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

Technical Summary

Oncorhynchus tshawytscha

Common name: Chinook Salmon, Okanagan population
Saumon chinook, population de l’Okanagan

Range of Occurrence in Canada: British Columbia (Okanagan River, Osoyoos Lake, coastal Pacific Ocean)


Extent and Area Information

Extent of occurrence (EO) (km2)
(freshwater phase in Canada)
About 16 km2 (Okanagan River and Osoyoos Lake)

specify trend (decline, stable, increasing, unknown)

Perhaps stable


are there extreme fluctuations in EO
(> 1 order of magnitude)?

Unlikely


Area of occupancy (AO) (km2)
(freshwater phase in Canada)

About 15 km2 (adults and juveniles)



specify trend (decline, stable, increasing, unknown)

Decline in suitable lake rearing habitat


are there extreme fluctuations in AO
(> 1 order magnitude)?

No


number of extant locations

One (river and lake)

specify trend in # locations (decline, stable, increasing, unknown)

None


are there extreme fluctuations in # locations (>1 order of magnitude)?

No


habitat trend: specify declining, stable, increasing or unknown trend in area, extent or quality of habitat

Improving in river; likely declining in lake



Population Information

generation time (average age of parents in the population) (indicate years, months, days, etc.)
4.5 years anadromous; 2 years nonanadromous


number of mature individuals (capable of reproduction) in the Canadian population (or, specify a range of plausible values)

Likely under 50 (includes anadromous and non-anadromous)



total population trend: specify declining, stable, increasing or unknown trend in number of mature individuals

Possibly stable at very low numbers



if decline, % decline over the last/next 10 years or 3 generations, whichever is greater (or specify if for shorter time period)

Historic collapse likely due to
overfishing and dams



are there extreme fluctuations in number of mature individuals (> 1 order of magnitude)?

No



is the total population severely fragmented (most individuals found within small and relatively isolated (geographically or otherwise) populations between which there is little exchange, i.e., ≤ 1 successful migrant / year)?

One population (Okanagan chinook is genetically and demographically isolated from all other chinook salmon populations in Canada; likely receives some gene flow from U.S.)



list each population and the number of mature individuals in each

Not available


specify trend in number of populations (decline, stable, increasing, unknown)

Not available


are there extreme fluctuations in number of populations (>1 order of magnitude)?

Not available



Threats (actual or imminent threats to populations or habitats)

- juvenile and adult mortality due to U.S. dams on Columbia River
- fisheries exploitation in ocean and rivers
- restricted spawning and rearing habitat in Canada due to dam
- deterioration in juvenile habitat (Osoyoos Lake); loss of adult spawning and early juvenile rearing habitat (river)
- predators including exotic fishes
- hatchery projects and potential outbreeding depression


Rescue Effect (immigration from an outside source): High

does species exist elsewhere (in Canada or outside)?
Yes (but this DU is genetically distinct from all other Canadian DUs)

status of the outside population(s)?

Candidate for Washington State listing
Is immigration known or possible?
Some U.S. hatchery adults have been observed in Canada. Genetic evidence suggests that fish from the U.S. population have contributed to reproduction in Canada.

would immigrants be adapted to survive here?

Unknown


is there sufficient habitat for immigrants here?

Possibly


Quantitative Analysis


Status and Reasons for Designation

Status: Threatened
Alpha-numeric code: Met criteria for Endangered, D1, but designated Threatened because of the rescue effect. Met criteria for Threatened: D1+2

Reasons for Designation: The Chinook salmon (Okanagan population) are the only remaining Columbia Basin population of Chinook salmon in Canada, and are geographically, reproductively and genetically distinct from all other Canadian Chinook salmon populations. They consist of anadromous salmon that migrate to and from the Pacific Ocean through the Columbia River, and also individuals that remain in Osoyoos Lake. The Chinook salmon (Okanagan population) was once large enough to support an important food and trade fishery prior to settlement by non-native people. The population used to occupy the area from Osoyoos Lake to Okanagan Lake, but McIntyre Dam has limited access to only the area below the dam and in Osoyoos Lake. As well as this habitat loss, the population was depleted by historic overfishing in the Columbia River and juvenile and adult mortality due to dams downstream on the Columbia River. Fisheries exploitation in the ocean, deterioration in the quality of the remaining Canadian habitat, and new predators and competitors such as non-native fishes also contributed to the current depleted state of the population. Genetic data show evidence of successful reproduction and maturation by individuals in this population, but also that this small population has genetic diversity similar to much larger populations in adjacent areas of the Columbia River basin, and is closely related to those populations. The genetic data, as well as the presence of fish of hatchery origin in the Canadian portion of the Okanagan River indicate that it is very likely that fish from elsewhere in the upper Columbia River basin have contributed reproductively to the population. With spawning numbers as low as 50 adults, the population is at risk of extinction from habitat loss, exploitation and stochastic factors, but may also be subject to rescue from populations in adjacent areas of the Columbia River basin.


Applicability of Criteria

Criterion A (Declining Total Population):
Not applicable as percent decline in recent past is unknown; however, it is known that the population has not recovered following a severe decline in the 1880s.

Criterion B (Small Distribution, and Decline or Fluctuation):
Not applicable as decline in recent past is unknown; however, it is known that the distribution is greatly reduced due to the construction of a Canadian dam.

Criterion C (Small Total Population Size and Decline):
Not applicable, as percent decline is unknown.

Criterion D (Very Small Population or Restricted Distribution):
Meets Endangered, D1 (fewer than 250 mature individuals). Current estimate of N is fewer than 50 individuals. Meets Threatened D1+2 (fewer than 1000 individuals and area of occupancy less than 20 km2). Estimated area of freshwater spawning and rearing habitat is 15 km2.

Criterion E (Quantitative Analysis):
Not applicable.