central Idaho and the high Uinta Mountains in Utah, Cascade Mountains and Sierra Nevada Mountains as far south as Arizona. Life cycle 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 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 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 , at which time they form shoals at the river margins. The juveniles grow quickly during their first two years of life. Conservation Arctic grayling are considered a secure species throughout their range. Although some populations at the southern extent of its native range have been extirpated, it remains widespread elsewhere and is not listed on the IUCN Red List of threatened species. The fluvial population in the upper Missouri river basin once merited a high priority for listing under the Endangered Species Act (ESA) 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 protects cooperating landowners from being prosecuted under the ESA "takings" clause so long as they fulfill specific obligations, spelled out in a contractual arrangement