In the day, a swordfish is deep in the ocean. The ocean's twilight zone is full of fish and other animals, and it is a rich place for swordfish to find food. Swordfish follow the daily twilight migration. They are deep by day and near the surface at night, feeding on the animals that rise and fall each day. As of November 2026, swordfish have been recorded from the surface down to a depth of 2,878 meters, so the deep ocean is part of their world.

A swordfish has a special heat-making muscle next to its eyes. The muscle warms its eyes and brain. A swordfish can keep its eyes and brain 10 to 15 degrees Celsius warmer than the water around it. That is 18 to 27 degrees Fahrenheit. Why does that help? Warm eyes help a swordfish see fast-moving animals, which helps it find food. Warm eyes let a swordfish process visual information more than ten times more quickly than cold eyes would, depending on how deep it dives.

How do scientists learn about a swordfish in deep water? They follow swordfish with satellite tags. The tags record depth, ocean temperature and light, and they send data when the fish breaks the surface. Researchers tagged five swordfish in late August off the coast of Miami. Swordfish tend to move to warmer waters in winter and cooler waters in summer, and some visit the Gulf of Maine in summer.

Swordfish live in oceans around the world, and some travel very long distances. Adults tracked in the western Atlantic migrated long distances between the Grand Banks and the Caribbean. A swordfish eats fish and squid. Scientists think a swordfish uses its sword when it hunts for food. At night a swordfish swims up to feed near the surface, and in the day it goes down again into deep water.

Words to know

• twilight: dim light
• muscle: moving body part
• Celsius: heat scale