How Fast Is a Sailfish? The Fastest Fish Claim Checked

Where does the 110 km/h figure come from?

The sailfish has been called the fastest fish in the sea for about a century. The famous figure of about 110 km/h (68 mph) is usually traced to fishing tests in Florida in the early 20th century, in which a hooked sailfish was reported to have pulled about 91 m (100 yd) of line off a reel in 3 seconds.

That is not a reliable way to measure swimming speed. The line can stretch, unwind in loops, or be dragged out by a fish that is leaping and thrashing at the surface rather than swimming in a straight line. The timing was done by hand. Even so, the number was repeated in record books and lists, and some sources raised it further, to as much as 125 km/h (78 mph) for sailfish and black marlin.

What did modern studies find?

Two studies published in 2015 and 2016, by largely the same international team of researchers, tested the claim directly.

In the first, published in Integrative and Comparative Biology in 2015, researchers filmed wild sailfish attacking sardine schools with high-speed cameras and recorded their movements with accelerometers, motion sensors similar to those in a smartphone. By measuring how fast the sailfish beat their tails, they estimated that the fish burst at about 7 m per second, or 25 km/h (16 mph), and never exceeded 10 m per second, or 36 km/h (22 mph).

The second study, published in Biology Open in 2016, measured how quickly sailfish swimming muscle can contract. Each tail beat pushes a fish forward by a certain distance, so the fastest possible tail beat sets a limit on speed. The researchers estimated a maximum of about 8.3 m per second, or 30 km/h (19 mph), for the sailfish they studied.

Why can’t sailfish swim 100 km/h?

The biggest obstacle is cavitation. When water flows very fast over a fin, the pressure on the surface can drop so low that bubbles of water vapor form. When these bubbles collapse, they release tiny shock waves that can damage the fin’s tissue, the same process that wears away ship propellers.

Theoretical work predicts that cavitation would start on fish fins at around 10–15 m per second, or 36–54 km/h (22–34 mph), near the surface where sailfish hunt. A sailfish sprinting at 110 km/h would be shredding its own fins.

Muscle sets another limit. Even the fastest-contracting fish muscle can only drive a tail beat so quickly, and the measurements suggest sailfish muscle is far too slow for the legendary speeds.

How do sailfish use speed when hunting?

Sailfish do not need extreme speed to catch sardines, because they rely on stealth and their bill. A hunting sailfish approaches a school slowly, raises its sail and slips its bill close to the prey before the sardines react. It then slashes or taps with the bill, which moves far faster than the body and reaches some of the highest accelerations recorded in any aquatic animal.

Sardines are slower than sailfish in a straight line, but they turn much more sharply. Speed alone would not help a large predator catch such agile prey, which is why the bill and the sail matter so much. Bursts of a few seconds are enough to close in, and the rest of the time sailfish cruise slowly to save energy.

How do sailfish compare with other fast fish?

The 2016 study also measured muscle speeds in three other ocean predators. Sailfish came out fastest in absolute terms, but because they are so large, they were the slowest of the four when speed was compared with body length.

Marlins, the swordfish and tunas such as the Atlantic bluefin tuna are all powerful, streamlined swimmers, and some of them keep their muscles or brains warmer than the surrounding water. None of them has a reliably measured top speed either. The claims for the swordfish are examined in How fast can a swordfish swim?

So what is the fastest fish?

Nobody knows for certain. Sailfish, marlins, swordfish, tunas, wahoo and the shortfin mako shark all remain candidates, but measuring the top speed of a large fish in the open ocean is extremely difficult, and the studies done so far have recorded far lower speeds than the old claims.

What is clear is that the sailfish is a superb hunter, not a 110 km/h torpedo. For how it compares with the fastest animals on land and in the air, see the fastest animals.