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COSEWIC assessment and update status report on the Chiselmouth in Canada


Habitat Requirements

Detailed habitat requirements of chiselmouth are poorly understood, but more is known about general habitat associations from which inferences on habitat requirements can be made. The factor that appears to limit chiselmouth distribution is stream temperature (Rosenfeld et al. 2001). Chiselmouth are absent from sites that have maximum temperatures below 20 °C or 2100 annual degree days. This is likely due to insufficient thermal conditions for growth and development of eggs, juveniles, adults, or their gonads at lower temperatures. In addition to an appropriate thermal regime, chiselmouth adults appear to require an abundance of deeper (greater than 1 m) faster flowing (water column velocities in the range of 40-80 cm s-1; Rosenfeld et al. 1998) habitat with boulder-cobble substrate that can be colonized by periphyton (as a food source). Thus adults will be restricted to streams with adequate suitable substrate as well as enough nutrients to support algal production. Based on location of captures, juveniles appear to require marginal, backwater, or side-channel habitat with slow current velocities for rearing. Juveniles are almost invariably collected in association with aquatic macrophytes, which are likely both a source of food (aquatic invertebrates – juveniles are insectivorous) and cover from predators. Juvenile chiselmouth are invariably collected in mixed schools with redside shiner, northern pikeminnow, and peamouth chub, likely reflecting similar juvenile habitat requirements and the advantage of schooling to avoid predation by larger fish.

Within a river system, chiselmouth appear to use primarily larger mainstem habitat; Rosenfeld et al. (2001) did not find chiselmouth at sites with a bankful channel width of less than 17 m. This suggests that there may be little direct dependence of riverine chiselmouth on small stream habitat, although clearly habitat change that degrades small stream habitat leading to cumulative impacts on mainstem habitat will negatively affect chiselmouth. However, small streams may be very important for some lake populations, since Moodie (1966) observed spawning in the small inlet stream to Wolfe Lake.

Spawning habitat and substrate for riverine populations is largely unknown, but is likely over coarse gravel-cobble-boulder substrate, as documented for closely related species (e.g. redside shiner, northern pikeminnow, peamouth chub) and lake populations of chiselmouth (Moodie 1966). Presence of juveniles in marginal habitat of larger rivers suggests that spawning takes place in riffles of mainstem river habitat rather than in smaller tributary streams, although this remains largely speculative.

Overwintering habitat of chiselmouth is poorly defined, but observations in the Blackwater drainage (Rosenfeld et al. 1998) suggest that fish appear to shift their distribution out of mainstem habitats in the fall (September-October, water temperatures below 6 °C) towards lakes tributary to rivers or deeper (8 m) backwater habitat on the mainstem. Adult peamouth chub, northern pikeminnow, largescale sucker (Catostomus macrocheilus), longnose sucker (Catostomus catostomus), rainbow trout (Oncorhynchus mykiss), and bull trout (Salvelinus confluentus) were also caught in deeper habitat, suggesting that many species of fish may overwinter in either lakes or larger deeper backwaters connected to mainstem habitat. Overwintering of chiselmouth in deeper water is supported by the observations of Moodie (1966) in Wolfe Lake, who found that by mid-October chiselmouth could no longer be found near the lake margins, and were only captured in deep-water habitat where they were previously absent.

Habitat requirements of chiselmouth in terms of water quality are undocumented. However, chiselmouth remain widespread in Oregon and Washington, indicating that they are likely not excessively sensitive to water quality impairment.

It is difficult to say what the minimum viable population size is for chiselmouth, since there are no reliable estimates of population size in any habitats. However, chiselmouth occur at relatively low densities in northern populations, and can typically be the least abundant fish collected; for instance, chiselmouth constituted approximately 2% of the total fish collected at 32 sites in the Blackwater drainage (Rosenfeld et al. 1998), and were generally less than 10% of the total fish catch at any of the subset of sites where they occurred. That being said, chiselmouth populations in the Blackwater drainage probably have adult populations numbering in the thousands, and there is no reason to expect that these populations are under any particular threat. Nevertheless, small populations are more subject to extinction from stochastic events than larger ones, and northern populations are likely more vulnerable to extinction than southern ones, although the degree of vulnerability is unclear and likely not large.

Although chiselmouth distribution in Canada is discontinuous, no populations stand out as being essential for the survival of other populations. In terms of uniqueness, Fraser basin populations are potentially more unique than Columbia basin populations, since they are likely smaller, more disjunct, and potentially isolated longer from source populations, although velocity barriers and dams downstream of Canadian populations in the Okanagan and Kettle Rivers would likely limit exchange with populations in the United States. Recolonization of extirpated Fraser drainage populations might also be slow because densities are low and it is unclear whether populations are consistently present in the mainstem. Than being said, nothing is know concerning movements of chiselmouth, colonization ability, and what constitutes a barrier to an adult fish, so that classification of Fraser basin populations as more unique or more vulnerable is purely speculative.


There is insufficient data to evaluate trends in habitat for individual populations as there are no reliable past or present estimates of habitat availability or quality for any Canadian population. Trends based on range contraction or expansion indicate that there has been no change in species distribution between recent sampling (last 5 years) and historically recorded distribution (last 40 years), suggesting that there have been no major changes in habitat quality; however, distributional data does not take into account changes in abundance within a range, and habitat quality has clearly degraded in certain river basins (e.g. Nicola river and Okanagan river), as a consequence of

resources extraction activities and general development (e.g. agriculture, livestock grazing, logging, channelization, water extraction).

In terms of long-term trends, assuming that there is no long-term habitat degradation (a questionable assumption for populations experiencing agricultural or forestry impacts), 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), if anything chiselmouth distribution is likely to be positively affected by global warming, although the outcome of global warming is always difficult to predict because of complex effects on flow regimes, prey species, disease, and competitors (Davis et al. 1998). 

Habitat trends in the United States are unknown, but likely similar to those in Canada, or worse (i.e. more habitat degradation associated with ongoing development in watersheds). However, American populations are apparently neither in decline or protected. 


Although chiselmouth occur throughout British Columbia, most of the river frontage where they occur is on crown land. Much of this land is subject to active resource extraction (e.g. logging in the Blackwater, upper Chilcotin, and Salmon rivers, extensive livestock grazing and agriculture in the Nicola and Okanagan rivers). None of this habitat is legally protected (e.g. in protected areas), and it is unclear how much of it will be secure in the future. Although land-use legislation exists to regulate resource extraction on this land base (e.g. B.C. Forest Practices Code), logging, agriculture, and livestock grazing may have cumulative impacts that will degrade habitats used by chiselmouth, but it is unclear whether these effects have resulted in population declines. While local impacts are likely, particularly in streams subjected to intensive agriculture (e.g. Nicola and Okanagan rivers), it seems unlikely that land-use impacts will seriously decrease chiselmouth populations throughout their range. It also seems likely that other species (e.g. salmonids) are more likely to experience negative impacts of habitat change before chiselmouth. However, it should be made clear that this remains speculative, given that details of the tolerance of chiselmouth to habitat change are poorly documented.