Head to head
Sailfish vs Blue Marlin
The sailfish and the blue marlin are both billfish, both blue-water predators armed with an elongated upper jaw, and both among the most sought-after game fish in the world. They are frequently confused in photographs. The quickest way to separate them is the dorsal fin: the sailfish carries an enormous sail that can stand taller than its body is deep, while the marlin's dorsal is a firm pointed crest.
| Sailfish | Blue Marlin | |
|---|---|---|
| Family | Istiophoridae (billfish) | Istiophoridae (billfish) |
| Dorsal fin | A vast sail, often taller than the body is deep | A firm, pointed crest, not sail-like |
| Size | Up to about 3 m and 60 kg | Females to 4 to 5 m and 500 kg or more |
| Bill | Long, slender, and round | Heavier, round, spike-like |
| Behaviour | Often hunts in loose groups around bait balls | Solitary, roaming the open ocean |
| Warm-bodied | Warms its eyes and brain | Warms its eyes and brain |
| Reputation | Billed as the fastest fish (figures disputed) | The largest and among the strongest game fish |
| Conservation | Vulnerable in places, generally more resilient | Vulnerable, slow to mature, heavily fished |
Sailfish vs Marlin: common questions
How do you tell a sailfish from a marlin?
The dorsal fin is the giveaway. A sailfish has a spectacular, membranous sail running most of the length of its back, far taller than the fin of any marlin, usually a deep cobalt-blue with dark spots. A marlin has a rigid, pointed dorsal fin that is tall at the front and tapers back, never forming a sail. Sailfish are also more slender, have longer pelvic fins, and rarely exceed 3 metres, while a big blue marlin is a much bulkier, heavier fish. Body colour and the vertical bars that both can flash when excited are not reliable, since both do it.
Is the sailfish really the fastest fish in the world?
That is the traditional claim, based on an old measurement of a hooked sailfish said to have pulled line off a reel at around 110 km/h. That figure has been repeated for decades but is not well supported. More recent research using high-speed cameras and accelerometers on wild billfish, including sailfish and marlin, has recorded burst speeds during hunting of only around 30 to 35 km/h, with the bill sweep itself moving much faster than the fish. Billfish are undoubtedly fast and accelerate hard, but the headline speed is almost certainly an overstatement, and no fish has been reliably clocked near 100 km/h.
What is the sail actually for?
Several uses have been proposed and more than one is probably true. When a sailfish approaches a school of small fish it raises the sail, which makes it look far larger and appears to help corral and confuse the prey so the fish can pick off individuals, sometimes cooperating with other sailfish. The sail may also work as a thermal panel, letting the fish warm up quickly at the surface after time in cooler water. It is kept folded flat in a groove along the back during fast swimming to cut drag, and raised for hunting, courtship, and threat displays.