What is the Biggest Fish in the World?

The Largest Living Fish Eat Only Tiny Plankton

Diver photographing a Whaleshark
Jones/Shimlock-Secret Sea Visions / Getty Images

What is the biggest fish in the world? It can be a trick question if you immediately think of whales, which are mammals, not fish. Even once you eliminate mammals, there comes the question of exactly what is a fish. There are plenty of other types of large aquatic life, including giant squid.

For the purpose of this question, we'll only consider a species to be a fish if they are vertebrates that live in the water, breathe primarily with gills, are cold-blooded, and have fins and scales. Also, let's consider only living species of fish, not extinct species that might be seen in fossil beds.

Narrowing it down in that way, we eliminate most aquatic life the average person wouldn't think of as fish. And now we discover that the world's biggest fish is a gentle giant that is bigger than a school bus.

The World's Biggest Living Species of Fish

The whale shark is the world's biggest fish, and it even sets the record as the largest living nonmammalian vertebrate on land or in the air or water. How big is it? The confirmed largest whale shark was 41.5 feet long and weight about 47,000 pounds. There are unconfirmed claims of individuals five feet longer and weighing an additional 19,000 pounds. School buses are regulated to be no longer than 40 feet and generally weigh much less. The whale shark lives in tropical oceans and they have very large mouths to filter the tiny plankton that is their only food. Their mouths can open up almost five feet wide, with over 300 rows of little teeth.

The second-largest fish is the basking shark, which grows to about 26 feet, but the largest ever accurately measured was 40.3 feet long and weighed over 20,000 pounds. But it was caught in 1851, before fishing reduced the population and lifespan so that basking sharks this large are no longer seen. It also is a plankton filter feeder with a very large mouth. It is a commercially harvested fish for food, shark fin, animal feed, and shark liver oil. The basking shark lives in temperate waters rather than tropical and it is often seen close to land.

Even though their size may make them look fearsome and inshore appearances may cause alarm, there is nothing to fear. Both species are filter-feeders and feed on tiny fish and plankton. Both whale sharks and basking sharks are cartilaginous fish.

Biggest Living Bony Fish and Freshwater Fish

The other type of fish is a bony fish. The largest bony fish is the ocean sunfish, growing as large as 10 feet across its body, 14 feet across its fins, and weighing over 5000 pounds. They mostly eat jellyfish and have a beak-like mouth.

Their size is rivaled by the largest freshwater bony fish, the beluga sturgeon that is a prized source of caviar. While beluga were once recorded as being as long as 24 feet, with increased fishing they are now generally up to less than 11 feet long.