San Francisco State University
Department of Geography


Geography 316:  Biogeography

In progress 09/27/2005

The Biogeography of  Rainbow Trout (Oncorhyncus mykiss)                             

by  Santiago Rengstorff,  student in Geography 316, Fall 2001       

Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Salmonitormes
Family: Salmonidae
Genus: Onchorhyncus
Species: Onchorhyncus mykiss 
  Rainbow trout figure 1. source:  Dr. Jeff  Goodwin

    Rainbow trout are streamlined, salmonid form fish.  They are cousins to the salmon.  The body shape and coloration vary and reflect habitat, age, sex and degree of maturity.  Rainbow trout body size may range from slender to thick.  The back may be blue- green to olive; coloration of each trout changes throughout the year.  Reds, violets, greens and yellows become richer as the breeding season gets closer (Greenhalgh 1989).  The body has black spots on the upper body above the lateral line and also on the upper fins and the tail and a brilliant reddish band along each side.  The lower sides are usually silver, fading to pure white beneath.  Rainbow trout have mouths that don’t go past the back of the eye, and they don’t have teeth at the base of the tongue.  The color of the rainbow trout usually blends in with their surroundings.  Rainbow trout usually mature at age 3 to 5 and grow to about 6 to 16 inches long, and can live up to 11 years.


Rainbow trout (Steelhead) Figure 2. Watercolour specimen study, Rod Sutterby


    The natural habitat of the rainbow trout is the cool waters of the Northern Hemisphere, but the trout have been introduced throughout the world.  Rainbow trout are native to the Pacific Coast of Northern America from Alaska to Mexico, the Pacific Ocean, and the eastern coast of Asia (Roberts 2001).  The rainbow trout now is stocked all over the United States, except in some of the southern most states, and in all the other continents.  The trout does not stock well in lakes in which a population of anadromous fishes exists already. Anadromous fish are fish that mature in the sea but migrate to fresh water to breed. There are two types of rainbow trout, the freshwater river rainbow trout and the part time river, part time sea-going rainbow trout, known as steelhead trout.



         Rainbow trout figure 3.  By John White.
 Natural History:  

  The diet of the rainbow trout varies; some eat almost everything that drifts by them like insect larvae and pupae.  Rainbow trout are carnivores, eating mostly aquatic insects; also they may eat crayfish, grasshopper, worms, salamanders, and other fish (including other trout).  Most of their diet consists of mayflies, caddis flies, stoneflies and their larvae, pupae, nymphs; also small mollusks, and algae.

            The rainbow trout will go in search of food if the stream isn’t carrying much food.  The trout will cruise the pool or go upstream in search of food.  They can cover a lot of water looking for food (Greenhalgh 1989).  Some rainbow trout eat so much they grow much faster than other trout in similar water.


    With this fast growth, rainbow trout reach sexual maturity earlier than other trout.  Rainbow trout may spawn first when they reach about 12- 16 inches in length, which is usually at the end of their second year (Roberts 2001).  During late winter or early spring, when water temperatures are rising, the maturing adult rainbow trout seek out the shallow gravel riffles or a suitable clear water stream.  Spawning occurs from late March through early July, depending on the location and conditions of the winter.  Most trout spawn in streams, rivers, and lakes with gravel bottoms and steady water flow.  Trout spawn in spring or autumn.  In early spring, rainbow trout move upstream to a spawning area, then the female choose a suitable site usually a shallow, gravel area at the beginning of a stretch of choppy water.  Then she turns on her side and beats her tail up and down, scooping out a shallow nest or redd.  When the redd is prepared, she positions herself over it.  Then the male courts her by swimming near her and shaking his body.  When the female is ready to spawn she moves to the bottom of the redd and presses her belly against the gravel.  Then 200 to 8,000 eggs are deposited in the redd, fertilized by a male and covered with gravel.  Hatching normally takes a few weeks to as much as four months, depending on water temperature.  A few more weeks may be required for the tiny fry to emerge from the gravel.  Then the small trout gather in groups and take shelter along the stream margins or protected lakeshore, feeding on crustaceans, plant material, and aquatic insects and their larvae.  The young trout eat mostly invertebrates (animals without backbones), including insects.  Rainbow trout then stay in a similar habitat for the first two or three years then move into the larger water of lakes and streams and turn to more of a diet of fish, salmon carcasses, eggs, and even small mammals. 

 Rainbow trout figure 4. Watercolour specimen study,  Rod Sutterby 

    The most primitive of the bony fish are trout and their relatives.  The Wisconsin episode, the most recent of the four Pleistocene glaciations (approximately 40,000 years ago) is thought to have been important in the current distribution of rainbow trout.  When these glaciers melted, huge lakes were left all over the current range of the western U.S.A.  When these lakes slowly receded, the subspecies of trout were isolated.  Rainbow trout were restricted south of the Columbia River until the late Pleistocene glacial events.  Then rainbow trout spread into the Columbia River drainage between 50,000 and 32,000 years ago.  When the glacial lakes of the Pleistocene receded, the subspecies became more and more isolated.  Both the California Golden Trout and the Sacramento Red-band Trout suspected to be the most primitive Oncorhyncus mykiss subspecies.  The origin of a rainbow trout-like subspecies is near the Gulf of California (Roberts 2001).

copyrighted :  not for public use
Michigan Interactive™

 Rainbow trout figure 5. From Fishweb Inc.


    The rainbow trout (Oncorhyncus mykiss) are native to the drainages of the United States Pacific Coast from Alaska to Mexico, the waters of the Pacific Ocean and the eastern coast of Asia.  This natural range goes from the Kuskokwim River region of Alaska to the Baja California Peninsula and the coastal rivers of Mexico (Greenhalgh 1989).  In this area, many of the rivers draining into the Pacific still contain rainbow trout, including the Mackenzie River of Oregon, the McCloud and Kern Rivers of California, the Babine River of British Columbia, and Kenai- Russia River system of Alaska.  But during the last 100 years, rainbow trout have been introduced into countless other waters throughout the world.  Today, many rivers in the USA and Canada and around the world have stocks of rainbow trout (and many other species of trout), but these aren’t native wild trout (Greenhalgh 1989).  The only continent that doesn’t have trout is Antarctica.

    There are six native subspecies of rainbow trout in the western United States and Pacific Ocean.  They are Oncorhyncus mykiss gairdneri (Columbia River red band trout), Oncorhyncus mykiss aguabonita (California golden trout), Oncorhyncus mykiss gilberti (Kern & Little Kern golden trout), Oncorhyncus mykiss stonei (Sacramento red band trout), Oncorhyncus mykiss irideus (Coastal rainbow trout), Oncorhyncus mykiss mykiss (Kamchatkan rainbow trout).

    The six subspecies are distributed throughout the drainages of the United States Pacific Coast from Alaska to Mexico, the waters of the Pacific Coast, and the eastern coast of Asia.

    Two subspecies of the rainbow trout live along the coastal regions of the rainbow trout’s native range.  The coastal rainbow trout (Oncorhyncus mykiss irideus) live along the Pacific Coast of North America, from the Kuskokwin River in Alaska to Rio del Presidio in Mexico.  The coastal rainbow trout also inhabit waters of inland North America west of the continental divide.  The Kamchatkan rainbow trout (Oncorhyncus mykiss mykiss) ranges from the Kamchatkan Peninsula, as far east as the Commander Islands of the Pacific Ocean, to as far south as the mouth of the Amur River on mainland Asia.

    Rainbow trout can be freshwater river trout or seagoing trout.  Seagoing rainbow trout are known as steelhead rainbow trout and are usually silvery color and bigger than river rainbow trout.  Steelhead trout live part time in the rivers and part time in the ocean.  The river rainbow trout live only in rivers their whole life.  They are smaller and blend in with the river.  Steelhead trout are born in rivers but later venture out to the ocean and adapt to the salt water.  Steelhead trout grow bigger and change color once they live in the ocean.  Steelhead trout stay in the ocean part of their life but come back to the river to spawn.  They are bigger than river rainbow trout and dominate the rivers when they return.  Both are carnivorous and are prized by anglers for their fighting nature when caught on a fishing line.  The steelhead is noticeably different than river rainbow trout in size and color.

    Map of Distribution:

  Rainbow trout map figure 6. From UC Davis.

    Other interesting issues:

        Some other common names for the rainbow trout are Bow, Red-band, Silver trout, Redsides and other names from other countries.

          Pollution is affecting the trout in many bad ways, including the loss of trout in many rivers throughout the world where they have been introduced.  Trout suffer directly from the toxic and chemical wastes being dumped directly into the rivers.  Also certain toxic wastes are dumped into rivers without being treated and in large amounts killing aquatic life and increasing the temperatures of these rivers. This effects organisms that are used to certain temperatures for sustainable life in that river.  Also wastes disrupt oxygen levels causing organisms to die and/ or become sick if they can’t adapt to these changes.  Also dams affect trout because they can’t migrate and these dams disrupt the usual patterns of rivers that trout are used to.  Trout aren’t used to higher or lower levels of water causing them confusion when spawning or feeding.  Dams prevent trout from accessing areas they usually go to find more food or going out to the sea and back.  As a result of man’s disturbance to nature many rivers have less trout and must have trout raised in fish hatcheries then brought to rivers.  These trout are raised eating different food and must adapt to life in a river.  Another problem is these hatchery trout conflict with the trout there by competing for survival.  Pesticide runoff, other chemicals, acid rain, or mercury from nearby mines enter rivers after heavy rainfall poisoning the trout making them unsafe for consumption.  Trout are important for rivers to maintain their ecosystem so when they or their diet are disturbed from pollution the river suffers with them.  Trout also are a food supply for humans and wildlife and a decrease in their numbers affects us and wildlife.  Healthy rivers attracts anglers and maintain a healthy ecosystem in that area and this should not be altered by pollution from nearby companies that dump their waste directly into the river before being treated or regulated.  Also trout fishing is major business that brings in a lot of money and is pastime for many, not to mention a food source for humans and wildlife.  It’s important to keep pollution out of rivers for the benefit of all.

     Becker, C. Dale and Neitzel, Duane, A.  WATER QUALITY IN NORTH AMERICAN RIVER SYSTEMS, 1992, Battelle Press, Columbus, Ohio.

    Canby, Thomas Y, August 1980. Water, Our Most Precious Resource, page 144.  National Geographic, Vol. 158, No. 2.

    Freeze, Allan R. 2000.  The Environmental Pendulum, University of California Press, Berkeley, Los Angeles, London.

    Gabrish, David M., Fishweb Inc. Figure 5., Available:

    Goodwin, Dr. Jeff, Figure 1., Available:

    Greenhalgh, Malcolm and Sutterby, Rod.1989, George Phillip Ltd, London.  From the book: The Wild Trout, The Natural History Of An Endangered Fish, [Online] Available:  Accessed 10/12/01, Figures 2 and 4 also from Sutterby and Greenhalgh's website, another link is

    Maitland, Peter S, 1978.  Biology of Fresh Waters.  A Halsted Press Book: New York- Toronto.

    Mills, Derek, 1971.  Salmon and Trout.  St. Martin’s Press:  New York.

    Netboy, Anthony, 1980.  The Columbia River Salmon and Steelhead Trout, Their Fight For Survival.  University of Washington Press: Seattle and London.

    Newman, Cathy, April 1989.  A Passion for Trout, page 64.  National Geographic, Vol. 189, No. 4.

    Roberts, Shane, 2001. Life history of rainbow and steelhead trout (Oncorhyncus mykiss) and subspecies, [Online] Available: Accessed 10/30/01.

    U.C. Davis,  Figure 6.  Available:

   White, John., Figure 3.  Available:

    Yapp, W. B., The Institute Of Biology, 1959. The Effects Of Pollution On Living  Material.  Church Army Press.  Cowley, Oxford, England.


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