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COSEWIC assessment and status report on the Chinook Salmon (Okanagan population) in Canada

COSEWIC Status Report
on the
Chinook salmon
Oncorhynchus tshawytscha
Okanagan population
in Canada

Species Information

Name and Classification

Chinook salmon (Salmonidae: Oncorhynchus tshawytscha Walbaum) is one of seven species of the genus Oncorhynchus native to North America (Healey 1991). Other common names include spring salmon, king salmon, tyee, and quinnat (Scott and Crossman, 1973). There are two names for chinook salmon in the Okanagan River basin that are used by aboriginal Okanagan peoples: Ntitiyix, meaning “king salmon”, and Sk’elwis, meaning “old king salmon”, which was used to refer to spawners later in the year (Vedan, 2002). The common name in French is saumon chinook.

Morphological Description

Chinook salmon (Figure 1) adults are distinguished from other Oncorhynchus species by their large size (up to 45 kg), and by: (1) the presence of small black spots on both lobes of the caudal fin; (2) black gums at the base of the teeth in the lower jaw; and (3) the large number of pyloric caeca (>100) (McPhail and Lindsey, 1970 cited in Healey, 1991; McPhail and Carveth, 1994).

Chinook fry and parr are distinguished by the presence of parr marks extending well below the lateral line, the deepest of which are deeper than the vertical eye diameter (McPhail and Carveth, 1994). The adipose fin is normally unpigmented in the centre, but edged with black. The anal fin is usually only slightly falcate, and the leading rays do not reach past the posterior insertion of the fin when folded against the body. The anal fin has a white leading edge, but the adjacent dark line present in coho salmon (O. kisutch) is absent. However, juvenile characteristics are highly variable, so proper identification often requires meristic and pyloric caeca counts (Healey, 1991).

Figure 1: Chinook Salmon Female from the Okanagan River Basin (2002 spawning season)

Figure 1: Chinook salmon female from the Okanagan River Basin (2002 spawning season).

Genetic Description

The life history of chinook salmon, especially its anadromy and homing to natal streams for reproduction, results in reproductive isolation, genetic differentiation, and the development of local adaptations. Healey (1991) provides a description of chinook life histories, some of which is summarized here.

The chinook salmon life history includes two largely discrete behavioural forms: “stream-type” and “ocean-type”. The stream-type form is typical of Asian populations and of northern and headwater populations in North America (Healey, 1991). In many areas, stream-type chinook are referred to as “spring chinook” because of the timing of their re-entry to freshwater. Stream-type chinook spend one or more years rearing in freshwater as juveniles before migrating to the ocean, where they exhibit extensive offshore migrations for one or more years before re-entering freshwater in the spring or summer (i.e., many months before spawning in their natal habitat). Occasionally some males (and, more rarely, females) of this form do not migrate, but rear entirely in their natal freshwater habitat (Healey, 1991). For the purposes of this document, chinook salmon in the Okanagan River Basin that are the offspring of anadromous parents but rear entirely in their natal freshwater habitat are referred to as “residuals”, whereas those whose parents were not anadromous but reared entirely in their natal freshwater habitat are referred to as “residents”. The ocean-type chinook form is typical of North American populations south of Alaska. Juveniles of this form usually migrate to the ocean during their first year following emergence, normally within the first three months. Ocean-type chinook spend nearly their entire life in the ocean, re-entering freshwater in the summer or fall, a few days or weeks before spawning (Healey, 1991). Depending on the timing of their spawning migration, ocean-type chinook are commonly referred to as “summer chinook” or “fall chinook”, although there is considerable variability and overlap between the migration timing of spring, summer and fall chinook (Chapman et al., 1995; Fish and Hanavan, 1948).

The following description of genetic relationships among North American chinook populations is mainly derived from a summary of extensive analyses and discussion presented by Myers et al. (1998). Populations in south-central and northwestern Alaska are genetically distinct from populations in southeastern Alaska, which are most similar to stream-type populations in northern B.C. Ocean-type chinook salmon populations from Vancouver Island, the lower Fraser River, and southern B.C. form a genetically distinct, though diverse, group. Populations in Puget Sound and along the Washington Coast also form distinct groups. Within the Columbia River basin there appear to be two large genetic groups: ocean-type and stream-type chinook. All populations south of the Columbia River appear to consist of ocean-type chinook. Genetic groupings in this southern area include the northern Oregon Coast, southern Oregon Coast to the lower Klamath River in northern California, a California coastal group, and the Sacramento and San Joaquin River populations.

Within these broad population groups there may be several sub-groupings based on genetic, geographic, or behavioural considerations. For example, within the Columbia River basin, the National Marine Fisheries Service (NMFS) has identified seven evolutionarily significant units (ESUs) of chinook: four ocean type and three stream-type (Myers et al., 1998). 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). The genetic basis for designating ESUs within the U.S. has relied on frequencies of protein variants (allozymes), or of naturally occurring mutations in minisatellite and microsatellite loci and mitochondrial DNA (Myers et al., 1998). The degree of reproductive isolation is inferred from an analysis of the pattern of genetic distances between populations (Myers et al., 1998). In addition to this research in the U.S., Fisheries and Oceans Canada (DFO 1999a, b) proposed recognizing five or six distinct units of chinook salmon within the Fraser River basin (Candy et al., 2002).

Designatable Unit: Okanagan chinook

The case for recognizing Okanagan chinook as a COSEWIC Designatable Unit (DU) is based on this population’s: (1) genetic differentiation from other Canadian chinook salmon populations; (2) geographic and reproductive isolation; and (3) unusual life history characteristics. These aspects of the Okanagan chinook population are described in the following paragraphs. This unique population of chinook salmon in Canada will be referred to as “Okanagan chinook."


The Okanagan chinook is genetically differentiated from all other Canadian chinook salmon populations. Chinook populations in North America have been grouped into ESUs (or equivalents) (e.g., Waknitz et al., 1995; Myers et al., 1998, Teel et al., 1999; Candy et al., 2002), with the units in the Columbia River basin found to be genetically distinct from those in the Fraser basin (or elsewhere in Canada), likely because of different glacial histories (Myers et al., 1998). The Canadian Okanagan chinook population is the only remaining Columbia River basin chinook population in Canada.

Examination of genetic relationships between the Okanagan chinook and other populations in the Columbia River basin has only recently begun. Beginning in 2000, tissue samples have been collected from a total of 36 unclipped (i.e., not hatchery marked) chinook captured in the Okanagan River or Osoyoos Lake. A single fish was sampled in each of 2000, 2002, and 2003, while three were sampled in 2004 and the remaining 28 in 2005. In addition, a tissue sample was collected from one fin-clipped (i.e., hatchery origin) chinook in 2005; although this sample was not included with the unclipped fish when compared to other populations (Anonymous 2006).

As detailed in Anonymous 2006, Okanagan chinook clustered with upper Columbia summer and fall run chinook salmon populations. The longer dendrogram branch length associated with the Okanagan River sample reflects the larger (Cavalli-Sforza Edward chord) distances between it and the other samples in the group (Figure 2).

Figure 2: Dendrogram of Cavalli-Sforza and Edwards (1967) Chord Distances Based on 12 Microsatellite Loci for Chinook Salmon Populations in the Columbia and Okanagan Basins

Figure 2: Dendrogram of Cavalli-Sforza and Edwards (1967) chord distances based on 12 microsatellite loci for chinook salmon populations in the Columbia and Okanagan Basins.

Reproduced from DFO MGL (2006). See Candy et al. (2002) for methods.

However, this distinctiveness of the Okanagan sample is attributable in part to the small sample size relative to both the Similkameen (N= 92) and Wenatchee (N=93) samples, and especially to the close familial relationships between the sampled fish in 2005 (DFO MGL 2006). Of the 28 fish sampled in 2005, 21 were closely related, either full or half siblings, to at least one other fish. The whole group of 28 chinook was the offspring of 11 fish of one sex and 18 fish of the other. Estimates of FST (a measure of genetic differentiation) between the Okanagan River sample and the Similkameen and Wenatchee Rivers were relatively low (mean FST = 0.016) despite the fact that the FST value was likely inflated by the presence of closely related fish in the Okanagan sample. Finally, in spite of the small numbers of spawners observed and their high degree of inter-relatedness in 2005 the allelic richness of the Okanagan sample (AR = 12.6) was comparable to other, larger populations in the Columbia River basin (mean AR = 11.9). Collectively, the genetic data indicate that the Okanagan River population is closely related to and likely derived at least in part from stray spawners from other populations in U.S. portions of the upper Columbia River drainage, a conclusion consistent with the presence of adipose-clipped chinook salmon in the river.

Nonetheless, an important implication of the close familial relationship between the chinook sampled on the spawning grounds in 2005 is that it provides strong evidence of successful out-migration, return and survival to spawning of Okanagan-produced chinook.

Geographic/Reproductive Isolation

The Canadian Okanagan chinook population is geographically and reproductively isolated from other Canadian chinook populations. The Okanagan River in British Columbia is the only portion of the Columbia River basin in Canada that is currently accessible to anadromous salmonids. Historically, prior to the construction of Grand Coulee Dam (completed in 1939), large numbers of salmon spawned as far upstream as the outlet to Lake Windermere, British Columbia (Fulton, 1968; Scholz et al., 1985; Chapman et al., 1995). The spawning grounds for the Canadian Okanagan chinook population are nearly 1000 km from the mouth of the Columbia River, and about 1400 km, by water, from the closest chinook salmon populations along the coast of BC. Although only a few kilometres would have separated the Okanagan and South Thompson (Fraser basin) chinook salmon (prior to the exclusion of chinook salmon from Okanagan Lake), this separation is believed to have existed since late in the last ice age. Presumably straying rates for all Columbia River basin chinook salmon populations (and those elsewhere) are very low, since Columbia River basin chinook salmon are in different ESUs than those in immediately adjacent coastal areas. Hatchery-produced chinook have been observed on the spawning grounds in the Okanagan River so clearly the Okanagan chinook population is influenced by chinook from neighbouring populations.

Life History Characteristics

The Okanagan chinook exhibits unusual life history characteristics when compared to chinook salmon in the U.S. portion of the Okanagan basin (Similkameen). The principal difference is the extended period of freshwater rearing by juveniles, with the additional possibility of freshwater maturation. The evidence of freshwater maturation is that: (1) seven young (mostly aged 1+) chinook captured in Osoyoos Lake in September 2003 were resorbing scales, consistent with scale resorption prior to reproduction in older anadromous salmon (ONAFD, unpublished data, 2005): (2) the stomach samples of six of these seven chinook contained sockeye fry, indicating piscivory (an adult characteristic), and the six also exhibited additional internal features of sexual maturation (ONAFD, unpublished data, 2005); and (3) in the 2005 Okanagan River samples, 4 of 17 female chinook were fully reproductively mature at three years of age, earlier than known for anadromous females.

Additional field assessment is required to determine whether these freshwater-rearing chinook salmon spawn in the Okanagan River. If they do spawn, these residuals/residents may have retained some of the population’s historic genetic lineage, bridging the period when the Grand Coulee Fish Maintenance Program (GCFMP) excluded anadromous fish from the Okanagan River. Under the GCFMP, most upstream migrating anadromous fish from the Upper Columbia River were captured in the fishway at Rock Island Dam for five years (1939-1943). The fish were then used as hatchery broodstock or were transported to major rivers upstream of Rock Island Dam (excluding the Okanagan) to spawn naturally. Hatchery out-plants were also released into selected rivers upstream of Rock Island Dam, with sockeye being the only anadromous salmon released directly into the Okanagan River. By 1944, it is possible that many of the returning runs of chinook to the Upper Columbia were the progeny of the mixed-stock hatchery and relocated stocks (Fish and Hanavan, 1948); however, some wild 6-year-old chinook may have returned to their natal streams in 1944 (Mullan, 1987) and continued existing lineages. It is also not known if any Okanagan chinook that were released upstream of the dam found their way to the Okanagan River.