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

Population Sizes and Trends

Search Effort

Search effort for R. cataractae populations has been moderate within the Canadian portion of the range. The earliest reliable records of R. cataractae in the Fraser Valley date from the 1950s (see Table 2). McPhail (pers. comm., 2006) reports that intensive sampling (using rotenone) in streams across the Fraser Valley in the 1960s did not reveal any populations other than those listed in Table 2 (the samples are not catalogued). Inglis et al. (1994) electrofished for Nooksack dace at 158 sites in 34 Fraser Valley streams (1 pass of 50-100 m per site) during the summer of 1992. They recorded no R. cataractae outside the Nooksack River tributaries, but sampled no streams on the north side of the Fraser River.

Estimates of population size have been hampered by a lack of sampling methods that are both non-destructive and effective. Pearson (2004) used CPUE in minnow traps to estimate relative abundance in the Nooksack River tributaries in 1999-2000 (minimum 10 sets in each of 74 reaches). He also attempted to quantify the size of the Bertrand Creek population using mark-recapture in two reaches in 2000-2001 (10-13 samples per reach, 32 traps per sample) but recapture rates were too low to permit an estimate.


Insufficient data exist to reliably calculate Nooksack dace population sizes, but a likely upper limit can be estimated. High quality habitat in Bertrand Creek supported an average of 1.9 Nooksack dace/m2 (n=20, SE=0.35, Inglis et al., 1994) and 1.4 /m2 (n=5, S.E.+0.24, McPhail, 1997) in the two available estimates. The riffle area in Bertrand Creek (Fig. 4) measured 3000 m2 in 1999 (Pearson, 2004). If all riffle areas were populated at 1.9/m2 the Bertrand population would be approximately 5700. This should be viewed as an upper limit for the breeding populations because much of the habitat is of lesser quality than where the density estimates were made and the samples would have included some yearling juveniles. Extending the calculations to Pepin Creek and Fishtrap Creek using riffle area yields a total population of 14 000 at these three locations (watersheds). However, actual densities in Pepin Creek and Fishtrap Creek are much lower than in Bertrand Creek according to a CPUE-based relative abundance model (Pearson, 2004). Applying the relative abundance ratios to the Pepin and Fishtrap Creel figures gives an adjusted total population estimate of approximately 6800 (Table 1). No data exist on current or historical abundances in the Brunette River, but extending the Bertrand Creek calculation would yield an unadjusted population of 38 295.

Fluctuations and Trends

No quantitative data exist on fluctuations or trends in abundance for any of the Canadian populations. Density in Bertrand Creek south of 16th Avenue appears to have remained high since the 1960s. McPhail (1997) reports ‘healthy’ populations in Pepin Creek and Fishtrap Creek in a 1993 survey, but density was very low in most reaches of these creeks by 1999-2000 (Pearson 2004). This corresponds with known losses of riffle habitat over the past 10 years in these creeks (see Habitat Trends). Continuing decline is also suggested by the apparent extirpation of Nooksack dace from headwater tributaries of Fishtrap and Bertrand Creek since the 1960s (McPhail 1997).

Rescue Effect

The three Nooksack tributary populations all straddle the United States border and individuals undoubtedly move across it, although downstream movement is more likely. A rescue effect benefiting Canadian populations is highly unlikely due to the very limited amount of suitable habitat in the Washington portion of these creeks (McPhail 1997), and its location downstream of the Canadian habitat.  This is a sedentary species; individuals hold very small home ranges (< 1 km), and are not likely to either traverse over 10 km of unsuitable habitat, or cross the divide between the Fraser and Nooksack watersheds (see Dispersal/Migration). A catastrophic event (e.g. chemical spill) that caused extirpation in the Canadian portion of a creek would also likely eliminate the corresponding American population.