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COSEWIC assessment and update status report on the Chiselmouth in Canada

Population Sizes and Trends

Chiselmouth are extremely abundant in many streams and rivers in their U.S. distribution. Chiselmouth were the most abundant species caught in the Yakima river (Patten et al. 1997) and are equally abundant in Oregon (Lassuy 1990). Densities of juveniles can be extremely high, and population size in larger rivers are unknown but probably on the order of tens of thousands of adults.

Chiselmouth occur at much lower densities and population sizes in British Columbia than in the southern (U.S.) part of their range. This is presumably because climatic conditions are harsher towards the northern end of the species distribution. In the more northern populations in B.C. chiselmouth are often one of the rarest species in the fish community, typically accounting for 2% or less of the fish caught within a drainage. The individuals recently collected in the Salmon(Muskeg) River north of Prince George represented only 3-4 individuals (including juveniles) of well over one thousand fish collected during fisheries inventory throughout the entire drainage basin. Low densities in the cooler northern rivers contrast with populations in the Nicola River, where chiselmouth are reported to be a dominant part of the fish community (R. L. Vadas, personal communication 1998). Populations may also be somewhat larger in the Kettle and Okanagan rivers of B.C. as well.

Population structure of chiselmouth is unclear. It remains unclear whether the Fraser River is a barrier to movement of adults between tributaries; although Haas (1998) suggests that populations are largely isolated, adults have been collected from the mainstem Fraser (Don McPhail, UBC Zoology, personal communication 1999), suggesting that some exchange of adults between tributary populations is possible. Similarly, it is unclear whether lake populations (e.g. in the Euchiniko drainage) are really distinct from river populations in the same drainage. Regardless, chiselmouth occur in at least eight large drainage basins that can probably be considered to represent separate populations or population complexes by conservative criteria – the Blackwater/Nazko/Euchiniko, the Salmon/Muskeg, the Similkameen, Okanagan, Kettle, Upper Chilcotin, Nicola, and Shuswap. However, distribution and population size within some of these drainages may in some instances be small, and fish may occur at very low densities.

There is no real data on population trends of chiselmouth in British Columbia. Chiselmouth appear to occur in the same drainage basins now as they did in previous surveys of several decades ago, suggesting that there has been no obvious range contraction, although this gives no insight into populations trends. Don McPhail (pers. comm.) indicates that a number of chiselmouth lake populations have been extirpated by provincial fisheries to reduce competition with stocked rainbow trout, and suggests that re-introduction might be appropriate1. To my knowledge no trends are documented for chiselmouth populations in Washington and Oregon where the species is more abundant and widespread.

As with long-term trends in species distribution, long-term trends in population size may be influenced strongly by climate change. Global warming may have positive effects on chiselmouth development and growth rate at various life stages, leading to higher densities and larger populations. However, this assumes no trends in habitat quality (i.e. no habitat degradation), and climate change effects may also be further complicated by changes in rainfall patterns and associated changes in rivers flows as well as complex interactions with predators, competitors, and disease that make net outcomes difficult to predict.

Given the absence of any reliable (or even unreliable) estimates of population size for Canadian populations, it is extremely difficult to speculate on the number of mature individuals in Canada. A population such as that in the Blackwater River (including the Nazko and Euchiniko Rivers and associated lakes) might contain at least 2000–5000 individuals. If there are eight drainage basins that can probably be considered to represent separate populations or population complexes by conservative criteria – the Blackwater/Nazko/Euchiniko, the Salmon/Muskeg, the Similkameen, Okanagan, Kettle, Upper Chilcotin, Nicola, and Shuswap – and at least 4 of these (the Blackwater, Okanagan, Kettle, and Nicola) are likely to have similar populations, then this yields a coarsely quesstimated cumulative population of 8 000–20 000 for the 4 populations, rounded up to 10 000–30 000 if the four likely smaller populations are included. Confidence in such a quesstimate is extremely low.

Although there are no reliable estimates of population sizes or trends, and consequently huge gaps in our knowledge of chiselmouth status and populations size, chiselmouth appear to be maintaining their range in British Columbia (and elsewhere), and there is no obvious reason to expect that populations have been declining in recent years. That being said, regular sampling of chiselmouth abundance at several index sites would be a good idea to create a baseline of information to assess population trends through time.

Chiselmouth remain apparently widespread in Washington and Oregon. The degree of local differentiation/adaptation of the Canadian populations of chiselmouth in the Columbia drainage is unclear, but in any case dams and natural barriers to movement would prevent dispersal into Canada. Fraser drainage populations are more fragmented than the Columbia river populations and have 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. It is also more likely that these populations are divergent from those in the U.S.


1 Following WW II into the 1970s the Province undertook a “lake rehabilitation” program that involved chemical treatment of lakes to remove unwanted species prior to introduction of “more desirable species” Alex Peden (a member of the Freshwater Fishes SSC) has initiated a compendium of the lakes involved, species removed, introductions and current status of populations vis à vis prior to treatment. Of the more than 50 lakes he has been able to identify to-date, two contained chiselmouth prior to treatment and do not now. This information is summarized in Appendix 1. R. Campbell, Co-chair ,COSEWIC Freshwater Fishes SSC.