Oncorhynchus mykiss is a very numerous and widespread species, which to the level of select subpopulations and subspecies is seen to be experiencing significant declines, as a result of habitat degradation from dams, sedimentation, over-fishing, pollution and invasive species, but overall is holding a stable population and faces no major global scale threats. The trend over the past 10 years or three generations as a whole is uncertain but probably relatively stable, or the species may be in decline but not fast enough to qualify for any of the threatened categories under Criterion A (reduction in population size). Oncorhynchus mykiss is, therefore, assessed as Least Concern. However, it should be noted that treatment at species-level masks many significant declines that are in progress and well documented.

Oncorhynchus mykiss is native to streams along the Pacific coast of North America from the Kuskokwim River drainage, Alaska, south to Otay river California, USA and continuing to northern Baja California, Mexico. It is also found in the upper Mackenzie River drainage (Arctic basin), Alberta and British Columbia, endorheic (i.e. having no outflow of water) basins of southern Oregon (Page and Burr 1991) and the Commander Islands (south western Bering Sea) (Miller 2005). Primary migration pathways are from the lower Mokelumne River through the Sacramento-San Joaquin Delta to the Pacific Ocean (Casey Del Real et al. 2012).

In Russia, this species is found on both coasts of the Kamchatka Peninsula with the northern range boundary near the Kinkil' River (Koval et al. 2015, Chereshnev 1996a). There is a relict population on Bol'shoy Shantar Island (southern Sea of Okhotsk) and it may occur in the Komondorskiye Islands east of Kamchatka. Individuals are occasionally captured in coastal streams on the Russian mainland as far south as the mouth of the Amur but native self-sustaining populations are sporadic and rare. 

At sea, Oncorhynchus mykiss occurs throughout the North Pacific above 40° N from the North American coast to the Sea of Okhotsk (Burgner et al. 1992). It is most abundant in the Gulf of Alaska and eastern part of the North Pacific, conforming to the 5°C isotherm in the north and 15°C isotherm in the south. Seasonal shifts in distribution are correlated with changes in water temperature (Sutherland 1973).

It is widely introduced and established in Canada and USA, including Arctic, Atlantic, Great Lakes, Mississippi River, and Rio Grande basins, and many suitable habitats all over the world (Lee et al. 1980, Page and Burr 2011). It is widely introduced for sport fishing and reared in commercial hatcheries for consumption, including the northern and central mountain regions of Mexico, where feral populations may be established (Miller 2006).

Oncorhynchus mykiss has an adult population that size greatly exceeds one million, represented by a very large number of occurrences (subpopulations). More recent analysis in Athabasca River watershed, Canada, has found populations declining over the past, with the total number of adults in all populations is estimated at 65,175 total mature individuals, with individual population estimates ranging from 45 to 9,497 mature individuals (CSAS 2018).

In a survey of populations in the contiguous U.S., Huntington et al. (1996) identified 28 healthy native stocks of winter Oncorhynchus mykiss (20 in Washington, seven in Oregon, one in California) and six healthy native stocks of summer Oncorhynchus mykiss, all in Oregon. Total annual abundance for all North American Oncorhynchus mykiss stocks was estimated at 1.6 million fish (Burgner et al. 1992). Of 867 Oncorhynchus mykiss stocks in British Columbia and the Yukon, Slaney et al. (1996) categorized nine as extirpated, eight as high risk, 10 as moderate risk, 143 as special concern, 282 as unthreatened, and 415 as of unknown status. Many winter Oncorhynchus mykiss populations are at very low levels; populations have declined in nearly all streams in central and southern California (see Nehlsen et al. 1991 for further details). Winter Oncorhynchus mykiss stocks from Siuslaw River north to Tillamook Bay, Oregon, have been declining since 1990 (Nehlsen et al. 1991). In the Illinois River (tributary to the Rogue River), Oregon, winter Oncorhynchus mykiss catches have declined since the mid-1970s (Nehlsen et al. 1991). Several winter populations in the Puget Sound area of Washington have also experienced declines (Nehlsen et al. 1991).

In Kamchatka, it is a relatively rare fish. Despite the ban on fishing, its numbers have recently declined sharply. There is no official fishing for this species, but because of its high value, it is caught in large quantities by poachers. A small population of freshwater individuals of this species of the Shantar Islands is also listed in the Red Book of the Russian Federation (2001) as category 3 relict freshwater population of the Shantarsky Islands. Although, no special measures have been taken to protect it. The population of remote, sparsely populated areas is rather high; whereas, near settlements it is rare, and the average size of fish in catches is small. There is a noticeable negative impact of harvesting is observed in the Bolshaya River basin (Tokranov and Sheiko 2006).

Oncorhynchus mykiss is capable of surviving in a wide range of temperature conditions. It does best where dissolved oxygen concentration is at least 7 ppm. It usually lives in freshwater, but some individuals forage at river mouths (Tokranov and Sheiko 2006, Bugaev 2007). Anadromous populations occur in coastal rivers. Resident populations now inhabit small headwater streams, large rivers, (non-frozen) lakes, or reservoirs; often in cool clear lakes and cool swift streams with silt-free substrate. In streams, deep low velocity pools are important wintering habitats (Sublette et al. 1990). Adult fish remain in the channel, and the young fish are widely dispersed throughout the river system. Spawning migration starts in spring during the period of intense ice and snow melting at water temperatures above 3°С. Spawning takes place in mid- to late June, when eggs are buried in the ground on rolls with rapid flow and sandy-pebble ground at water temperatures of 6-16°С (Tokranov and Sheiko 2006, Bugaev 2007). It usually requires a gravel stream riffle for successful spawning. Salinity of 8 ppt is the upper limit for normal development of eggs and alevins (Morgan et al. 1992). Lake populations move to tributaries to spawn. Females immediately leave spawning sites, whereas males stay there for 2-3 days. Most of the individuals mature at an age of 5-6 years. This species spawns annually, and between 1-4 times in its life. Fecundity is 0.61-2.60 thousand eggs with a diameter of 3.8-5.7 mm. The diet of juveniles is very wide and includes amphibious and air insects, and fish eggs. Adults are predatory, eating mainly fish, but also small mammals (rodents). It reaches a length of 90 cm, and a body weight of 2.75 kg, with life expectancy of 10 years (Tokranov and Sheiko 2006, Bugaev 2007).

Oncorhynchus mykiss on a range-wide scale, is not significantly threatened. However, many subspecies and populations face serious threats of pollution, over fishing, sports fishing, hatcheries, major dams, estuary alteration, logging and alien species (Casey Del Real et al. 2012).

Declines in winter Oncorhynchus mykiss stocks from the Siuslaw River north to Tillamook Bay, Oregon, may have resulted from deterioration of ocean feeding conditions, widespread use of hatchery stock, predation by marine mammals, and ocean drift-net fishing (Nehlsen et al. 1991). Declining winter catches on the Illinois River (tributary to the Rogue River), Oregon, since the mid-1970s have been attributed to water withdrawal for irrigation (Nehlsen et al. 1991). In the Columbia River basin, winter stocks are threatened by habitat degradation, main stem passage problems, and interactions with hatchery fish (Nehlson et al. 1991). Declines in several Oncorhynchus mykiss winter populations in the Puget Sound area of Washington have resulted from habitat degradation (e.g., water quality problems, siltation, and sedimentation); predation by sea lions has been reported as a problem for the Lake Washington population (Nehlsen et al. 1991). Whirling disease has caused population declines in some areas. The disease is caused by a protozoan pathogen (inadvertently introduced from Europe) and involves tubifex worms as an alternate host. Brown trout (Salmo trutta) are unaffected by the protozoan and serve as a reservoir.

Tokranov and Sheiko (2006) noted the impacts of settlements on this species, where individuals are rare, and the average size of fish in catches is small. This is likely due to increased harvesting pressure, with a noticeable negative impact of harvesting is observed in the Bolshaya River basin.

Oncorhynchus mykiss commercially and internationally fished, as well as utilised for sports fishing. This species is fished throughout its entire range and utilised in hatcheries and captive breeding programmes for human consumption.

In Kamchatka, it is one of the main objects of sports fishing, ecological tourism and amateur fishing (Bugaev 2007).

Oncorhynchus mykiss is currently of relatively low conservation concern and does not require significant additional protection or major management, monitoring, or research action. However, many subspecies and populations are of high conservation concern.

A management concern in western North America is keeping Rainbow Trout out of waters inhabited by native salmonids, such as Cutthroat Trout and Gila Trout, so that the native species maintain their genetic integrity. Due to the precipitous declines of Oncorhynchus mykiss in some populations of and an apparent shift towards the non-anadromous life history forms, the connection between anadromous and non-anadromous Oncorhynchus mykiss and their management as a single or separate population has profound implications for conservation and recovery (Busby et al. 1996, Zimmerman and Reeves 2000, McEwan 2001). Since anadromous and non-anadromous trout may form an interbreeding population (Seamons et al. 2004, Araki et al. 2007) with females producing progeny with opposite life history traits (Viola and Schuck 1995, Riva-Rossi et al. 2007, Zimmerman et al. 2008), Oncorhynchus mykiss management may need to include protection of non-anadromous forms and the connectivity between the resident and anadromous fish (McEwan 2001).

Parts of the distribution in Russia (River Ozernaya and Kuril'koye Lake) are within the limits of Kronotskiy Nature Reserve (N. Bogutskaya pers. comm. 2020).

While focus on species-level status assessments are an important first step, the IUCN Species Survival Commission (SSC) Salmon Specialist Group (SSG) emphasizes the need to characterize status of Pacific Salmon at a more granular, population-level scale (identified as “subpopulations” in the IUCN Red List Guidelines) to provide meaningful guidance to stem the loss of biodiversity across the natural range of the species. There are many examples of declines in wild Pacific Salmon in both North America and Asia, particularly in the southern portion of their range given the degree of degradation and fragmentation of habitat there and the more immediate risk of climate change impacts. At the same time, there are large-scale ocean drivers that appear to be affecting species broadly across the North Pacific, regardless of their freshwater origin. Two excellent examples exist of assessment approaches and policies in the US (Waples 1991) and Canada (DFO 2005, COSEWIC 2018) that establish an effective framework for Pacific Salmon conservation. These efforts involve identifying population units based on a variety of criteria including examination of traits that are important in the evolutionary process and future adaptation. In these examples, assessments are conducted at a more granular, population-level, resulting in listings for individual population units, with identification of needed conservation actions specific to each unit. An example of assessing range-wide status of the species and at the individual subpopulation level in the IUCN Red List now exists for Oncorhynchus nerka (Rand 2011). While the amount of effort required to rigorously assess the species is substantial, we encourage efforts like this applied to the other species in the genus.