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COSEWIC assessment and update status report on the Chiselmouth in Canada

Distribution

Global Range

Chiselmouth are endemic to the west coast of North America, where they are restricted to the Fraser and Columbia river basins and Malheur Lake in Oregon (Scott and Crossman 1973, Wydowski and Whitney 1979). Chiselmouth are abundant and widespread throughout Oregon streams and rivers (Lassuy 1990) as well as Washington State, where Patten et al. (1970) reported chiselmouth to be the most abundant fish in the Yakima River in Washington (a major tributary of the Columbia). Chiselmouth are also present in Idaho and the northeast corner of Nevada (Columbia River tributaries). The combined area of the drainages occupied by chiselmouth in the United States is large, well over 20 000 km2, and the linear distance of stream and river channels occupied is in the order of hundreds or thousands of kilometers.

Canadian populations of chiselmouth are more disjunct and appear to occur at much lower densities than in the central and southern part of the species range, and consequently may be ecologically and genetically distinct from populations in the U.S.


Canadian Range

Chiselmouth are restricted to the warmer interior rivers and lakes in British Columbia. The total area of drainage basins occupied in Canada is well in excess of 5000 km2. However, this statistic is somewhat misleading, because only a fraction of the habitat available in each basin is suitable for chiselmouth. Nevertheless, the linear distance of streams and rivers occupied is approximately four hundred kilometers (based on very rough visual estimates of channel length from topographic maps), and close to a dozen lake populations occur in the same drainages. Extremely rough back-of-the-envelope calculations give an approximate wetted area of 40 km2 for river habitat and 90 km2 of wetted lake habitat area (excluding the very large Okanagan Lake; total lake area increases to 440 km2 when Okanagan Lake is included).

Fraser basin populations of chiselmouth occur in the Blackwater River drainage west of Quesnel (including the tributary Euchiniko and Nazko rivers), in the Nicola river (Vadas 1998) as well as Nicola Lake, Vidette Lake, and Mara Lake, the upper Chilcotin, the Muskeg river (tributary to the Salmon river near Prince George, and the northernmost recorded occurrence of chiselmouth), and the Shuswap River (Fig. 2). Chiselmouth have also been reported from the mainstem Fraser between Quesnel and Prince George (McPhail unpublished). Columbia basin populations occur in the Okanagan river (including Skaha, Osoyoos and Okanagan Lakes), the Kettle river, and Wolfe and Missezula lake in the Similkameen drainage. This information is summarized in Table 1 (from Rosenfeld et al. 2001). The earlier record of chiselmouth from Lake Windermere in the Kootenay-Columbia drainage basin has not been confirmed. Although the specimen is correctly identified as a chiselmouth (Peter Troffe, Royal B.C. Museum, personal communication 1999), it is possible that the fish samples may have been mislabeled or confused during the original sampling survey (Don McPhail, UBC Zoology, personal communication 1999). Chiselmouth were not collected during a 1998 inventory in Lake Windermere specifically targeted at them (Radridge 1998), however the most effective gear for collecting adults (2 – 2 ½ inch gill nets) was not used. 


Figure 2: North American Distribution of Chiselmouth

Figure 2: North American Distribution of chiselmouth.

Adapted from Scott and Crossman (1973).


Figure 3: Canadian Distribution of Chiselmouth

Figure 3: Canadian distribution of chiselmouth. 

From Rosenfeld et al. 2001. White dots represent collection sites on major drainages where chiselmouth are present, and black areas represent drainage basins where chiselmouth are present within the Fraser and Columbia Rivers basins with a probability of greater than 0.5 based on habitat-based logistic regression models (from Rosenfeld et al. 2001). Black dots represent sites where chiselmouth were absent, and grey areas represent major drainages with an estimated probability of chiselmouth occurrence less than 0.5.

Table 1: Sites and Corresponding Drainage Basins where Chiselmouth have been Collected in British Columbia
SiteMaximum temp.
(°C)
Channel width
(m)
UTM
Easting
UTM
Northing
Blackwater R.20.16010U 04705505900450
Euchiniko R.21.09010U 04412005911500
Nazko R.21.52110U 04585005873400
Muskeg R.22.02010U 04903746034860
Shuswap23.48511U 03574265601119
Upper Chilcotin24.43210U 04531845775573
Kettle R.24.519311U 04006425429442
Okanagan R.24.54511U 03136505442000
Nicola R.25.61710U 06584185553912

Rosenfeld et al. 2001.

Although there have been no formal or quantitative estimates of chiselmouth populations in Canada, fish catches during sampling indicate that chiselmouth are likely most abundant in the Nicola and Okanagan river populations. Chiselmouth appear to be less abundant (i.e. fewer were caught for a given sampling effort) in the more northerly and likely colder drainages – particularly the upper Chilcotin and Muskeg rivers.

Recent sampling throughout the known Canadian range of chiselmouth indicates that the range does not appear to have changed relative to earlier occurrences, i.e. there are no apparent trends in chiselmouth area of occupancy at the scale of major drainages. Although the absence of chiselmouth from Lake Windemere could be interpreted as a range contraction, it could equally be an original labeling error; similarly, the new record of chiselmouth in the Muskeg River near Prince George is likely the result of more intensive sampling rather than a range expansion. There have unquestionably been a number of deliberate local extirpations of chiselmouth from lakes which were chemically treated (up until the 1960s) by the provincial fisheries branch to remove “coarsefish” to facilitate stocking of monoculture rainbow trout for recreational angling. While these extirpations likely permanently eliminated isolated populations that could not be naturally recolonized, the total number of lakes that were treated is unknown. Identification of these lakes as potential sites for reintroduction may be useful, particularly if chiselmouth undergo declines in the future (although there is at present no reason to expect this).

In terms of long-term trends, in the absence of long-term habitat degradation (a questionable assumption), future range expansions and contractions will likely be related to climate change. Since the major limitation on chiselmouth distribution appears to be an adequate thermal regime (discussed below under habitat requirements and limiting factors and threats), chiselmouth distribution may be positively affected by global warming.