How many salmon?

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How Many Salmon?

Lesson at a glance:

Students will use their math skills to learn just how difficult a salmon's life cycle really is.

Relevant National Curriculum Standards and Benchmarks:

Math standard (3): Uses basic and advanced procedures while performing the processes of computation

Benchmark: Upper Elementary (Grades 3-5): Adds, subtracts, multiplies and divides whole numbers and decimals.
Benchmark: Upper Elementary (Grades 3-5): Determines the effects of addition, subtraction, multiplication and division on size and order of numbers.
Benchmark: Middle School/Jr. High (Grades 6-8): Selects and uses appropriate computational methods for a given situation.

Math standard (6): Understands and applies basic concepts of statistics and data analysis

Benchmark: Upper Elementary (Grades 3-5): Understands that data represents specific pieces of information about real world objects or activities.

Math standard (9): Understands the general nature and uses of mathematics

Benchmark: Upper Elementary (Grades 3-5): Understands that numbers and operations performed on them can be used to describe things in the real world and predict what might occur.

Science standard (4): Knows about the diversity and unity that characterize life

Benchmark: Upper Elementary (Grades 3-5): Knows that plants and animals progress through life cycles of birth, growth and development, reproduction, and death; the details of these life cycles are different for different organisms.

Science standard (7): Understands how species depend on one another and on the environment for survival

Benchmark: Upper Elementary (Grades 3-5): Knows that changes in the environment can have different effects on different organisms.
Benchmark: Upper Elementary (Grades 3-5): Knows that all organisms (including humans) cause changes in their environments, and these changes can be beneficial or detrimental.
Benchmark: Middle School/Jr. High (Grades 6-8): Knows how an organism's ability to regulate its internal environment enables the organism to obtain and use resources, grow, reproduce, and maintain stable internal conditions while living in a constantly changing external environment.
Benchmark: Middle School/Jr. High (Grades 6-8): Knows factors that affect the number and types of organisms an ecosystem can support.
Benchmark: High School (Grades 9-12): Knows ways in which humans can modify ecosystems and cause irreversible effects.

Science standard (8): Understands the cycling of matter and flow of energy through the living environment

Benchmark: Upper Elementary (Grades 3-5): Knows that the transfer of energy (e.g., through the consumption of food) is essential to all living organisms.

Geography standard (8): Understands the characteristics of ecosystems on Earth's surface

Benchmark: Upper Elementary (Grades 3-5): Knows ways in which humans can change ecosystems.

Geography standard (14): Understands how human actions modify the physical environment

Benchmark: Upper Elementary (Grades 3-5): Knows ways people alter the physical environment.
Benchmark: Upper Elementary (Grades 3-5): Knows ways the environment is stressed by human activities.
Benchmark: Middle School/Jr. High (Grades 6-8): Understands the environmental consequences of people changing the physical environment.
Benchmark: Middle School/Jr. High (Grades 6-8): Understands the ways in which human changes in the physical environment in one place can cause changes in other places.

These national standards and benchmarks are summarized at http://www.mcrel.org/standards-benchmarks/

Materials:

Scratch paper and a pencil

A calculator (optional)

Background:

A salmon's life cycle
The salmon life cycle begins as an egg buried in loose gravel in a cool stream with lots of oxygen. When the salmon first hatch they are called alevins. They wait in the gravel until they finish the last of their yolk then hurry to the surface for a quick gulp of air to fill their swim bladders.

The little salmon, now called fry, are whisked downstream with the current towards the ocean. Some species hang out in their native streams for months or even years, hiding in snags and other slack waters and feeding. Others ride the current to estuaries where they will wait while their bodies go through necessary changes for life in the salty oceans.

As the young fry near the estuary they become smolts. Their scales grow, they become more silvery to help blend into the ocean environment. The smolts feed like mad in the estuary, trying to become as big as they can before braving the treacherous waters of the Pacific ocean.

Salmon at sea might migrate thousands of miles or they may stay right off the coast. They may spend anywhere between 2 and 8 years feeding in the ocean before returning to their native rivers and streams to spawn. Their homing abilities are legendary and not entirely understood.

On the way back salmon make another stop in coastal estuaries. Like last time, their bodies go through chemical changes so they can survive in the freshwater of their home streams. This time they also stop eating and adopt their flashier, spawning colors. Males may get hooked beaks and humped backs. The journey home is a daunting one, only the strongest will survive to reproduce.

Those salmon who make it will go through the final task of their lives. The females will build gravel nests with their tails called redds. and the males will fight for the opportunity to fertilize the females eggs. Although some steelhead and cutthroat will live to spawn again, most anadromous salmon will die after spawning. Their bodies feed the stream environment.

Perils of the journey
The journey to and from the spawning grounds is dangerous indeed.

Not even the egg is safe. Some predators prefer eggs because they are high energy sources of food. Careless hikers, unleashed dogs and other disturbances can destroy redds and either crush eggs or loose them to drift downstream. too much erosion can cause silt to smother the redds and suffocate the eggs. Streamside vegetation and responsible forestry can help reduce this hazard.

When the fry emerge from the gravel they are very vulnerable to predators. They are small and need to learn very quickly how to hide. Predators are natural but around dams predators gather to await salmon fry that are spilled over the top or rocketed through bypasses. Young salmon must also be wary of dam turbines, and irrigation water diversions. When the plant life along the streamside is removed the temperatures in the stream increase, the flow is reduced and their are fewer insects to eat. Pollution from cities and farms also takes its toll.

At sea, adult salmon must deal with not only the larger oceanic predators like sharks and killer whales but also with commercial fisheries.

And the trip back upstream can be even more dangerous. Adults have to face natural predators like bear and eagles plus both commercial and recreational fishers. Dams bar the path of migrating salmon. Those who make it up the fish ladders may become confused or even sick in the warmer slack waters above the dam. Pollution, erosion and water diversions play key roles in preventing adults from successfully spawning by destroying good salmon spawning habitat.

Activity:

Print out the "How Many Salmon?" worksheet. Have your students fill in the blanks to learn more about the salmon's risky life cycle. Answers: 105,000 | 52,000 | 48,000 | 44,500 | 29,500 | 18,500 | 7,000 | 1,600 | 900 | 650 | 12

Summary:

With your students (or kids or friends), discuss the following questions:

What kinds of natural hazards did the salmon face?
What kind of human made hazards did the salmon face?
How would your numbers change if you removed all the human-made hazards?

Extensions:

Discuss what you can do to help more salmon make it home to spawn
Have students research individual hazards (e.g., dams, predators, fishing) and report back to the class.
Have students write a narrative about the journey of one salmon.