Arctic grayling (Thymallus arcticus) is a species of freshwater fish in the salmon family (family Salmonidae) of order Salmoniformes. It comprises five subspecies native to the Nearctic and Palearctic ecozones. T. arcticus is widespread throughout the Arctic and Pacific drainages in Canada, Alaska, and Siberia, as well as the upper Missouri River drainage in Montana. In the U.S. state of Arizona, an introduced population is found in the Lee Valley and other lakes in the White Mountains. They were also stocked at Toppings Lake by the Grand Tetons.
The scientific name of the arctic grayling is Thymallus arcticus. It was named in 1776 by German zoologist Peter Simon Pallas from specimens collected in Russia. The name of the genus Thymallus first given to grayling (T. thymallus) described in the 1758 edition of Systema Naturae by Swedish zoologist Carl Linnaeus originates from the faint smell of the herb thyme, which emanates from the flesh.
Arctic grayling grow to a maximum recorded length of 76 cm (30 in) and a maximum recorded weight of 3.8 kg (8.4 lb). Of typical thymalline appearance, the Arctic grayling is distinguished from the similar grayling (T. thymallus) by the absence of dorsal and anal spines and by the presence of a larger number of soft rays in these fins. There is a dark midlateral band between the pectoral and pelvic fins, and the flanks may possess a pink iridescence. T. a. arcticus has been recorded as reaching an age of 18 years.
Arctic grayling are widespread in Arctic ocean drainages from Hudson Bay, Canada to Alaska and in Arctic and Pacific drainages to central Alberta and British Columbia in Canada. They do not occur naturally in the Fraser and Columbia river basins. There are remnant native populations of fluvial arctic grayling in the upper Missouri river drainage in the Big Hole river and Red Rock basin. Fluvial arctic grayling have been reestablished in the upper Ruby river, a tributary of the Beaverhead river. The native range formerly extended south into the Great Lakes basin in Michigan. They occur naturally in the Arctic ocean basin in Siberia from the Ob to Yenisei drainages and in European Russia in some tributaries of Pechora river. Lake dwelling forms of Arctic grayling have been introduced in suitable lake habitats throughout the Rocky Mountains, Cascade Mountains and Sierra Nevada Mountains as far south as Arizona.
Several life history forms of Arctic grayling occur: fluvial populations that live and spawn in rivers; lacustrine populations that live and spawn in lakes; and potamodromous populations that live in lakes and spawn in tributary streams.
The Arctic grayling occurs primarily in cold waters of mid-sized to large rivers and lakes, returning to rocky streams to breed. The various subspecies are omnivorous. Crustaceans, insects and insect larvae, and fish eggs form the most important food items. Larger specimens of T. arcticus become piscivorous and may even take small aquatic mammals, such as lemmings. The immature fish feed on zooplankton and insect larvae.
Spawning takes place in the spring. Adult fish seek shallow areas of rivers with fine, sand substrate and moderate current. Males are territorial and court females by flashing their colourful dorsal fins; the fins are also used to brace receptive females during the vibratory release of milt and roe. The fish are nonguarders: the eggs are left to mix with the substrate. Although the Arctic grayling does not excavate a nest, the highly energetic courtship and mating tends to kick up fine material which covers the zygotes. The zygote is small (approximately 3 mm or 0.1 in in diameter) and the embryo will hatch after two to three weeks. The newly hatched embryo remains in the substrate until all the yolk has been absorbed. They emerge at a length of around 12 to 18 mm (0.5 to 0.7 in), at which time they form shoals at the river margins. The juveniles grow quickly during their first two years of life.
Arctic grayling are considered a secure species throughout their range. One fluvial population in the upper Missouri river basin is a candidate for listing under the Endangered Species Act. Although some populations at the southern extant of its native range have been extirpated, it remains widespread elsewhere and is not listed on the IUCN Red List of threatened species.
The upper Missouri River basin population once merited a high priority for Endangered Species Act (ESA) listing by the US Fish & Wildlife Service (FWS). This unique southernmost population is now extirpated from all areas of the basin with the exception of the Big Hole River watershed. In preparation for an ESA listing, the US FWS began implementing a "Candidate Conservation Agreement with Assurances" (CCAA). This agreement would protect cooperating landowners from being prosecuted under the ESA "takings" clause so long as they fulfil specific obligations, spelled out in a contractual arrangement and intended to restore the dwindling population. The CCAA, however, is now in doubt. On 25 April 2007, the FWS removed Big Hole River grayling from ESA candidacy based on arguments that (1) the rarer fluvial populations should not have been "lumped together" with the more common lake-dwelling populations and (2) the Montana grayling populations are insignificant and their loss would be inconsequential given the presence of thriving populations in Alaska.
On 8 September 2010 the U.S. Fish and Wildlife Service announced a revised 12-month finding on the petition to list the upper Missouri River Distinct Population Segment (Missouri River DPS) of Arctic grayling \ as endangered or threatened under the Endangered Species Act finding that listing was warranted but precluded in the current fiscal year by higher priority listings. The Service has added the species to the candidate species list and will work on developing a proposed rule for listing as priorities allow.
On August 19, 2014, the FWS determined not to list the grayling due to the effectiveness of the CCAA. 
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- FWS (2004). U.S. Fish and Wildlife Service Species Assessment and Listing Priority Assignment Form for fluvial Arctic grayling (distinct population segment of the Upper Missouri River), commonly called Montana Arctic grayling. November 30, 2004.
- EcoRover blog about Big Hole River grayling
- Pat Munday's rating of Big Hole Watershed Committee effectivness in Big Hole River grayling restoration
- Length and age at maturity of Arctic grayling in the Snake River during 2003 / by Alfred L. DeCicco and Andrew D. Gryska. Hosted by Alaska State Publications Program.
- Summer abundance of Arctic grayling in the Chena River, 2005 by Klaus G Wuttig; Steven M Stroka; Alaska. Division of Sport Fish.; Alaska. Division of Commercial Fisheries. Hosted by Alaska State Publications Program.