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COSEWIC assessment and update status report on the Chiselmouth in Canada

Update
COSEWIC Status Report
on the
Chiselmouth
Acrocheilus alutaceus
in Canada
2003

Species Information

Name and Classification

Acrocheilus alutaceus (Agassiz and Pickering) is the only living member of it’s genus. The cyprinid fauna west of the Rockies is relatively depauperate in Canada, and chiselmouth is a member of a small group of closely related four minnow species that are widespread throughout the Fraser and Columbia River basins, the other three species being northern pikeminnow (Ptychocheilus oregonensis), peamouth chub (Mylocheilus caurinus), and redside shiner (Richardsonius balteatus). Although these species are ecologically well differentiated – chiselmouth is an algivore, pikeminnow is a piscivore, and peamouth and redside shiners are planktivores – chiselmouth is known to hybridize with northern pikeminnow and peamouth chub (Patten 1960, Stewart 1966).


Description

Chiselmouth (Fig. 1) are a relatively large cyprinid, reaching maximum sizes of up to 30 cm and a maximum recorded age of 6 years in Canada (Moodie 1966) and up to 22 years in Oregon (Lassuy 1990). Chiselmouth are uniquely adapted to herbivory, and have a chisel-like lower jaw that they use to scrape algae off of hard substrata (boulders, cobble, submerged wood). In addition to their distinctive lower jaw, chiselmouth have an unusually narrow caudal peduncle and large deeply forked caudal fin, suggesting adaptation to higher water velocities which is consistent with the habitat use observed for adult riverine fishes.

Although chiselmouth are readily distinguished from adults of other cyprinids, juveniles are difficult to distinguish from young redside shiners, peamouth, and pikeminnow with which they typically school. Scott and Crossman (1973) provides a more detailed description of chiselmouth morphology and a key for identifying adults; McPhail and Carveth (1993) and Rosenfeld et al. (2001) provide a key for separating juvenile chiselmouth, redside shiners, peamouth, and northern pikeminnow.


Figure 1: Chiselmouth Acrocheilus alutaceus

Figure 1: Chiselmouth, Acrocheilus alutaceus.

Male, 663 mm, British Columbia, Missizoula Lake, July 24-25, 1959; B.C. 60-221. [Drawing by Anker Odum, from Scott and Crossman (1973) by permission of the authors.]