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COSEWIC assessment and update status report on the Chiselmouth in Canada


Executive Summary

Acrocheilus Alutaceus

Species Information

Acrocheilus alutaceus (Agassiz and Pickering) is the only living member of it’s genus. The common name Chiselmouth (Bouche coupante in french) reflects its square scleritized lower lip.



Chiselmouth occur in the Fraser and Columbia drainage basins in western Canada and the Pacific Northwest United States. Although chiselmouth are abundant throughout their American range, they have a much more limited distribution in British Columbia, and are restricted to the warmer interior of the province where they typically occur at lower densities than populations in Washington and Oregon.



Chiselmouth are primarily a large river fish, although they can occur at high densities in lakes as well. In both habitats chiselmouth will only occur in productive systems with enough algal growth on rocks to support adult fishes. Consequently, their distribution in B.C. is largely restricted to interior lakes and rivers. 

In rivers, adult chislelmouth usually occur in deeper (greater than a meter) faster water over a boulder-cobble substrate that will support algae for adults to feed on. Juvenile fish occur in slower weedy marginal or backwater areas, where they feed on aquatic and terrestrial insects and occur in mixed schools with juvenile northern pikeminnow (Ptychocheilus oregonensis), peamouth chub (Mylocheilus caurinus), and redside shiner (Richardsonius balteatus).



Chiselmouth are unique in western Canada in that they are the only native freshwater fish species west of the Rockies specifically adapted to eating algae. Although chiselmouth will opportunistically feed on invertebrates like most other minnows, their lower lip is straight and hard and adapted to scraping algae off of boulders or other hard surfaces (Moodie and Lindsey 1972), and algae constitutes the primary diet of adults (see Scott and Crossman (1973) and B.C. Fish facts for more general information on biology). Chiselmouth are a relatively large minnow, attaining a maximum size of up to 30 cm. Although they sometimes hybridize with northern pikeminnow and peamouth chub, chiselmouth are readily distinguished from both species by their distinctive lower lip.

Reproduction is in the spring over coarse substrate. Spawning habitat in rivers in Canada is undocumented, but is possibly coarser substrate in mainstem habitat. However, spawning in smaller tributaries is also a possibility, since Moodie (1966) observed spawning of the Wolfe Lake population in a small inlet stream.


Population Sizes and Trends

Populations of chiselmouth in British Columbia tend to be disjunct (non-continuous) and often occur at low densities. Population sizes are poorly documented, but range from being likely very large (Nicola river basin) to very small (Salmon river basin near Prince George). Population trends are unknown, but there is no reason to believe that they are declining, although most watersheds where chiselmouth are present have experienced some degree of habitat degradation associated with forestry or agriculture. There appear to be no recent major extirpations relative to past collections (although the B.C. Fisheries Branch has historically chemically treated some lakes to remove “coarsefish” for stocking with rainbow trout, resulting in several local extirpations from small lakes; Don McPhail pers. comm.), but there have been no reliable estimates of population size in the past, and none in the present beyond point resampling to verify current distribution (presence/absence).


Limiting Factors and Threats

The large-scale distribution of chiselmouth in Canada is likely limited by temperature, i.e. colder rivers in B.C. provide inadequate thermal conditions for growth and development of eggs, juveniles and adults. Where temperatures are adequate, populations are probably limited by the availability of adequate deeper mainstem habitat with boulder/cobble substrate to support algae, and slower vegetated nearshore habitat for juvenile rearing. Threats primarily relate to habitat degradation from sedimentation of clean substrate used for adult foraging, loss of marginal and backwater habitats for juvenile rearing, and siltation of spawning habitat.


Special Significance of the Species

Chiselmouth are unique among Canadian fishes in both their body morphology and feeding mode. They are ecologically unique in that they are the sole primarily herbivorous freshwater fish native to western Canada. Canadian populations also encompass the northern range of the species global distribution.


Existing Protection or Other Status Designations

Although chiselmouth are blue-listed in B.C. (identified as a species of concern), they have no protection. Chiselmouth are unlisted in the U.S.