Many books say a sailfish can swim 68 miles an hour. The number came from a Florida fishing trial in the early 1900s, when a hooked sailfish reportedly pulled out line in three seconds. Sailfish and black marlin were once estimated to swim faster than 30 meters a second, about 67 miles an hour, but newer work doubts these old estimates, as of November 2026.

One reason for doubt is physics. Fish may not be able to swim faster than about 10 to 15 meters a second, which is 22 to 34 miles an hour, as of November 2026. Bubbles that form on their fins can collapse and cause damage; a swordfish is estimated to reach over 62 miles an hour, but a 2007 study suggested that speed may be beyond the limits of any fish.

Muscle is another limit. A muscle can twitch only so fast. In a 2016 study, the fastest twitch took about 68.5 milliseconds, which limits the tail beat to about 7.4 beats a second. From that, the study estimated top speeds of 8.3 meters a second for sailfish, 6.2 for barracuda, 5.6 for little tunny and 4.0 for dorado; the sailfish figure is about 19 miles an hour, as of November 2026.

Other methods agree. Earlier tag studies found that blue marlin rarely exceeded 2 meters a second, and a video study of sailfish hunting sardines found an upper limit of 8.19 meters a second. Biologist Paolo Domenici said that sailfish measured at most 8 to 10 meters a second, which is 18 to 22 miles an hour, as of November 2026.

Some big numbers are not top speeds at all. As of November 2026, a tracked sailfish was logged speeding up at a rate that, kept up for two seconds, would equal 78 miles an hour. As of November 2026, bluefin tuna logged an acceleration of 3.27 g in a 2015 study. Kept up for two seconds, it would equal 144 miles an hour, a made-up figure to show how fast it could be. Both the sailfish and bluefin records come from brief bursts of acceleration, not speeds that were kept up.

The swimmers that are fast are built for it; the fastest swimmers tend to have long, streamlined bodies and powerful tails; the shortfin mako is considered the fastest shark and can reach about 43 miles an hour, as of November 2026; a blood vessel system called countercurrent exchange lets it keep its body warmer than the sea.

Other swimmers have records too. Dall's porpoise can swim up to about 34 miles an hour, and a leatherback sea turtle can swim up to about 22 miles an hour, which makes it the fastest swimming reptile, as of November 2026. Even so, a falcon in a dive is faster. In a model, a falcon's top speed in level flight was 29 meters a second, but in a vertical dive its terminal speed reached 123 meters a second, over four times faster.

Words to know

• twitch: quick squeeze
• acceleration: speeding up
• countercurrent: opposite flow
• streamlined: smooth, slim