Nooksack dace (Rhinichthys cataractae) COSEWIC assessment and status report: chapter 2

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Species Information

The Nooksack dace is a streamlined minnow, nearly round in cross-section, with a triangular head and a bulbous snout that overhangs the mouth. The pectoral fins are large, paddle-shaped, and used as hydrofoils in swift currents. Body colouration is grey-green above a dull, brassy lateral stripe and dirty white below it. Distinct pale marks occur on the back at each end of the dorsal fin. A black stripe is limited to the head in front of the eyes in adults, but continues down the flanks to the tail in juveniles. Males have slightly longer pectoral fins but the sexes are not otherwise distinguishable. The Nooksack dace is genetically distinct from other forms of R. cataractae in the Fraser and Columbia basin and physically separable from them in having fewer, larger scales. The largest recorded Canadian specimen measured 114 mm from snout to tail fork and weighed 16.1 g. The Nooksack dace is believed to be a subspecies of the longnose dace (Rhinichthys cataractae), but may be a separate species.

Distribution

Nooksack dace are restricted to rivers and streams in northwestern Washington State and British Columbia’s Fraser Valley. Populations have been confirmed in four Canadian streams: Bertrand Creek, Pepin Creek, Fishtrap Creek and the Brunette River. Some, but not all, of the R. cataractae in two other watersheds, the Coquitlam and Alouette Rivers, carry Nooksack dace mtDNA markers, but it is uncertain if this indicates past hybridization between the Nooksack and Columbia-Fraser forms of R. cataractae or their present coexistence in these watersheds.

Habitat

Nooksack dace are habitat specialists dependent on stream riffles (shallow, moderately turbulent, flowing water). They rarely occur in reaches with less than 10 percent riffle by length or in reaches where long stretches of deep pool habitat separate riffles. Adult densities are highest in depths of 10 to 20 cm, at water velocities between 20 and 35 cm/s, over loose gravel, cobble or boulder substrates. Juveniles occupy shallow (10-20 cm), calm, pools with fine substrates at the downstream end of riffles during their first summer. In Canada Nooksack dace are associated with small to moderate sized channels (1-10 m in width), but this probably reflects available habitat in occupied watersheds rather than a preference.

Biology

Nooksack dace spawn at night between April and early July and may spawn more than once in a season. The young emerge from the gravel in mid-summer and inhabit shallow, marginal pools with sand or mud substrates where they feed on zooplankton. After approximately 4 months (about 45 mm body length) they move into riffle habitat. Lifespan is four to six years and sexual maturity is attained at the end of the second summer. Their life history characteristics (small body size, short generation time) should permit rapid population growth leading to early recovery from small–scale disturbances, and rapid expansion into nearby restored or created habitats. Most adults appear to range less than 50 m annually. Nooksack dace are largely inactive at temperatures below 11^o C, but forage normally at temperatures in excess of 20^o C. Adults feed primarily on aquatic insects and are likely eaten by coastal cutthroat trout (Oncorhynchus clarkii clarkii), rainbow trout (O. mykiss), and prickly sculpin (Cottus asper). Juveniles are probably taken by these species as well as by juvenile coho salmon (O. kisutch).

Population Sizes and Trends

Insufficient data exist to reliably estimate total population size, but available evidence suggests that it is less than 10 000. Density appears to have remained relatively high since the 1960s in lower Bertrand Creek, but to have declined in Pepin Creek and Fishtrap Creek. Continuing decline is also suggested by the apparent extirpation of the species from headwater tributaries of Fishtrap Creek and Bertrand Creek since the 1960s. The recently discovered Brunette River population has not been assessed.

Limiting Factors and Threats

Canadian populations of Nooksack dace are limited by the availability of their primary habitat, high quality riffle habitat, and most of the identified population threats relate to its loss or degradation. Imminent threats likely to cause harm or population impacts include: lack of water in late summer (causing riffle loss through drying), physical destruction of riffle habitat (dredging, channelization, etc.), sediment accumulation in riffles, and riffle loss to beaver ponding. Imminent threats of uncertain impact include toxicity from urban storm sewer effluent, low dissolved oxygen in late summer, predation by introduced species, and habitat fragmentation by physical barriers or patches of degraded/destroyed habitat. The relative magnitude of threats varies among watersheds.

Special Significance of the Species

The Nooksack dace is a member of the ‘Chehalis fauna’, a group of fishes that evolved through geographic isolation during the Pleistocene glaciations in an ice-free refuge in present-day Washington State. It is of considerable scientific interest in the study of evolutionary biology and biogeography.

Existing Protection

The Nooksack dace was assigned Endangered status by COSEWIC in 1997 and the species was subsequently listed under Schedule 1 of the Species at Risk Act (SARA). As a federally listed species it is protected from harm or capture in all Canadian waters. Its habitat is also provided some protection by the federal Fisheries Act. The Recovery Team has proposed 21.3 km of the Nooksack River tributaries as critical habitat in a draft recovery strategy under SARA, but has not defined the species ‘residence’ under the Act.