Kokanee

By Bill and Lori Jollymore's In Species
Oncorhynchus nerka

The kokanee is generally similar to the anadromous sockeye except in ultimate length and weight. The permanent freshwater form of this species, the kokanee, was originally described as a separate species. Later it was realised that it and the anadromous form were the same species but they were given subspecific status.

A silvery, trout-like looking fish usually without dark spots on the back sides, or dorsal and caudal fins (occasionally a few spots on the outer edge of the tail and sometimes kokanee in Williston Reservoir have fine black spots on the back). Long anal fin base (13 or more bony rays), hind margin slants backwards. Fry (25-50 mm) and parr (50-500 mm) lack a dark stripe on the leading edge of the dorsal fin.

Commonly, kokanee develop a red colouration on the body as they approach spawning condition. Often the red is more intense in males than in females and males typically develop a green head (olive-drab in females). The jaws of males become elongated with the upper jaw hooked and with enlarged teeth. Males also develop a fleshy hump anterior to the dorsal fin. The body of native kokanee in the Finlay River is a light burgundy colour along the top and sides with a white belly and a dirty grey head.

Adult kokanee live in the offshore habitat of lakes. Here, they feed in the food-rich middle or upper strata at dawn and dusk and migrate down into the cool hypolimnion at night and during the day. Basically, juvenile kokanee use the same habitat as adults. In some lakes, however, after juveniles reach a threshold size they move inshore to forage during the day. On entering their nursery lake, the fry of some kokanee populations immediately move offshore; others remain inshore and forage for variable amounts of time. These differences in fry behaviour are probably related to food availability, temperature, and predation risk.

Kokanee spawn in the fall, usually when water temperatures drop below 12°C (September or October). Spawning occurs in both streams and in lakes. Entry into streams or onto shore-spawning sites occurs at night. In streams, females select the spawning site (usually in a riffle below a pool or on the outside of bends) and dig redds in the gravel. After spawning, the female moves slightly upstream and begins gentle digging movements which do not move gravel but force the eggs into interstices at the bottom of the nest. The digging movements gradually become more vigorous and gravel is displaced downstream over the nest. Like other Pacific salmon, kokanee die after spawning.

In lakes, kokanee usually spawn inshore in areas where there is upwelling or some subsurface flow. The site is cleaned of sand and silt by the female's attempts to dig and the fertilized eggs fall into interstices between the gravel and cobbles. The depth of known lake-spawning sites usually is less than 10 m. Egg number also varies among populations and ranges from about 200 to about 1500. As in most fish, development rate is a function of incubation temperature (45 days from fertilization to emergence at 14 °C and 73 days at 8 °C).

Upon emergence, kokanee spawn in streams and migrate to a nursery lake before starting to feed. At hatching, kokanee alevins average about 15.5 mm in length, and on emergence fry are about 19.5 mm in fork length. Growth rate is influenced by temperature and food availability, but by the end of their first summer fry range from about 40-50 mm in length. Kokanee continue to grow over the winter and are about 180 mm by the end of their second growing season. Sexual maturity typically is reached at the end of the third (2+) or fourth (3+) summer. Size at maturity also varies and kokanee in Williston Reservoir are 25 to 30 cm in length when they return to the streams to spawn.

Migrating kokanee fry usually do not begin feeding until they reach their nursery lake. Once fry start feeding, their primary prey is crustacean zooplankton, and zooplankton remains their major food throughout life. The species of zooplankton, however, change with the seasons and lakes. These common food items include: Cyclops, Bosmina, Daphnia, Diaptomus, and sometimes chironomids. Sockeye salmon are native to western North America and eastern Asia. In western North America, anadromous sockeye originally ranged from the Sacramento River in California, to the Mackenzie River and other Beaufort Sea tributaries in the north. The region of maximum abundance, however, was much narrower (the Columbia system north to the Kuskokwim River). The freshwater resident form of sockeye (kokanee) has a North American distribution (Columbia system to the upper Yukon system) similar to sockeye but is not widespread in Alaska.

Postglacially, the non-migratory form of sockeye (kokanee) evolved in many of B.C.'s lakes. Natural populations of kokanee are widely scattered in the province, and from the Fraser system northwards many of these populations still are in contact with anadromous sockeye. The only two native kokanee populations in the upper Peace system are from southern stocks at Arctic Lake in the headwaters of the Parsnip River and Thutade Lake in the headwaters of the Finlay River. Thus, kokanee appear to have colonized the upper Peace system at least twice - Arctic Lake is adjacent to Pacific Lake (a Fraser tributary) and Thutade Lake is adjacent to Skeena tributaries.
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