Nooksack dace (Rhinichthys cataractae) COSEWIC assessment and status report: chapter 5
Update
COSEWIC Status Report
on the
Nooksack dace
Rhinichthys cataractae ssp.
in Canada
2007
Species Information
The Nooksack dace (Figure 1) is believed to be a subspecies of the longnose dace (Rhinichthys cataractae), although it may constitute a separate species (J.D. McPhail, pers. comm., 2006). It is a member of the ‘Chehalis fauna’, a group of fishes that diverged from the Columbia fauna during the Pleistocene Epoch through geographic isolation in a glacial refuge in present-day Washington State (McPhail, 1967; McPhail, 1997). It is one of several closely related daces of uncertain taxonomic relationship found in the Pacific Northwest. The most widespread form is found in the Columbia and Fraser River systems. Divergent forms include the Umpqua dace (R. evermanni Snyder) of the Umpqua drainage and the undescribed Millicoma dace of the Coos drainage, both in Oregon, in addition to the Nooksack dace (Bisson and Reimers 1977, McPhail, 1967). None of the forms are known to occur in sympatry.
Figure 1: A Male Nooksack Dace (May 20, 1999, Pepin Brook, UTM 10U 539071 5428930)
R. cataractae morphology reflects a preference for fast flowing riverine habitats. The body is streamlined and nearly round in cross-section. The head is triangular with a bulbous snout overhanging the mouth and a slight hump at the nape. The eyes are small relative to head length. Pectoral fins are large, paddle-shaped, and used as hydrofoils in swift currents, pelvic fins are small and the caudal fin is shallowly forked with rounded lobes. Body colouration is grey-green above a dull, brassy lateral stripe and dirty white below it. The swim bladder is small and poorly developed (Scott and Crossman, 1973). There are distinct pale marks on the back at the anterior and posterior base of the dorsal fin and a distinct black stripe on the head in front of the eyes, which in juveniles continues down the flanks to the tail. Males have slightly longer pectoral fins but the sexes are not otherwise distinguishable (McPhail, 1997). Relative to other R. cataractae populations in the Fraser and Columbia river basins, the Nooksack dace has a more slender caudal peduncle and larger scales that are fewer in number (50-59 vs 60-73 on the lateral line; McPhail,1967; Bisson and Reimers, 1977). The largest recorded Canadian specimen measured 114 mm (snout to tail fork) and weighed 16.1 g (Pearson, 2004). Mean values for key morphological features (lateral line scale and dorsal fin ray counts) in the recently identified Coquitlam River and Alouette River populations are intermediate between the two forms and show higher variation. At present, however, sample size is insufficient to determine if distributions are unimodal or bimodal (J.D. McPhail, pers. comm. 2006).
Nooksack dace from the Nooksack drainage are distinguishable from the Columbia-Fraser R. cataractae by allelic frequency for one allozyme (Pgi-1 slow allele; McPhail and Lindsey, 1986) and genetic distance calculated from mtDNA variation. A detailed mtDNA sequencing study of the R. cataractae species group is in progress (J.D. McPhail pers. comm., 2006). The group includes the Umpqua dace, R. evermanni, (n=15; endemic to Oregon’s Umpqua River) and several distinct forms of putative R cataractae, including the Nooksack dace (n=20), the Millicoma dace, (n=5; endemic to Oregon’s Coos River; Bisson and Reimers, 1977), and the more widespread Columbia-Fraser form (n=5 x 6 sites in BC and WA). Early results show that the cytochrome b gene (1141 bp) and control region (892 bp) in Nooksack dace mtDNA differ from Columbia-Fraser R. cataractae at approximately 2% of sites, suggesting that divergence between Nooksack and Columbia River-Fraser River longnose dace occurred about 2 million years ago, prior to the Pleistocene Epoch. This exceeds the level of divergence between the longnose sucker, Catostomus catostomus and the Salish sucker, Catostomus sp. (1.15%; McPhail and Taylor, 1999, 2000) and is similar to that between speckled dace, R. osculus, the leopard dace, R. falcatus, and the Umatilla dace R. Umatilla (1.3-2.8%; McPhail and Taylor, unpub. data). There are small (0.3-0.4%) differences between populations of Nooksack dace from river systems on the Olympic Peninsula, the Puget Sound lowlands, and the Fraser Valley. The Willipa River, the only occupied drainage to the south of the Chehalis River, contains a population slightly more divergent from other Nooksack dace populations, differing at approximately 1% of sites (McPhail and Taylor, unpub. data.).
The Canadian population structure has yet to be fully clarified. Until 2004 Nooksack dace were believed restricted to tributaries of the Nooksack River, with northwestern longnose dace occupying all Fraser River tributaries. Recent genetic and morphometric work has revealed, however, that all R. cataractae of the Brunette River (a Fraser River tributary) are Nooksack dace (McPhail and Taylor, unpubl. data). Preliminary sampling also indicates that the Nooksack dace mitochondrial genome is found in a high frequency of individuals from two neighbouring watersheds, the Coquitlam River (47%, n=30) and the Alouette River (28%, n = 32) but is absent in the Norrish Creek population (n=30), which is somewhat further east (Fig. 3). It is not clear if this is evidence of historical hybridization or of sympatry with occasional hybridization. Clarifying the situation is the critical step in determining the taxonomic status of Nooksack dace. If populations of northwestern longnose and Nooksack dace maintain themselves as separate, sympatric entities in the Coquitlam and Alouette Rivers, full species status is probably warranted. Conversely, if these populations consist of introgressed hybrids a subspecies designation would be appropriate (McPhail pers. comm., 2006). A conservative nuclear marker for Nooksack dace has yet to be developed, but is needed.
Nooksack dace populations occupy two independent drainages within British Columbia, the lower Fraser River and the Nooksack River system, which enters the Strait of Georgia from Washington State. Dispersal between the drainages is extremely unlikely, although brief connections between tributary headwaters do occur during flood events in some years. However, there is no data to support distinguishing the populations as separate designatable units in accordance with the COSEWIC guidelines for recognizing designatable units below the species level (COSEWIC 2006).
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