Chapter 1:  How Salmon Evolved and Adapted

 Picture of Coho (Silver) Salmon

Key Concepts

Salmon and related species have evolved and changed over millions of years. Genetic diversity allowed different species and populations to adapt and survive under different conditions in various streams, lakes, and rivers. The genetic variety among these fishes is crucial to the survival of Alaska’s salmonids.


Chapter Objectives


Students will be able to explain:

·        how salmon and related fishes evolved over the ages;

·        some of the evolutionary benefits salmon have gained by adaptations such as being anadromous;

·        why genetic diversity and healthy habitats are essential to the survival of wild salmon in Alaska.


Terms Students Should Understand


salmonids (sal-MON-ids) – salmon and the other species related to them, such as trout and whitefish

Oncorhynchus (on-kor-IN-cus) – Latin name for the genus that includes salmonids

anadromous (an-AD-ruh-mus) – a term describing fish that spawn in fresh water and spend part of their lives in salt water


Background for Teachers


How did salmonids evolve?

Between 50 and 100 million years ago, when the land masses which now surround the Atlantic Ocean were still joined together, a small fish, possibly resembling today’s Arctic grayling, lived in the cold waters of what was to become Northern Europe. It was the Age of the Dinosaurs.

Scientists believe something happened to a group of these small fish that caused the numbers of their chromosomes to double in each cell. That change left all descendents of these fish with twice as many chromosomes in each cell as other members of similar ancient fish.

This doubling of chromosomes is called tetratriploidization. It is a component of evolution in many species, and allows increased diversity and adaptability. All the extra genetic material salmon gained may be why salmon are able to live part of their life in fresh water and part in salt water.

Scientists have identified a few species of ancient fishes that were most likely salmonids. The earliest of these fossils identified to date is Eosalmo driftwoodensis, which lived about 50 million years ago. Another early salmonid was Smilodonichthys, the sabertoothed salmon (page 8).  Smilodonichthys existed between 10 and 15 million years ago, during the middle Miocene epoch. From evidence in the fossil record, we know it was a filter feeder which probably used its large tooth in displays or fighting when breeding. It grew to a length of more than 6 feet and had a single large fang extending beyond the snout.

How do we know salmon evolved and didn’t start out the way they are now?

The fossil record provides many examples of earlier salmonid species, and the study of DNA allows us to find the relative times in the past when species diverged.

Why should we worry that salmon won’t evolve and adapt to changes in the environment today?

Pacific salmon are very adaptable, but today too much climate change may be happening too fast. It takes hundreds of thousands of years for a species to evolve. Our present species of salmon have managed to survive in the North Pacific during the past six million years. Individual stocks must have faced, and sometimes been destroyed by, glaciers, volcanoes, rock slides, huge flood events, massive changes in sea level caused by continent-wide ice sheets, and other natural forces. Over a number of generations, one stock might perish, but others could adapt and colonize new habitat as opportunity occurred.

Today, human activities and other natural cycles are causing rapid changes in habitat, water quality, and water quantity. Climate change may modify freshwater and ocean conditions on a large scale. Salmon cannot adapt or evolve to so many changes over so short a time.

What physiological changes must take place for a young salmon that grew up in fresh water to survive in salt water?

Salmon just emerged from the gravel are called fry. Pink and chum salmon fry migrate very quickly to estuary areas and out to sea, Their period of change from freshwater to saltwater species is very brief. This change, called smoltification, takes longer in other Pacific salmon and involves much greater change in the young fish.

Fry of chinook, coho, and sockeye salmon spend more time in fresh water (See chart on p. 17 of Alaska’s Wild Salmon). When they migrate to the ocean they must first re-orient themselves from seeking cover and hiding in the stream margins, to swimming in the surface of flowing waters. They continue to swim upstream, but the increased current speed and large flows of spring soon wash fry into estuary areas.

 Estuaries are very fertile mixing zones, where the sea and fresh waters mingle. Here the young smolt undergo physiological changes that adapt them to life in saltwater. They become tolerant of increased amounts of salt, they change color and loose their parr marks, and they grow rapidly. Within a few weeks or months at the most, surviving salmon smolts are ready to head to the sea, where they will feed and grow into adults.

Questions For Discussion

1. What are some of the evolutionary adaptations that help salmon and other fish live successfully in water?

Fish have streamlined bodies that create little resistance in the water, gills to extract oxygen, scales for flexibility of movement and protection.

2. From the perspective of a salmon, why would you, as a species, go through all of the changes required to survive? 

It was not a choice for salmon to do as they do. Remember that they evolved in an ever-changing world. They had to adapt to survive. Some populations of salmonids did not migrate to the sea. Some became extinct. Others became the cutthroat trout, Dolly Varden char and rainbow trout we know as freshwater fish. (It is interesting to note that fully a third of all rainbow trout migrate to the ocean and return to fresh water as steelhead.  Dollies and cutthroats also may migrate to the sea for brief periods.)

What all anadromous fishes find in the ocean is an enormously rich smorgasbord of food (more than could be found in their natal fresh waters). There must be advantages to each species of salmonids in the way they live. Each has adapted to fill its specific niche during all stages of its life cycle. For Pacific salmon, it is an advantage to breed in fresh water and migrate to salt water.

Ideas for Activities

1. Have students do a web search using “evolution of fishes” and similar terms in a search engine.


2.  The study of glaciers is a great way to help students understand the changing surface of the earth.

See web site in Resources, this chapter.

3. Have students use modeling clay to make "fossil" imprints of natural objects.


Resources Especially for Teachers

BBC Evolution Web site

Biology and Evolutionary Theory

These three web sites have good information for teaching about evolution.

Joseph Taylor III. 1999. Making Salmon: An Environmental History of the Northwest Fisheries Crisis.  Seattle: University of Washington Press.

This is perhaps the best documented work on the history of the salmon crisis in the Pacific Northwest. Informative and a good read!

Salmon & Survival

The primary purpose of this site is to address the failure of hatcheries to solve the crisis of declining salmon stocks, but in the process it provides good information about how salmon evolved and adapted—and why that ability is important to their survival.


This web site from the South Dakota Alliance for Distance Education offers several lesson plans for teaching about glaciers.

Resources for Students and Teachers


Biology and Evolutionary Theory

This site introduces the basics of evolution.

Matsen, B. and R. Troll. 1994. Planet Ocean, A Story of Life, the Sea, and Dancing to the Fossil Record. Ten Speed Press. 133 pp.

See also Web site:

Many students will be attracted by the highly visual “Weird Science” approach to evolution and weird fish by Ketchikan artist Ray Troll.

Jim Licatowich. 1999. Salmon Without Rivers:  A History of the Pacific Salmon Crisis.  Washington, D.C.: Island Press.

Chapter 1 of this book traces the evolution of salmon and their adaptation to the changing geologic landscape in the Pacific Northwest.

Physical Geography

This site on basic physical geography at Okanagan University College in British Columbia has good basic information on such topics as natural selection and evolution, and factors influencing distribution of species.

California's Salmon and Steelhead

This site contains the full text of California’s Salmon and Steelhead edited by Alan Lufkin and published by University of California Press, 1996. Chapter 6, entitled “Why All the Fuss About Preserving Wild Stocks?” has an excellent subsection on ‘Natural Selection’ and the genetic importance of wild fish. A short search route is to type the book title into Google or another search engine.

Entering “Pacific wild salmon, evolution and adaptation” into a web search engine will yield a great deal of information.  Adding additional descriptors such as “survival” and others can focus the search.  The following website is an example: Preserving Salmon Biodiversity

You can also reach this article on Preserving Salmon Biodiversity by searching the archives on the American Scientist web site.

Looking Ahead


Ask students how they or members of their family or community distinguish between different species of fish they may catch. Why is it important to be able to tell what species a fish belongs to?


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