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COSEWIC assessment and update status report on the Chiselmouth in Canada

Summary of Status Report

The primary limiting factors for chiselmouth in Canada appear to be i) the availability of rivers or lakes with suitably warm temperatures to permit growth and development of different life stages ii) aquatic systems with sufficient productivity (nutrients) to support sufficient algal growth for adult fish to feed on iii) the availability of aquatic systems (large rivers or lakes) with hard substrates (e.g. boulders or cobbles) for periphyton to grow on, and iv) the availability of adequate clean spawning substrate, slow-water juvenile rearing habitat, and deep-water overwintering habitat. The largest potential threat to chiselmouth appears to be the cumulative impacts of habitat change within a watershed (e.g. from agriculture, forestry, livestock grazing, etc.). Although these threats are present they do not yet appear to be pervasive for most populations (based on very limited data), but tolerance of chiselmouth to habitat change is poorly understood.

There have been no apparent changes in chiselmouth distribution at the provincial scale since the original collections of chiselmouth in B.C. This indicates that the range of chiselmouth in B.C. has been largely stable, or at least presents no evidence to anticipate that there has been a range contraction. However, it is impossible to evaluate trends because there is no historic or current information on population size for any populations in B.C.. Although northern populations appear to occur at lower densities than populations in warmer rivers and may therefore be at higher risk from stochastic events, there is no reason to anticipate that local populations have been declining, although habitat for some populations has been subject to degradation from local impacts such as livestock grazing or agriculture, or extensive watershed development (e.g. Nicola and Okanagan basins).

Despite the fact that there does not appear to be any discernable trend in chiselmouth populations, or any specific reason to anticipate one, in the absence of reliable information on population sizes or trends a conservative assessment of chiselmouth status must conclude that the species remains data deficient. Any inferences about population trends are speculative in the complete absence of baseline data, and some level of monitoring of representative populations is essential to assess present and future status. Provincial or federal fisheries agencies responsible for stewarding fish biodiversity should establish index sites and a long-term monitoring program to assess and monitor the status of chiselmouth and other species at risk or of concern in British Columbia.