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Chapter 2: Pacific Salmon Biology
Key Concepts
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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.
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Chapter Objectives
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Students will be able to explain:
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the life cycle of salmon and their stages of development;
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the importance of salmon at each stage in their life cycle
to the entire ecosystem;
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differences among the five species of wild salmon, and
rainbow and cutthroat trout;
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why these differences are so important to the survival
of Alaska’s wild salmon;
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how salmon expand into new areas and why it is important
that they do.
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Terms Students Should Understand
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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:
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variability of the individual, and
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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.
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Background for Teachers
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Why are healthy wild salmon runs important
to the Alaska environment as a whole?
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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.
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Questions For Discussion
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1. Why is it important that each female
salmon builds several redds, each of which may be fertilized by a different
male?
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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.
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2. What would happen if all salmon returned
to their home streams at the same time?
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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.
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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?
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“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.
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4. The second paragraph on page 14 describes
the habitat qualities each species of salmon needs for reproduction:
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clean and well oxygenated water;
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gravel of the correct size and depth;
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constant cold water temperature;
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water velocity of the correct speed; and
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appropriate water depth.
What kinds of things might disrupt one or
more of these qualities and cause spawning or egg rearing to fail?
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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.
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Ideas for Activities
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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.
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Be sure they include information on:
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which stage or stages of salmon the animal feeds
on;
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how it’s equipped to catch and consume the fish;
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what else it eats (Note that that often changes with
the seasons); and
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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?)
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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.
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A great resource for this is the Alaska
Department of Fish and Game Wildlife Notebook pages at:
Wildlife Notebook
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3. Have students dissect a salmon.
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See the study guide in Getting Into a Fish
in Appendix A.
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Resources Especially for Teachers
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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
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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 www.wa.gov/wdfw/hab/salmonwild
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C. Groot and L. Margolis. 1998. Pacific
Salmon Life Histories. University of British Columbia Press
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This is the most authoritative biology text
on Pacific salmon.
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Video: The Miracle of the Scarlet Salmon-NATURE
educational programming
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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.
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Resources for Students and Teachers
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National
Geographic
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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.
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Type “salmon life histories” or “Pacific
salmon decomposing” into a web search engine.
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ADFG
Teacher Resources
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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.
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Looking Ahead
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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.
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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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