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COSEWIC assessment and update status report on the Chiselmouth in Canada

Limiting Factors and Threats

Temperature appears to be the major factors limiting chiselmouth distribution in British Columbia. Chiselmouth do not appear to occur in streams with maximum temperatures of less than 20 °C or 2100 annual degree days (Rosenfeld et al. 2001), suggesting temperature-related constraints on juvenile or adult growth or egg development. Riverine populations also occur primarily in larger rivers rather than small streams, suggesting that the availability of the larger substrate and flows associated with larger streams also limit chiselmouth distribution. This does not appear to be an artifact of a positive correlation between stream size and temperature, since limited data suggests that small warm streams do not harbor chiselmouth. Smaller streams may not provide the appropriate combination of larger substrate with periphytic growth to support adults, and slow weedy marginal habitat for juvenile rearing, whereas both of these habitats may be present in larger intermediate-gradient rivers with marginal and off-channel habitat. Lack of suitable river habitat in Canada (relative to the U.S.) is probably due to both colder average temperatures and steeper river gradients that preclude the development of suitable off-channel or marginal habitat.

Lake populations probably also require an abundance of hard substrate in the littoral zone to support periphyton for adult grazing. Presence of suitable small streams for spawning may also limit abundance of lake populations (Moodie 1966), since chiselmouth are probably incapable of spawning successfully in lakes. Limitation of riverine populations by suitable spawning habitat is also possible, although this is difficult to evaluate because of lack of information on spawning habitats used by chiselmouth in larger rivers.

System productivity also likely limits both distribution and abundance of chiselmouth. Adults likely require substantial periphytic growth on rocks, which is absent from low productivity systems characteristic of coastal streams as well as cold-water glacier fed interior streams. The same is also likely true for lakes, where temperature and productivity also likely co-limit the presence and abundance of chiselmouth.

Although chiselmouth do not appear to be directly threatened by any specific anthropogenic or environmental impacts, like most lotic fishes they are likely to be sensitive to sedimentation that covers periphyton or fills interstices of spawning substrate. They are also probably sensitive to loss of marginal, backwater, or off-channel rearing habitat that is likely critical for juvenile survival. Lake populations that spawn in small inlet streams may be especially vulnerable to sedimentation of spawning habitats, since small spawning streams may be adversely affected by habitat degradation associated with forestry, livestock grazing, or urbanization.

There appear to be no specific threats to habitat used by chiselmouth, beyond the cumulative effects of habitat degradation in a watershed associated with logging, agriculture, or livestock grazing. In some instances these effects can be substantial (e.g. Vadas 1998), but it is unclear how they affect chiselmouth, since their sensitivity to habitat degradation is unclear, as is their tolerance to impaired water quality. However, persistence of chiselmouth in the U.S. portion of their range suggests that the species is not excessively sensitive to perturbation, and is likely less sensitive to changes in water quality than salmonids, although this is largely speculative and not based on hard data.

Since chiselmouth distribution and density appear to be temperature-limited, climate warming may have indirect positive effects on chiselmouth distribution, although this remains speculative. One clearly negative impact of global warming is that warmer water temperatures will also permit colonization and survival of a broader range of exotic species in B.C. freshwaters. Even in the absence of any warming trend, it is likely that exotic species will begin to have negative impacts on native fauna in the near future as development progresses and pathways of introduction (e.g. international trade) expand. For example, an exotic tapeworm has been found in Oregon infecting chiselmouth as well as other species (Bend Bulletin, Sept. 30 2001). That being said, all native fauna are likely susceptible to negative impacts of exotics, and specific impacts are difficult to predict. 

At present the specific identifiable threats to chiselmouth in British Columbia are those associated with habitat degradation, either from local impacts of development, range, and agriculture, or from more diffuse effects at a landscape scale (e.g. in the Okanagan valley; Scudder and Smith 1998). However, the degree of impact of these effects remains speculative in the absence of reliable data on chiselmouth population trends and corresponding changes in habitat for any Canadian populations.