How do scientists follow a fish that spends its days in deep water, far from anyone's sight? They use satellite tags that record depth, ocean temperature and light, and that send data when the fish breaks the surface. In one study, researchers tagged five swordfish in late August off the coast of Miami, and one tag on each swordfish beams back the precise location when the fish surfaces each day. Researchers say swordfish commonly reach 10 feet, or 3 meters, in length. Each tag records depth, temperature and light, three measurements that turn a fish out of sight into a set of numbers.

What do the tags show? Swordfish tagged off Florida typically spent the day about 550 meters, a third of a mile, deep; in a tagging study off Miami, swordfish spent the day in the dark waters of the twilight zone, roughly 3,000 feet deep. These numbers come from different studies and different fish, so they do not match exactly, and neither is the last word on how deep a swordfish goes. Swordfish hunt in water as cold as three degrees Celsius, several hundred meters below the surface, and the ocean twilight zone is home to an abundance of fish and invertebrate life. Tagged swordfish rise toward the surface at night, along with the animals they feed on, and they return to deep water by day, a daily trip known as the twilight migration.

Tags also show how far swordfish go. In one tagging study, juvenile swordfish off the Azores kept to that area, while adults tracked in the western Atlantic migrated long distances between the Grand Banks and the Caribbean, and NOAA says North Atlantic swordfish migrate thousands of miles each year. Swordfish range from about 60 degrees north to 45 degrees south, and they tend to move to warmer waters in winter and cooler waters in summer. Swordfish live in oceans around the world, and the places that tags have reached cover only part of that range.

Not every question can be answered by a tag. Researchers at the University of South Florida planned to compare the mechanical properties of the bill in two billfish species, blue marlin and swordfish. Another study says swordfish can use their sword to subdue prey so it is easier to eat. Squid and cuttlefish eaten by swordfish often show slash marks on their bodies, and a study found that most large fish eaten by swordfish had been slashed, while small prey were eaten whole. In the wild, a swordfish is deep in the ocean in the day, and it swims up near the surface at night to feed. Swordfish live in oceans around the world.

Words to know

• satellite: space-based
• tagging: marking fish
• mechanical: about force