Marlins and swordfish are often confused due to their similar appearances and shared ocean habitat. While both are powerful, migratory fish with elongated, pointed snouts, they are distinct species with different biological classifications, physical attributes, and behaviors.
Not the Same Species
Marlins and swordfish belong to separate biological families, highlighting their evolutionary divergence. Marlins are members of the Istiophoridae family, which includes several species of marlins, sailfish, and spearfish. In contrast, the swordfish is the sole member of its family, Xiphiidae.
Distinct Physical Characteristics
One of the most noticeable differences between marlins and swordfish lies in their “bill” or rostrum. A swordfish possesses a long, flat, and pointed bill, resembling a broadsword, which is why they are sometimes called broadbills. Marlins, on the other hand, have a rounder, more spear-like bill, which is typically shorter in proportion to their body than a swordfish’s bill.
Their dorsal fins also present clear distinguishing features. Swordfish have two distinct dorsal fins; the first is tall and crescent-shaped, while the second is much smaller and positioned further back on their body. Marlins feature a single, long dorsal fin that extends along a significant portion of their back, often appearing sail-like.
Body shape and coloration provide additional cues. Swordfish generally have a stout, rounded, or cylindrical body, often appearing black, brown, purplish, or bluish on their upper side, fading to a lighter silvery underside. Marlins tend to have more elongated, streamlined, and muscular bodies. Blue marlins, for instance, display deep blue backs with silvery-white bellies, often marked with pale, cobalt-colored stripes along their sides. Adult swordfish also lack scales and teeth, a trait not shared by marlins.
Regarding size, blue marlins can grow larger, sometimes reaching up to 16 feet in length and weighing close to 2,000 pounds. Swordfish typically grow up to 15 feet long and can weigh around 1,400 pounds.
Habitat and Behavior Differences
Swordfish primarily inhabit deeper, cooler ocean waters during the day, often descending to depths between 650 and 1,970 feet, and sometimes even deeper. They undertake daily vertical migrations, ascending to shallower, warmer surface waters at night to feed. This behavior is supported by specialized organs near their eyes that warm their brain and eyes, allowing them to hunt effectively in dimly lit, cold environments. Marlins, by contrast, generally prefer warmer surface waters, typically remaining closer to the ocean’s top layers or diving to depths of about 100 meters.
Both species are highly migratory predators that use their bills to subdue prey. Swordfish employ a slashing technique with their broadbills to stun or injure fish and squid before consuming them. Their diet includes a variety of fish, squid, and crustaceans. Marlins also use their spear-like bills to stun prey, often targeting schooling fish such as mackerel, tuna, and dolphinfish near the surface.
Swordfish migrate seasonally, moving to warmer waters in winter and cooler waters in summer, in addition to their daily vertical movements. Marlins also undertake latitudinal migrations, moving to higher latitudes during warmer periods and returning to lower latitudes when waters cool. While both are powerful open-ocean predators, swordfish are often observed individually or in loose aggregations, whereas marlins can be solitary hunters, sometimes concentrating in areas with abundant prey.