Sailfish postcard, via Boston Public Library, Print Department.
Sailfish postcard, via Boston Public Library, Print Department.

Sailfish Use Inescapable Face- Swords to Wound, Then Kill

Meet the sailfish—a predator that combines teamwork, ninja-like stealth, record-breaking speed, chameleonic colour changes, and a weapon that looks like a sword, works like a sword, and is mounted on its face.

It is surely one of the most spectacular hunters in the ocean. Thanks to a new study by Jens Krause, we now have a much better idea of its technique, and how it uses that distinctive pointed snout.

Sailfish typically grow to around 3 metres in length and are among the fastest of fish, reaching speeds of up to 68 miles per hour (110 kilometres per hour). Like their relatives, the swordfishes and marlins, their upper jaws end in a sharp, protruding bill. Many people assumed that the bills are used to attack prey, but others have claimed that they are too fragile; instead, they might help the fish to swim faster by cutting down on drag.

Krause became captivated by sailfish after watching a sequence in the classic 2001 documentary Blue Planeta sequence in the classic 2001 documentary Blue Planet, in which a hundred-strong team take out a school of smaller fish. He wanted to see these hunts for himself and in 2011, he got his wish. “I took a trip organised by Shark Diver magazine,” he says. “They claimed it was possible to observe these animals. It was, and next year, I gathered a group of scientists to film them seriously.”

By using frigate birds and pelicans as spotters, the team found several groups of hunting sailfish. They jumped in the water, and captured several hours of high-speed and high-definition video. “It’s quite scary,” says Krause. “They do come very close to you, but they’re very accurate and careful, so they never made any contact with the divers.”

The bill isn’t a piercing weapon; it’s a slashing one. Krause’s team saw that a sailfish would swim up, insert its bill within the sardines, and flick it sideways to hit one or more targets. The sardines are none the wiser. The high-speed videos revealed that fish close to the bill don’t react any differently than the ones far away. The bill, which is so obvious to us, is actually a stealth weapon! It’s so thin that it’s hard to see and barely disturbs the surrounding water, allowing the sailfish to thrust it into the sardine school without being detected.

Now, the sardines are in serious trouble. When a sailfish flicks its bill, it either gives a gentle tap that stuns an individual fish, or a violent slash that hits several at once. During a slash, the tip of the bill can cover 6 metres and turn through 575 degrees in a single second. That’s much faster than a sardine can swim, and the bill’s acceleration (among the highest of any aquatic back-boned animal) outmatches the sardine’s reflexes. It’s hard to detect and impossible to avoid.

Krause thinks that the sailfish combine these tactics in a brutal way. They start by chasing schools of fish using their legendary speed, and they erect their eponymous sails to corral their prey. Gradually, they split large schools into smaller ones.

Then, they start slashing to inflict heavy wounds. “They don’t just attack a school and remove individuals, like dolphins or sharks would,” says Krause. “They rough these fish up for many hours. They keep them pinned, go in, and hit multiple individuals over and over again. In smaller schools, virtually every fish has been injured many times. They’re slow and exhausted. That’s when the sailfish start with the tapping. The tapping is targeted harvesting of individuals that have already been roughed up,” he adds.

The sailfish also work in teams. “We’ve seen up to 40 sailfish surrounding just 50 to 100 sardines, although maybe they started with 1,000,” says Krause. They always took turns to attack, presumably to avoid injuring each other with their sharp bills. No one knows how they coordinate their movements, but it might have something to do with their ability to change their colours. When they attack, they switch from silvery to almost black, and their flanks blaze with orange spots and electric blue bars. Perhaps these are signals to other fish, which say, “I’m up now; stand back.”

Krause’s study clearly shows that the sailfish uses its bill for hunting. “There has always been anecdotal information about bill use in feeding, but as far as I know this is the first systematic investigation. Recreational fishermen that fish for marlins and sailfish often use artificial lures or natural baits towed on the surface behind the boat,” says Richard Brill from the Virginia Institute of Marine Science. “I personally have seen blue and white marlin strike lures with their bills immediately before they grab it in their mouths.”

Brill adds that fishermen often catch billfishes whose bills are broken or missing, but that still seem healthy. There is no way of knowing whether most fish with such injuries starve and die, but these catches tell us that at least some individuals can survive without their bills.

Krause thinks that their phenomenal speed might help. He’d sometimes see a lone sardine breaking off from the school and trying to flee. When that happened, the sailfish simply chased it down through sheer speed, and swallowed it. This might explain why fishermen have sometimes found whole, uninjured fish in the bellies of billfish.

Reference: Domenici, Wilson, Kurvers, Marras, Herbert-Read, Steffensen, Krause, Viblanc, Couillaud & Krause. 2014. How sailfish use their bills to capture schooling prey. Proc Roy Soc B.