Chinook salmon (Okanagan population) COSEWIC assessment and status report: chapter 2

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Species Information

The chinook salmon (Oncorhynchus tshawytscha Walbaum) is one of six species of the Pacific salmon (genus Oncorhynchus) native to North America. This report assesses the status of the chinook salmon population within the Okanagan River basin in British Columbia as a COSEWIC Designatable Unit (DU).

The case for recognizing Canadian Okanagan chinook salmon as a 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, including evidence of extended freshwater rearing and possible freshwater maturation. This unique population of chinook salmon in Canada will be referred to as “Okanagan chinook."

Distribution

Spawning populations of chinook salmon are found in streams and rivers from northern Hokkaido (Japan) to the Anadyr River (Russia) on the Asian coast, and from central California to at least Kotzebue Sound (Alaska), on the North American coast. The Okanagan chinook appears to exist only in the Okanagan River of Canada (a tributary to the Columbia River). Its current northern limit is the McIntyre Dam (near Oliver, BC), and its southern limit may be the north basin of Osoyoos Lake, immediately north of the BC border with Washington State. In such case the entire breeding population of Okanagan chinook is in Canada, although anadromous individuals will migrate through the U.S. Columbia River to and from the Pacific Ocean.

Habitat

Chinook salmon are born in fresh water and grow in streams, lakes, estuaries, and/or the ocean. Sexually mature or maturing fish migrate to their natal stream to spawn, following which the adults die. They spawn in a broad range of stream flows, water depths, and substrate sizes, but spawn preferentially in areas with intra-gravel water flow. In the ocean, chinook may remain in coastal areas or complete extensive offshore migrations.

Adult anadromous Okanagan chinook migrate from the Pacific Ocean, up the Columbia River (past nine mainstem U.S. dams), and into Osoyoos Lake and the Okanagan River in Canada. The accessible portion of the Okanagan River ends at the McIntyre Dam, and spawning occurs between the dam and Osoyoos Lake. During migration, anadromous adults may hold in the Okanagan River below the Similkameen confluence or Osoyoos Lake until spawning temperatures are favourable. At fry emergence, Okanagan chinook may either rear in the Okanagan River or in Osoyoos Lake for a varying length of time before anadromous individuals migrate as smolts through the Columbia River to the Pacific Ocean. Non-anadromous individuals (residuals/residents) remain in Osoyoos Lake. Available spawning and rearing habitat in Canada could support many times more Okanagan chinook than have been observed for several decades.

Biology

Anadromous Okanagan chinook enter the Okanagan River in June/July and likely hold until spawning in October. Peak spawning occurs generally in the third week of October, when water temperatures are about 10 °CC-14 °C. It is unknown whether spawning also occurs in early July when temperatures are also favourable. Eggs incubate through the winter and fry emerge between January and May.

Fry rear in the Okanagan River and/or Osoyoos Lake for a period ranging from weeks to a year or more. Anadromous migrants exit Osoyoos Lake probably during April/May or in early July. The marine phase of their life history ranges from 1–3 years with adults returning primarily as four- or five-year-olds. Some Okanagan chinook appear not to migrate but instead come to maturity in Osoyoos Lake. Their reproductive success is unknown.

Population Sizes and Trends

The historic population of anadromous Okanagan chinook was large in size and supported a significant food and commercial/economic trade fishery by the native Okanagan peoples. However, the current population of anadromous individuals (when enumerated) is now only 5-25 adults. In addition, there may be a freshwater maturing segment of the population, but their numbers are hard to estimate and are probably also very low. The Okanagan chinook has been historically persistent, but with such low numbers its future persistence is unlikely.

Limiting Factors and Threats

Limiting factors include habitat issues and exploitation by various fisheries. Habitat issues include: (1) direct losses of migrating juveniles and adults to injury and predation at the mainstem dams and their impoundments; (2) indirect losses due to migration delays; (3) loss of access to habitat upstream of McIntyre Dam; (4) water quality issues in spawning and rearing habitats; and (5) ecological effects of exotic species including several competitive and predatory fish species, Eurasian milfoil (a plant) and Mysis relicta (a planktonic crustacean) in Osoyoos Lake.

Analysis of habitat between Osoyoos Lake and McIntyre Dam indicates that habitat availability does not limit actual spawning activity. While the rate of egg-to-fry survival is unknown, live alevins and fry have been observed in many areas of the spawning grounds. However, there is little suitable river rearing habitat for juveniles due to channel modifications and high summer water temperatures. While juvenile rearing habitat is available in Osoyoos Lake, it may be severely limited in some years due to high water temperatures in the epilimnion and anoxic conditions in the hypolimnion.

Habitat impacts in the U.S. Columbia River can be severe. An estimated 80-85% of anadromous adults survive the upstream migration through dams and impoundments, but only some 43% of anadromous juveniles survive the outward migration.

Anadromous adults are captured in various fisheries, including marine and freshwater. Total fishing mortality for Columbia River summer chinook (Okanagan chinook migrate with these) during the 1990s averaged 31.8%, but increased to a high of 76.4% in 2003 (marine and freshwater). Freshwater fisheries have steadily increased their exploitation of summer chinook, and those that have escaped marine fisheries and lower Columbia River fisheries and enter the upper reaches of the Columbia River, such as the Okanagan chinook, have experienced an expanding fishery from a couple of percent in 2000 to 30% in 2005. The combined mortality resulting from human exploitation and habitat problems such as dams poses a threat to the existence of Okanagan chinook.

The neighbouring chinook salmon populations in the U.S. portion of the Okanagan basin are considered by state fisheries agencies to be “of special concern” (ocean-type) or extirpated (stream-type). The U.S. Colville Confederated Tribes have a hatchery program aimed at reintroducing stream-type chinook into the U.S. portion of the Okanagan basin, and strays from this program could further threaten the integrity of Okanagan chinook.

Special Significance of the Species

The Okanagan chinook is the only remaining population of Columbia River basin chinook salmon that spawns in Canada. This population shows evidence of extended freshwater rearing, a trait that is uncommon in the chinook salmon of the U.S. portion of the basin. Furthermore, this extended freshwater rearing may include reproduction without migration to the Pacific Ocean, although this has yet to be well documented. Okanagan chinook are also significant for their contributions to First Nations communities, especially as an important food and commercial trade species for aboriginal harvest. There are numerous aboriginal fishing stations along the Okanagan River that are not utilized because of the lack of Okanagan Chinook.

Existing Protection or Other Status Designations

In May 2005, COSEWIC assessed the Okanagan chinook as Endangered in an Emergency Assessment. Provincial and federal statutes and policies exist to protect fish and their freshwater and marine habitats.