How Many Fishes Are In The World?

Written by Kathryn Dueck
Published: July 5, 2022
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As movies like Finding Nemo demonstrate, there is almost no limit to the variety and wonder of the aquatic world. Fish are among the most populous creatures on the earth and are a vital link in almost every food chain. They provide anglers with a challenge and pet owners with companionship. On top of that, they make a tasty meal!

Find out how many fish are in the world and more as we explore these incredibly diverse creatures.

Types of Fish

Fish are aquatic vertebrates, which means they have an internal backbone and live underwater.


What makes a fish a fish? First of all, fish are aquatic vertebrates, which means they have an internal backbone and live underwater. Most fish are cold-blooded and long in the body. They also have gills for respiration, fins for movement, and swim bladders to keep them afloat. Aside from this, fish vary greatly in size, shape, habitat, and diet.

Taxonomists typically group fish into 3 superclasses: cartilaginous fishes (Chondrichthyes), bony fishes (Osteichthyes), and jawless fishes (Agnatha). Cartilaginous fishes, as the name suggests, have skeletons made of cartilage. Bony fishes have skeletons made of bone. Jawless fishes, or roundmouth fishes, have no jaws and include the eerie-looking lampreys with their suction-cup mouths.

Another way to classify fish is by water type. Saltwater fish make their homes in seawater such as that found in oceans and seas. These are also called marine fish or sea fish. Common habitats include reefs, seagrass beds, and the deep sea.

Examples of saltwater fish are:

Freshwater fish live in water with a salinity of 1.05% or less. This includes most lakes, rivers, and ponds. They can be further divided into coldwater, coolwater, and warmwater fish.

Examples of freshwater fish are:

Some aquatic mammals look like fish but don’t actually qualify. These include whales, dolphins, and porpoises.

How Many Fishes Are in the World?

Part of a school of Wrought Iron Butterflyfish in Hachijojima Island. The typical environment of this species of butterfly fish is near saltwater coral reefs.

There are over 33,000 species of fish in the world, divided into cartilaginous fishes, bony fishes, and jawless fishes.


Over 33,000 fish species exist in the world. And more are being discovered all the time, which means that in the future, that number could skyrocket. After all, humans have only explored about 20% of the ocean; who knows what resides in the remaining 80%!

Scientists estimate that 3.5 trillion fish live in all the oceans combined. About half of all fish species make their homes in freshwater.

The World’s Largest Fish

Biggest Fish in the World: Whale Shark

Whale sharks are the biggest fish in the world.

©Onusa Putapitak/

The largest species of fish in the world is the whale shark. Whale sharks typically grow to about 40 feet in length, with a weight of 30,000 pounds or 15 tons. Despite their name, they are not whales at all. Unlike most sharks, they don’t use their teeth. Instead, they are passive feeders who swim with their mouths open to snare small fish and plankton in the water. After filtering out the water, whale sharks then swallow their prey.

Generally, whale sharks are peaceful and docile creatures. They are known for their curiosity and willingness to interact with divers. Some divers have even held onto their fins and gone for a ride with no problems from their host.

The largest whale shark ever measured stretched a length of 41.5 feet. It weighed in at a whopping 47,000 pounds or 23.5 tons!

The World’s Smallest Fish

Photocorynus spiniceps
Photocorynus spiniceps

is the smallest fish in the world.

©Emőke Dénes / CC BY-SA 2.5 – License

The world’s smallest fish is a type of anglerfish, Photocorynus spiniceps. The males are much smaller than the females, only measuring a few millimeters on average. The Guinness Book of World Records lists the smallest specimen as only 6.2 millimeters in length!

The males of this species attach themselves permanently to a female, who can grow up to 50 millimeters in length. The male then feeds off the female host its whole life, parasitically benefiting from the nutrients she imbibes.

Fish Diet and Predators

Fish eat a diverse range of foods. Most are omnivores and will eat whatever is most convenient, including other fish. Sharks, for example, are apex predators and rely mostly on the flesh of other fish to survive. They may also attack large mammals like seals or sea lions. Contrary to popular belief, humans are not their preferred source of sustenance.

Some fish are filter feeders, which means they have adapted to filter out small fish and other organisms from water. These fish commonly ingest plankton and krill.

Depending on their size, fish face various predators in their habitats. Sharks, seals, and sea lions all eat fish as the main part of their diet. Reptilian predators include crocodiles and turtles. Some smaller fish predators are barracuda, anglerfish, and tigerfish.

Humans, of course, catch and eat fish all over the world. Some of the best-tasting fish are catfish, largemouth bass, and salmon. Anglers also fish for sport, hoping to catch the next biggest fish. Commercial fishing has depleted fish stocks worldwide, leading to increasing government regulations in an attempt to allow populations to replenish themselves.

Best Aquarium Fish

Wave maker for your aquarium

Fish make great pets as they are typically low-cost and low-maintenance.

©Sharjil Khan/

For beginners, freshwater fish are likely the better option for an aquarium pet. They require less maintenance and are generally easier to care for. However, those wanting a more exotic experience might want to consider a saltwater tank. This way, potential pet owners will have more varieties of fish to choose from.

There are a number of reasons people might choose a fish as a pet, such as:

  • Beauty: Fish can be stunning creatures. With a wide array of colors and body shapes, an aquarium can be a source of natural beauty in a home or business. Most people find it very calming to sit and watch fish swim.
  • Convenience: Fish are relatively easy to care for. They require far less attention and care than other pets such as dogs or cats. They can be a great way to teach young children responsibility without adding too much pressure.
  • Low cost: The biggest fish-related expense is usually the aquarium itself. After that, the cost of maintaining a fish is typically very low. Some rare or exotic fish can be expensive; however, most fish at your local pet store are reasonably priced. Also, vet bills tend to be low or nonexistent.
  • Relatively short-term commitment: Fish don’t live very long relative to other pets, usually between 1-5 years. This is a fact of fish ownership that young children, in particular, may find difficult to process. However, if you are the type of pet owner who prefers not to be locked into a 10 to 15-year commitment, a fish may be the perfect pet.

Some of the most common aquarium fish are goldfish, guppies, betta fish, angelfish, and clownfish. More exotic fish include the wolf cichlid, vampire tetra, and discus.

Whether you like the taste of fish or just like watching them in an aquarium, there are no limits to the variety you might encounter!

The photo featured at the top of this post is © Fiona Ayerst/

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About the Author

Kathryn Dueck is a writer at A-Z Animals where her primary focus is on wildlife, dogs, and geography. Kathryn holds a Bachelor’s Degree in Biblical and Theological Studies, which she earned in 2023. In addition to volunteering at an animal shelter, Kathryn has worked for several months as a trainee dog groomer. A resident of Manitoba, Canada, Kathryn loves playing with her dog, writing fiction, and hiking.

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