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The pearl dace (Margariscus margarita Cope, 1868) is a cyprinid fish native to Montana. Its phylogenetic relationships are unclear. Classified as a member of the genus Semotilus since 1960 (Bailey et al. 1960), it is likely more closely related to Couesius, Phoxinus, or possibly Hemitremia, than to Semotilus (Cavender and Coburn 1993). Until the phylogeny is clarified, Cope's original genus name Margariscus has been resurrected for the species (Robbins et al. 1991).
Pearl dace have an elongate, almost cylindrical body, with a relatively small eye, and a moderately small mouth that does not reach to below the front of the eye. The pelvic and pectoral fins are rounded, the dorsal, anal, and pelvic fins each normally have 8 rays, and the caudal fin is forked. The dorsal fin insertion is posterior to the pelvic fin insertion. Pearl dace usually have a small flattened barbel concealed in the groove of the upper lip on one or both sides; however, barbels may be absent in some individuals (Gould and Brown 1968). Color is dark olivaceous on the back, grading to silvery along the sides, with a silver-gray to white belly. A dark lateral band is sometimes present, which is more distinct on smaller specimens, and may terminate with a distinct spot at the base of the caudal fin. Males may be brilliantly colored from late autumn until mid-summer, with a rich orange-red flush along the flanks below the lateral band (Langlois 1929; Brown 1971; Scott and Crossman 1973).
Distribution and Habitat
The range of pearl dace includes most of Canada, from Nova Scotia to eastern British Columbia, north to the southern Northwest Territories. In the United States, pearl dace occur from Maine south to Virginia, and across the northern tier states from New Hampshire to Montana, with relict populations in South Dakota, Nebraska, and Iowa (Menzel and Boyce 1973; Lee et al. 1980). Although widely distributed, pearl dace are not normally abundant (Fava and Tsai 1974). In five of seven collections from four Montana streams, pearl dace numbered less than 20 individuals and comprised 5 % or less of the total number of fish captured (Bramblett, unpublished data).
Montana contains the southwestern periphery of the continental range of the pearl dace. In Montana, pearl dace occur only in the Missouri and Saskatchewan River basins (Figure 1). Most known pearl dace localities are in south-flowing tributaries to the Missouri River downstream of the confluence of the Milk River, in the Milk River drainage, or on the Blackfeet Indian Reservation in Glacier County and in Glacier National Park (Schultz 1941; Gould and Brown 1968; Brown 1971; Holton and Johnson 2003; Stash 2001; Bramblett, unpublished data; Robbin Wagner, U. S. Fish and Wildlife Service, personal communication, January 2004).
Two previously reported localities for pearl dace in the lower Yellowstone River (Gould and Brown 1968; Brown 1971; Holton and Johnson 2003) were probably attributable to misidentified creek chubs. The Gould and Brown (1968) collection was reexamined and the putative pearl dace was found to be a creek chub (William R. Gould, Montana State University, personal communication, January 2004). Other surveys have failed to find pearl dace in the Yellowstone River basin in Montana (Elser et al. 1980; Bramblett, unpublished data). Pearl dace appear to be a glacial relict in Montana as they are most commonly found in formerly glaciated portions of the plains regions.
Life History, Habitat and Ecology
In Montana, pearl dace reach 38-51 mm total length in their first year, and 76-101 mm in two years; the largest specimen from Montana is 101 mm (Brown 1971). Pearl dace elsewhere reach lengths of 158 mm, and ages of 4 years (Scott and Crossman 1973). Pearl dace typically reach sexual maturity at age 1, and females grow faster and liver longer than males; more females reach ages 3 and 4 than do males (Fava and Tsai 1974; Stasiak 1978; Cunningham 1995).
The only description of spawning behavior of pearl dace is from two small, clear-water streams in western lower Michigan (Langlois 1929). Pearl dace there spawned during early June in water temperatures of 17.2-18.3 C, in current velocities ranging from calm to strong currents, over sand or gravel substrates. The male's pectoral fins were larger than the females, and the first seven rays were studded with paired rows of small tubercles. Brightly colored males maintained small central territories about 200 mm wide over the stream bottom, and no two territories were closer than 1.8 m.
Pearl dace occur in lakes, cool bog ponds, creeks, and cool springs (Scott and Crossman 1973). Little habitat-related information exists for this species in Montana. At four stream locations where pearl dace were captured in northeastern Montana, average stream widths ranged from 5.4 to 11.8 m, average thalweg depths ranged from 0.4 to 1.4 m, substrates ranged from 53 to 100 % fine substrate (< 0.06 mm), and aquatic macrophytes were sparse to very heavy (<10 % to >75 % coverage; Bramblett, unpublished data). Eleven fish species were associated with pearl dace in seven collections from four sites on four Montana streams (Table 1).
Table 1. Fish species co-occurring with pearl dace in seven collections (Bramblett, unpublished data) from four streams in Montana during 2000-2003.
|COMMON NAME||SCIENTIFIC NAME||# SAMPLES WHERE PRESENT|
|Fathead Minnow||(Pimephales promelas)||7|
|Brook Stickleback||(Culaea inconstans)||7|
|White Sucker||(Catostomus commersoni)||6|
|Brassy Minnow||(Hybognathus hankinsoni)||6|
|Iowa Darter||(Etheostoma exile)||3|
|Northern Redbelly Dace||(Phoxinus eos)||3|
|N. Redbelly Dace x Finescale Dace||(P. eos x P. neogaeus)||3|
|Lake Chub||(Couesius plumbeus)||2|
|Longnose Dace||(Rhinichthys cataractae)||1|
|Black Bullhead||(Ameiurus melas)||1|
|Northern Pike||(Esox lucius)||1|
Pearl dace appear to prefer cool to cold water temperatures. In Canada, pearl dace were more often found to co-occur with brook trout (Salvelinus fontinalis) and mottled sculpin (Cottus bairdi) at water temperatures of 15.8 to 16.6 C, than with smallmouth bass (Micropterus dolomieu) and rock bass (Ambloplites rupestris) at 20.8 to 21.5 C (Becker 1983). The August upper lethal temperature was 31.1 C (Becker 1983). In the southernmost part of their range in Maryland and Virginia, pearl dace were found in streams that were cool in summer and warm in winter, with substantial spring-water input (Tsai and Fava 1982). In Montana, pearl dace were captured in streams with daytime water temperatures in July-September ranging from 9.6 to 23.1 C (Bramblett, unpublished data).
Pearl dace eat zooplankton, terrestrial and aquatic insects, and small fish on occasion. Plant materials, including detritus, algae, and diatoms have been observed in the stomachs of pearl dace (Scott and Crossman 1973; Tallman and Gee 1982; Becker 1983).
Although fairly tolerant to acidification, pearl dace may be poor competitors. In a lake acidification experiment, the pearl dace population recovered rapidly from low pH levels, but later declined as fathead minnow abundance increased (Mills et al. 2000). Pearl dace were usually much less abundant than fathead minnow in lakes where these two species co-occurred. Pearl dace were classified as moderately intolerant to phosphorus loading and turbidity in lakes of the northeastern US, but intolerant to watershed disturbance, shoreline disturbance, and introduced species (Whittier and Hughes 1998).
Pearl dace undoubtedly serve as forage for piscivorous fishes and other animals across their range, but their role as forage is undocumented (Becker 1983). However, pearl dace appear to be vulnerable to population-level effects from predation. Ontario lakes with smallmouth bass contained no pearl dace, whereas similar lakes without smallmouth bass contained pearl dace (MacRae and Jackson 2001). Pearl dace rarely co-occurred with predators, particularly introduced species, in lakes in the northeastern US (Whittier et al. 1997; Whittier and Hughes 1998; Findlay et al. 2000).
Status and Threats
The status of pearl dace is classified at the State level by Montana Department of Fish, Wildlife and Parks and the Montana Natural Heritage Program as S2: Imperiled because of rarity or because of other factors(s) demonstrably making it very vulnerable to extinction throughout its range (Carlson 2003). Its limited distribution in Montana renders it vulnerable to extirpation from the state. Moreover, limited evidence exists that it may be declining in certain areas. For example, no pearl dace were collected during 1999-2003 in four streams known to be occupied based on specimens in the Montana State University Vertebrate Collections from the 1940s and 1950s. All four of these streams were located in northeastern Montana and also contained an introduced piscivore, the northern pike (Esox lucius). In contrast, just one of seven collections from four streams where pearl dace were captured in 1999-2003 had northern pike present (Bramblett, unpublished data). These observations conform with the aforementioned vulnerabilities to predation and competition (Whittier et al 1997; Findlay et al. 2000; MacRae and Jackson 2001).
Pearl dace are too small to be sought by anglers for sport or food, but are undoubtedly collected on occasion by anglers seeking bait minnows. The extent of bait harvest is unknown. Also unknown is the extent of "bait-bucket" introductions of potential predators or competitors of fishes into waters containing pearl dace.
Pearl dace have declined or are vulnerable to extirpation in other parts of their geographic range. In the Red River of the North basin, pearl dace have undergone moderate reductions in range (Koel and Peterka 1998). Many historical collection sites in the Nebraska sandhills no longer contain pearl dace; the declines are attributed to anthropogenic disturbances in headwater streams and wet meadows (Cunningham 1995). Pearl dace are ranked as S3 (vulnerable to extinction) or worse in 15 of the 30 (50 %) states or provinces in its range (NatureServe 2004).
Given their preference for cool to cold water temperatures, their geographic range across northern Montana, anthropogenic stressors that increase water temperatures may increase the likelihood of extirpation of pearl dace from the Montana. For example, if water temperatures in prairie streams currently approach the upper lethal limit of 31.1 C (Becker 1983), small increases in temperature may lead to local extirpations.
The S2 ranking is appropriate for the pearl dace in Montana given its somewhat limited geographic range in the State, its possible decline in some areas, the low number at which it typically occurs, its vulnerability in other states and provinces, and the potential threats from introduced piscivores and anthropogenic disturbances.
The pearl dace is classified as a non-game wildlife species by Montana Fish, Wildlife and Parks. The primary management task is to monitor the status of the species in Montana. Fish surveys supported by voucher specimens should be conducted in streams across the range of the species to better determine its geographic range. Streams with historical collection records could be sampled to determine if the species is still extant there. Further introductions or expansions of the ranges of introduced piscivores, especially northern pike, should be discouraged.
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