Marlin Identification Guide
How to identify a marlin and separate it from swordfish and sailfish.
Read the full Marlin encyclopedia entry →
Marlin are large, fast billfish of the open ocean, prized for their spear-like bills and high dorsal fins.
Key identification features
- A long, round, spear-like bill (not flattened like a swordfish's).
- A pointed first dorsal fin that can fold down into a groove along the back.
- Pelvic fins present (absent in swordfish).
- A streamlined body, often with faint vertical bars or stripes that can flash when feeding.
Telling species apart
- Blue Marlin: cobalt-blue back, pointed dorsal that is lower than the body depth; rounded pectoral fins.
- Black Marlin: rigid pectoral fins that cannot fold flat against the body — a unique feature.
- Striped Marlin: prominent blue vertical stripes and a taller dorsal fin.
- White Marlin: rounded fin tips and spotted fins.
Where you'll see one
Marlin roam warm open oceans worldwide. The round spear bill, foldable pointed dorsal, and pelvic fins separate marlin from swordfish; fin shape and stripes distinguish the species.
Frequently asked questions
How is a marlin different from a swordfish?
A marlin has a round spear-like bill, a foldable pointed dorsal fin, and pelvic fins, while a swordfish has a flat blade-like bill and no pelvic fins.
How do I identify a Black Marlin?
Black Marlin have rigid pectoral fins that cannot be folded flat against the body, unlike other marlin species.