Fish Facts

Fish [fĭsh]

Cold-blooded vertebrates with gills and fins, living wholly in water.

What Is a Fish?

Anemone fish, clown fish, underwater photo

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

  • have a backbone
  • breathe oxygen, 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.

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 marine invertebrates - animals without backbones.

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.

Fish gills

How Many Kinds of Fish Are There?

Blue Tang Fish Paracanthurus Hepatus Swimming In Water

Fish have been on earth for more than 500 million years. There were fish in the water even before dinosaurs roamed the earth! Today, there are three main types 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 30,000 different species of fish in the world. That's more than all the amphibians, reptiles, birds, and mammals combined! Every year, 200 to 300 new fishes are discovered and named. It is estimated that there may be as many as 15,000 fish species yet to be identified. Fish scientists, called ichthyologists, often reclassify fish as different species as they learn more about them.


Fish come in all shapes and sizes. The smallest fish is the stout infantfish, measuring about 1/4 of an inch (8 mm) long when fully grown. Whale sharks can grow to 42 ft (16 m) in length. The heaviest bony fish is the ocean sunfish, which can weigh up to 5,000 pounds. An adult white sturgeon, the largest freshwater fish in North American, can weigh more than 400 pounds, reach more than 12 feet in length, and live over 100 years.

Diagram of fish parts

When it comes to shape, 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 an efficient shape for speeding through the water. Airplanes have a similar streamlined shape for moving through air. But not all fish have the same shape! Rays have flat, triangular bodies, while inflated puffer fish are round like spiky balloons, and eels look more like snakes. Fish are a very diverse group of animals!

Where Can Fish Be Found?

Fish underwater in a coral habitat

Fish can be found in almost every type of water habitat. They can live high up in mountain streams, in hot desert springs, in drying mud, in topical ponds, and deep in the ocean. Some fish thrive in warm waters, while others need cold water to live. The Antarctic icefish can survive in water below 0° C (32° F) because their blood contains special chemicals that keep their bodies from freezing.

Some fish, such as trout and catfish, live in freshwater, which includes lakes, rivers, ponds, and streams. Other fish, like tuna and cod, live in the saltwater of the oceans, coral reefs, and bays. And some live in both! Read about one of Idaho's special fish, the salmon, which live part of their life in freshwater and part in saltwater.

What Do Fish Eat?

Photo of freshwater carp
Freshwater carp

The diet of a fish depends on what type it is. Some fish are herbivores who eat aquatic plants and algae. Others are carnivores, who eat other fish, insects, crabs, frogs, snakes, and even squid or octopus. Some sharks are predators who hunt for prey. Many fish are omnivores, feeding on both plants and animal life.

Fish and Humans

Colorful fish from the spieces Symphysodon discus in aquarium

Fish are important to people for many reasons. They are a source of food all over the world. For some people, fishing is their job and the way they support themselves, while for others, fishing is a fun hobby. Some people keep fish as pets, and others like to admire beautiful fish while underwater or at zoos or aquariums.

Fish are an essential part of the food chain all over the earth. Everything that affects the ecology of our planet ultimately affects humans too, so maintaining fish populations is important. It is estimated that about 1,000 species of fish are currently in danger of extinction. Some fish species are threatened due to human activity. People sometimes overfish certain populations of fish, such as cod and tuna. Overfishing hurts the entire species because the remaining fish can't reproduce fast enough to preserve the population. Another threat to fish is their habitat being destroyed. Fish that depend on coral reefs can become threatened when the reefs are destroyed, and fish living in rivers can be threatened if a dam is built or the water is polluted. Fish that require shade and cold water will suffer if logging removes trees and the water becomes warmer.

Fish Facts

Fish Lifespans

whip coral goby

The Coral Reef Pygmy Goby lives only 5-6 weeks, while the Greenland Sharks have an average lifespan of 272 years! Scientists determine a fish's age in much the same way they figure out the age of a tree. Like a tree, fish are always producing new layers of growth. 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). During times of slow growth, the layers of growth in these areas are closer together and darker. In the summer when fish are growing more quickly, these layers of growth are spread out, resulting in a lighter area.

Fish Scales

fish scale

Overlapping scales help to protect fish from injuries and attacks from predators. Scales also help reduce water resistance when the fish is swimming. Often, fish scales are covered with a layer of slime which helps the fish move more quickly through the water. There are four kinds of scales. Go to the Australian Museum to learn more about the different types of fish scales and see photos.

Fish Defense

school of fish

Fish protect themselves from predators by swimming together in large groups called schools. Another way that many fish protect themselves is to use camouflage to blend in with their environment. Fish will adapt their colors to match the sand, and some even have scales that look like plants found in coral reefs. Some flatfish use camouflage to make themselves look like a rock on the ocean floor.

Fish Senses

Fish are believed to have a good sense of sight, touch, and taste. They also have a specialized sense organ called the lateral line which is similar to radar, helping them navigate and find their way in dark or muddy water.

Fossil fish

Living fossil fish, Coelacanth

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 species and other fossil fish.

Strange Fish

  • Almost all fish obtain oxygen through their gills underwater, but a few, such as the lungfish, can breathe air on land.
  • Almost all fish lay eggs, but a few, such as sharks, give birth to live young.
  • Almost all fish are cold-blooded, but a few, such as the opah, are warm-blooded.
  • Almost all fish must remain in the water at all times, but the mudskipper can use its fins to walk on land.
  • Seahorses are the only fish that swim in an upright position.
  • "Cleaner fish" help other fish by removing parasites and dead skin from their scales.
  • Flying fish propel themselves out of the water at speeds of 37 mph, reach heights of over 4 feet, and glide distances of up to 700 feet.
  • Sharks are the only fish who have eyelids. Sharks have to keep swimming all the time or they will sink. Most fish have a swim bladder that works like a flotation device to keep them buoyant, but sharks do not.

Idaho's Native Fish Species

Photo of cutthroat trout in a man's hand
Cutthroat trout

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.

Photo of cutthroat trout being released
Cutthroat trout

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 lawnmowers.

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?

Fishes in a lake

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.

Photo of a Brook trout
Brook trout

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.

Photo of a 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.

Striped catfish in school

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.

Cutthroat 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!

Photo of Cutthroat trout in the Salmon River
Cutthroat trout

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

Kokanee salmon spawning

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. Discover why fish need trees and why trees need fish.

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.