Fish: Facts

What Is a Fish?


A fish is a vertebrate, an animal with a backbone, which has adapted to life in the water. All fish:

  • have a backbone
  • breathe air, at least part time, using gills
  • are protected by scales
  • have a simple heart
  • have a streamlined body and use fins for swimming

What other animals are vertebrates? Mammals (such as cats, horses, and elephants), reptiles (such as snakes and lizards), amphibians (such as toads and frogs), and birds also have a backbone.

Common Starfish, Caswell Bay, Wales [Credit: Pam Brophy]

Not all underwater animals are fish. Whales and dolphins are mammals. A mammal is a vertebrate that produces milk to feed their young.

Jellyfish, starfish, and octopus are invertebrates — animals without a backbone.

How Do Fish Breathe?


All animals need oxygen to survive. Fish have special organs called gills. Gills are located on the sides of a fish's head. They are made of thin sheets or membranes.

After fish open their mouths and take in water, they pump it to gills. Fish absorb dissolved oxygen as water passes through gill membranes. Once oxygen is absorbed, water flows out through the gill openings.

How Many Groups of Fish Are There?


There are three main groups of fishes:

  1. Jawless fishes, like hagfishes and lampreys
  2. Cartilaginous fishes, like sharks, skates and rays
  3. Bony fishes, like tuna, eels, and trout

There are over 27,000 different species of fish in the world. That's more that all the amphibians, reptiles, birds, and mammals combined! About 200 to 300 new fishes are discovered and named a year.

Fish come in all shapes and sizes. The smallest fish is the stout infantfish (Schindleria brevipinguis), measuring about 7 mm (0.3 in) long. Whale sharks can grow to 12m (40 ft) in length. An adult Pacific sturgeon can weigh 400 pounds, reach more than 20 feet in length, and live over 100 years.

Where Can Fish Be Found?

Small Fish

Fish can be found in almost every type of underwater habitat. The Antarctic icefish can survive in water below 0° C (32° F) because their blood contains special chemicals that keep their bodies from freezing. Fish are found in hot desert springs, in dried mud in tropical ponds, and deep in the ocean.

Some live in freshwater, some in saltwater, and some in both! Read about one of Idaho's special fish, the salmon, which live part of their life in freshwater and part in saltwater.

Discover why fish need trees.

Fish Facts


Fish Shapes

In general, fish are streamlined with a pointed anterior (snout), a pointed posterior (rear), and a broad propulsive tail. This torpedo-shaped body is typical of the fastest-swimming fishes. Streamlined means that it is a efficient shape for speeding through the water. Airplanes have a similar streamlined shape for moving through air.

How long do fish live?

Some fish live for a few weeks, and some can live over 50 years.

How do you know the age of a fish?

Scientists look at growth rings on a fish's scales and/or the ringlike structures found in a fish's otoliths (small bones of the inner ear). Read more...


All about scales

Scales help protect fish. There are four kinds of scales. Go to the Australian Museum to learn more about the different types of fish scales and see photos.

Oldest living fish

The Coelacanth is a living fossil that has changed little in its 400 million years on Earth. This ancient fish gives scientists a window into what the Earth was like a long time ago. Don't miss the story of this fascinating fish! Read about other fossil fish.

Did you know?

Most fish are colorblind.

Idaho's Native Fish Species

Cutthroat Trout [Credit: Timothy Knepp, U.S. Fish and Wildlife Service]

Of all the thousands of different kinds of fish in the world, only about 100 species are found in Idaho. And of those 100, only 39 were originally native to Idaho and not brought in from somewhere else.

Some of Idaho's best-known native species are cutthroat trout — the Idaho State Fish — as well as rainbow trout, bull trout, steelhead trout, chinook salmon, kokanee salmon, sockeye salmon, whitefish, and white sturgeon.

What Is a Native Species?


We say animals are native if their species has lived in the same place for a very long time. For example, cutthroat trout have been in Idaho for over one million years, so they are definitely native to Idaho. There are 39 species of fish that are native to Idaho.

Native animals are important parts of the environment. They all do important jobs, or fill niches, in their habitat. For example, deer have an important job. They are nature's lawn mowers.

Can you think of a job that one of Idaho's native fish does? Read about salmon's job.


Check out the Idaho Fish and Game Department's Game Fish Identification site, with photos, descriptions, life histories, feeding habits and angling techniques.

Where Do Native Fish Live?

Clean Stream

Most of our native fish live in cold, clean water. Cold water is different from warm water. Cold water can retain more oxygen than warm water. All fish need oxygen to breathe, just like you do, but they get it from the water. Oxygen in water is called dissolved oxygen. Cold water can hold a lot of oxygen, so fish that live in cold water habitats usually have no problem getting all the oxygen they need. Cold water is also usually a bit cleaner and clearer.

Trout like clear water, but they don't like to see their neighbors. Trout can live pretty close to each other. They just need enough food and a “wall” between themselves and other fish. Fallen logs or rocks make nice “walls” between neighbors. Good cold water habitats need fallen logs, rocks or other plants to give fish hiding places. Usually cold water is moving. The Boise River, Salmon River and Selway River are examples of cold water habitats in Idaho.

It's important to keep our water clear and clean. Learn all about the importance of healthy streams for fish.

Check out where other native Idaho fish live and what a riffle is!

Are All of the Fish in Idaho Native?

Not anymore. A long time ago, all of the fish in Idaho were native. But when settlers moved here, they brought non-native species to our state, such as brook trout, brown trout, catfish, crappie and blue gill.

If you moved to Idaho from another state or country, you are a “non-native.” Sometimes, a plant or animal species can be a non-native too! People have moved both plants and animals to Idaho, sometimes on purpose and sometimes by mistake.

Why Did People Introduce Non-Native Fish in Idaho?

Most non-native fish were introduced into Idaho on purpose because people liked to catch them. When the settlers moved here they were used to catching certain fish, like brook trout and catfish.

When they arrived in Idaho, those species of fish weren't here. Instead of changing what kind of fish they caught, the settlers decided to bring their favorite fish here. They let them go in Idaho's rivers and lakes, and now those non-native fish live here and reproduce on their own.

Where Do Non-Native Fish Live?


Many, but not all, of the non-native fish in Idaho like warm water habitats like shallow ponds and reservoirs. Warm water usually has less oxygen in it. The warmer the water gets the harder it is for water to hold onto oxygen. Warm water usually has fish living in it that have interesting ways to get the oxygen they need.

Catfish, called brown bullheads, are a species of non-native fish that you can find living in warm water. Bullheads can breathe through their skin. They can even use their air bladders as an emergency lung by coming up to the surface of the water and gulping air. They hold the air in their air bladders then “burp” the air out. The air can pass over their gills, so they can get oxygen out of the air. Pretty amazing!

Warm water is usually murky. This is where the catfish's whiskers come in handy. Their whiskers help them find their way around and “smell” the water for food. Water warm is usually still.

Non-native fish live in cold water, too. Some non-native fish like to live in cold, clean streams and lakes. Our native fish prefer the same habitats. Usually one species of fish wins, or out-competes, the other species of fish for habitat.

Cuttroat Trout

Cutthroat trout are Idaho's state fish! They are found from the Rocky Mountains west to the Pacific Ocean and from Alaska south to New Mexico. Cutthroat may live in small mountain streams, large rivers like the Snake River, or huge desert lakes. There are even cutthroat trout that spend part of their lives in the ocean!

Some cutthroats live in very cold places like Stanley, Idaho while other cutthroats live in hot deserts. There are cutthroat trout that never get larger that six inches long. Other cutthroats may weigh more than 40 pounds. There are silvery cutthroat, golden cutthroat and cutthroat that look like rainbow trout. Some cutthroat have large spots; some have tiny spots. Still others have almost no spots at all.

There are at least 14 subspecies, or types, of cutthroat trout living on the planet. But no matter what their names, shapes, sizes, colors or lifestyles, cutthroat all have one thing in common: the bright red slash marks under their throats, which is how they got the name cutthroat.

Native Fish Have Important Jobs


Salmon are fertilizer. Salmon swim all the way to the ocean and then back to Idaho's streams, where they spawn, or lay their eggs. They die very soon after spawning, and their decaying bodies fertilize our rivers, streams, lakes and forests. Microorganisms, tiny little animals, help decompose salmon flesh. These tiny animals then become food for small fish, even baby salmon. These small fish then become food for birds and other fish. When animals like bears and otters drag salmon into the woods to eat them, they are bringing nutrients into our forests.

Salmon are the only way to move nutrients from the ocean back to our ecosystems in Idaho. All day, everyday, Idaho's rivers are carrying nutrients to the ocean. Without salmon, it would be a one-way trip for these essential elements.

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