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COSEWIC Assessment and Update Status Report on the Nooksack dace in Canada

Limiting Factors and Threats

Populations in Canada are probably limited by the availability of high quality riffle, their primary habitat, and most of the identified population threats relate to its loss or degradation (McPhail 1997, Pearson 2004). The following sections are adapted from a recent comprehensive threats assessment for Nooksack dace (Pearson, 2004; Pearson et al. 2006).

Imminent Threats Likely to Cause Harm or Population Impacts

Lack of water in late summer is the most serious threat to the largest known population, that of Bertrand Creek. Riffle area is reduced by 80 to 100% in the best habitats during the most productive time of year (Pearson unpubl.). Aquifer draw-down by local wells is estimated to have reduced baseflow by 24% since 1960 (Golder and Associates, 2004) and significant pumping from the creek occurs for irrigation, but has not been quantified (Pearson pers. obs.). The Brunette River also has inadequate summer flows due to the high proportion of its watershed that is impermeable (41%, Lavkulich et al., 1999).

Physical destruction of habitat has likely been the most serious threat to Nooksack dace in Canada historically. As the ‘high spots’ in a stream, riffle habitats tend to be targeted for removal or alteration in drainage projects, which are common in the urban and agricultural landscapes that dominate these watersheds. Both authorized and illegal alterations occur annually in these watersheds (McPhail 1997; Pearson pers. obs.).

Sediment accumulation in riffles clogs the spaces between and under coarse riffle substrate where Nooksack dace spawn, forage and rest. It also inhibits the flow of oxygenated water through the substrate to eggs. Where reed canary grass (Phalaris arundinacea), an invasive species, occurs in the channel, sod forms. This narrows the channel, greatly reducing riffle area. Significant sediment deposition originating from bank erosion, urban storm drains, or gravel mining operations occurs in all four Nooksack dace streams (Pearson pers. obs.). 

Riffle lost to beaver ponds is an imminent threat to one population, that of Pepin Creek. An estimated 600 m2 of riffle (10% of total available habitat for population) was inundated due to beaver damming between 1999 and 2001 (Pearson 2004). Beaver activity poses no threat to Nooksack dace in other watersheds within the Canadian range. Removal of any dams in Pepin Creek will require full consideration of effects on other species, particularly Salish sucker and salmonid populations, and authorization under the Fisheries Act.

Imminent Threats of Uncertain Impact

Toxicity is a known problem in the Brunette River, where levels of copper, lead, zinc, and manganese commonly exceed federal guidelines for aquatic life in both water and sediments (Hall et al., 1998). Similar conditions presumably prevail in the urbanized headwaters of Fishtrap Creek and Bertrand Creek, but their impacts on Nooksack dace are unknown.

Severe hypoxia is documented from some reaches of all four watersheds. In some highly eutrophic reaches oxygen levels remain low (<2 mg/l) throughout the year, while hypoxic episodes are limited to late summer. Riffles tend to have higher oxygen levels than other stream habitats during episodes of hypoxia (due to water turbulence), but little data exist and critical levels for the species are unknown.

Increased predation by introduced species is a concern, as all occupied watersheds are known to contain introduced predators (see predation above). In some watersheds they have coexisted with Nooksack dace for at least ten years (Pearson, 2000), but their impacts are unknown. All would undoubtedly prey upon Nooksack dace given the opportunity, but there is little habitat overlap. These predators thrive in warm water littoral zones (Scott and Crossman, 1973; Corkran and Thoms, 1996) and are rarely found in riffles. Lack of water could, however, force Nooksack dace out of riffles and into pools where predation risk is likely to be much higher.

Habitat fragmentation likely has some long-term impacts to Nooksack dace populations, but the magnitude is difficult to assess. All occupied streams contain some physical barriers (e.g. perched culverts, beaver dams, agricultural weirs) and are fragmented, at least seasonally by one or more of the threats discussed above. On a larger scale, connections between watersheds during floods were undoubtedly more common prior to the extensive dyking and drainage works of the past century. Most barriers and habitat fragmentation in Nooksack dace watersheds date from 50 to 130 years ago, and surviving populations have shown some resilience (Pearson, 2004). The effects of less movement between populations/metapopulations and reduced ability to colonize new habitat, however, may occur over longer time frames.