Western silvery minnow (Hybognathus argyritis) COSEWIC assessment and status report: chapter 6

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Biology

General

The western silvery minnow is a small cyprinid species that generally lives to an age of 4+ years and attains FLs of up to 140 mm (Watkinson et al. MS 2007). Size of fish caught in the Milk River in 2005 and 2006 with estimated ages of 1+, 2+, and 3+ had corresponding FLs of 51−63, 67−88 mm, and 95−114 mm (Watkinson et al. MS 2007). During this same study, 100 western silvery minnows were collected from the Milk River in Montana in May 2006. These minnows had a multi-modal distribution in length over the sample range with the suggestion of two peaks in the 26−74 mm (FL) range. The maximum frequency observed was for fish of 58−60 mm FL. The length-weight relationship for these specimens was expressed by the equation: Log W = 3.3878 (Log L) - 5.6199, where W is weight in grams and L is fork length in millimetres. Both sexes appear to reach sexual maturity at 2+ years or at the beginning of their third year of life. Female eastern silvery minnow start spawning at age one, 50 to 55 mm standard length (SL) and males start spawning during their second year of life (Raney 1939).

The growth rate of the Milk River western silvery minnow was slower than reported for the Mississippi silvery minnow (Becker 1983; Taylor and Miller 1990) but more rapid than reported for the eastern silvery minnow (Raney 1942), and similar to the plains minnow (Taylor and Miller 1990). The growth of young-of-the-year (YOY) Mississippi silvery minnows is rapid, reaching an average length of 52−69 mm total length (TL) by September in two Wisconsin rivers (Becker 1983; Taylor and Miller 1990). Becker (1983) reported only one female Wisconsin fish of 107 mm TL that lived to 3 years. Eastern silvery minnow (H. regius Girard, 1856) hatched in late April were 31 mm TL on 20 June and 45 mm TL by 15 July (Raney 1942). By the end of their second summer the average TL was 80 mm and by the end of their third summer the average TL was 82 mm for males and 88 mm for females (Raney 1942). The 140 mm FL western silvery minnows caught by Watkinson et al. (MS 2007) were the largest reported FL for a western silvery minnow sampled in Canada. The ages obtained for western silvery minnows were higher than those of closely related Hybognathus species (Watkinson et al. MS 2007). This is not an uncommon observation for fish species sampled at the extreme northern extent of its range.

Sizes for newly hatched western silvery minnow larvae have not been determined. Raney (1939) found newly hatched eastern silvery minnow larvae to be 6 mm TL in July and about 51 mm by August. Larvae stages for the eastern silvery minnows have been described and illustrated by Mansueti and Hardy (1967).

Reproduction

In Canadian waters, spawning for the western silvery minnow appears to take place between late May and early July when water temperatures range between 13.6 and 26.8°C (Watkinson et al. MS 2007). Females with mature eggs were collected from the Milk River in May 2006 (Watkinson et al. MS 2007). In July of 2005, when water temperatures were >20°C, large females were collected, but had limited numbers of bound mature eggs. Eddy and Underhill (1974) also reported that Hybognathus nuchalis (= argyritis) spawn in May and June in Montana.

Spawning in the plains minnow is protracted from April to August (Gilbert 1980) and there is evidence to suggest that the same may be the case for the eastern silvery minnow (Scott and Crossman 1973) and the central silvery minnow (Forbes and Richardson 1920).

In the previous western silvery minnow status report, Houston (1998a) suggested that like the eastern silvery minnow (Raney 1939), the western silvery minnow may spawn in heavily vegetated backwaters in slower moving reaches of the Milk River. It is possible that the western silvery minnow utilizes shallow backwaters with little or no current and silt substrate for spawning, which is similar to the spawning habitat of pond-raised eastern silvery minnows that Raney (1942) propagated, or as reported for the Mississippi silvery minnow (Eddy and Underhill 1974). However, unlike the spawning habitat of the eastern silvery minnow, the Milk River is a hydrologically dynamic, turbid prairie river with little or no aquatic vegetation due to the highly mobile bed. The western silvery minnow is more likely to have a similar spawning strategy to that of the Rio Grande silvery minnow or the plains minnow. Both of these closely related species are pelagic, broadcast spawners (pelgophils) that produce nonadhesive, semi-buoyant eggs that remain in suspension as long as there is current (Platania and Altenbach 1998).

Of the western silvery minnows collected in the Watkinson et al. (MS 2007) study, the smallest mature female was age 2+ with a FL of 81 mm collected in July 2005. Other mature females collected ranged in size from 82–127 mm FL. The fecundities of 11 of these fish varied based on female size with the smallest female of 81 mm FL having 2924 eggs and the largest female of 127 mm FL having 19 573 eggs. Compared to the fecundity counts of the eastern silvery minnow in Raney (1942), the larger western silvery minnow produced a significantly greater number of eggs.

Physiology

Species tolerances of poor water quality, high or low temperatures, high turbidity, and low dissolved oxygen levels for the western silvery minnows are not known. However, due to the types of the habitats they inhabit, western silvery minnows appear to be very tolerant of high turbidity and possibly of high temperatures and low dissolved oxygen levels. Matthews and Maness (1979) found the plains minnow, a closely related Hybognathus species, to be more tolerant of low dissolved oxygen levels and higher temperatures (40°C) than many other cyprinid species. The brassy minnow has been found to be tolerant of water temperature up to 35.5°C and dissolved oxygen levels of 0.03 mg/L (Scheurer et al. 2003).In addition, Buhl (pers. comm. 2007) found the Rio Grande silvery minnow was able to tolerate living in 100% effluent for extended periods of time (21–28 days) with 0% mortality. In general, these findings suggest that most Hybognathus species are very hardy fish and can tolerate extreme conditions.

Dispersal/migration

No information regarding movement patterns or dispersal ability is available for the western silvery minnow. Raney (1939) noted that adult eastern silvery minnows migrated to inshore waters of lakes and larger rivers in the spring to spawn, but it is not clear how far these fish migrated. As broadcast spawners they probably move up river to spawn, allowing the eggs to disperse some distance downstream (Pollard, pers. comm. 2008). Similarly, no information is available regarding the ability of the species to disperse and recolonize new or empty habitats. The fact that the western silvery minnow has likely undergone fairly regular drought conditions in the past and still persists in the Milk River suggests that it has the ability to disperse short distances into reaches of the river that may have temporarily been devoid of water.

Diet

All species of the genus Hybognathus have pharyngeal taste buds or papillae arranged in a pattern that suggests a filtering apparatus for trapping diatoms and other small food items (Hlohowskyj et al. 1989). Stomach content analysis of Milk River specimens (Watkinson et al. MS 2007) found that western silvery minnows fed largely on bacillariophytes (35%), chlorophytes (26%), plant remains (23%), and cyanophytes (10%) in May 2006. Smaller quantities of carbon, fungi, chrysophytes, pollen, zooplankton remains, heterocysts, rotifers, and protozoans were also found. Similar gut contents were found for the eastern silvery minnow (Gascon and Leggett 1977), the Mississippi silvery minnow (Forbes and Richardson 1920), and the plains minnow (Gilbert 1980). All species apparently ingest silt and bottom ooze from the backwaters and pools that they inhabit, filtering out and digesting the algae, diatoms, and organic matter.

The diet of YOY western silvery minnows is unknown. In Lac Memphremagog, Quebec the diet of 0+ eastern silvery minnow was found to change from cladoceran (82% by volume), rotifers (8.4%) and chironomids (7%) to organic detritus (95%) and cladocerans (3%), as the average FL increased from 32 to 44 mm (Gascon and Leggett 1977). Individuals greater than 40 mm FL fed almost exclusively on organic detritus, except in June when 46% of their diet consisted of cladocerans.

Interspecific interactions

The predators, parasites, and diseases of the western silvery minnow are not known. The silvery minnows in North America were found to have three species of parasites associated with them including three trematode species, one protozoan, and the larval form of the cestode, Ligula intestinalis. (Hoffman 1967). Seventeen fish species have been documented as co-occurring with western silvery minnow in the Milk River (Table 1). Piscivore species such as sauger (Sander canadensis (Griffith & Smith, 1834)), burbot (Lota lota (Linnaeus, 1758)), northern pike (Esox lucius Linnaeus, 1758), and yellow perch (Perca flavescens (Mitchill, 1814)) may negatively impact western silvery minnow populations in the Milk River. In addition, if a low flow condition occurred during the summer western silvery minnows could be exposed to aquatic, avian, and terrestrial predators. 

Behaviour/adaptability

The western silvery minnow is a schooling fish and has been found to school with flathead chubs (Platygobio gracilis (Richarson, 1836)) (Watkinson et al. MS 2007). They have maintained populations in the Milk River, where drought, low and high water temperatures, and low dissolved oxygen concentrations are common.