Chapter 2:  Pacific Salmon Biology

 Picture of spawning Sockeye (Red) Salmon

Key Concepts


Over millions of years Alaska’s wild salmon have developed a complex life cycle that allows them to thrive and expand into new areas. Each of the five species has developed distinct physical characteristics, different habitat needs, and different timetables for spawning and rearing. All five species are a crucial part of the food web that binds together Alaska’s land and oceans.


Chapter Objectives


Students will be able to explain:

·        the life cycle of salmon and their stages of development;

·        the importance of salmon at each stage in their life cycle to the entire ecosystem;

·        differences among the five species of wild salmon, and rainbow and cutthroat trout;

·        why these differences are so important to the survival of Alaska’s wild salmon;

·        how salmon expand into new areas and why it is important that they do.


Terms Students Should Understand


redd - the nest or depression a female salmon makes in the gravel of a river or stream (for depositing her eggs).

milt – the fluid containing sperm that male salmon produce.

alevin - the first life stage of a salmon after hatching from the egg. At this point a yolk sac is still attached to the young fish’s abdomen, providing a ready made source of food.

fry - a young salmon just emerged from the gravel. The young fish must now find food in the surrounding fresh water and hide from predators.

smolt – a young salmon living in an estuary area prior to outmigrating to the ocean.

parr marks – dark brown vertical stripes on the sides of salmon fry before they become smolts. These markings help provide camouflage to protect the young fish in their rearing areas. They can also be used to identify the different species. Pink salmon do not develop parr marks.

genes – long strands of genetic material responsible for passing on hereditary information in reproduction.

genetic diversity – the mix of genetic materials within a species or stock that allows for:

·        variability of the individual, and

·        adaptability to different conditions by members of the population.

straying – the behavior in which some adult salmon spawn in streams where they were not hatched. Straying assures continued diversity in genetic materials among populations of wild salmon.  Researchers have found that up to one-third of chinook salmon may stray when they return to fresh water to spawn.


Background for Teachers


Why are healthy wild salmon runs important to the Alaska environment as a whole?

Salmon benefit many ecosystems. One study shows that more than 137 species of wildlife rely upon salmon for nutrients at one or more stages of the salmon life cycle. (“Pacific Salmon and Wildlife” by the Washington Department of Fish and Wildlife – See Resources later in this chapter) Without salmon, Alaska would have greatly decreased numbers of bears, sea lions, river otters, bald eagles, mergansers, kingfishers, arctic char, and other animals.

Research has traced isotopes of nitrogen and carbon, two important nutrients, from dead salmon into the ecosystems along rivers. Salmon may provide 18% of the nitrogen in streamside trees, 25-30% of the nitrogen and carbon found in insects, and 25-40% of the nitrogen and carbon in young salmon, which feed on the insects.

Questions For Discussion


1. Why is it important that each female salmon builds several redds, each of which may be fertilized by a different male? 

By depositing their eggs in more than one redd, female salmon increase the possibility that at least some of the eggs will survive to the next life stage. Having more than one male fertilize those eggs assures increased genetic diversity in the population.

The forces ranged against survival of a salmon’s spawn are huge. From disturbance of the gravel by other spawning fish, to flood events that scour streambeds, to predation by dozens of other animals, the chances of survival from egg to fry is very small. As a general rule, in nature only 10 percent of any given life stage of wild salmon can be expected to survive to the next life stage. One estimate in Salmonids in the Classroom suggests that for every 2,500 eggs, two adult salmon return to spawn.

2. What would happen if all salmon returned to their home streams at the same time? 

Salmon partition spawning times as well as areas of the stream. This use of the entire season for returning and spawning makes the most efficient use of any system’s limited resources.

If all salmon returned at the same time, stocks could be mixed, habitats would be strained, and the biological and chemical resources of the systems could be taxed beyond the fishes’ ability to survive. Here’s just one example: When too many salmon return to spawn and die in small systems at the same time, their rotting carcasses may use all available oxygen in the water. This would leave other fish and benthic macroinvertebrates (animals without backbones that are larger than ½ millimeter. They live on the bottom of waters, under rocks, mud, and weeds. They include crustaceans such as crayfish, mollusks such as clams and snails, aquatic worms and the immature forms of aquatic insects such as stonefly and mayfly nymphs) in the system without the oxygen they need to survive.

Encourage students to explore other possible examples.

3. Bald eagles, seals, Dolly Varden trout, and other animals eat great quantities of mature salmon, young fry, or eggs. Why do we no longer offer bounties to kill these predators? How do we know they are not destroying our salmon runs?

“Our” salmon are theirs as well. Natural predation has always occurred. As we have learned more about the natural relationships between salmon and all their predators, resource managers have come to value the complete cycle of life involved with salmon and learned to limit our harvest to take only the surplus of salmon not needed to both renew the runs and recharge the ecosystems.

4. The second paragraph on page 14 describes the habitat qualities each species of salmon needs for reproduction:

·        clean and well oxygenated water;

·        gravel of the correct size and depth;

·        constant cold water temperature;

·        water velocity of the correct speed; and

·        appropriate water depth.

What kinds of things might disrupt one or more of these qualities and cause spawning or egg rearing to fail?

Students might suggest extreme hot or cold weather, too much or too little rainfall, human activities disrupting water flow, changes in the gravel layer, siltation, water depth, etc.

Ideas for Activities


1. Ask each student to choose an animal from the Pacific Salmon Food Web on page 19, then use words or drawings to describe in as much detail as they can how that animal gets salmon for food.

Be sure they include information on:

·   which stage or stages of salmon the animal feeds on;

·   how it’s equipped to catch and consume the fish;

·   what else it eats (Note that that often changes with the seasons); and

·   how it passes along nutrients from salmon to other animals or parts of the ecosystem. (Is it a predator on salmon, for example, and prey for some other animal?)

2. Ask students to determine what species of salmon are found in waters near their community. They could research when the fish normally spawn, what parts of water bodies they use, and some characteristics of places where they spawn. They could gather information from their own experience and by interviewing parents, elders, fishermen and –women, biologists, and others.

A great resource for this is the Alaska Department of Fish and Game Wildlife Notebook pages at:

Wildlife Notebook

3. Have students dissect a salmon.

See the study guide in Getting Into a Fish in Appendix A.

Resources Especially for Teachers


C. Jeff Cederholm and David H. Johnston. 2000. Pacific Salmon and Wildlife: Ecological Contexts, Relationships, and Implications for Managers. Olympia: Washington Department of Fish and Wildlife

This study looks at the relationship between salmon and all fish, plants, and wildlife in Washington state. It is a clear, easy to read summary of the interrelationships between salmon and their habitats. The Washington Department of Fish and Wildlife web site posts an abstract and a link to the complete study at

C. Groot and L. Margolis. 1998. Pacific Salmon Life Histories. University of British Columbia Press

This is the most authoritative biology text on Pacific salmon.

Video: The Miracle of the Scarlet Salmon-NATURE educational programming

This video follows one salmon from the egg through all life stages and its return to spawn. Good for all ages. Available from Jon Lyman, Alaska Department of Fish and Game, (907) 465-6186.

Resources for Students and Teachers


National Geographic

This “Pacific Salmon” lesson plan on the “expeditions”portion of the National Geographic Society web site can easily be adapted for middle school students. The site links to a slide show on the salmon life cycle, a simulation game, and puzzles.

Type “salmon life histories” or “Pacific salmon decomposing” into a web search engine.


ADFG Teacher Resources

This web site links to the Wildlife Notebook series pages on salmon. You can also reach it through the Publications link on the Alaska Department of Fish and Game home page.

Looking Ahead


Ask students to find topical maps of Alaska that show rivers, streams, surrounding oceans, the continental shelf, and other features, then on the basis of what they’ve learned about salmon biology, see if they can suggest some geographic reasons why Alaska can support many healthy stocks of wild salmon.

Use any good map of geographic features. Or see Alaska in Maps: A Thematic Atlas, produced for Alaska schools. Maps in this reference show rivers, lakes, coastline, the continental shelf, ocean currents, and other features that relate to salmon biology and habitats.

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