductions.