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Home › Species › Rainbow Trout
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Trout and Salmon · Salmonidae

Rainbow Trout

Oncorhynchus mykiss

Native to the Pacific slope and stocked almost everywhere else, the rainbow trout is globally secure while its wild steelhead runs are among the most imperilled fish in the country.

Rainbow Trout (Oncorhynchus mykiss)
Rainbow Trout. Raver, Duane, Public domain. Source.

The rainbow trout is the most widely distributed trout on earth and, for a great many American anglers, the first fish they ever caught. It is native to the Pacific slope of North America, from the Kuskokwim River in Alaska south into Baja California, and inland through the Columbia and upper Mackenzie basins. Everywhere else it swims — which is to say nearly everywhere — it was put there, beginning in the 1870s, by hatchery crews, railroads, milk cans and volunteers with buckets.

That history produces a species with two faces. Globally, the rainbow trout is secure and abundant. Locally, its wild anadromous populations — the fish we call steelhead — include one run listed as endangered and ten listed as threatened under the Endangered Species Act. Both statements are true at once, and holding them together is the key to understanding this fish.

Identification

A rainbow trout is bluish-green to dark olive across the back, silver to white on the sides and belly, scattered with black spots over the body and fins, and marked along the flank with the reddish-pink stripe that gives the fish its name. The scales are small, the adipose fin is present, and the tail is distinctly forked with rows of small spots running along the rays. Idaho Fish and Game notes the white leading tips on the pelvic and anal fins, a character that holds up well in the hand.

Three confusions come up constantly on the water.

Against cutthroat trout, two characters are reliable. Cutthroat carry the red-orange slash beneath the lower jaw, though Washington’s identification guide warns it can be invisible on bright sea-run fish. The character that never fails is the tongue: cutthroat have basibranchial teeth on the tongue, and rainbow trout do not. Spotting helps too — cutthroat spots are larger and cluster toward the back and tail, while rainbow spots run more evenly from head to tail.

Against brown trout, look at the tail and the spots. Brown trout carry red or orange spots, often haloed in pale blue, on a square, largely unspotted tail. Rainbow spots are black, and the forked tail is spotted along the rays. Brown trout also lack the pink flank stripe entirely.

Against a cuttbow — a rainbow × cutthroat hybrid — field identification is genuinely unreliable, and it is worth saying so plainly. When Idaho Fish and Game researchers tested appearance against genetics in Palisades Creek, phenotype correctly identified 94% of Yellowstone cutthroat but only 79% of hybrids and 71% of rainbow trout. Hybrids usually show a muted, intermediate throat slash and mixed spotting, and siblings from one pairing can look markedly different from one another.

Range and distribution

The native range runs the Pacific slope from Alaska to northwestern Mexico, plus the upper Mackenzie drainage in Alberta and British Columbia, and Kamchatka in the Russian Far East. Introductions began in the late 1800s and have since reached, by the U.S. Geological Survey’s account, nearly every U.S. state, from Alabama to Hawaii and Puerto Rico. Michigan’s first fish came from California eggs imported in 1876 and released into the Au Sable River.

Two points of precision are worth keeping straight, because they are frequently garbled. Idaho lists rainbow trout as native — those are interior redband trout, a native inland form, not a contradiction of the continental range. And in much of the country the fish present are maintained entirely by stocking: Missouri’s Department of Conservation notes that most of its rainbow populations depend on continuous stocking, with only small self-sustaining pockets. In Texas, summer heat prevents reproduction almost everywhere, and the only self-sustaining population in the state lives in McKittrick Canyon in the Guadalupe Mountains.

Habitat and water temperature

Rainbow trout want cold, clear, well-oxygenated water over clean gravel. Beyond that generality, the thermal numbers are unusually well established, and they explain more about where fish will be on a given day than any other single variable.

Research by Bear, McMahon and Zale, published in the Transactions of the American Fisheries Society, put optimal growth at 13.1 °C (55.6 °F). Growth declines significantly by 20 °C (68 °F) and falls to almost nothing at 24 °C (75 °F). The ultimate upper incipient lethal temperature over a 60-day exposure is 24.3 °C (75.7 °F); in the same study, survival was essentially complete at 20 °C and 22 °C, fell to 73% at 24 °C, and reached 2% at 26 °C. A review by Richter and Kolmes adds the behavioural thresholds that matter to anglers: adult migration is blocked at 21–22 °C (70–72 °F), and fish actively avoid water above 22 °C (72 °F).

That last figure is the practical one. When a mainstem climbs past the low seventies, trout do not simply feed less — they leave. They stack into groundwater-fed tributaries, spring inflows and cold seeps, which is why late-summer fishing concentrates so sharply at tributary mouths. Oregon State University notes that Klamath Basin redband trout survive otherwise lethal summer conditions precisely this way.

One caution on applying these numbers: interior and desert redband trout tolerate considerably more heat and less oxygen than coastal rainbows. Do not carry coastal thermal figures into desert water.

Tailwaters invert the whole picture. Cold water drawn from the bottom of a reservoir creates rainbow habitat hundreds of miles outside anything the species could otherwise occupy — Lake Taneycomo below Table Rock, the tailraces below Canyon Reservoir in Texas, and dozens more.

Life history

Rainbow trout spawn in spring across most of their range, and this is the cleanest contrast with the fall-spawning brown and brook trout. The female digs a shallow redd in gravel riffles, eggs are deposited and fertilised, and the nest is covered over. Incubation runs about 21 days at 55 °F, though Michigan’s Department of Natural Resources gives a range of 20 to 100 days depending on temperature. Successful incubation requires gravel largely free of fine sediment, which is why sedimentation is such a direct lever on trout recruitment.

Regional variation is real. Missouri documents wild trout in Ozark spring branches spawning from late December into early February — constant-temperature spring systems and tailwaters shift timing away from the typical pattern entirely.

The species is polymorphic, and this is the single most useful fact about it. Resident rainbow trout and anadromous steelhead are the same species, not merely close relatives. Idaho Fish and Game puts it plainly: the two forms live in the same waters, interbreed, and their offspring can adopt either life history. A resident female can produce anadromous young; a steelhead can produce fish that never leave. Which path a given fish takes emerges from the interaction of its genetics, its growth and condition, and its environment.

Unlike most Pacific salmon, rainbow trout are iteroparous — they do not necessarily die after spawning. Steelhead may spawn one to five times, most often once or twice.

Steelhead: the anadromous form

Steelhead are born in fresh water, rear there for one to three years, then migrate to the ocean — or, in the Great Lakes, to open lake water — before returning to spawn. At sea they turn silver and grow far beyond what their stream-resident siblings ever will. Telling a returning steelhead from a large resident rainbow can require examining scales or the chemical signature of the otoliths.

The distinction anglers care most about is run timing, and it is not really about the calendar. It is about sexual maturity at the moment the fish enters the river.

Summer-run (stream-maturing) fish return between May and October while still sexually immature. They need months in fresh water to mature, overwintering in the river and spawning the following spring. Inland Columbia basin rivers are almost entirely summer-run. Because these fish hold for months, summer steelhead fishing is a holding-water fishery.

Winter-run (ocean-maturing) fish return from November through April with gonads already developed and spawn soon after arrival. They spawn nearer the ocean and travel shorter distances. Coastal streams are predominantly winter-run, and winter fishing tracks fresh arrivals rather than resident fish.

Run timing turns out to have a strong genetic basis. NOAA Fisheries’ 12-month finding on northern California summer-run steelhead identifies the Greb1L gene region as associated with the trait, yielding summer-run homozygotes, winter-run homozygotes and heterozygotes. NOAA concluded that summer-run fish do not constitute a separate distinct population segment, in part because siblings from a single mating can express different run types.

In the Snake and Columbia systems, managers overlay a further distinction on summer-run fish. A-run steelhead spend one year at sea and return at roughly 23 to 26 inches and 4 to 6 pounds, arriving June through August. B-run fish spend two years at sea and come back at 31 to 34 inches and 10 to 13 pounds, arriving from late August, with most bound for the Clearwater River.

Two other forms are worth knowing. Half-pounders return to fresh water after only a single summer at sea, overwinter, and go back out — a pattern documented in the Klamath basin. Kelts are post-spawn survivors, and their persistence is one measure of a healthy population.

Great Lakes steelhead are not anadromous in the strict sense but behave identically. They enter spawning streams from late October through early May, with fall-run fish spawning in March and spring-run fish in April, and feed in the open lakes chiefly on alewives and rainbow smelt.

Diet

Fry begin on zooplankton and water fleas. Stream juveniles move onto drifting aquatic insects — caddisflies, mayflies and midges — and larger stream fish add terrestrial insects, snails and small fish. In lakes and the Great Lakes the shift is decisive: adult steelhead feed primarily on baitfish, and Michigan’s DNR notes they work surface scumlines where thermal bars concentrate food. Ocean fish take fish, eggs, crustaceans and molluscs.

Angling

Almost everything about how rainbow trout are fished follows from temperature and forage. Optimal growth near 55 to 59 °F, measurable loss by 68 °F, and active avoidance above 72 °F mean that a live gauge reading maps directly onto where fish will be holding — and, in a hot August, onto the cold tributary mouths they have retreated into.

Stream fish keyed to drifting caddis, mayflies and midges are the reason drift presentations and matched imitations dominate river fishing. Lake and Great Lakes fish keyed to alewife and smelt are the reason presentation changes so completely on open water.

For steelhead, method follows water condition. Oregon’s Department of Fish and Wildlife describes plunking — a stationary presentation near the bottom — as productive specifically when water is high and fish hold in soft water near the bank. At normal flows fish spread into the main current, and presentations that drift at current speed near the bottom take over. Swung flies on single- or double-handed rods, bobber-and-jig, drift fishing and diving plugs all trace back to the same problem: putting something in front of a fish that has stopped feeding in earnest.

It is worth knowing where the giants actually come from. The IGFA record table for this species is dominated not by classic river habitat but by reservoirs and canals — Lake Diefenbaker in Saskatchewan, the Tekapo and Ohau canals in New Zealand, Argentina’s Lake Strobel.

Conservation

The rainbow trout’s conservation story runs in two directions, because the species is both protected and a threat.

Where it is native and anadromous, it is in trouble. NOAA Fisheries lists the Southern California steelhead DPS as endangered and ten further DPSs as threatened. Snake River steelhead must pass up to eight mainstem dams, against three or four for mid-Columbia fish, producing consistently lower life-cycle survival. The current five-year geometric mean abundance of natural-origin steelhead passing Lower Granite Dam is 12,605 fish; the Clearwater drainage alone may historically have produced 40,000 to 60,000 adults.

Where it has been introduced, it is frequently the problem. The USGS records that hybridization with rainbow trout has been a major factor in the decline of native cutthroat trout across the Lahontan drainage and the Rocky Mountain rivers, and links the virtual extinction of the Alvord cutthroat to rainbow introduction. The thermal asymmetry compounds it: rainbow trout tolerate 4.7 °C more heat than westslope cutthroat, so a warming stream steadily favours the invader over the native.

Hatchery propagation carries its own risks, which NOAA states directly: genetic effects, reduced fitness, altered life-history traits, competition, amplified predation and disease transfer. Mainstem dams can worsen straying of hatchery fish into wild spawning areas.

Rainbow trout are also the salmonid most susceptible to whirling disease, caused by the parasite Myxobolus cerebralis, which invades the head and spinal cartilage of young fish. First detected in the United States in 1958 and now reported in more than twenty states, it produced losses of up to 90% of wild rainbow trout in several Colorado and Montana streams during the 1993–94 outbreaks, including a 90% reduction in the Madison River population.

Where to catch it

518 waterways on GuidedFloats list rainbow trout, across 44 states. Each page carries live gauge readings, access notes, and the guides who fish it.

Alabama 1

Alaska 13

Arizona 17

Arkansas 8

California 31

Colorado 22

Connecticut 9

Delaware 3

Georgia 6

Hawaii 1

Idaho 29

Indiana 1

Iowa 3

Kentucky 2

Maine 2

Maryland 10

Massachusetts 7

Michigan 9

Minnesota 4

Missouri 24

Montana 21

Nebraska 6

Nevada 16

New Hampshire 20

New Jersey 4

New Mexico 15

New York 18

North Carolina 14

Ohio 4

Oklahoma 4

Oregon 20

Pennsylvania 31

Rhode Island 11

South Carolina 2

South Dakota 13

Tennessee 4

Texas 1

Utah 16

Vermont 17

Virginia 12

Washington 20

West Virginia 16

Wisconsin 12

Wyoming 19

References

  1. Rainbow Trout (Oncorhynchus mykiss) - Species Profile — USGS Nonindigenous Aquatic Species
  2. Steelhead — NOAA Fisheries
  3. Steelhead: Conservation and Management — NOAA Fisheries
  4. Comparative Thermal Requirements of Westslope Cutthroat Trout and Rainbow Trout (Bear, McMahon and Zale 2007) — Transactions of the American Fisheries Society, via Montana State University
  5. Maximum Temperature Limits for Chinook, Coho, and Chum Salmon, and Steelhead Trout in the Pacific Northwest (Richter and Kolmes 2005) — Reviews in Fisheries Science, hosted by NOAA
  6. Salmon and Trout: The Identification Guide — Washington Department of Fish and Wildlife
  7. Distinguishing Yellowstone Cutthroat Trout, Rainbow Trout, and Hybrids by Field-Based Phenotypic Characteristics (Meyer et al. 2017) — Idaho Department of Fish and Game
  8. Rainbow trout and steelhead - what is the difference? — Idaho Department of Fish and Game
  9. What is a Steelhead? — Idaho Department of Fish and Game
  10. 12-Month Finding on a Petition to List Summer-Run Steelhead in Northern California — NOAA Fisheries
  11. Snake River Basin Steelhead Species and Critical Habitat Summary Update — NOAA Fisheries
  12. Rainbow Trout - Field Guide — Missouri Department of Conservation
  13. Rainbow Trout (Oncorhynchus mykiss) — Texas Parks and Wildlife Department
  14. Rainbow trout — Michigan Department of Natural Resources
  15. Steelhead — Michigan Department of Natural Resources
  16. Rainbow trout (Oncorhynchus mykiss) — Washington Department of Fish and Wildlife
  17. Whirling Disease — USGS Fish Health Program
  18. Redband Rainbow Trout — Oregon State University, College of Agricultural Sciences
  19. How to fish for steelhead — Oregon Department of Fish and Wildlife
  20. Oncorhynchus mykiss - Rainbow Trout or Steelhead — NatureServe Explorer