Recommended for grades 4-6

Key Elements of Lesson 4:

  • Students will explore how salmon are adapted for water, including breathing, positioning, motion & balance, protection from disease, and the senses.
  • Students will learn the external and internal anatomy of salmon and the function of each body part and organ.
  • Students will compare and contrast salmon with human anatomy.

Outcomes of Lesson 4:

  • Students will be able to draw, identify and describe the purpose of each part of a salmon’s external anatomy.
  • Students will be able to describe how salmon use their fins to control their balance, direction and movement in the water.
  • Students will be able to identify and describe the function of a salmon’s internal organs.
  • Students will observe a salmon dissection.
  • Students will be able to describe differences and similarities between human and salmon internal anatomy.

Learning Galleries and Activities:

This is a mature, adult chinook salmon. Let's focus on the important external body parts...
Study these internal organs of a salmon. In this case, a female salmon. When you're ready, click next to take the quiz...


  • Present the diagrams as overheads and quiz your students during the presentation. (PROVIDE A PDF OF THE LABELLED ANATOMY.)
  • If you have fish in your classroom: have your students observe them and try to determine how each fin is used by the fish to move, balance and turn.
    • Salmon swim mainly by lateral movements of the tail (caudal) fin, while the paired fins are held closely against the body and the remaining unpaired fins are spread out to keep the fish in a vertical position. This is a good way for students to learn the names of the fins.
  • Print out & test your students: once your students have had time to study the anatomy of salmon, print out the PDF’s of our un-labelled diagrams and test them by having them fill in the labels away from their computer. (PROVIDE PDF OF INTERIOR/EXTERIOR ANATOMY WITHOUT LABELS.)

Salmon External Anatomy

Salmon (and all fish) have special fins and features which help them survive in their watery habitat. 

Salmon — just like you — need oxygen to live. You have lungs that remove oxygen from the air. Salmon have gills (located under the gill cover pictured here) that remove oxygen from the water. Water flows in through their mouth, passes through the gills where carbon dioxide from the fish is exchanged for oxygen from the water. Just like humans expel carbon dioxide when we breathe, salmon expel carbon dioxide when the water flows out through the gill vents.

Salmon are streamlined. This hydrodynamic shape allows them to move fast, slipping through the water with the least resistance, using the least amount of energy. This shape allows them to make their way up rapidly flowing rivers against strong current. Compare this to other fish, such as a flounder, whose flat shape allows it to hide on a sandy or muddy bottom.

Fins allow salmon to move, aim and balance:

  • A salmon propels itself forward using its caudal fin, or tail fin;
  • Balances and turns with help of its dorsal, pelvic and anal fins; and
  • Turns and brakes using its pectoral fins; however…
  • The purpose of the adipose fin, located between the dorsal and caudal fins, is less understood. There is evidence to support that it detects turbulence in the water and provides feedback to the caudal fin to swim more efficiently. Most hatcheries clip the adipose fins on Chinook and coho to mark the fish as hatchery fish. In these cases, an unclipped adipose fin indicates a natural-origin (non-hatchery) Chinook or coho. In fact, it is often illegal for anglers (people who fish) to keep unclipped salmon, to protect threatened wild runs. 

Salmon have evolved to avoid being seen by predators. Adults in the sea are white on the bottom, so to a predator looking up from below (such as a shark or orca), a salmon will blend in with the light of the sky. Salmon are dark on top, so a predator looking down (such as an osprey, eagle or heron) cannot discern them from the darkness of the water below. And finally, salmon are silver on the sides. Silver helps them blend with reflected light in the water when viewing them from the side. You can think of their silver color as something like an “invisibility cloak,” allowing them to blend in with the background. But rather than magic, it uses the science of reflected light in the water column.

Salmon fry, growing up in streams and lakes, frequently have vertical bars which help them blend with the grasses and sticks found in those environments.

Salmon, like most fish, have scales that cover their body. Scales protect their body like clothes protect your body. The scales, in fact, slide over each other, allowing the fish to move and turn. 

Like most other fish, salmon also have a protective slime that coats their body. The slime is essential, protecting them from fungi, disease, and parasites. Fishing note: when handling a fish that you intend to release back into the water, be mindful of the importance of this slime to the fish’s survival. Wet your hands before handling the fish (this reduces the friction of your hands across the fish’s body, preserving the slime).


  1. Smell – Salmon have an extremely keen sense of smell. This sense is based on two nostrils (also called nares — pronounced “nair-eez”) located between their snout and their eyes. They are for smelling purposes only — salmon do not breathe through their nares. Salmon’s sense of smell is much more sensitive than even that of a dog, and salmon use this ability to “smell their way” back to the home streams to spawn.
  2. Sight – Like many predators, salmon have well-developed sight. Salmon can use their eyes independently. One eye may be looking up, while the other is looking ahead and down!
  3. Taste – Just like you, fish taste what they eat. They have taste buds in their mouths and on their lips.
  4. Hearing (for salmon, this also includes “feeling”) – Salmon have two ways of hearing and sensing sound and movement around them. First, they sense (feel) sound pressures and vibrations in the water through a series of nerves located along their lateral line (see image). It enables them to feel the movement of another fish or animals (for example, humans) entering the water. Second, salmon have internal ears, with which they can detect pressure changes caused by sounds at a farther distance. 
Every stream has a unique odor, consisting of the dissolved minerals, plankton and even pollution that is unique to every stream.
Some fish have interesting adaptations: Catfish have taste buds outside of their mouth, including their whiskers (barbels). Catfish can taste their food before they even put it in their mouths!

Learning Gallery: Learn the External Anatomy of Salmon (Click Image to Launch)

Helpful note: fins in all fish are always of two kinds: 

  • Paired – which includes the pectoral and pelvic fins; and 
  • Unpaired – which includes the dorsal, caudal (tail), adipose and anal fins. 

Salmon Internal Anatomy


The gills consist of two components which have entirely different puporses:

  1. Gill filaments are for breathing.
  2. Gill rakers are for filtering food.

Gill filaments are blood-filled membranes (thin sheets of tissue) that absorb oxygen and discharge carbon dioxide. Similar to what lungs do for humans and other land animals.

We discuss gill rakers below.

Food Intake

Grabbing food
Most salmon use their small teeth to aid them in grabbing and swallowing prey. Young salmon eat insects, larval and other small fish in freshwater, and small fish and squid in saltwater. Larger salmon eat herring, anchovy and sand lance. They don’t chew their food — they swallow it whole. The food digests in the stomach, pyloric caeca (like human small intestine) and then the large intestine before exiting through the vent, located near the anal fin.

Filter feeders use their gill rakers
Some salmon do not use their teeth to grab food, but instead filter tiny organisms from the water (such as plankton and krill) with their gill rakers, and swallow it. Sockeye — but also pink and chum — specialize in filter feeding. Sockeye are almost exclusively filter feeders, and have more-and-smaller gill rakers. Young filter feeders rely on ___, while larger salmon filter shrimp and krill.

Processing Food


  1. The food that a salmon consumes passes down its esophagus to the stomach where digestion begins.
  2. The digesting food passes through the pyloric caeca to the large intestine, where it is further digested and nutrients are absorbed into the bloodstream.
  3. Waste is excreted as feces and sometimes urine (in saltwater, salmon don’t typically urinate much at all)  through the vent.

Stomach (digestion)
The stomach is a sac-like digestive organ, fed by the esophagus (after the mouth). Enzymes in the stomach begin the digestive process. The stomach opens into the intestine.

Pyloric caeca (digestion and some absorption)
Part of the digestive system, in the vicinity of where the stomach and intestine meet. While it is not entirely understood, the pyloric caeca absorbs some nutrients into the blood stream.

Intestine (continued digestion and absorption)
The intestine receives partially digested food from the pyloric caeca and continues the digestive process while also transferring nutrients to the blood stream. It passes waste along to the vent.

Urine, feces and reproductive products (eggs or milt) all exit the body via this opening just in front of the anal fin.

Blood Circulation

This two-chambered organ is the pump that moves blood around in the fish, much as in humans, for two important purposes:

  1. Delivers oxygen (from the gill filaments) and nutrients (from the small intestine) to internal organs, and
  2. Removes carbon dioxide from internal organs and expel it from the body via the gill filaments (similar to how our lungs expel carbon dioxide to the air).

The liver is a large organ in salmon and performs several functions:

  1. It is responsible for filtering dead cells from the blood stream.
  2. It makes essential amino acids from digested food, which build protein.
  3. It makes and releases bile — a substance that breaks down fatty foods into fatty acids, which the intestine is then able to absorb into the body.

The spleen produces and stores red blood cells, which are the vehicles in the blood stream that:

  1. Carry oxygen from the gill filaments to organs throughout the body, and
  2. Carry carbon dioxide from organs back to the gill filaments to be expelled.

The kidney performs two important roles in salmon:

  1. It filters nitrogen waste from the bloodstream. 
  2. It maintains the balance of electrolytes (salts) in the blood, which we discuss in the next section.
Regulating salt

While salmon and humans might seem worlds apart, we both require a very specific balance of salts in our bodies. Salmon (and other anadromous fish) are fascinating in this regard because of their unique ability to migrate between fresh and salt water. They must maintain the same salt balance, regardless of whether they are in fresh or salt water. While in fresh water, they must retain enough salt in their system. While in salt water, the opposite is true: they must limit the amount of salt in their system. This unique ability is called osmoregulation, and it relies on two organs:

  1. Gill filaments: Gill filaments are permeable — they allow oxygen from the water to flow in to the bloodstream and carbon dioxide from the blood stream to flow out to the water. This permeability enables them to perform another essential function: 1) to pass water out of the bloodstream when they are in saltwater, and 2) to pass water into the bloodstream when they are freshwater. Let’s examine how this works. When they are in saltwater, they: A) pass water out of their gills, and — to prevent dehydration — they B) avoid peeing as much as they can and C) drink lots of water (yes, salmon do drink). When they are in freshwater, they: A) absorb water through their gills and — to prevent too much water flowing into their bloodstream — they B) don’t drink water, and C) pee frequently to prevent bloating.  All of this also relies on the special behavior and function of the kidney.
  2. Kidney:
    1. While in fresh water, a salmon’s kidney works to ensure a high enough concentration of salts in the blood. During this time, salmon excrete very low-salt urine as the kidney works to retain salts.
    2. In salt water, the opposite is true: the kidney dilutes the concentration of salt in the blood. During this time, salmon excrete minimal but highly salty urine as the kidney works to maintain a lower concentration of salts in the body compared to the surrounding seawater.

Urinary bladder
Urine is collected from the kidney and exits via the vent. Urine contains nitrogen and salt (the amount of which depends on whether they are is fresh or salt water, as we describe above).

Sensory organs

The brain is a central nervous system, similar to the human brain, where salmon interpret sensory inputs, make decisions, and control their movements.

Otoliths are bony components — more like small rocks/stones — that sit in a gel-filled cavity inside a salmon’s head. Salmon have six otoliths which — much like a human’s inner ear — provide a sense of balance as well as pick up sound waves. What is particularly fascinating about otoliths is that they store information about a salmon’s past, much like the rings of a tree trunk. Rings of growth are laid down in a salmon’s otoliths at regular intervals, and their size, appearance and chemical composition tell scientists a great deal about that salmon’s life, including:

  1. How old the salmon is
  2. What it ate
  3. When it migrated from fresh to salt water
  4. What water temperatures were like throughout its lifetime.

Lateral line (discussed under “External Anatomy” because it is both external and internal.)


Ovary (in the female) and Testes (in the male)
The ovary produces eggs in the female. When the eggs are ‘ripe’, hens excrete them through the vent. The testes produce milt (sperm and suspending fluids) which the male excretes through the vent. Interestingly, these organs are virtually invisible in salmon during the ocean phase, and only becomes apparent when they approach spawning.

Consider this:
A salmon’s body is denser than water. This means it “should” sink in the water.  Does it? The answer is that almost all salmon sink when they die.

Why do some dead salmon float “belly up?” Some salmon actually float again after a few days, and here’s why: bacterial decomposition starts working on the carcass immediately after they die, and the byproduct of that bacteria is gas. The abdominal cavity (the salmon’s belly, not the swim bladder) gradually fills with gas, and the fish floats belly up. As the fish continues to decompose, gas eventually accumulates in the tissue, so it’s possible for dead fish to float even after they’ve “popped” (the belly is punctured). Gross, huh?


Swim bladder
While salmon control their upright position in the water using their paired fins, salmon maintain their buoyancy (their floating depth in the water) by controlling their gas-filled swim bladder. It allows them to effortlessly float at a depth of their choosing. When they swim deeper, the pressure squeezes this bladder. The swim bladder is connected to their esophagus, and salmon can reduce it by burping, or increase it by gulping air at the surface.  

Nervous System

Spine and spinal cord
Just like in humans, the spine of a salmon is a skeletal structure that contains and protects the spinal cord, which are a bundle of nerves that run from that brain to the rest of that body. The spine is made up of bones called vertebrae, therefore the spine is also referred to as the “vertebral column.” ASIDE BOX: the spinal cord is the defining characteristic of vertebrates, as opposed to invertebrates, such as jellyfish or squid or insects (some of which you will introduced to in fascinating detail in Lesson 11 – The Aquatic Ecosystem). 

Learning Gallery: Dissection School!

Recommended grade level: ____?



Learning Activity: Apples to Oranges, Humans to Salmon

With your new understanding of salmon anatomy, take some time to think about what physical features humans and salmon have in common — and how they differ. Think both internal and external — and don’t be afraid to think “outside the box.” Be logical but allow yourself to think creatively as you evaluate the questions. We may live in different worlds, but we share similar needs. For example, we both balance ourselves upright as we move, but accomplish it in different ways.  

Open to a new page in your Salmon Journal and title it:

Lesson 5: How Our Anatomies Make Us Different and Similar

Below that, write & underline:

1. Humans and salmon have body parts in common:

Write down external and internal body parts that we have in common with salmon.

Next, write & underline:

2. Humans and salmon actually do things in common:

Write down things that both humans and salmon do in their everyday lives. With each one, include how we may differ in how we accomplish it, and whether we use differing body parts to do so.

Next, write & underline:

3. Five interesting ways in which humans and salmon are anatomically different:

Then list five interesting ways that humans and salmon are anatomically different. (Try to think of ways that aren’t as obvious as others.)

Class Discussion:

  • Share your list with the class. Bonus points for originality!

Learning Activity: Take the Fish Motion Quiz


  1. Salmon use their ______ to propel forward through the water.

  2. Salmon use their ______ to control their direction.

  3. ______ 

  4. Their ______ helps salmon control buoyancy (how they float) in the water column.

  5. The ______ is likely used by salmon to help maintain stability in turbulent water. However, hatcheries use it by _____ to distinguish hatchery from naturally spawned salmon.

  6. A female salmon has ______ and a male salmon has ______.

  7. A salmon’s sense of smell is located in its _______.

  8. In your Journal, explain how a salmon uses its pectoral fins to guide direction. (Consider how a rower maneuvers a row boat.)

  9. You’re a fish on a mission. Go straight (using your ___). Take a left at the boulder using your ____. Oh no, bigger fish, let’s get out of here using your ____ to move fast, and your ___ to turn sharp left and under a log.


Learning Activity: Draw a Salmon and Label the External Anatomy

On a new page of your Salmon Journal:

  1. Draw a full-page diagram of the external anatomy of a salmon.
  2. Label all the parts you have learned.

Learning Activity: Labelling and Coloring for Young Learners

Print out our salmon diagram. Color the salmon and label the exterior anatomy parts. When you’re finished, tape it or glue it in your Salmon Journal as a fold-out.