Start with what swordfish eat. In a California Current study, 299 swordfish were sampled, and the 292 stomachs that held food contained remains from 60 kinds of prey. The study used swordfish sampled from 2007 to 2014, a span of years that is part of what the result describes, so the findings belong to that period and that place. The diet consisted mainly of squid and other cephalopods, but it also included fishes from near the surface and from the deep, and jumbo squid and a squid called Gonatopsis borealis were the most important prey. Another study found that most large fish eaten by swordfish had been slashed, while small prey were eaten whole. Each study covers one place and one period, so a careful reader asks how far each result reaches before treating it as true of every swordfish everywhere. Other sources list fishes such as Atlantic mackerel, barracudinas, silver hake, redfish, herring and lanternfishes among the foods of swordfish.
The eyes are a different kind of evidence. Vision researcher Kerstin Fritsches of the University of Queensland in Australia worked on swordfish eye warming with colleagues in the United States and Sweden. A swordfish has a heat-making muscle next to its eyes that can keep its eyes and brain 10 to 15 degrees Celsius warmer than the water around it, and warm eyes can process visual information more than ten times more quickly, depending on how deep the fish dives. In the same line of work, a swordfish retina warmed to 20 degrees Celsius could detect objects moving five times as quickly as objects it can spot at 10 degrees Celsius. Such results describe the speed of response of an eye, and a test on a retina is not the same thing as watching a swordfish hunt, so the finding applies most directly to the retina that was tested.
The sword is the hardest question, and the answer rests on indirect evidence. Squid and cuttlefish eaten by swordfish often show slash marks on their bodies. Swordfish can use their sword to subdue prey so it is easier to eat, and they have no teeth and swallow food whole, which limits the size of prey they can handle. Researchers at the University of South Florida planned to compare the mechanical properties of the bill in two billfish species, blue marlin and swordfish. Taken together, the marks, the mouth and the shape of the bill point toward a tool for slashing and subduing prey, but scientists still say they think, because the evidence is the marks and the structure. The bill may be one-third of the fish's total length, and one source says it streamlines the fish for swimming as well as helping it catch prey, so the same structure may do more than one job.
Tags show where a swordfish goes, and every tagging result comes from the particular fish that were followed. Researchers tagged five swordfish in late August off the coast of Miami, and in a tagging study off Miami, swordfish spent the day in the dark waters of the twilight zone, roughly 3,000 feet deep. In a tagging study off the Azores, juvenile swordfish kept to that area, while adults tracked in the western Atlantic migrated long distances between the Grand Banks and the Caribbean. Because each study follows only certain fish, a careful reader asks how far its result reaches before applying it to every swordfish in the ocean.
Numbers also differ from source to source. As of November 2026, the largest published weight for a swordfish is 650 kilograms, about 1,400 pounds, while NOAA Fisheries (as of November 2026) says North Atlantic swordfish reach a maximum size of about 1,165 pounds. As of November 2026, the International Game Fish Association all-tackle record for a swordfish caught on rod and line is 1,182 pounds, which is a different kind of figure from the largest published weight of 650 kilograms, since one is a single catch by a person with rod and line and the other is the largest weight that has been published. Female swordfish are larger than male swordfish, and nearly all swordfish over 300 pounds are female. Behind all the numbers is an animal that lives in oceans around the world, spends the day deep in the ocean and rises toward the surface at night to feed, a hunter of the open sea that scientists are still learning to follow.
Words to know
- retina: back of eye
- evidence: proof clues
- subdue: calm down
- cephalopods: squid and kin
