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Kokanee salmon
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Salmoniformes
Family:	Salmonidae
Genus:	Oncorhynchus
Species:	O. nerka
Binomial name
Oncorhynchus nerka (Walbaum, 1792)

The kokanee salmon (Oncorhynchus nerka), also known as the kokanee trout, little redfish, silver trout, kikanning, silver salmon, Kennerly’s salmon, Kennerly’s trout, or Walla,^[1] is the non-anadromous form of the sockeye salmon (meaning that they do not migrate to the sea, instead living out their entire lives in freshwater). There is some debate as to whether the kokanee and its sea-going relative are separate species; geographic isolation, failure to interbreed, and genetic distinction point toward a recent divergence in the history of the two groups. The divergence most likely occurred around 15,000 years ago when a large ice melt created a series of freshwater lakes and rivers across the northern part of North America. While some members of the salmon family (salmonids) went out to sea (anadromous), others stayed behind in fresh water (non-anadromous).^[2][3] The separation of the sockeye and the kokanee created a unique example of sympatric speciation that is relatively new in evolutionary terms. While they occupy the same areas and habitats during the breeding season, when ocean-going sockeye salmon return to freshwater to spawn, the two populations do not mate with each other in some regions, suggesting speciation.^[4][5]

Distribution

The kokanee species can be found in the northern United States of Alaska, Washington, Idaho, and Oregon. It can also be found up through British Columbia and the Yukon in Canada. Subdivisions of the kokanee are found in Japan and Russia. The kokanee was introduced to the northeastern United States and some southern states, including New England, New York, Montana, North Dakota, California, Wyoming, Utah, and Nevada. It was also introduced to parts of Canada.^[6][5]

Genetic and morphological diversity

There is debate as to whether the kokanee has enough genetic distinction to be classified as a subspecies or a separate species from the sockeye salmon. Genetic evidence suggests that the evolution of landlocked sockeye has occurred more than once with different kokanee populations, showing genetic differences between populations. Most evidence points to events that land-locked the lake-type sockeye (which spawns in streams and lakes, rather than tributaries like the sea-type sockeye, but is still anadromous and spends most of its adult life out at sea), which evolved into the non-anadromous form.^[3] Genetic evidence from kokanee in the Fraser River drainage and Columbia River drainage shows that ancestors of the kokanee came from the lake-type sockeye. The data also suggests that the kokanee may have evolved back into a lake-type anadromous form at some point in recent history, although there is a lack of interbreeding between it and sockeye in the drainage systems. It is also important to note that genetic distinction between sockeye and kokanee that cohabitate varies from region to region, with some populations showing distinct divergence, but others showing very little divergence.^[7] Studies done in Okanagan Lake in British Columbia and Lake Sammamish in Washington State suggested that the genetic diversity between the lake-type sockeye and the kokanee marks the divergence of two species because cohabitating sockeye and kokanee did not interbreed, despite the fact that interbreeding was possible.^[4][8]

There is some morphological divergence between the kokanee and sockeye. The most noticeable difference is size. The kokanee is smaller than the sockeye due to less nutrient availability in freshwater. Other morphological differences include higher gill raker counts in the kokanee, failure to develop the changes that allow the sockeye to transition between fresh and salt water, and the kokanee's poor swimming ability compared to the sockeye. Egg sizes are also a difference. Due to its size, the kokanee should theoretically have smaller eggs in order to increase fecundity. In reality, the kokanee egg size varies, even though the energetic cost of larger eggs can limit the fecundity of kokanee populations.^[9] Studies have found that kokanees can extract carotenoids (which provide red pigmentation during breeding) from food better than sockeyes due to sexual selection and mate choice.^[10] Again, the degree of morphological variation, such as gill raker count, can vary from population to population.

The Japanese kokanee, also known as the kunimasu salmon or black kokanee, is considered a subspecies of the sockeye salmon by some and occurs naturally in Lakes Akan and Chimikeppu on Hokkaido Island.^[5] The creation of a dam caused the extermination of the fish by changing lake acidity. As a result, the kokanee was believed to have gone extinct in the 1940s. In 2011 a few fish were found in an isolated lake on Mount Fuji.^[11]

The Japanese kokanee varies from its sea-going sockeye relative in a few ways. The black kokanee breeds in March at a depth of 30–40 feet, while sockeye salmon breed in the fall and have a different number of gill rakers than the kokanee. In addition, the black kokanee is much darker in color than the sockeye or any other kokanee population. The fish has been introduced to different lakes around Japan for commercial fishing practices. It seems that commercial captive breeding programs, the introduction of non-native kokanee populations from Canada, and a population crash have decreased the genetic distinctiveness between the black kokanee and its sockeye relatives. These factors have also caused a decrease in native kokanee populations.^[5]

Identification, life cycle, and development

Kokanee grows to an average size of 23–30 centimetres (9–12 in) with an average weight of 0.45 kilograms (1 lb). It can also reach a size of up to 51 centimetres (20 in) and weigh 1.4–2.3 kilograms (3–5 lb). The largest kokanee, caught in Washington State, weighed 2.83 kilograms (6.25 lb). Adult kokanee can be found in open water where the thermocline is around 10 °C (50 °F). They can have anywhere between 29–40 gill rakers.^[12][6] As a freshwater-based fish, they are much smaller than their sea-going sockeye relatives due to less food availability.^[13] Size is the most significant morphological distinction between the kokanee and the Sockeye, but gill raker count can differ from Sockeye salmon as well. The main food source of this fish is plankton. “Kokanee have blue backs and silver sides and unlike other salmon and trout, except chum salmon, sockeye and kokanee lack distinct dark spots on their backs and tail fins. In addition, when compared to other trout, they have finer scales, larger eyes, and deeply forked tail”.^[12]

The typical life cycle of the kokanee is similar to that of other salmon. They are born in a stream and migrate down to a lake where they will spend most of their adult lives. Kokanee typically live for four years in a lake before heading back to spawning grounds to spawn and die. However, population longevity can vary between 2–7 years. A kokanee can spawn in a variety of different time periods called runs. Individual populations can have multiple runs associated with the kokanee in a lake and occur from August to February. Some kokanee have been seen spawning in April.^[14] The female kokanee creates a nest called a redd. She will lay around 1000 eggs, depending on food availability. Eggs hatch within 110 days, and the juveniles swim out to the lake.^[14][6]

During spawning, the males turn bright red and develop a humped back and an elongated jaw similar to the male Sockeye salmon. Females also dawn a dark red hue during the breeding season, which also corresponds with the breeding season of Sockeye salmon.^[15]

Competition with introduced lake trout can lead to a decline in Kokanee populations during the summer. Lake trout are predatory and will eat young Kokanee. Predation by Lake Trout accounted for 83 percent of the 88 percent decline in Kokanee populations in Lake Chelan, Washington.^[16] Other factors such as pollution, habitat loss, and warming global temperatures put the Kokanee at risk in some areas.^[15]

Conservation

Some kokanee populations have declined in the US and Canada, while others thrive. The kokanee’s status is variable among different locations under the Endangered species Act. They are listed as Endangered in the Snake River in Idaho and threatened in Ozette Lake in Washington State.^[6] Other kokanee populations in Washington State have shown genetic distinction, but attempts by the Lake Sammamish Kokanee Work Group to get the Lake Sammamish kokanee listed as a separate species and therefore endangered failed the US Fish and Wildlife criteria to be listed as a distinct species.^[17] The current IUCN red list standing of the kokanee is listed under the sockeye salmon (Oncohynchus nerka) is of least concern.^[18]

Even so, King County Washington in partnership with US Fish and Wildlife has issued conservation measures to save the fish that once numbered in the thousands. As few as 150 fish spawned in 2007–2008 marking a critical need for conservation of the state’s kokanee population. Currently, restoration of streams, habitats, hatchery breeding, and a ban on fishing for kokanee has caused an increase in native kokanee populations.^[19][20]

Lakes in Canada have also seen a decline in native kokanee, with numbers dropping from 2,800 fish to just 88 fish in 2007 in the Kluane National Park and Reserve. The park has outlawed fishing of kokanee, and it is illegal to possess a kokanee salmon. Conservation efforts have been largely successful with 4,660 kokanee spawning in the park in 2015. It is unclear as to why the kokanee population crashed in the mid and late 2000s.^[15]

References