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Earth's oceans are full of fast fish, but crowning the fastest fish isn't as simple as it might sound. Determining top speed of fish in the wild is challenging since both the fish and the water are moving, sometimes together and sometimes in opposite directions. There are also different metrics to compare: swimming speed versus leaps into the air, for example, or absolute speed (which favors larger fish) versus body lengths per second.
Although not all experts agree which fish is fastest, a few speedy species seem to be in a league of their own. Here is a closer look at those fish, all of whom deserve recognition for the incredible feats they perform on a regular basis — especially considering the limitations of their watery habitat, which is about 700 times denser than air at sea level.
Sailfish
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Sailfish hunt sardines off the coast of Mexico. Rodrigo Friscione / U.S. National Ocean Service / Public Domain
Widely cited as the fastest fish in the ocean, the sailfish belongs to a group of big, swift predators known as billfish. Billfish use their long bills not to spear their prey, but to slash and injure. Sailfish have been clocked at 68 miles per hour (109 kph), according to the U.S. National Ocean Service, but there's an asterisk. During speed trials at Florida's Long Key, a hooked sailfish took out 100 yards (91 meters) of fishing line in 3 seconds, according to the ReefQuest Centre for Shark Research. That's equivalent to 68 mph, but the sailfish was leaping as it fled, so that may not reflect its true swimming speed.
Recent research has also cast doubt on the reputed speed of sailfish. A 2016 study published in Biology Open, for example, measured how quickly sailfish muscles could twitch in response to electrical stimulus, then used that to calculate their top speed. The results suggest sailfish can't exceed 10 to 15 meters per second (22 to 34 mph), and as the authors added, that's also roughly the speed at which cavitation should begin to damage their fins.
Nonetheless, sailfish are still among the ocean's fastest sprinters, not to mention skillful leapers. And they also achieve impressive speeds in another way: when a sailfish slashes its bill back and forth through a school of sardines, the tip can accelerate at 130 meters per second squared, according to a 2014 study published in the Proceedings of the Royal Society B, which noted this is "one of the highest accelerations ever recorded in an aquatic vertebrate." Who needs to swim 68 miles per hour if you can do that?
Marlin
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A white marlin leaps from the water off the coast of North Carolina. Dominic Sherony / Flickr / CC BY-SA 2.0
Marlins are the most biodiverse of the billfish, with about 10 different species scattered around the planet, including the blue, black, striped, and white marlins. Some marlin species are threatened by overfishing, often becoming entangled in fishing gear meant for other species.
Like sailfish, they are large predators — some measuring 16 feet (5 meters) long and weighing more than 1,400 pounds (635 kg) — with a long rostrum used for hunting. They are also strong leapers and fast swimmers, and at least one species, the black marlin, is sometimes cited as a contender for the fastest fish on Earth. The BBC has reported, for example, that a black marlin stripped line from a reel at 120 feet per second, equating to about 80 mph (129 kph), while the ReefQuest Centre reports marlins can leap at 50 mph (80 kph). Some experts consider those speeds unlikely, but nonetheless, marlins are famously fast and powerful swimmers, as immortalized by the blue marlin in Ernest Hemingway's "The Old Man and the Sea."
Swordfish
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A swordfish swims about 2,300 feet deep off the coast of Tampa, Florida. NOAA Southeast Deep Coral Initiative and Pelagic Research Services
The third group of billfish is the swordfish, a single species and the sole member of its taxonomic family, Xiphiidae. Found in warm waters of the Atlantic, Pacific, and Indian oceans, swordfish are big, powerful swimmers and capable of incredible leaps.
Swordfish are famous for their namesake "sword," but they also share the billfish family's penchant for speed. They can reportedly swim at more than 60 mph (100 kph), although that faces doubts similar to those raised for sailfish and marlin. Swordfish are undoubtedly fast swimmers, however, even if they have been overhyped. And while their speed is largely due to strength and body shape, scientists have also discovered another factor that makes swordfish so fast: oil.
According to a 2016 study published in the Journal of Experimental Biology, MRI scans revealed a complex organ in the upper jaws of swordfish that features an oil-producing gland connected to capillaries, which "communicate with oil-excreting pores in the skin of the head." This lets a swordfish secrete oil when water moves past its head, creating what researchers suspect is a "super-hydrophobic layer" that reduces drag and helps the fish swim more efficiently to reach high speeds.
Tuna
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A yellowfin tuna swimming towards a school of mackerel. Rodrigo Friscione / Getty Images
There are 15 different species of tuna around the world, including some surprisingly large and powerful predators. Yellowfin and bigeye tuna can grow to roughly 8 feet (2.4 meters) long and weigh 440 pounds (200 kg), for example, while some bluefin tuna measure nearly 15 feet (4.6 meters) long and weigh up to 2,000 pounds (900 kg).
Tuna are strong, fast swimmers, but similar to the billfish, their top speeds are commonly inflated based on anecdotes or unreliable accounts. While some sources claim tuna can swim up to 75 mph (120 kph), research suggests that's unlikely. A 1964 study concluded yellowfin tuna can swim at about 46 mph (74 kph), and a 1989 study found the giant Atlantic bluefin tuna probably has a maximum speed of about 33 mph (53 kph). According to the aforementioned 2016 study in Biology Open, the little tunny (a common tuna species also known as bonita) may max out at about 16 mph (25 kph). Like the billfish, tunas' top speeds may be limited by the effects of cavitation on their fins.
Mako Shark
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The shortfin mako shark is commonly cited as the fastest shark alive today. Its top speed is as difficult to pinpoint as that of many other fast fish, but it has been reliably clocked at 31 mph (50 kph), according to the ReefQuest Centre for Shark Research, which also cites a claim of burst speeds up to 46 mph (74 kph). According to one account from New Zealand, in which researchers enticed a shortfin mako to chase a baited camera pulled by their boat, the shark at one point accelerated from a dead stop to cover more than 100 feet (30 meters) in only two seconds. That suggests it may have reached 68 mph (109 kph) during its sprint, although the ReefQuest Centre advises taking this lone finding with a grain of salt.
Regardless of its exact top speed, the shortfin mako deserves its reputation as a toothy torpedo. It makes a living by chasing down some of the other fastest fish in the ocean, including tunas, bonitos, mackerels, and swordfish. It's also famous for its acrobatic leaps while hunting, and in some cases has leapt into or even smashed through the boats of anglers trying to reel it in. Shortfin mako sharks are potentially dangerous to humans, although reports of attacks are relatively rare, and as with all sharks, we're far more dangerous to them overall. Due mainly to threats from fishing, both as bycatch and a target species, the shortfin mako shark is listed as endangered by the International Union for Conservation of Nature.
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