A-z - Animals

How many fish are there in the world?

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As movies like Finding Nemo demonstrate, the variety and wonder of the underwater world is virtually limitless. Fish is one of the most abundant organisms on earth and is an important link in almost all food chains. They provide challenges for anglers and companionship for pet owners. Most importantly, they make a delicious meal!

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

type of fish

How many fish are there in the world 1
Fish are aquatic vertebrates, which means they have an internal backbone and live underwater.

© iStock.com/FtLaudGirl

What makes a fish a fish? First, fish are aquatic vertebrates, meaning they have an internal backbone and live underwater. Most fish are cold-blooded and have long bodies. They also have gills for breathing, fins for locomotion, and swim bladders for keeping them afloat. Beyond that, fish vary widely in size, shape, habitat, and diet.

Taxonomists generally divide fish into 3 superclasses: Chondrichthyes, Osteichthyes, and Agnatha. As the name suggests, cartilaginous fishes have skeletons made of cartilage. The skeletons of bony fishes are made of bones. Jawless or round-mouthed fishes lack jaws, including the monstrous lamprey with its sucker-like mouth.

Another way to classify fish is by water type. Saltwater fish make their home in seawater, such as the seawater in the ocean. These are also known as sea fish or ocean fish. Common habitats include coral reefs, seagrass beds, and the deep ocean.

Examples of saltwater fish are:

  • shark
  • eel
  • Certain species of bass and halibut
  • cod

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 cold water fish, cold water fish and warm water fish.

Examples of freshwater fish are:

  • catfish
  • Largemouth Bass and Smallmouth Bass
  • Vitiligo
  • rainbow trout
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Some aquatic mammals look like fish but don't actually qualify. These include whales, dolphins and porpoises.

How many species of fish are there in the world?

Part of a school of wrought iron butterflyfish on Hachijojima. The typical environment for this butterflyfish is near saltwater coral reefs.
There are more than 33,000 species of fish in the world, divided into cartilaginous, bony and jawless fishes.


There are more than 33,000 species of fish in the world. And more are being found all the time, meaning that in the future, that number could skyrocket. After all, humans have only explored about 20% of the ocean; who knows what the other 80% is!

Scientists estimate that 3.5 trillion fish live in all oceans. About half of all fish make their home in freshwater.

world's largest fish

The largest fish in the world: the whale shark
Whale sharks are the largest fish in the world.

© Onusa Putapitak/Shutterstock.com

The largest fish in the world is the whale shark. Whale sharks typically grow to about 40 feet long and weigh up to 30,000 pounds or 15 tons. Despite their name, they are not whales at all. Unlike most sharks, they don't have teeth. Instead, they are passive predators, swimming with their mouths open to catch small fish and plankton in the water. After filtering the water, the whale shark swallows its prey.

Generally, whale sharks are peaceful and docile creatures. They are known for their curiosity and willingness to interact with divers. Some divers even grab their fins and go for a ride without any problems with their owners.

The largest whale shark ever recorded measured 41.5 feet. It weighs a whopping 47,000 pounds or 23.5 tons!

world's smallest fish

light thorns
Photocorynus spiniceps is the smallest fish in the world.

© Emőke Dénes / CC BY-SA 2.5 – License

The smallest fish in the world is a species of anglerfish, Photocorynus spiniceps . Males are much smaller than females, averaging only a few millimeters. The smallest specimen in the Guinness Book of World Records is only 6.2mm long!

Males of this species are permanently attached to females, who can reach a length of 50 mm. The male then feeds on the female host for life, benefiting parasitically from the nutrients she absorbs.

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Fish Diets and Predators

Fish eat a variety of foods. Most are omnivores and will eat whatever is most convenient, including other fish. Sharks, for example, are top predators and rely primarily on the meat of other fish to survive. They may also attack large mammals such as seals or sea lions. Contrary to popular belief, humans are not their preferred food source.

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

Depending on their size, fish face a variety of predators in their habitat. Sharks, seals and sea lions all eat fish as their main diet. Predators of reptiles include crocodiles and turtles. Some smaller fish predators are barracuda, anglerfish, and tigerfish.

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

Best Aquarium Fish

Make waves for your aquarium
Fish make great pets because they are usually inexpensive and low maintenance.

©Sharjil Khan/Shutterstock.com

For starters, freshwater fish may be better choices for aquarium pets. They require less maintenance and are generally easier to care for. However, those looking for a more exotic experience might consider a saltwater tank. That way, potential pet owners will have a wider variety of fish to choose from.

There are many reasons why people choose fish as pets, such as:

  • Beautiful: Fish can be amazing creatures. Aquariums come in a variety of colors and sizes and can be a source of natural beauty in your home or business. Most people find it calming to sit and watch the fish swim by.
  • 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 stress.
  • Low cost: The biggest expense associated with fish is usually the aquarium itself. After that, the cost of keeping fish is usually very low. Some rare or exotic fish can be expensive; however, most fish at your local pet store are reasonably priced. Additionally, veterinary costs are often low or non-existent.
  • Relatively Short-Term Commitment: Fish don't live very long relative to other pets, usually between 1-5 years. This is a fact of keeping fish that young children in particular may find overwhelming. However, if you're the kind of pet owner who doesn't like being locked into a 10- to 15-year commitment, a fish might be the perfect pet.
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Some of the most common aquarium fish are goldfish, guppies, bettas, angelfish, and clownfish. More exotic fish include wolf cichlids, vampire fish, and discus.

Whether you just love the smell of fish or enjoy viewing them in an aquarium, the variety you may encounter is limitless!


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More from AZ Animals

featured image

great white shark
Great white shark underwater, Gansbaai, South Africa.

© Fiona Ayerst/Shutterstock.com

about the author

Catherine Duke

I am a freelance writer with experience in both fiction and non-fiction. When I'm not writing on the page, I enjoy reading, hiking outside, and playing with my dog.

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