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COSEWIC assessment and update status report on the Chiselmouth in Canada

Biology

General

Key life stages are eggs, which are deposited over clean coarse substrate (Moodie 1966), juvenile life stage (requires slow weedy marginal habitat), adult (coarse substrate in deeper mainstem habitat), and overwintering (deeper lake habitat tributary to mainstem habitat). Chiselmouth do not seem more vulnerable at any of these life stages than other cyprinids.


Reproduction

Chiselmouth are relatively large for a cyprinid, and can live for up to 22 years (Lassuy 1990), although the maximum age recorded for a Canadian population is 6 years (Moodie 1966) and average adult age in the Wolfe Lake population studied by Moodie was 4-6 years. Moodie reported that males were sexually mature at age 3, and females usually at age 4. Later age of reproduction is a factor that contributes to the vulnerability of a species, however because chiselmouth are not subject to sport or commercial harvest this is of lesser concern.

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 (Moodie 1966, summarized in Scott and Crossman 1973). Observation of hybrids (based on intermediate morphology; Patten 1960, Stewart 1966) indicates that spawning may take place in similar habitats for these species. 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 is largely speculative. Lake populations will use small tributary streams for spawning (Moodie 1966). Moodie (1966) also observed spawning only at temperatures of 17 °C or higher, and 6 females had average egg counts of 6200. Further details of reproductive behaviour and requirements are lacking, particularly for riverine populations.


Survival

There is little information on population age structure, stability, or survival rate of different life stages and what influences mortality. What information exists is for the lake population studied by Moodie (1966, summarized in Scott and Crossman 1973). Although Moodie (1966) observed higher growth rates of chiselmouth in the Okanagan River, it is unclear whether this difference reflects intrinsic differences in habitat quality or density dependent effects.

Growth potential of the more northern populations is likely low, since temperatures and shorter growing seasons likely limit growth and development of individuals. Periodic recruitment failure for northern populations is possible, since this has been documented for other cool-water fish species near the northern limits of their range (Shuter et al. 1980, Shuter and Post 1990), but there are no data to support this for chiselmouth.


Physiology

Chiselmouth are a coolwater cyprinid, and it is likely that minimal thermal thresholds for development and growth at different life stages (eggs, juvenile, adult growth and gonad development) limit the distribution of the species in B.C., although it is unclear which particular life stage may be limiting. Absence of populations in water bodies with maximum temperatures below 20 °C or 2100 annual degree days (Rosenfeld et al. 2001) supports the supposition of temperature limitation.


Movements/Dispersal

Nothing is know concerning movements of chiselmouth, colonization ability, and what constitutes a barrier to an adult fish. It is unknown to what degree the Fraser River is a partial or complete barrier to adults from populations in tributaries; adults have been collected from the mainstem Fraser (Don McPhail, UBC Zoology, personal communication 1999), suggesting that some exchange of adults between tributarypopulations is possible.

Natural immigration from southern populations in the U.S. (Columbia basin) into Canadian populations is unlikely. There is a natural velocity barrier in the Kettle River at Cascade, B.C., that would prevent natural upstream dispersal, and there is also a barrier on the Okanagan River at Okanagan Falls. Fraser drainage populations are more fragmented with apparently relatively disjunct distributions. Populations in the Blackwater/Nazko/ Euchiniko, Salmon/Muskeg, Upper Chilcotin, Nicola, and Shuswap rivers are relatively isolated from one another and cannot be naturally colonized from outside of Canada.


Nutrition and Interspecific Interactions

System productivity (availability of nutrients influencing algal production) may likely be a limiting factor for chiselmouth populations, insofar as growth rates would be slower in less productive streams. This could be one reason why populations are restricted to the more productive interior streams and lakes of B.C., unlike other cyprinid species (e.g. redside shiner). However, system productivity is highly correlated with water temperature (interior streams are both warmer and more productive), so that it is difficult to separate trophic and temperature effects.

Competition with other species may be a limiting factor in certain populations or habitats, although this is entirely speculative. Competition may be less likely to be important for adults, since their feeding mode and resource consumption is relatively unique in the B.C. fish fauna (although it is likely that sucker – largescale and bridgelip – also feed on periphyton on hard substrata). Juvenile chiselmouth occur in mixed schools with redside shiner, peamouth chub, and northern pikeminnow, and presumably consume similar resources, so that there is likely more scope for competition at the juvenile life stage, but this is speculative as well.


Behaviour/Adaptability

Chiselmouth are relatively highly specialized trophically and morphologically. Specialization of their mouthparts as adults requires the presence of periphyton on hard substrates in sufficient quantities to support adult fish. However, chiselmouth remain omnivorous, and consume invertebrates as well as algae as adults – in fact they can be caught fly fishing or with live bait (e.g. worms). Similarly, chiselmouth are flexible in their habitat requirements insofar as they may occur in either lakes or streams, providing that suitable hard substrata is present for adults to feed on, and spawning and rearing habitat are also present. 

Chiselmouth do not appear to be unusually susceptible to disturbance or stochastic perturbations, although this is based on limited information and is largely speculative. Because northern chiselmouth populations occur at low densities and populations are somewhat disjunct, they may be more subject to stochastic disturbance and have a lower probability of recovery or recolonization than populations or species that occur at higher densities or have more continuous distributions.

Moodie and Lindsey (1972) reported aggressive behaviour of chiselmouth held in aquaria, but it is unclear whether the species is territorial in the wild (seems unlikely but would be interesting to verify).