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How to Tell Cabo’s Billfish Apart: Marlin, Sailfish & More

Six billfish can show up behind a boat off Cabo San Lucas, and most of them look alike when they’re lit up and moving fast. Color fools people; fin structure doesn’t, so this guide follows the marks the FAO billfish catalog and IGFA use to tell them apart.

Striped marlinStriped marlinBlue marlinBlue marlinBlack marlinBlack marlinPacific sailfishPacific sailfishShortbill spearfishShortbill spearfishSwordfishSwordfish

Quick key: check in this order

  1. Is the bill flat and broad, with no pelvic fins and a tall dorsal that never folds?Swordfish. It’s also the only one with a single big keel on each side of the tail base.
  2. Is the first dorsal a sail, far taller than the body is deep along most of its length?Pacific sailfish. Its pelvic fins are also the longest of any billfish.
  3. Is the bill barely longer than the lower jaw, with the vent well ahead of the anal fin?Shortbill spearfish. In every other billfish the vent sits just in front of the anal fin.
  4. Is the front of the dorsal as tall as the body is deep (or nearly), on a slim, flat-sided fish?Striped marlin. Even in the biggest fish that lobe is at least 90% of body depth.
  5. Dorsal clearly lower than body depth: do the pectoral fins stick out stiffly and refuse to fold flat?Black marlin. IGFA notes the pectorals are rigid in adults over about 120 lb.
  6. Low, pointed dorsal and pectorals that fold flat against the side?Blue marlin. On a small fish, rule out a young black marlin: it has a low, blunt dorsal, very short pelvic fins and a lower jaw that curves down.

Side by side

Striped marlinBlue marlinBlack marlinPacific sailfishShortbill spearfishSwordfish
First dorsal vs. body depthEqual or taller; pointedLower than body depth; pointedLowest of all; often under halfSail, far taller than bodyAbout equal, then even heightTall, crescent-shaped, doesn’t fold
Pectoral finsFold flat, even after deathFold flat, except the largest fishRigid in adults; won’t foldFold flatShort and narrowRigid and fixed
Pelvic finsAbout as long as pectoralsShorter than pectoralsVery short, rarely over 30 cmLongest of any billfishSlender, about twice pectoral lengthNone
BillLong, roundLong, stout, roundStout; lower jaw curves downLong, slenderVery short, barely past lower jawLongest; flat and broad
Bars on bodyStrongest of all; persist after deathPale cobalt; fade after deathUsually none; may show when feedingPale bars or rows of spotsFaint or noneNone
Lateral lineSingle, straight, clearly visibleLooped network; hidden in adultsSingle; obscured in big fishSingle, well visibleSingle, visible, arches over pectoralNone in adults
BodySlim, strongly compressedNot compressed; high napeHeavy shoulders; high napeSlender, fairly compressedSlim, fairly compressedRounded; adults have no scales
Vent (anus)Just ahead of anal finJust ahead of anal finNear anal finNear anal finWell ahead of anal finNear anal fin
IGFA all-tackle record494 lb (224.1 kg)1,376 lb (624.14 kg), Pacific1,560 lb (707.61 kg)221 lb (100.24 kg), Pacific110 lb 3 oz (50 kg)1,182 lb (536.15 kg)

Swipe sideways on a phone to see all six.

Species by species

Striped marlin

Striped marlin

Kajikia audax
  • First dorsal with a pointed front lobe as tall as the body is deep or taller (at least 90% in the largest fish)
  • Slim, strongly compressed body with a single, straight lateral line you can see
  • Pointed pectoral and anal fins; the pectorals fold flat, even after death
  • About 15 rows of cobalt stripes that are usually the boldest of any marlin and stay visible after death

Common mistake: A lit-up blue or black marlin can show bars too, so judge by the dorsal: on a striper it is as tall as the body is deep, on the other two it’s clearly lower.

Blue marlin

Blue marlin

Makaira nigricans
  • Heavy body that isn’t compressed, with a high nape (the back rises steeply behind the head)
  • First dorsal pointed at the front but lower than the body is deep
  • Pectorals fold flat against the side, except in the very largest fish
  • Cobalt back; any light blue or lavender bars are fainter than a striper’s and fade soon after death

Common mistake: A blue marlin under about 120 lb can be confused with a young black marlin, whose pectorals may still fold, so check for the black’s blunter, lower dorsal, very short pelvic fins and down-curved lower jaw.

Black marlin

Black marlin

Istiompax indica
  • Pectoral fins rigid and sickle-shaped in adults: they can’t be folded flat without breaking the joints
  • Lowest first dorsal of any billfish, usually under half the body depth and not pointed
  • Very short pelvic fins, almost never over 30 cm (12 in), whatever the size of the fish
  • Mass concentrated in the head and shoulders, with a high nape and a lower jaw that curves down

Common mistake: The name misleads: alive it is dark slate-blue above and silver below, and after death it turns whitish, so never call black versus blue by color; check the pectorals.

Pacific sailfish

Pacific sailfish

Istiophorus platypterus
  • First dorsal is a sail, much taller than the body is deep along most of its length
  • Slate or cobalt-blue sail scattered with black spots
  • Pelvic fins the longest of any billfish, reaching nearly to the vent
  • Long, slender bill, usually a little over twice the length of the lower jaw

Common mistake: A sailfish swimming fast keeps the sail folded down, so it can pass for a small marlin; wait for it to raise the sail, or look for the very long pelvic fins.

Shortbill spearfish

Shortbill spearfish

Tetrapturus angustirostris
  • Very short bill, under 15% of body length and barely longer than the lower jaw; IGFA says it can look broken off at the base
  • Vent set well ahead of the anal fin, about the anal fin’s height or more; in other billfish it sits just in front
  • First dorsal about as tall as the body is deep at the front, then nearly even in height to the end
  • Short, narrow pectorals; slender pelvic fins about twice as long; bars faint or absent

Common mistake: It is easily logged as a small striped marlin (FAO says it is strongly oceanic and very rarely taken by sportfishing), so on any small, slim billfish check the bill length and where the vent is.

Swordfish

Swordfish

Xiphias gladius
  • Bill the longest of any billfish, flat and broad in cross-section, not round
  • No pelvic fins at any size; adults have no scales and no teeth
  • Tall, crescent-shaped first dorsal and rigid pectorals that don’t fold
  • One large keel on each side of the tail base (marlins and sailfish have two), plus the biggest eyes of any billfish

Common mistake: At the surface you may see only a dorsal and tail sticking out, which can look like a basking marlin, so look for the flat sword and the dark brown to bronze color.

Tips for identifying them

Expect fins to be hidden. Billfish fold their pelvic fins into a groove on the belly, and filmed sailfish kept the dorsal and pelvic fins folded down while swimming and raised them only near prey (Marras et al., 2015). A fish crossing the spread at speed may show no useful fins at all, so don’t call it until the dorsal goes up: in a jump, as it slows near the boat, or alongside before release.

Don’t trust the color of a lit-up fish. Striped marlin make their stripes more contrasting right before they strike a bait ball and dim them again afterwards (Burns et al., 2024), black marlin can show light blue bars when feeding or leaping, and a blue marlin’s bars fade soon after death. Bars tell you the fish is excited. The dorsal height, the pectorals, the pelvic length and the bill tell you which species it is.

Practical advice for the boat: glare off the water flattens colors and hides how high the dorsal is, so wear polarized glasses, and if you can, watch from the side the fish is on rather than looking down into the sun. The most reliable check is the dorsal’s height against the body’s depth, so make it as the fish swims level next to the boat, not while it is twisting on the leader.

In tournaments, get the evidence in the photo or video. IGFA asks record applicants for extra photos of fins and other details on fish that are hard to identify, and the same habit protects a release in a tournament. Film the fish side-on with the dorsal raised, show whether the pectorals lie flat or stick out, and on a small, slim fish get the bill and the vent area in the shot. Those details separate a black from a blue and a spearfish from a striped marlin when the fish is no longer there to check.

Questions

Can you tell a black marlin from a blue marlin by color?

No. Despite the name, a live black marlin is dark slate-blue above and silver below, and after death it turns whitish. A blue’s bars fade soon after death as well. The reliable check is the pectoral fins: rigid in an adult black, fold flat in a blue.

Do striped marlin always show their stripes?

No. IGFA notes the stripes may or may not be prominent, but they are usually the boldest of any marlin and stay visible after death. Drone footage of striped marlin hunting sardine schools showed that they brighten their stripes just before attacking and dim them afterwards.

Is the blue marlin’s ‘chicken-wire’ lateral line a useful field mark?

Not on an adult at the boat. The FAO catalog and IGFA both say it is clear on young fish but buried in the skin of adults, and seen only once the skin or scales are removed. FAO also describes Pacific fish as having simpler loops than Atlantic ones, and taxonomists still disagree on whether they are one species or two.

Can a small black marlin fold its pectoral fins?

It can. IGFA’s identification guide says the pectorals are rigid in black marlin over about 120 lb. On a smaller fish, look at the low, blunt dorsal, the very short pelvic fins and the down-curved lower jaw before calling it a blue.

By Max VargasTop Anglers, Cabo San Lucas · international judge at big game fishing world championships · reviewed October 2026

Sources

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