Swordfish are fast-moving predators that can use their sharp bills to devastating effect. A fishing boat captain unfortunately got that firsthand experience while trying to catch a swordfish last Friday in Hawaii.

The man, whom Hawaiian authorities identified as 47-year-old Randy Llane, died after being impaled in the chest by a swordfish he had just speared. The accident occurred in the Honokohau small boat harbor ( map ) on the Big Island of Hawaii, when Llane jumped into the water to go after the 6-foot (1.8-meter) long fish.

It's not unheard of for a swordfish to attack an animal or an object like a boat or research submersible, says Heidi Dewar , a fisheries research biologist with NOAA Fisheries in La Jolla, California. But reports of swordfish injuring or killing people are incredibly rare, she says.

"Normally, you wouldn't see a swordfish swimming around in the harbor," Dewar says. These are open-ocean animals that are more often found over deep water.

Dewar isn't sure what this particular swordfish was doing in such shallow water when it struck Llane. "It may have been ill," the biologist says. Or the fish could have been chasing something near the surface and ended up in the harbor. "It's really hard to say."

Defensive Sideswipe

Experts also aren't sure why the swordfish struck Llane, but "it was probably a defensive behavior," says Philip Motta , a fish biologist at the University of South Florida in Tampa.

Swordfish use their bill to catch food and probably in defense as well, he explains. That bill looks like a flattened oval in cross section and it has incredibly sharp edges—similar to a metal sword. The animals swipe their heads from side to side to cut apart prey like squid and fish, says Motta. (Read about how strong a swordfish's sword really is .)

And a sideswipe is likely how the swordfish in the Hawaiian harbor struck Llane, Motta explains.

Dewar says that the average beachgoer needn't worry about running into a swordfish, since shallow water isn't their normal habitat. But if people do see one, "it's inadvisable to jump into the water with these animals," she says.