Why Do Sailfish Have a Sail? Herding and Hunting Sardines

What is a sailfish’s sail?

The sail is the sailfish’s first dorsal fin, greatly enlarged. It is supported by 42–49 slender rays and runs along most of the length of the back, with the tallest rays near the middle. When raised, it is much taller than the fish’s body is deep. It is dark blue to black and speckled with small black spots.

Other billfish, such as marlins and spearfishes, have a tall front lobe on the dorsal fin, but none has a fin anything like the sailfish’s sail. The sail is not stiff like a shark’s fin. It is a thin membrane stretched between rays, rather like a folding fan, and the fish can raise it, lower it or hold it halfway.

When do sailfish raise the sail?

Most of the time the sail is kept folded down into a groove along the back. With the sail and the long pelvic fins tucked away, the sailfish becomes a smooth, streamlined shape that slips through the water with little drag.

The sail goes up mainly when the fish is hunting, excited or alarmed. Divers and underwater filmmakers often see sailfish circling a school of baitfish with their sails fully raised. Spawning females have also been seen swimming slowly near the surface with the sail above the water, followed by males.

How does the sail help sailfish hunt?

The best evidence comes from studies of Atlantic sailfish hunting schools of sardines off Mexico’s Yucatán Peninsula, filmed with high-speed cameras. They suggest the sail helps in several ways.

• Steadiness. When a sailfish swings its bill, the rest of the body tends to wobble. Spreading the sail and the other fins increases resistance to sideways movement, which seems to keep the head steady. A steadier head makes the bill harder for the sardines to notice as it creeps closer.
• Turning and braking. Raised fins probably act like control surfaces, helping a large fish turn sharply and slow down around agile prey.
• Looking bigger. A raised sail makes a sailfish appear much larger from the side. Many observers think this helps groups of sailfish keep sardines bunched together, although this has not been measured directly.

The sail does not catch fish by itself. The actual capture is done with the bill.

Hunting sardines as a group

Sailfish often hunt in groups that surround a school of sardines. Rather than charging in together, they take turns: one sailfish attacks, then pulls back while another moves in. A 2016 study called this proto-cooperation, a simple form of teamwork that needs no planning between individuals.

Only about 24% of attacks end with a sardine being caught. However, multiple sardines are injured in about 95% of attacks, losing scales and skin to the bill. Injured sardines swim more slowly and drift toward the back of the school, where they are easier targets. Every sailfish in the group therefore benefits from the others’ attacks. A mathematical model in the same study suggested that groups of up to about 70 sailfish gain from hunting this way.

There is a further twist. Individual sailfish tend to slash consistently to the left or to the right, and the tiny teeth on their bills show more wear on one side. Fish with a strong preference catch more prey, but on their own they would become predictable. In a group with a mix of left- and right-handed hunters, the sardines cannot predict which way the next attack will come from.

The bill: tapping and slashing

Once a sailfish’s bill is close to or inside the school, it either taps an individual sardine or slashes sideways through the school with a powerful sweep of the head. The researchers who filmed these attacks found that the bill reaches some of the highest accelerations ever recorded in an aquatic vertebrate.

This explains why sailfish do not need extreme speed to feed. Their approach is slow and stealthy, and the deciding moment is the flick of the bill, not a high-speed chase. The real top speed of sailfish is discussed in How fast is a sailfish?

Color changes and signals

Sailfish can change color within seconds, flashing bright blue stripes and dark bars along their sides as they hunt. Together with the raised sail, these color changes make a hunting sailfish look very different from a cruising one.

Scientists think the displays may help members of a hunting group keep track of one another, or may confuse the prey. The way females hold the sail out of the water while spawning hints that it may also play a role in communication. These ideas are reasonable, but none has been confirmed by experiments.

Other ideas about the sail

Because the sail contains many blood vessels, some biologists have suggested it might help a sailfish warm up or cool down at the surface after diving or sprinting. This is possible but unproven. Sailfish already have a special heater organ that warms their eyes and brain, a feature shared with other billfish such as the swordfish.

Overall, the evidence points to the sail being mainly a hunting tool: a fin that can be spread wide for control and display when stalking a school of fish, and folded away when the sailfish needs to be fast and streamlined.