Fish
Sailfish
Istiophorus platypterus · also called sailfish, Indo-Pacific sailfish, Atlantic sailfish
The sailfish is the ocean’s poster predator — a slim, bill-nosed billfish that raises a huge dorsal sail, strobes electric-blue bars across its body, and slashes into schools of sardines alongside a dozen others of its kind. It is routinely called the fastest fish in the sea, though that claim is more legend than measurement.
Overview
Sailfish live in warm and temperate waters of every ocean, near the surface down to about 200 m, and migrate long distances with the seasons. They are carnivores, hunting sardines, anchovies, mackerel, and squid, usually in coordinated groups. They grow to about 3.5 m including the bill but stay light — built for acceleration, not endurance.
About that top speed
The famous “110 km/h” comes from one 1920s reel test and almost certainly overstates the truth. Modern camera and tag studies put sailfish bursts closer to 35 km/h — still elite, in the class of marlin and tuna, but the century-old record is a myth.
The sail
The oversized dorsal fin folds flat into a groove when the fish moves fast, so it is not for swimming. It goes up during hunts — making the fish look bigger and helping herd bait into a ball — and in displays to other sailfish, and it may help shed heat at the surface.
Colour and the group hunt
Hunting sailfish strobe blue-and-silver bars in the instant before they strike, apparently signalling their move to the others so a dozen bill-armed predators lunging at the same bait ball don’t injure each other. Each fish slashes sideways through the school; injuries pile up, and the group feeds far better than a lone sailfish could.
Status
The sailfish is Least Concern and fairly productive, but taken heavily as bycatch and by sport and artisanal fisheries, with some regional stocks overfished.
Sailfish: common questions
Is the sailfish really the fastest fish in the ocean?
It is usually named the fastest, but the famous figure is shaky. The often-quoted "68 mph" (about 110 km/h) traces back to a single 1920s experiment at a Florida fishing camp, where a hooked sailfish pulled line off a reel and the speed was estimated from how fast the line ran out — a method that overstates true swimming speed. More recent work using high-speed cameras and accelerometer tags suggests sailfish burst at more like 30 to 35 km/h, and that fin and body structure would make much higher speeds physically implausible because the water would cavitate and damage the fish. So the honest answer is — the sailfish is one of the fastest fish over a short burst, in the same league as marlin, wahoo, and tuna, but the "100 km/h" claim is a myth.
What is the sail on a sailfish for?
Not for speed — the sail is folded flat into a groove along the back whenever the fish is swimming quickly, which is a strong clue that it creates drag. It is raised in two situations. During group hunts, sailfish put their sails up as they approach a school of sardines, which appears to make the fish look bigger and helps herd the bait into a tight ball; it may also reduce the prey's ability to judge the predator's speed. Sailfish also raise the sail in aggressive and courtship displays toward each other. A third proposed function is thermoregulation — the sail is richly supplied with blood vessels and could dump or absorb heat at the surface.
How do sailfish hunt together?
Groups of a few to a dozen or more sailfish will surround a school of sardines and drive it to the surface, where the bait forms a dense ball with nowhere to go. The sailfish then take turns — one fish at a time moves in and sweeps its long bill sideways through the edge of the ball, a fast slash that strikes several sardines and leaves them stunned, scaled, or injured. The attacker and its neighbours then pick off the disabled fish. Filming has shown that individual attacks only catch a fish about a quarter of the time, but because injuries accumulate in the school, the group as a whole feeds efficiently, and each sailfish does better hunting cooperatively than it would alone.
Why does a hunting sailfish flash colours?
A sailfish can change colour extremely fast, turning its normally dark flanks into bright silver crossed with glowing blue vertical bars, and it does this most dramatically in the seconds before it slashes into a bait ball. Researchers think it is a signal to the other sailfish in the hunting group. When several predators with metre-long bills are lunging at the same tight cluster of prey, there is a real risk of striking each other. A sailfish that "lights up" just before its attack seems to be announcing its move, so the others hold back for a moment, reducing collisions and injuries among the hunters. The colour drains away again within a second or two after the strike.