The 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. One research team asked how much that zone matters to predators. Fishermen supplied small tissue samples from their catches. The researchers analyzed the fish bits for essential amino acids to learn where the fish had been feeding. They found that the twilight zone is a major food destination for three predatory fish: bigeye tuna, yellowfin tuna and swordfish.

A different study asked what swordfish eat. In a California Current study, the diet of swordfish consisted mainly of squid and other cephalopods, but it also included fishes from near the surface and from the deep. Jumbo squid and a squid called Gonatopsis borealis were the most important prey. Other sources add that swordfish eat fishes such as Atlantic mackerel, barracudinas, silver hake, redfish, herring and lanternfishes, and that they feed at many depths, from near the surface into deep water.

Hunting in cold, dim water takes special equipment. Swordfish hunt in water as cold as three degrees Celsius, several hundred meters below the surface. A swordfish has a heat-making muscle next to its eyes that warms its eyes and brain, and it can keep them 10 to 15 degrees Celsius warmer than the water around it. Warming the eyes and brain is an energetically costly adaptation, so it costs a swordfish a lot of energy. In return, warm eyes let a swordfish process visual information more than ten times more quickly than cold eyes would, depending on how deep it dives.

As of November 2026, a swordfish lives about 9 years, and all of this fits into that life. Young swordfish hatch about 4 millimeters long and live near the surface. A female swordfish may carry from 1 million to 29 million eggs, and the eggs are fertilized outside the mother's body. Sources give slightly different ages for when swordfish can have young: NOAA says females can reproduce between 4 and 5 years old, and the Florida Museum says swordfish reach maturity at 5 to 6 years. Swordfish live in oceans around the world, deep by day and near the surface at night.

Words to know

• twilight: dim light
• amino: building block
• cephalopods: squid and kin
• fertilized: made ready