What Is a Rainbow Trout?

Fishing - What Is a Rainbow Trout

The rainbow trout, Oncorhynchus mykiss, is an invasive fish species that has been introduced to at least 45 countries and every continent except Antarctica. The introductions have negatively affected native fish populations.

Rainbow trout are olive green dorsally, with brassy to purple iridescence and dark black spots on the back and sides. They are also marked with a wide red, pink or purple band on their mid-sides.


The rainbow trout (Oncorhynchus mykiss) is a member of the family Salmonidae and is native to Western North America. It is also found in Japan, Southeast Asia, Africa, and parts of Europe and Hawaii.

In many areas, it is not uncommon to see large populations of this fish thriving, especially in freshwater streams and rivers. In fact, this species is considered a non-native pest in some regions and can be detrimental to other native fishes by interbreeding, predation, competition and spreading whirling disease.

Rainbow trout are distinguished from other salmonids by a pink-tinged lateral band down the middle of each side, a dark spot on their dorsal and caudal fins, and a spotted body. It is one of the most popular trout species and has been stocked around the world.

They are opportunistic feeders and feed on aquatic invertebrates such as mayflies, caddis flies, stoneflies and their larvae, and small baitfish. They are secondarily fed by zooplankton, worms, daphnia and fish eggs.

Stocking of these fish has spanned many regions worldwide, ranging from the Sierra Nevada in California to the Amazon River in Peru, and from Alaska to South America. Their popularity as sport fish has made them a symbol of man’s relationship with nature.


The habitat of rainbow trout is a key consideration for anglers. The fish prefers clear, cold water and complex stream habitat containing a lot of riffles and runs with rocky bottoms. In addition, the water should be well-oxygenated and free of silt.

Instream trout are primarily opportunistic feeders and feed on a variety of invertebrates, including mayflies, stoneflies, caddisflies and midges. They also eat fish eggs and small baitfish.

Trout in lakes and reservoirs typically feed on zooplankton, fish eggs, small fish, worms (aquatic and terrestrial annelids), and crustaceans. They also eat insects and daphnia.

Some rainbow trout populations migrate to the ocean during adulthood and return to freshwater for spawning, while others live their entire lives in freshwater. In the latter case, they are called steelhead.

Spawning occurs in spring, triggered by rising water flows and temperatures. The female excavates a nest, known as a redd, in gravel.

Unlike brook and brown trout, most rainbows migrate to their natal stream to spawn. Males arrive first and aggressively guard the female and nest from other males and predators.

During spawning, females deposit thousands of eggs in a series of nests. They then fertilize the eggs by rubbing them with saliva. Then, the eggs hatch and the larvae grow into juveniles. As the larvae mature, they eat zooplankton, fish and insect eggs. They become fully mature in their second or third year.


The rainbow trout is a cold-water species that spawn in the spring and early summer as water temperatures rise. They spawn on gravel beds called redds, where the female trout removes a layer of gravel to form a nest for her eggs.

Spawning is a complex and important process that takes place over the course of several days. A male trout releases sperm or milt to fertilize the eggs, while the hen fish digs over the eggs with her tail and throws up gravel to cover them.

She will spawn repeatedly until all her eggs are laid. The resulting embryos hatch into alevins, which live down in the gravel for 14 – 30 days, depending on the temperature of the water. The alevins eventually become fry, move out of the gravel, and feed on small insects.

When the young fish have grown to about 12 inches in length, they shift their diets and eat larger prey items such as worms, crustaceans, mollusks, and fish eggs. These trout are opportunistic feeders and will feed on almost anything in the stream they can find.

Because of their reliance on instinct, rainbow trout can be extremely aggressive during their spawning period. They are often harassed by other trout and can be difficult to control, especially when anglers are pursuing them.

Life Expectancy

The life expectancy of a rainbow trout depends on many factors, including where they live and their environmental conditions. In wild rivers, trout usually live from four to six years; in lakes, they can last longer, sometimes up to 11 years.

The best habitats for trout include cool freshwater streams with lots of gravel rock, natural shading and plenty of sunlight. They prefer waters ranging from 54 to 66 degrees Fahrenheit, but can tolerate temperatures as low as 77 degrees in some populations.

They are opportunistic feeders that can take aquatic insects in drifting water flow and terrestrial insects and crustaceans that fall into the stream. In addition, they are also known to consume fish eggs and small fish.

As they grow, trout become more specialized and can feed on smaller fish, especially salmon. Some will also eat freshwater shrimp, which contain the carotenoid pigments responsible for the pink coloration often visible in wild trout.

A young trout is called a fry. After a month, they can swim and feed. They are then called a fingerling and, after two months, a parr.

During periods of rapid growth and aging, trout experience high levels of metabolic activity. This leads to changes in oxidative stress and machinery repair. The trout’s mitochondria are key organelles that help to control metabolism, and a high level of oxidative stress can reduce the health of these organelles.

Favorite Foods

A rainbow trout’s diet consists of a variety of prey items. These include insects, small fish, worms, crayfish, snails, mussels, and small baitfish.

As they grow, rainbow trout begin to focus on larger options. They will often feed on nymphs, midges, caddis flies, and mayflies throughout their lives, but as they mature, they start to focus more on larger prey items like minnows and sculpins.

In the fall, shoals of newly hatched minnows are abundant in lakes and rivers, and rainbow trout will gorge on them whenever possible. During the spawning season, trout eggs are also a common food source.

Another common food choice for rainbow trout is moss and algae. These are not the first things that come to mind when you think of a fish’s favorite foods, but they do occur in many streams and lakes that rainbows call home.

The current will occasionally pull moss and algae into a rainbow’s mouth. While a trout will not eat them on purpose, they will partake out of a quick reaction when a nymph or other emerger is caught off the bottom.

Trout will eat anything alive and the right size, so it doesn’t matter what you throw at them. They’ll gulp down something as small as a zebra midge larva or as large as an adult mouse. Keeping an eye on their preferences and knowing the top choices they’re feeding on will help you increase your catch rate.


A rainbow trout can be a very large fish. They range from 8 inches to 24 inches and can weigh up to 48 pounds. They are commonly stocked into lakes and larger streams to provide anglers with the opportunity to catch them.

Rainbow trout are opportunistic feeders and will eat a variety of different types of food. In addition to aquatic insects, they also consume zooplankton, crustaceans, mollusks, fish eggs, and small minnows.

The size of a rainbow trout can vary depending on the habitat, gender, age, and maturity of the fish. Adult stream rainbows are typically 0.5 to 2.5 kilograms (1 and 5 lbs). Lake-dwelling trout are much bigger, with males reaching 9 kg (20 lb).

They have a bluish-green to the olive-green body and a silvery white belly. They also have a broad reddish stripe along their lateral line that extends from their gills to their tail, which is most pronounced in breeding males.

They are smooth-feeling fish with a soft, rubbery skin that can be easily caught on a three- to six-weight rod. They are a popular species to target with dry flies, nymphs, and streamers. They are also quite responsive to a variety of lures, including spinners, spoons, plugs, and jigs in the 1’’-3’’ size range.

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