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COSEWIC assessment and status report on the coho salmon (Interior Fraser population) in Canada

COSEWIC Status Report
on the
Coho salmon
Oncorhynchus kisutch
Interior Fraser Population
in Canada

Species Information

Name and Classification

Coho salmon (Oncorhynchus kisutch Walbaum) (Fig. 1) is one of seven species of the genus Oncorhynchus native to North America. Other species are sockeye (O. nerka), chum (O. keta), pink (O. gorbuscha), and chinook (O. tshawytscha) salmon and steelhead (O. mykiss) and cutthroat (O. clarki) trout.

While the common name most frequently used for this species is coho, they are sometimes referred to as silver salmon, sea trout, hooknose, or bluebacks, the latter term usually referring to small coho caught early in their final marine year. The French common name is saumon coho.


Coho and other Pacific salmon can be distinguished from trout and char by the presence of 12 or more rays in the anal fin. The anal fin of juvenile coho is sickle-shaped and its leading edge is longer than its base. Adult coho can be differentiated from other salmon by the presence of white gums at the base of the teeth in the lower jaw. As well, black spots, when present on the caudal fin, occur usually on the upper lobe only (Fig. 1a). Sexual dimorphism develops as coho salmon become sexually mature. Male coho become darker and often bright red, their upper jaw develops an elongated hooked snout, and their teeth become enlarged. Females are usually less brightly coloured and their upper jaw development is less extreme than males (Fig. 1b). Adult coho usually weigh from 2 - 5 kg (45 - 70 cm in length) and only rarely exceed 9 kg. Jacks (precocious males) are common in some populations, are usually less than 30 cm in length, and often superficially resemble small females. More detailed descriptions of coho salmon are provided in Scott and Crossman (1973), Hart (1973), Pollard et al. (1997), and Sandercock (1991).

Figure 1a: Adult Coho Salmon (marine phase)

Figure 1a: Adult coho salmon (marine phase).

Figure 1b: Adult Coho Salmon (male and female) Showing Spawning Morphology and Colouration

Figure 1b: Adult coho salmon (male and female) showing spawning morphology and colouration.

By H. Heine.

Nationally Significant Population

Coho salmon warrant more than one status designation. In the United States, the National Marine Fisheries Service proposed six evolutionarily significant units (ESUs) for coho salmon (Fig. 2) extending from central California to southern British Columbia (Weitkamp et al. 1995). An ESU is a population or group of populations that is substantially reproductively isolated from other populations and represents an important component of the evolutionary legacy of the species (Waples 1991).

Most of British Columbia (BC) was covered by ice 15 000 years ago (Fulton 1969), after which a period of global warming began (Roed 1995). During the period of glaciation, anadromous salmon were able to exist in several glacial refugia including the lower two-thirds of the Columbia River, which was ice-free. As the ice retreated, much of the Fraser River drained through the Okanagan watershed, entering the ocean via the Columbia River. At this time, the Fraser canyon was blocked with ice near Hell’s Gate (Fig. 3). It was during this period that coho salmon (and other species) colonized the interior Fraser/Thompson River watershed. Fish entered by postglacial lake connections in the Okanagan-Nicola areas and by upper mainstem Fraser/Columbia connections (Northcote and Larkin 1989). Coho in the Columbia are extinct upstream of the Deschutes River (Fig. 2) (Nehlsen 1997). In contrast to the inland dispersal pattern found for most interior Fraser fish populations, many fish now found in the lower Fraser River watershed, including coho salmon, colonized along the coast via the sea. The Fraser canyon remains a velocity barrier for many species of fish, resulting in a discontinuous distribution of many species and populations within species (McPhail and Lindsey 1986).

Figure 2: Coho Salmon Evolutionarily Significant Units

Figure 2: Coho salmon evolutionarily significant units.

From Weitkamp et al. 1995). Dark shaded areas in inset show locations of extinct populations in Washington, Oregon, and California (from Ecotrust 1999).

Figure 3: Approximate Distribution of 5 Subpopulations of Coho Salmon (North Thompson, South Thompson, Lower Thompson/Nicola, Fraser Canyon, and Upper Fraser) Within the Interior Fraser River Watershed

Figure 3: Approximate distribution of 5 subpopulations of coho salmon (North Thompson, South Thompson, Lower Thompson/Nicola, Fraser Canyon, and upper Fraser) within the interior Fraser River watershed.

Distribution of coho in the upper Fraser is not well known as indicated by the areas in the upper Fraser where coho are suspected to occur but have not been confirmed.

Results from earlier work documenting the genetic uniqueness of interior Fraser coho (Small et al. 1998a, 1998b, Shaklee et al. 1999) were confirmed by Beacham et al. (2001) and Irvine et al. (2000, 2001) who examined larger data sets. Co-ancestry coefficients (FST values)1 were used to produce a dendrogram illustrating the relatedness of coho from samples taken in the entire Fraser River watershed (Fig. 4). Interior Fraser coho were genetically distinct from fish in the lower Fraser (FST @0.02). Coho from the Fraser Canyon (Nahatlatch River) appeared to be more closely related to lower Fraser River coho than other interior Fraser coho, implying that some genetic exchange may have occurred between the canyon and the lower river. Samples taken in the major basins (Fig. 3, North Thompson, South Thompson, and lower Thompson/Nicola) grouped together. Fish from upper Fraser sites (Bridge and McKinley) did not pair with a recent sample from the Fraser Canyon. Irvine et al. (2000) describe the genetics of interior Fraser coho in more detail, and Beacham et al. (2001) discuss the population structure for BC coho salmon resulting from the analysis of ~28 000 coho salmon, mostly from sites in BC.

Since Fraser coho salmon that spawn upstream of the Fraser canyon are substantially reproductively isolated from other coho salmon, and they constitute an important component in the evolutionary legacy of the species by virtue of their Columbia River heritage, they constitute an ESU. Within the interior Fraser, there appear to be at least five separate subpopulations (North Thompson, South Thompson, lower Thompson/Nicola, Fraser canyon, and upper Fraser) which should perhaps be considered as separate conservation/management units. Genetic data suggest considerable genetic exchange among individual tributaries within these subpopulations, and less genetic exchange among subpopulations. Additional baseline sampling and analysis is required to finalize the delineation of subpopulations within the total interior Fraser coho population.

In summary, interior Fraser coho originated from populations that survived glaciation in Columbia River refugia. Coho in the mid-upper Columbia River watershed that may have been similar genetically to interior Fraser coho are now extinct. Interior Fraser River watershed coho salmon are genetically unique and can be readily distinguished from coho from the lower Fraser and other areas of Canada. The Fraser River canyon appears to be a natural boundary that separates many fish populations into upper and lower Fraser units. The Fraser is the largest river in BC draining over 220 000 km2, about one-quarter of the province (Northcote and Atagi 1997) and the interior Fraser constitutes most of this large drainage basin. Interior Fraser River (Southern Mountain Ecozone) coho are a nationally significant population that occupy ~25% of the natural freshwater range of coho salmon within Canada.

Figure 4: Neighbor Joining Dendrogram of Fraser River Coho Salmon Populations Based on FST Values Calculated from Six Microsatellite Loci and Two MHC Class I and Class II Loci

Figure 4: Neighbor joining dendrogram of Fraser River coho salmon populations based on Fst values calculated from six microsatellite loci and two MHC class I and class II loci.

From Irvine et al. 2001). Heavy line separates Fraser River populations into those from the interior Fraser and the lower Fraser. FST scale is shown on bottom left.


1 FST is the correlation of genes of different individuals in the same population. The higher the value (maximum 1), the more closely related individuals are to one another other than they are to individuals in other samples.